Sunday, January 25, 2009
Multimotor Applications
Many applications require more than one motor. In some instances, one drive can supply two or more motors. When this happens, the current rating of the motors added together cannot be greater than the current rating of the drive. Other applications require multiple motors and drives. A spinning machine for producing synthetic fibers, illustrated below, is one example of a multimotor, multidrive application. Various motors run the extruder, spinning pumps, preparation rolls, godets, traversing devices and winders. One drive may supply all the spinning pump motors or all the godet motors or individual motor control for each pump or godet may be used.
Constant Horsepower Applications
Constant horsepower applications require a constant force as radius changes. A lathe, for example, starts out with a certain diameter object. As the object is cut and shaped the diameter is reduced. The cutting force must remain constant. Another example of a constant horsepower application is a winder where radius increases as material is added to a roll and decreases as material is removed
Relationship of Speed, Torque, and Horsepower
Applications, such as lathes, that are driven in a continuous circular motion are sometimes referred to as spindle drives. Horsepower will remain constant in a spindle drive application. The surface speed in feet per minute (FPM) is equal to 2π times the radius (in feet) of the material times the speed in RPM. Surface speed will remain constant as the material is shaped and the radius reduced. Torque is equal to force times radius.
Horsepower is equal to torque times speed
The drive increases the speed (RPM) of the material as the radius is reduced. If the cutting tool has cut away half of the radius, for example, the RPM must double to maintain a constant surface speed (FPM). Reducing the radius by half will cause a corresponding reduction in torque. A doubling of speed (RPM) and a reduction of torque by half cause’s horsepower to remain constant.
A smaller radius requires less torque to turn. Because torque decreases with a smaller radius, motors operating a constant horsepower application can be run above base speed. A 60 Hz motor, for example, could be run at 90 Hz when the radius is at minimum. RPM must
a decrease in torque means horsepower will remain constant
Relationship of Speed, Torque, and Horsepower
Applications, such as lathes, that are driven in a continuous circular motion are sometimes referred to as spindle drives. Horsepower will remain constant in a spindle drive application. The surface speed in feet per minute (FPM) is equal to 2π times the radius (in feet) of the material times the speed in RPM. Surface speed will remain constant as the material is shaped and the radius reduced. Torque is equal to force times radius.
Horsepower is equal to torque times speed
The drive increases the speed (RPM) of the material as the radius is reduced. If the cutting tool has cut away half of the radius, for example, the RPM must double to maintain a constant surface speed (FPM). Reducing the radius by half will cause a corresponding reduction in torque. A doubling of speed (RPM) and a reduction of torque by half cause’s horsepower to remain constant.
A smaller radius requires less torque to turn. Because torque decreases with a smaller radius, motors operating a constant horsepower application can be run above base speed. A 60 Hz motor, for example, could be run at 90 Hz when the radius is at minimum. RPM must
a decrease in torque means horsepower will remain constant
Variable Torque Applications
A variable torque load implies that torque and horsepower increase with an increase in speed. Overloads, as a rule of thumb, are not possible at lower speeds. Peak torques are typically limited to rated torque. Fans and pumps are examples of variable torque. A pump used on a chilled water system is shown below.
Variable Torque Pumps
There are several types of pumps. The most common pump is the end-suction centrifugal pump illustrated below. There are variations of the centrifugal pump. Turbine and propeller pumps are examples. This section deals with variable torque loads. The faster a centrifugal pump turns the more fluid it pumps and the more torque it requires. It should be noted that not all pumps are variable torque. Reciprocating, positive displacement pumps are constant torque.
Horsepower
Calculating horsepower for a pump application is an involved process that requires someone with a thorough knowledge of the application and pumps. The following information is for illustration only. There are three related horsepower calculations involved in pump applications: liquid, mechanical and electrical
Hydraulic Head
Hydraulic head is the difference in hydraulic pressure between two points, which actually includes elevation, pressure and velocity. An increase in pump speed would cause increases in pressure and velocity which increases the hydraulic head
Liquid Horsepower
Liquid horsepower is the hydraulic power transferred to the pumped liquid. The following formula can be used to calculate liquid energy.
Liquid Energy in ft-lb = Total Head x (Gallons x Weight)
Water weighs 8.34 pounds per gallon. If 50 gallons of water per minute were required to be moved through 100 feet of head the energy required would be 41,700 ft I lb/minutes.
100 feet x (50 gallons x 8.34) = 41,700 ft-lb/minute
If the pumps speed were increased so that 100 gallons of water were being pumped through 100 feet of head the energy would be 83,400 ft lb/minute. Twice the energy would be required. The hydraulic head would, in actuality, also increase.
100 feet x (100 gallons x 8.34) = 83,400 ft-lb/minute
The common method of expression is horsepower. One horsepower is equal to 33,000 ft-lb/minute.
Therefore, 41,700 ft-lb/minute is 1.26 HP and 83,400 ft-lb/minute is 2.53 HP.
Mechanical Horsepower
Mechanical horsepower is the horsepower input to the pump and is equal to the liquid horsepower divided by the pump’s efficiency. If the liquid horsepower is 2.53 and the pump is 75% efficient the brake horsepower is 3.4 HP.
Electrical Horsepower
Electrical horsepower is the horsepower required to run the motor driving the pump and is equal to the mechanical horsepower divided by the motor’s efficiency. If the motor is 90% efficient the electrical horsepower is 3.78 HP. It can be seen that with an increase of pump speed there is a corresponding increase in electrical horsepower
Torque, HP, and Speed
The speed on a pump is increased by increasing the AC drive frequency (F) to the motor. Torque (T) is affected by flux (Φ) and working current (IW). The drive will maintain appropriate flux by adjusting the voltage and frequency ratio dependent on speed. During acceleration, working current will increase causing a corresponding increase in torque. In this application, torque increases in proportion to the speed squared. This is due to the increase in hydraulic head as the pump works harder to pump more fluid. Horsepower increases in proportion to the speed cubed due to an increase of torque and speed. The pump cannot be operated above the rated frequency of the motor
(60 Hz) because the drive will no longer be able to provide constant flux. As a result, the motor will be unable to supply rated torque. The load’s torque requirements increase while the motor’s ability to supply torque decreases
Fans
This same principle applies to fan applications. The horsepower of a fan is determined by dividing the product of air flow (in cubic feet per minute) and pressure by the product of the constant 6356 and fan efficiency. Increasing the speed of the fan increases air flow and pressure, requiring the motor to work harder (IW increases). Torque and horsepower increase
Variable Torque Pumps
There are several types of pumps. The most common pump is the end-suction centrifugal pump illustrated below. There are variations of the centrifugal pump. Turbine and propeller pumps are examples. This section deals with variable torque loads. The faster a centrifugal pump turns the more fluid it pumps and the more torque it requires. It should be noted that not all pumps are variable torque. Reciprocating, positive displacement pumps are constant torque.
Horsepower
Calculating horsepower for a pump application is an involved process that requires someone with a thorough knowledge of the application and pumps. The following information is for illustration only. There are three related horsepower calculations involved in pump applications: liquid, mechanical and electrical
Hydraulic Head
Hydraulic head is the difference in hydraulic pressure between two points, which actually includes elevation, pressure and velocity. An increase in pump speed would cause increases in pressure and velocity which increases the hydraulic head
Liquid Horsepower
Liquid horsepower is the hydraulic power transferred to the pumped liquid. The following formula can be used to calculate liquid energy.
