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.