Sunday, January 25, 2009

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.


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