Liquid Energy in ft-lb = Total Head x (Gallons x Weight)
Water weighs 8.34 pounds per gallon. If 50 gallons of water per minute were required to be moved through 100 feet of head the energy required would be 41,700 ft I lb/minutes.
100 feet x (50 gallons x 8.34) = 41,700 ft-lb/minute
If the pumps speed were increased so that 100 gallons of water were being pumped through 100 feet of head the energy would be 83,400 ft lb/minute. Twice the energy would be required. The hydraulic head would, in actuality, also increase.
100 feet x (100 gallons x 8.34) = 83,400 ft-lb/minute
The common method of expression is horsepower. One horsepower is equal to 33,000 ft-lb/minute.
Therefore, 41,700 ft-lb/minute is 1.26 HP and 83,400 ft-lb/minute is 2.53 HP.
Mechanical Horsepower
Mechanical horsepower is the horsepower input to the pump and is equal to the liquid horsepower divided by the pump’s efficiency. If the liquid horsepower is 2.53 and the pump is 75% efficient the brake horsepower is 3.4 HP.
Electrical Horsepower
Electrical horsepower is the horsepower required to run the motor driving the pump and is equal to the mechanical horsepower divided by the motor’s efficiency. If the motor is 90% efficient the electrical horsepower is 3.78 HP. It can be seen that with an increase of pump speed there is a corresponding increase in electrical horsepower
Torque, HP, and Speed
The speed on a pump is increased by increasing the AC drive frequency (F) to the motor. Torque (T) is affected by flux (Φ) and working current (IW). The drive will maintain appropriate flux by adjusting the voltage and frequency ratio dependent on speed. During acceleration, working current will increase causing a corresponding increase in torque. In this application, torque increases in proportion to the speed squared. This is due to the increase in hydraulic head as the pump works harder to pump more fluid. Horsepower increases in proportion to the speed cubed due to an increase of torque and speed. The pump cannot be operated above the rated frequency of the motor
(60 Hz) because the drive will no longer be able to provide constant flux. As a result, the motor will be unable to supply rated torque. The load’s torque requirements increase while the motor’s ability to supply torque decreases
Fans
This same principle applies to fan applications. The horsepower of a fan is determined by dividing the product of air flow (in cubic feet per minute) and pressure by the product of the constant 6356 and fan efficiency. Increasing the speed of the fan increases air flow and pressure, requiring the motor to work harder (IW increases). Torque and horsepower increase
Constant Torque Applications
A constant torque load implies that the torque required to keep the load running is the same throughout the speed range. It must be remembered that constant torque refers to the motor’s ability to maintain constant flux (Φ). Torque produced will vary with the required load. Peak torques in excess of 100% can occur at any speed, including zero speed. One example of a constant torque load is a conveyor similar to the one shown below. Conveyors can be found in all sorts of applications and environments, and can take many styles and shapes.
Conveyors are made up of belts to support the load, various pulleys to support the belt, maintain tension, and change belt direction, and idlers to support the belt and load.
Motor Speed
The speed and horsepower of an application must be known when selecting a motor and drive. Given the velocity in feet per minute (FPM) of the conveyor belt, the diameter in inches of the driven pulley, and the gear ratio (G) between the motor and driven pulley, the speed of the motor can be determined. The following formula is used to calculate conveyor speed.
If, for example, the maximum desired speed of a conveyor is
750 FPM, the driven pulley is 18” in diameter, and the gear ratio between the motor and driven pulley is 4:1, the maximum speed of the motor is 638.3 RPM. It would be difficult to find a motor that would operate at exactly this speed. An AC drive can be used with an eight-pole motor (900 RPM). This would allow the conveyor to be operated at any speed between zero and the desired maximum speed of 750 FPM.
Another advantage to using AC drives on a conveyor is the ability to run different sections of the conveyor at different speeds. A bottle machine, for example, may have bottles bunched close together for filling and then spread out for labeling. Two motors and two drives would be required. One motor would run the filling section at a given speed and a second motor would run the labeling section slightly faster spreading the bottles out.
Horsepower
Calculating motor horsepower is complicated with many variables, which is beyond the scope of this course. Someone with knowledge of, and experience with conveyor operation would be required to accurately calculate the required horsepower. The horsepower required to drive a conveyor is the effective tension (Te) times the velocity (V) of the belt in feet per minute, divided by 33,000.
Effective tension (Te) is determined by several forces:
• Gravitational weight of the load
• Length and weight of belt
• Friction of material on the conveyor
• Friction of all drive components and accessories
- Pulley inertia
- Belt/chain weight
- Motor inertia
- Friction of plows- Friction of idlers
• Acceleration force when new material is added to Conveyor
If the effective tension of a conveyor were calculated to be
2000 pounds and the maximum speed is 750 FPM, then the required horsepower is 45.5.
Starting torque of a conveyor can be 1.5 to 2 times full load torque. A motor capable of driving a fully loaded conveyor may not be able to start and accelerate the conveyor up to speed.
AC drives can typically supply 1.5 times full load torque for starting. An engineer may need to choose a larger motor and drive in order to start and accelerate the conveyor
Torque, HP, and Speed
The speed on a conveyor is increased by increasing the AC drive frequency (F) to the motor. Torque (T) is affected by flux (Φ) and working current (IW). The drive will maintain constant flux by keeping the voltage and frequency ratio constant. To do this the drive increases voltage and frequency in proportion. During acceleration working current will increase, however, causing a corresponding increase in torque. Once at its new speed the working current and torque will be the same as its old speed. The conveyor cannot be operated above the rated frequency of the motor (60 Hz) without losing available torque. Since torque is proportional to (volts/Hz)2 any increase in speed will cause available torque to decrease by the square. As a result, the motor will be unable to supply rated torque. Horsepower (HP) is affected by torque and speed. There will be a corresponding increase in horsepower as speed (RPM) increases.
Conveyors are made up of belts to support the load, various pulleys to support the belt, maintain tension, and change belt direction, and idlers to support the belt and load.
Motor Speed
The speed and horsepower of an application must be known when selecting a motor and drive. Given the velocity in feet per minute (FPM) of the conveyor belt, the diameter in inches of the driven pulley, and the gear ratio (G) between the motor and driven pulley, the speed of the motor can be determined. The following formula is used to calculate conveyor speed.
If, for example, the maximum desired speed of a conveyor is
750 FPM, the driven pulley is 18” in diameter, and the gear ratio between the motor and driven pulley is 4:1, the maximum speed of the motor is 638.3 RPM. It would be difficult to find a motor that would operate at exactly this speed. An AC drive can be used with an eight-pole motor (900 RPM). This would allow the conveyor to be operated at any speed between zero and the desired maximum speed of 750 FPM.
Another advantage to using AC drives on a conveyor is the ability to run different sections of the conveyor at different speeds. A bottle machine, for example, may have bottles bunched close together for filling and then spread out for labeling. Two motors and two drives would be required. One motor would run the filling section at a given speed and a second motor would run the labeling section slightly faster spreading the bottles out.
Horsepower
Calculating motor horsepower is complicated with many variables, which is beyond the scope of this course. Someone with knowledge of, and experience with conveyor operation would be required to accurately calculate the required horsepower. The horsepower required to drive a conveyor is the effective tension (Te) times the velocity (V) of the belt in feet per minute, divided by 33,000.
Effective tension (Te) is determined by several forces:
• Gravitational weight of the load
• Length and weight of belt
• Friction of material on the conveyor
• Friction of all drive components and accessories
- Pulley inertia
- Belt/chain weight
- Motor inertia
- Friction of plows- Friction of idlers
• Acceleration force when new material is added to Conveyor
If the effective tension of a conveyor were calculated to be
2000 pounds and the maximum speed is 750 FPM, then the required horsepower is 45.5.
Starting torque of a conveyor can be 1.5 to 2 times full load torque. A motor capable of driving a fully loaded conveyor may not be able to start and accelerate the conveyor up to speed.
AC drives can typically supply 1.5 times full load torque for starting. An engineer may need to choose a larger motor and drive in order to start and accelerate the conveyor
Torque, HP, and Speed
The speed on a conveyor is increased by increasing the AC drive frequency (F) to the motor. Torque (T) is affected by flux (Φ) and working current (IW). The drive will maintain constant flux by keeping the voltage and frequency ratio constant. To do this the drive increases voltage and frequency in proportion. During acceleration working current will increase, however, causing a corresponding increase in torque. Once at its new speed the working current and torque will be the same as its old speed. The conveyor cannot be operated above the rated frequency of the motor (60 Hz) without losing available torque. Since torque is proportional to (volts/Hz)2 any increase in speed will cause available torque to decrease by the square. As a result, the motor will be unable to supply rated torque. Horsepower (HP) is affected by torque and speed. There will be a corresponding increase in horsepower as speed (RPM) increases.
Applications
When applying an AC drive and motor to an application it is necessary to know the horsepower, torque, and speed characteristics of the load. The following chart shows typical characteristics of various loads.
Loads generally fall into one of three categories:
Constant Torque
The load is essentially the same throughout the speed range. Hoisting gear and belt conveyors are examples.
Variable Torque
The load increases as speed increases. Pumps and fans are examples.
Constant Horsepower
The load decreases as speed increases. Winders and rotary cutting machines are examples.
Loads generally fall into one of three categories:
Constant Torque
The load is essentially the same throughout the speed range. Hoisting gear and belt conveyors are examples.
Variable Torque
The load increases as speed increases. Pumps and fans are examples.
Constant Horsepower
The load decreases as speed increases. Winders and rotary cutting machines are examples.
Siemens MASTERDRIVE
Siemens MASTERDRIVES provide an excellent solution for industrial applications worldwide. In addition to standard air cooled units, water cooled versions can be used in areas with high ambient temperature or where external air cooling is unavailable. MASTERDRIVES can be used for variable-speed control on motors rated from 1 to 5,000 HP. MASTERDRIVES are available for all major worldwide 3-phase supply voltages: 380-460, 500-575, and 660-690 volts. The Siemens
MASTERDRIVES can also be referred to by a model series number, 6SE70.
Versions
There are two versions of the MASTERDRIVES product: vector control (VC) and motion control (MC).
Vector Control (VC)
One mode of operation in the MASTERDRIVES is vector control (VC), which is the focus of this part of the course. In the past, the dynamic response of a DC motor was generally considered significantly better than an AC motor. An AC motor, however, is less expensive and requires less maintenance than a DC motor. Using a complex mathematical motor model and proprietary internal computer algorithms vector control is able to exert the necessary control over an AC motor so that its performance is equal to that of a DC motor. Vector control, flux vector, and field orientation are terms that describe this specialized control technique of AC drives.
Vector control drives have 4-quadrant operation and control torque and speed continuously through zero speed, and can hold a motor stationary against an applied torque. Speed control is exact, even with varying loads. Speed control reaction time is ≤ 45 ms without tacho feedback, and ≤ 20 ms with tacho feedback. Maximum torque is available up to base speed. Torque control reaction time is ≤ 10 ms in torque control with feedback.
Motion Control (MC)
A second mode of operation available on the MASTERDRIVES is motion control (MC). Servo drives are designed to operate with a specific motor and are designed to achieve speed precision and fast response to a speed change. Servo applications typically have rapid start-stop cycles, require zero speed holding torque and high accelerating torque from zero speed, and are used positioning applications. In a packaging machine, for example, material may have to start and stop at various positions along a conveyor system.
AC - AC (Converter)
The terms AC - AC and DC - AC refers to hardware methods of configuring MASTERDRIVES. AC - AC in the MASTERDRIVE
VC family refers to a single drive, connected to an AC source, controlling an AC motor, an AC motor with a tacho, or Multimotor applications
DC - AC (Inverter)
The MASTERDRIVE VC can also be configured so that a common DC bus supplies power to several AC inverters.
Common DC bus systems also allow single and Multimotor combinations. This is referred to as DC-AC. An advantage to this system is that energy regenerated by one inverter can be consumed by another inverter on the same bus.
Braking Choices
The dynamics of certain loads require four-quadrant operation. Torque will always act to cause the rotor to run towards synchronous speed. If the synchronous speed is suddenly reduced, negative torque is developed in the motor. This could occur, for example when a stop command is initiated and the drive tries to slow down to bring the motor to a stop. The motor acts like a generator by converting mechanical power from the shaft into electrical power which is returned to the AC Drive. This is known as regeneration, and helps slow the motor. Braking occurs in quadrants II and IV. When equipped with an optional braking unit, Siemens MASTERDRIVEs are capable of four-quadrant operation.
One method of dealing with negative torque and the current it produces is controlled deceleration. Voltage and frequency is reduced gradually until the motor is at stop. This would be similar to slowly removing your foot from the accelerator of a car. Many applications, however, require the motor to stop quicker, and the drive must be capable of handling the excess energy produced by motor when this is done.
Rectifier Regenerative
Front End Another method of dealing with excessive regeneration is with a rectifier regenerative front end. Diodes in the converter section are replaced with SCRs and a second regenerative bridge is added. An SCR functions similarly to a diode rectifier, except that it has a gate lead, which is used to turn the SCR on. This allows the control logic to control when the converter bridge and regen bridge are turned on.
A simplified block diagram provides a clearer view of the regen process. When the motor needs motoring energy to accelerate or maintain speed against the inertia of a load, the converter bridge is turned on. When the motor is in the regenerative mode, it acts like a generator, supplying electrical energy back to the DC link. When the DC link voltage reaches a predetermined level the motoring SCRs are switched off and the regen (generating) SCRs are switched on.
This allows the excess energy to be returned to the AC line in the form of AC current.
ACTIVE FRONT END
An ACTIVE FRONT END (AFE) is another option available to control regenerative voltage. With this option the diodes in the converter bridge are replaced with IGBT modules and a Clean Power Filter. The IGBT, controlled by control logic, operates in both motoring and regenerating modes
Harmonics are created by electronic circuits, such as the nonlinear loads of adjustable speed drives. Harmonics can cause problems to connected loads. The base frequency is said to be the fundamental frequency or first harmonic. Additional harmonics that are superimposed on the fundamental frequency are usually whole number multiples of the first harmonic. The fifth harmonic of a 60 Hz power supply, for example, is 300 Hz (60 x 5).
A distinct advantage of Siemens MASTERDRIVES equipped with AFE and a Clean Power Filter is they are optimally harmonized with each other to eliminate harmonics and provide a clean power supply. In addition, the Siemens AFE allows for capacitive KVAR production which effectively compensates for other inductive loads in an industrial plant. This helps reduce the overall utility bill.
Programming and Operating Sources
Access is gained to the MASTERDRIVE VC for programming operating parameters and motion profiles from the following sources:
Operator Control Panel (OP1S)
Parameterization Unit (PMU)
Various Serial Interfaces
PC Based Software (Simovis)
PMU, OP1S, and HMI Panels
The MASTERDRIVE can be programmed and operated by the PMU, OP1S, or other SIMATIC HMI device such as the TP170A (shown), TP170B, OP27, or MP370.
Parameters, such as ramp times, minimum and maximum frequencies, and modes of operation are easily set. The changeover key (“P”) toggles the display between a parameter number and the value of the parameter. The up and down pushbuttons scroll through parameters and are used to select a parameter value, once the “P” key sets the parameter. The
OP1S has a numbered key pad for direct entry. The TP170A uses a touch-sensitive screen for control and monitoring.
Serial communication is available through RS232 or RS485 connections. The OP1S can be mounted directly on the PMU or up to 200 meters away. An additional 5 volt power supply is required for remote operation over 5 meters. The TP170A is powered from the drive and standard PROFIBUS connections
MASTERDRIVES can also be referred to by a model series number, 6SE70.
Versions
There are two versions of the MASTERDRIVES product: vector control (VC) and motion control (MC).
Vector Control (VC)
One mode of operation in the MASTERDRIVES is vector control (VC), which is the focus of this part of the course. In the past, the dynamic response of a DC motor was generally considered significantly better than an AC motor. An AC motor, however, is less expensive and requires less maintenance than a DC motor. Using a complex mathematical motor model and proprietary internal computer algorithms vector control is able to exert the necessary control over an AC motor so that its performance is equal to that of a DC motor. Vector control, flux vector, and field orientation are terms that describe this specialized control technique of AC drives.
Vector control drives have 4-quadrant operation and control torque and speed continuously through zero speed, and can hold a motor stationary against an applied torque. Speed control is exact, even with varying loads. Speed control reaction time is ≤ 45 ms without tacho feedback, and ≤ 20 ms with tacho feedback. Maximum torque is available up to base speed. Torque control reaction time is ≤ 10 ms in torque control with feedback.
Motion Control (MC)
A second mode of operation available on the MASTERDRIVES is motion control (MC). Servo drives are designed to operate with a specific motor and are designed to achieve speed precision and fast response to a speed change. Servo applications typically have rapid start-stop cycles, require zero speed holding torque and high accelerating torque from zero speed, and are used positioning applications. In a packaging machine, for example, material may have to start and stop at various positions along a conveyor system.
AC - AC (Converter)
The terms AC - AC and DC - AC refers to hardware methods of configuring MASTERDRIVES. AC - AC in the MASTERDRIVE
VC family refers to a single drive, connected to an AC source, controlling an AC motor, an AC motor with a tacho, or Multimotor applications
DC - AC (Inverter)
The MASTERDRIVE VC can also be configured so that a common DC bus supplies power to several AC inverters.
Common DC bus systems also allow single and Multimotor combinations. This is referred to as DC-AC. An advantage to this system is that energy regenerated by one inverter can be consumed by another inverter on the same bus.
Braking Choices
The dynamics of certain loads require four-quadrant operation. Torque will always act to cause the rotor to run towards synchronous speed. If the synchronous speed is suddenly reduced, negative torque is developed in the motor. This could occur, for example when a stop command is initiated and the drive tries to slow down to bring the motor to a stop. The motor acts like a generator by converting mechanical power from the shaft into electrical power which is returned to the AC Drive. This is known as regeneration, and helps slow the motor. Braking occurs in quadrants II and IV. When equipped with an optional braking unit, Siemens MASTERDRIVEs are capable of four-quadrant operation.
One method of dealing with negative torque and the current it produces is controlled deceleration. Voltage and frequency is reduced gradually until the motor is at stop. This would be similar to slowly removing your foot from the accelerator of a car. Many applications, however, require the motor to stop quicker, and the drive must be capable of handling the excess energy produced by motor when this is done.
Rectifier Regenerative
Front End Another method of dealing with excessive regeneration is with a rectifier regenerative front end. Diodes in the converter section are replaced with SCRs and a second regenerative bridge is added. An SCR functions similarly to a diode rectifier, except that it has a gate lead, which is used to turn the SCR on. This allows the control logic to control when the converter bridge and regen bridge are turned on.
A simplified block diagram provides a clearer view of the regen process. When the motor needs motoring energy to accelerate or maintain speed against the inertia of a load, the converter bridge is turned on. When the motor is in the regenerative mode, it acts like a generator, supplying electrical energy back to the DC link. When the DC link voltage reaches a predetermined level the motoring SCRs are switched off and the regen (generating) SCRs are switched on.
This allows the excess energy to be returned to the AC line in the form of AC current.
ACTIVE FRONT END
An ACTIVE FRONT END (AFE) is another option available to control regenerative voltage. With this option the diodes in the converter bridge are replaced with IGBT modules and a Clean Power Filter. The IGBT, controlled by control logic, operates in both motoring and regenerating modes
Harmonics are created by electronic circuits, such as the nonlinear loads of adjustable speed drives. Harmonics can cause problems to connected loads. The base frequency is said to be the fundamental frequency or first harmonic. Additional harmonics that are superimposed on the fundamental frequency are usually whole number multiples of the first harmonic. The fifth harmonic of a 60 Hz power supply, for example, is 300 Hz (60 x 5).
A distinct advantage of Siemens MASTERDRIVES equipped with AFE and a Clean Power Filter is they are optimally harmonized with each other to eliminate harmonics and provide a clean power supply. In addition, the Siemens AFE allows for capacitive KVAR production which effectively compensates for other inductive loads in an industrial plant. This helps reduce the overall utility bill.
Programming and Operating Sources
Access is gained to the MASTERDRIVE VC for programming operating parameters and motion profiles from the following sources:
Operator Control Panel (OP1S)
Parameterization Unit (PMU)
Various Serial Interfaces
PC Based Software (Simovis)
PMU, OP1S, and HMI Panels
The MASTERDRIVE can be programmed and operated by the PMU, OP1S, or other SIMATIC HMI device such as the TP170A (shown), TP170B, OP27, or MP370.
Parameters, such as ramp times, minimum and maximum frequencies, and modes of operation are easily set. The changeover key (“P”) toggles the display between a parameter number and the value of the parameter. The up and down pushbuttons scroll through parameters and are used to select a parameter value, once the “P” key sets the parameter. The
OP1S has a numbered key pad for direct entry. The TP170A uses a touch-sensitive screen for control and monitoring.
Serial communication is available through RS232 or RS485 connections. The OP1S can be mounted directly on the PMU or up to 200 meters away. An additional 5 volt power supply is required for remote operation over 5 meters. The TP170A is powered from the drive and standard PROFIBUS connections
Siemens MICROMASTER
Siemens offers a broad range of AC drives. In the past, AC
drives required expert set-up and commissioning to achieve
desired operation. The Siemens MICROMASTER offers “out of
the box” commissioning with auto tuning for motor calibration,
flux current control, vector control, and PID (Proportional-
Integral-Derivative) regulator loops. The MICROMASTER is
controlled by a programmable digital microprocessor and is
characterized by ease of setup and use.
Features The MICROMASTER is suitable for a variety of variable-speed applications, such as pumps, fans, and conveyor systems. The
MICROMASTER is compact and its range of voltages enable the MICROMASTER to be used all over the world.
MICROMASTER 410 The MICROMASTER 410 is available in two frame sizes (AA and AB) and covers the lower end of the performance range. It has a power rating of 1/6 HP to 1 HP. The MICROMASTER 410 features a compact design, fanless cooling, simple connections, an integrated RS485 communications interface, and easy startup.
MICROMASTER 420 The MICROMASTER 420 is available in three frame sizes (A, B, and C) with power ratings from 1/6 HP to 15 HP. Among the features of the MICROMASTER 420 are the following:
• Flux Current Control (FCC)
• Linear V/Hz Control
• Quadratic V/Hz Control
• Flying Restart
• Slip Compensation
• Automatic Restart
• PI Feedback for Process Control
• Programmable Acceleration/Deceleration
• Ramp Smoothing
• Fast Current Limit (FCL)
• Compound Braking
MICROMASTER 440 The MICROMASTER 440 is available in six frame sizes (A - F) and offers higher power ranges than the 420, with a corresponding increase in functionality. For example, the 440 has three output relays, two analog inputs, and six isolated digital inputs. The two analog inputs can also be programmed for use as digital inputs. The 440 also features Sensorless Vector Control, built-in braking chopper, 4-point ramp smoothing, and switchable parameter sets.
Design In order to understand the MICRO Master’s capabilities and some of the functions of an AC drive we will look at the 440. It is important to note; however, that some features of the MICROMASTER 440 are not available on the 410 and 420. The MICROMASTER has a modular design that allows the user configuration flexibility. The optional operator panels and PROFIBUS module can be user installed. There are six programmable digital inputs, two analog inputs that can also be used as additional digital inputs, two programmable analog output, and three programmable relay output.
Operator Panels There are two operator panels, the Basic Operator Panel (BOP) and Advanced Operator Panel (AOP). Operator panels are used for programming and drive operation (start, stop, jog, and reverse).
BOP Individual parameter settings can be made with the Basic Operator Panel. Parameter values and units are shown on a 5-digit display. One BOP can be used for several units
AOP The Advanced Operator Panel enables parameter sets to be read out or written (upload/download) to the MICROMASTER. Up to ten different parameter sets can be stored in the AOP. The AOP features a multi-line, plain text display. Several language sets are available. One AOP can control up to 31 drives.
Changing Operator Panels Changing operator panels is easy. A release button above the panel allows operator panels to be interchanged, even under power.
Parameters A parameter is a variable that is given a constant value.
Standard application parameters come preloaded, which are good for many applications. These parameters can easily be modified to meet specific needs of an application. Parameters such as ramp times, minimum and maximum frequencies, and operation modes are easily set using either the BOP or AOP.
The “P” key toggles the display between a parameter number and the value of the parameter. The up and down pushbuttons scroll through parameters and are used to set a parameter value. In the event of a failure the inverter switches off and a fault code appears in the display
Ramp Function A feature of AC drives is the ability to increase or decrease the voltage and frequency to a motor gradually. This accelerates the motor smoothly with less stress on the motor and connected load. Parameters P002, P003 and P004 are used to set a ramp function. Acceleration and deceleration are separately programmable from 0 to 650 seconds. Acceleration, for example, could be set for 10 seconds and deceleration could be set for 60 seconds.
Smoothing is a feature that can be added to the acceleration/ deceleration curve. This feature smoothes the transition between starting and finishing a ramp. Minimum and maximum speeds are set by parameters P012 and P013.
Analog Inputs The MICROMASTER 440 has two analog inputs (AIN1 and AIN2), allowing for a PID control loop function. PID control loops are used in process control to trim the speed. Examples are temperature and pressure control. Switches S1 and S2 are used to select a 0 mA to 20 mA or a 0 V to 10 V reference signal. In addition, AIN1 and AIN2 can be configured as digital inputs.
In the following example AIN1 is set up as an analog reference that controls the speed of a motor from 0 to 100%. Terminal one (1) is a +10 VDC power supply that is internal to the drive. Terminal two (2) is the return path, or ground, for the 10 Volt supply. An adjustable resistor is connected between terminals one and two. Terminal three (3) is the positive (+) analog input to the drive. Note that a jumper has been connected between terminals two (2) and four (4). An analog input
cannot be left floating (open). If an analog input will not be used it must be connected to terminal two (2). The drive can also be programmed to accept 0 to 20 mA, or 4 to 20 mA speed reference signal. These signals are typically supplied to the drive by other equipment such as a programmable logic controller (PLC).
Digital Inputs The MICROMASTER 440 has six digital inputs (DIN1 - DIN6). In addition AIN1 (DIN7) and AIN2 (DIN8) can be configured as digital inputs. Switches or contacts can be connected between the +24 VDC on terminal 9 and a digital input. Standard factory programming uses DIN1 as a Start/Stop function. DIN 2 is used for reverse, while DIN3 is a fault reset terminal. Other functions, such as preset speed and jog, can be programmed as well.
Thermistor Some motors have a built in thermistor. If a motor becomes overheated the thermistor acts to interrupt the power supply to the motor. A thermistor can be connected to terminals 14 and 15. If the motor gets to a preset temperature as measured by the thermistor, the driver will interrupt power to the motor. The motor will coast to a stop. The display will indicate a fault has occurred. Virtually any standard thermistor as installed in standard catalog motors will work. Snap-action thermostat switches will also work
Analog Outputs Analog outputs can be used to monitor output frequency, frequency set point, DC-link voltage, motor current, motor torque, and motor RPM. The MICROMASTER 440 has two analog outputs (AOUT1 and AOUT2).
Relay Output There are three programmable relay outputs (RL1, RL2, and RL3) on the MASTERDRIVE 440. Relays can be programmed to indicate various conditions such as the drive is running, a failure has occurred, converter frequency is at 0 or converter frequency is at minimum
Serial Communication The MICROMASTER 440 has an RS485 serial interface that allows communication with computers (PCs) or programmable logic controllers (PLCs). The standard RS485 protocol is called USS protocol and is programmable up to 57.6 K baud. Siemens
PROFIBUS protocol is also available. It is programmable up to 12 M baud. Contact your Siemens sales representative for information on USS and PROFIBUS protocol.
Current Limit The MICROMASTER 440 is capable of delivering up to 150% of drive rated current for 60 seconds within a period of 300 seconds or 200% of drive rated current for a period of 3 seconds within a period of 60 seconds. Sophisticated speed/ time/current dependent overload functions are used to protect the motor. The monitoring and protection functions include a drive over current fault, a motor overload fault, a calculated motor over temperature warning, and a measured motor over temperature fault (requires a device inside the motor).
Low Speed Boost We learned in a previous lesson that a relationship exists between voltage (E), frequency (F), and magnetizing flux (Φ).
We also learned that torque (T) is dependent on magnetizing flux. An increase in voltage, for example, would cause an increase in torque.
Some applications, such as a conveyor, require more torque to start and accelerate the load at low speed. Low speed boost is a feature that allows the voltage to be adjusted at low speeds.
This will increase/decrease the torque. Low speed boost can be adjusted high for applications requiring high torque at low speeds. Some applications, such as a fan, don’t require as much starting torque. Low speed boost can be adjusted low for smooth, cool, and quiet operation at low speed. An additional starting boost is available for applications requiring high starting torque.
Control Modes The MICROMASTER has four modes of operation:
Linear voltage/frequency (410, 420, 440)
Quadratic voltage/frequency (410, 420, 440)
Flux Current Control (FCC) (440)
Sensorless vector frequency control (440)
Closed loop vector control (440 with encoder option card)
Linear Voltage/Frequency The MICROMASTER can operate utilizing a standard V/Hz curve. Using a 460 VAC, 60 Hz motor as an example, constant volts per hertz is supplied to the motor at any frequency between 0 and 60 Hz. This is the simplest type of control and is suitable for general purpose applications.
Quadratic Operation A second mode of operation is referred to as a quadratic voltage/frequency curve. This mode provides a V/Hz curve that matches the torque requirements of simple fan and pump applications.
Flux Current Control Stator current (IS) is made up of active and reactive current. The reactive current component of stator current produces the rotating magnetic field. The active current produces work. Motor nameplate data is entered into the drive. The drive estimates motor magnetic flux based on the measured reactive stator current and the entered nameplate data. Proprietary internal computer algorithms attempt to keep the estimated magnetic flux constant.
If the motor nameplate information has been correctly entered and the drive properly set up, the flux current control mode will usually provide better dynamic performance than simple V/Hz control. Flux current control automatically adapts the drive output to the load. The motor is always operated at optimum efficiency. Speed remains reliably constant even under varying load conditions.
Sensorless Vector Control In the past, the dynamic response of a DC motor was generally considered significantly better than an AC motor. An AC motor, however, is less expensive and requires less maintenance than a DC motor. Using a complex mathematical motor model and proprietary internal computer algorithms vector control is able to exert the necessary control over an AC motor so that its performance is equal to that of a DC motor. Vector control, flux vector, and field orientation are terms that describe this specialized control technique of AC drives.
Vector control systems facilitate independent control of flux producing and torque producing elements in an induction motor. Sensorless vector control calculates rotor speed based on the motor model, calculated CEMF, inverter output voltage, and inverter output current. These results in improved dynamic performance compared to other control methods.
When motor speed is calculated at very low speeds, based on a small CEMF and known corrections for stator resistance, slight variations in stator resistance and other parameters will have an effect on speed calculation. This makes vector control without a tachometer impractical below a few hertz.
Siemens Sensorless vector control drives do operate smoothly to low speed. Sensorless vector control drives will produce full torque below a few hertz, and 150% or more torque at all speeds.
There are some complicated techniques used to accomplish this low speed torque with sensorless vector control. Expert setup and commissioning may be required to achieve desired operation at low speed.
Parameters for static torque, flux adaptation, slip compensation, and other concepts are complex and beyond the scope of this course.
Single-Quadrant Operation In the speed-torque chart there are four quadrants according to direction of rotation and direction of torque. A single-quadrant drive operates only in quadrants I or III (shaded area). Quadrant I is forward motoring or driving (CW). Quadrant III is reverse motoring or driving (CCW). Reverse motoring is achieved by reversing the direction of the rotating magnetic field. Motor torque is developed in the positive direction to drive the connected load at a desired speed (N). This is similar to driving a car forward on a flat surface from standstill to a desired speed. It takes more forward or motoring torque to accelerate the car from zero to the desired speed. Once the car has reached the desired speed your foot can be let off the accelerator a little. When the car comes to an incline a little more gas, controlled by the accelerator, maintains speed.
Coast-to-Stop To stop an AC motor in single-quadrant operation voltage and frequency can simply be removed and the motor allowed to coast to a stop. This is similar to putting a car in neutral, turning off the ignition and allowing the car to coast to a stop.
Controlled Deceleration Another way is to use a controlled deceleration. Voltage and frequency are reduced gradually until the motor is at stop.
This would be similar to slowly removing your foot from the accelerator of a car. The amount of time required to stop a motor depends on the inertia of the motor and connected load. The more inertia the longer it will take to stop.
DC Injection Braking The DC injection braking mode stops the rotating magnetic field and applies a constant DC voltage to the motor windings, helping stop the motor. Up to 250% of the motor’s rated current can be applied. This is similar to removing your foot from the accelerator and applying the brakes to bring the car to a stop quickly
Compound Braking Compound braking uses a combination of the controlled deceleration ramp and DC injection braking. The drive monitors bus voltage during operation and triggers compound braking when the bus exceeds a set threshold point. As the motor decelerates to a stop a DC voltage is periodically applied to the motor windings. The excess energy on the bus is dissipated in the motor windings. This is similar to alternately applying the brakes to slow a car, then allowing the mechanical inertia of the engine to slow the vehicle until the car is brought to a stop.
Four-Quadrant Operation The dynamics of certain loads may require four-quadrant operation. When equipped with an optional braking resistor the Siemens MICROMASTER is capable of four-quadrant operation. Torque will always act to cause the rotor to run towards synchronous speed. If the synchronous speed is suddenly reduced, negative torque is developed in the motor. The motor acts like a generator by converting mechanical power from the shaft into electrical power which is returned to the AC drive. This is similar to driving a car downhill. The car’s engine will act as a brake. Braking occurs in quadrants II and IV.
Pulsed Resistor Braking In order for an AC drive to operate in quadrant II or IV, a means must exist to deal with the electrical energy returned to the drive by the motor. Electrical energy returned by the motor can cause voltage in the DC link to become excessively high when added to existing supply voltage. Various drive components can be damaged by this excessive voltage. An optional braking resistor is available for the Siemens MICROMASTER. The braking resistor is connected to terminals B+ and B-. The braking resistor is added and removed from the circuit by an IGBT. Energy returned by the motor is seen on the DC link.
When the DC link reaches a predetermined limit the IGBT is switched on by the control logic. The resistor is placed across the DC link. Excess energy is dissipated by the resistor, reducing bus voltage. When DC link voltage is reduced to a safe level the IGBT is switched off, removing the resistor from the DC link. This is referred to as pulsed resistor braking.
This process allows the motor to act as a brake, slowing the connected load quickly.
Distance to Motor All motor cables have line-to-line and line-to-ground capacitance. The longer the cable, the greater the capacitance. Some types of cables, such as shielded cable or cables in metal conduit have greater capacitance. Spikes occur on the output of all PWM drives because of the charging current of the cable capacitance. Higher voltage (460 VAC) and higher capacitance (long cables) result in higher current spikes. Voltage spikes caused by long cable lengths can potentially shorten the life of the inverter and the motor.
The maximum distance between a motor and the MICROMASTER, when unshielded cable is used, is 100 meters (328 feet). If shielded cable is used, or if cable is run through a metal conduit, the maximum distance is 50 meters (164 feet). When considering an application where distance may be a problem, contact your local Siemens representative.
Enclosures The National Electrical Manufacturers Association (NEMA) has specified standards for equipment enclosures. The MICROMASTER is supplied in a protected chassis and a NEMA
Type 1 enclosure.
Ambient Temperature The MICROMASTER is rated for operation in an ambient temperature of 0 to 40° C for variable torque drives and 0 to
50°C for constant torque drives. The drive must be derated to operate at higher ambient temperatures.
Elevation The MICROMASTER is rated for operation below 1000 meters (3300 feet). At higher elevations the air is thinner, consequently the drive can’t dissipate heat as effectively and the drive must be derated. In addition, above 2000 meters (6600 feet) the supply voltage must be reduced.
drives required expert set-up and commissioning to achieve
desired operation. The Siemens MICROMASTER offers “out of
the box” commissioning with auto tuning for motor calibration,
flux current control, vector control, and PID (Proportional-
Integral-Derivative) regulator loops. The MICROMASTER is
controlled by a programmable digital microprocessor and is
characterized by ease of setup and use.
Features The MICROMASTER is suitable for a variety of variable-speed applications, such as pumps, fans, and conveyor systems. The
MICROMASTER is compact and its range of voltages enable the MICROMASTER to be used all over the world.
MICROMASTER 410 The MICROMASTER 410 is available in two frame sizes (AA and AB) and covers the lower end of the performance range. It has a power rating of 1/6 HP to 1 HP. The MICROMASTER 410 features a compact design, fanless cooling, simple connections, an integrated RS485 communications interface, and easy startup.
MICROMASTER 420 The MICROMASTER 420 is available in three frame sizes (A, B, and C) with power ratings from 1/6 HP to 15 HP. Among the features of the MICROMASTER 420 are the following:
• Flux Current Control (FCC)
• Linear V/Hz Control
• Quadratic V/Hz Control
• Flying Restart
• Slip Compensation
• Automatic Restart
• PI Feedback for Process Control
• Programmable Acceleration/Deceleration
• Ramp Smoothing
• Fast Current Limit (FCL)
• Compound Braking
MICROMASTER 440 The MICROMASTER 440 is available in six frame sizes (A - F) and offers higher power ranges than the 420, with a corresponding increase in functionality. For example, the 440 has three output relays, two analog inputs, and six isolated digital inputs. The two analog inputs can also be programmed for use as digital inputs. The 440 also features Sensorless Vector Control, built-in braking chopper, 4-point ramp smoothing, and switchable parameter sets.
Design In order to understand the MICRO Master’s capabilities and some of the functions of an AC drive we will look at the 440. It is important to note; however, that some features of the MICROMASTER 440 are not available on the 410 and 420. The MICROMASTER has a modular design that allows the user configuration flexibility. The optional operator panels and PROFIBUS module can be user installed. There are six programmable digital inputs, two analog inputs that can also be used as additional digital inputs, two programmable analog output, and three programmable relay output.
Operator Panels There are two operator panels, the Basic Operator Panel (BOP) and Advanced Operator Panel (AOP). Operator panels are used for programming and drive operation (start, stop, jog, and reverse).
BOP Individual parameter settings can be made with the Basic Operator Panel. Parameter values and units are shown on a 5-digit display. One BOP can be used for several units
AOP The Advanced Operator Panel enables parameter sets to be read out or written (upload/download) to the MICROMASTER. Up to ten different parameter sets can be stored in the AOP. The AOP features a multi-line, plain text display. Several language sets are available. One AOP can control up to 31 drives.
Changing Operator Panels Changing operator panels is easy. A release button above the panel allows operator panels to be interchanged, even under power.
Parameters A parameter is a variable that is given a constant value.
Standard application parameters come preloaded, which are good for many applications. These parameters can easily be modified to meet specific needs of an application. Parameters such as ramp times, minimum and maximum frequencies, and operation modes are easily set using either the BOP or AOP.
The “P” key toggles the display between a parameter number and the value of the parameter. The up and down pushbuttons scroll through parameters and are used to set a parameter value. In the event of a failure the inverter switches off and a fault code appears in the display
Ramp Function A feature of AC drives is the ability to increase or decrease the voltage and frequency to a motor gradually. This accelerates the motor smoothly with less stress on the motor and connected load. Parameters P002, P003 and P004 are used to set a ramp function. Acceleration and deceleration are separately programmable from 0 to 650 seconds. Acceleration, for example, could be set for 10 seconds and deceleration could be set for 60 seconds.
Smoothing is a feature that can be added to the acceleration/ deceleration curve. This feature smoothes the transition between starting and finishing a ramp. Minimum and maximum speeds are set by parameters P012 and P013.
Analog Inputs The MICROMASTER 440 has two analog inputs (AIN1 and AIN2), allowing for a PID control loop function. PID control loops are used in process control to trim the speed. Examples are temperature and pressure control. Switches S1 and S2 are used to select a 0 mA to 20 mA or a 0 V to 10 V reference signal. In addition, AIN1 and AIN2 can be configured as digital inputs.
In the following example AIN1 is set up as an analog reference that controls the speed of a motor from 0 to 100%. Terminal one (1) is a +10 VDC power supply that is internal to the drive. Terminal two (2) is the return path, or ground, for the 10 Volt supply. An adjustable resistor is connected between terminals one and two. Terminal three (3) is the positive (+) analog input to the drive. Note that a jumper has been connected between terminals two (2) and four (4). An analog input
cannot be left floating (open). If an analog input will not be used it must be connected to terminal two (2). The drive can also be programmed to accept 0 to 20 mA, or 4 to 20 mA speed reference signal. These signals are typically supplied to the drive by other equipment such as a programmable logic controller (PLC).
Digital Inputs The MICROMASTER 440 has six digital inputs (DIN1 - DIN6). In addition AIN1 (DIN7) and AIN2 (DIN8) can be configured as digital inputs. Switches or contacts can be connected between the +24 VDC on terminal 9 and a digital input. Standard factory programming uses DIN1 as a Start/Stop function. DIN 2 is used for reverse, while DIN3 is a fault reset terminal. Other functions, such as preset speed and jog, can be programmed as well.
Thermistor Some motors have a built in thermistor. If a motor becomes overheated the thermistor acts to interrupt the power supply to the motor. A thermistor can be connected to terminals 14 and 15. If the motor gets to a preset temperature as measured by the thermistor, the driver will interrupt power to the motor. The motor will coast to a stop. The display will indicate a fault has occurred. Virtually any standard thermistor as installed in standard catalog motors will work. Snap-action thermostat switches will also work
Analog Outputs Analog outputs can be used to monitor output frequency, frequency set point, DC-link voltage, motor current, motor torque, and motor RPM. The MICROMASTER 440 has two analog outputs (AOUT1 and AOUT2).
Relay Output There are three programmable relay outputs (RL1, RL2, and RL3) on the MASTERDRIVE 440. Relays can be programmed to indicate various conditions such as the drive is running, a failure has occurred, converter frequency is at 0 or converter frequency is at minimum
Serial Communication The MICROMASTER 440 has an RS485 serial interface that allows communication with computers (PCs) or programmable logic controllers (PLCs). The standard RS485 protocol is called USS protocol and is programmable up to 57.6 K baud. Siemens
PROFIBUS protocol is also available. It is programmable up to 12 M baud. Contact your Siemens sales representative for information on USS and PROFIBUS protocol.
Current Limit The MICROMASTER 440 is capable of delivering up to 150% of drive rated current for 60 seconds within a period of 300 seconds or 200% of drive rated current for a period of 3 seconds within a period of 60 seconds. Sophisticated speed/ time/current dependent overload functions are used to protect the motor. The monitoring and protection functions include a drive over current fault, a motor overload fault, a calculated motor over temperature warning, and a measured motor over temperature fault (requires a device inside the motor).
Low Speed Boost We learned in a previous lesson that a relationship exists between voltage (E), frequency (F), and magnetizing flux (Φ).
We also learned that torque (T) is dependent on magnetizing flux. An increase in voltage, for example, would cause an increase in torque.
Some applications, such as a conveyor, require more torque to start and accelerate the load at low speed. Low speed boost is a feature that allows the voltage to be adjusted at low speeds.
This will increase/decrease the torque. Low speed boost can be adjusted high for applications requiring high torque at low speeds. Some applications, such as a fan, don’t require as much starting torque. Low speed boost can be adjusted low for smooth, cool, and quiet operation at low speed. An additional starting boost is available for applications requiring high starting torque.
Control Modes The MICROMASTER has four modes of operation:
Linear voltage/frequency (410, 420, 440)
Quadratic voltage/frequency (410, 420, 440)
Flux Current Control (FCC) (440)
Sensorless vector frequency control (440)
Closed loop vector control (440 with encoder option card)
Linear Voltage/Frequency The MICROMASTER can operate utilizing a standard V/Hz curve. Using a 460 VAC, 60 Hz motor as an example, constant volts per hertz is supplied to the motor at any frequency between 0 and 60 Hz. This is the simplest type of control and is suitable for general purpose applications.
Quadratic Operation A second mode of operation is referred to as a quadratic voltage/frequency curve. This mode provides a V/Hz curve that matches the torque requirements of simple fan and pump applications.
Flux Current Control Stator current (IS) is made up of active and reactive current. The reactive current component of stator current produces the rotating magnetic field. The active current produces work. Motor nameplate data is entered into the drive. The drive estimates motor magnetic flux based on the measured reactive stator current and the entered nameplate data. Proprietary internal computer algorithms attempt to keep the estimated magnetic flux constant.
If the motor nameplate information has been correctly entered and the drive properly set up, the flux current control mode will usually provide better dynamic performance than simple V/Hz control. Flux current control automatically adapts the drive output to the load. The motor is always operated at optimum efficiency. Speed remains reliably constant even under varying load conditions.
Sensorless Vector Control In the past, the dynamic response of a DC motor was generally considered significantly better than an AC motor. An AC motor, however, is less expensive and requires less maintenance than a DC motor. Using a complex mathematical motor model and proprietary internal computer algorithms vector control is able to exert the necessary control over an AC motor so that its performance is equal to that of a DC motor. Vector control, flux vector, and field orientation are terms that describe this specialized control technique of AC drives.
Vector control systems facilitate independent control of flux producing and torque producing elements in an induction motor. Sensorless vector control calculates rotor speed based on the motor model, calculated CEMF, inverter output voltage, and inverter output current. These results in improved dynamic performance compared to other control methods.
When motor speed is calculated at very low speeds, based on a small CEMF and known corrections for stator resistance, slight variations in stator resistance and other parameters will have an effect on speed calculation. This makes vector control without a tachometer impractical below a few hertz.
Siemens Sensorless vector control drives do operate smoothly to low speed. Sensorless vector control drives will produce full torque below a few hertz, and 150% or more torque at all speeds.
There are some complicated techniques used to accomplish this low speed torque with sensorless vector control. Expert setup and commissioning may be required to achieve desired operation at low speed.
Parameters for static torque, flux adaptation, slip compensation, and other concepts are complex and beyond the scope of this course.
Single-Quadrant Operation In the speed-torque chart there are four quadrants according to direction of rotation and direction of torque. A single-quadrant drive operates only in quadrants I or III (shaded area). Quadrant I is forward motoring or driving (CW). Quadrant III is reverse motoring or driving (CCW). Reverse motoring is achieved by reversing the direction of the rotating magnetic field. Motor torque is developed in the positive direction to drive the connected load at a desired speed (N). This is similar to driving a car forward on a flat surface from standstill to a desired speed. It takes more forward or motoring torque to accelerate the car from zero to the desired speed. Once the car has reached the desired speed your foot can be let off the accelerator a little. When the car comes to an incline a little more gas, controlled by the accelerator, maintains speed.
Coast-to-Stop To stop an AC motor in single-quadrant operation voltage and frequency can simply be removed and the motor allowed to coast to a stop. This is similar to putting a car in neutral, turning off the ignition and allowing the car to coast to a stop.
Controlled Deceleration Another way is to use a controlled deceleration. Voltage and frequency are reduced gradually until the motor is at stop.
This would be similar to slowly removing your foot from the accelerator of a car. The amount of time required to stop a motor depends on the inertia of the motor and connected load. The more inertia the longer it will take to stop.
DC Injection Braking The DC injection braking mode stops the rotating magnetic field and applies a constant DC voltage to the motor windings, helping stop the motor. Up to 250% of the motor’s rated current can be applied. This is similar to removing your foot from the accelerator and applying the brakes to bring the car to a stop quickly
Compound Braking Compound braking uses a combination of the controlled deceleration ramp and DC injection braking. The drive monitors bus voltage during operation and triggers compound braking when the bus exceeds a set threshold point. As the motor decelerates to a stop a DC voltage is periodically applied to the motor windings. The excess energy on the bus is dissipated in the motor windings. This is similar to alternately applying the brakes to slow a car, then allowing the mechanical inertia of the engine to slow the vehicle until the car is brought to a stop.
Four-Quadrant Operation The dynamics of certain loads may require four-quadrant operation. When equipped with an optional braking resistor the Siemens MICROMASTER is capable of four-quadrant operation. Torque will always act to cause the rotor to run towards synchronous speed. If the synchronous speed is suddenly reduced, negative torque is developed in the motor. The motor acts like a generator by converting mechanical power from the shaft into electrical power which is returned to the AC drive. This is similar to driving a car downhill. The car’s engine will act as a brake. Braking occurs in quadrants II and IV.
Pulsed Resistor Braking In order for an AC drive to operate in quadrant II or IV, a means must exist to deal with the electrical energy returned to the drive by the motor. Electrical energy returned by the motor can cause voltage in the DC link to become excessively high when added to existing supply voltage. Various drive components can be damaged by this excessive voltage. An optional braking resistor is available for the Siemens MICROMASTER. The braking resistor is connected to terminals B+ and B-. The braking resistor is added and removed from the circuit by an IGBT. Energy returned by the motor is seen on the DC link.
When the DC link reaches a predetermined limit the IGBT is switched on by the control logic. The resistor is placed across the DC link. Excess energy is dissipated by the resistor, reducing bus voltage. When DC link voltage is reduced to a safe level the IGBT is switched off, removing the resistor from the DC link. This is referred to as pulsed resistor braking.
This process allows the motor to act as a brake, slowing the connected load quickly.
Distance to Motor All motor cables have line-to-line and line-to-ground capacitance. The longer the cable, the greater the capacitance. Some types of cables, such as shielded cable or cables in metal conduit have greater capacitance. Spikes occur on the output of all PWM drives because of the charging current of the cable capacitance. Higher voltage (460 VAC) and higher capacitance (long cables) result in higher current spikes. Voltage spikes caused by long cable lengths can potentially shorten the life of the inverter and the motor.
The maximum distance between a motor and the MICROMASTER, when unshielded cable is used, is 100 meters (328 feet). If shielded cable is used, or if cable is run through a metal conduit, the maximum distance is 50 meters (164 feet). When considering an application where distance may be a problem, contact your local Siemens representative.
Enclosures The National Electrical Manufacturers Association (NEMA) has specified standards for equipment enclosures. The MICROMASTER is supplied in a protected chassis and a NEMA
Type 1 enclosure.
Ambient Temperature The MICROMASTER is rated for operation in an ambient temperature of 0 to 40° C for variable torque drives and 0 to
50°C for constant torque drives. The drive must be derated to operate at higher ambient temperatures.
Elevation The MICROMASTER is rated for operation below 1000 meters (3300 feet). At higher elevations the air is thinner, consequently the drive can’t dissipate heat as effectively and the drive must be derated. In addition, above 2000 meters (6600 feet) the supply voltage must be reduced.
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