Motor Performance at Low Speeds

Typical Questions

If the motor rated frequency is 60 Hz, and we use the variable frequency drive to run this motor continuously at 5 to 10 Hz frequency, will we damage the motor winding? Will it overheat?

The answer to that question lies mostly in how much torque the motor will produce at those slow speeds. At very light loads, the motor will be fine. At heavy loads, it may well overheat, since the motor’s fan is not moving enough air to control its temperature.

Another important but smaller factor is the ambient temperature around the motor. If the motor is running in a refrigerated warehouse at 2°C, it will be able to produce more torque without overheating than if it is running in an area that is very warm (near 40°C, for example). A third consideration is motor cleanliness. Clearly, a motor buried in dust, paper pulp, or other dirt will run warmer than a clean motor under the same load conditions.
A motor run a very low speed must either be specifically engineered and designed for such a low speed or a cooling system must be added. Typical cooling systems include external blowers or fans. The difference in price between a TEFC motor and a TEBC motor is rarely negligible. Up to about 30hp, a less expensive option would be a TENV motor. Above that hp level, if you choose TEBC, cost may become a greater issue.
If a Variable Frequency Drive is used, is there any need to use gear box? For example, the motor base speed is 1750 RPM and the required speed is 80 RPM. If Variable Frequency Drive is used, is a gearbox required? Can the motor be connected to the load shaft directly without a gearbox?
If you apply a gearbox to a motor as a means of getting a reduces shaft speed, you will get an increase in torque as the shaft speed is reduced. The gearbox is like a mechanical transformer. Step the speed down and step the torque up. Your motor power rating (horsepower) remains the same.
If you use a variable frequency drive to reduce the speed of the motor shaft, the torque will remain at the rated torque of the motor. This means that, effectively, the motor power rating is reduced. To achieve a power output of 10HP at 10Hz with a 60Hz motor, you would need to use either a 10HP motor and a 6:1 gearbox or a 60HP motor and a variable frequency drive, ignoring efficiencies of gearbox, etc., and ignoring the additional cooling required when running the motor at low speed. In a constant torque application, a variable frequency drive can be used to replace a gearbox. That is not the case in a constant power application.

Your stated requirement to operate at 80 RPM (less than 4% of rated motor speed) would require that you run the motor, without a gearbox, at less than 5HZ. Unless your load is extremely constant, you can expect to experience speed and torque variations.
A better choice would be to use a low ratio gearbox (3:1 or 4:1) and a variable frequency drive to control the motor speed at a higher value. 15 Hz should be considered minimum regarding continuous motor speed.
The decision to use a gearbox is determined by the available torque from the motor at that slow speed and the torque required to drive the load. To find motor rated torque, you use the formula HP = T(ft-lbs) x RPM/5250.
Solving for torque, T(ft-lbs) = 5250 x HP/RPM (nameplate or base speed of the motor), you get the rated torque for the motor. This is the torque available from the motor base speed down to one-third or possibly one-quarter of motor nameplate speed.
For a 4-pole motor (1750 RPM nameplate speed), an approximate value of 3 ft-lbs per horsepower is possible. At 5Hz, it is unlikely that you would be able to get 15% of rated torque continuously unless the motor is located in a very cold place.
If the load requires not more than this 15% level of torque, then no gearbox is required. If it requires more than the 15% level, you need a gearbox to convert some of the available motor speed into load torque.
Below one-quarter of motor nameplate speed, you will have to increase the rated voltage supplied to the motor to achieve rated torque. As long as the motor is fully loaded, the increase in voltage will not create additional losses. If the motor load decreases, the increase in voltage will create additional core losses, causing more heating of the motor.

The use of a gear box will create additional losses in the motor/drive system. Keeping losses to a minimum requires a knowledgeable tradeoff in the selection of the motor, variable frequency drive, and gearbox. If the application does not require higher speed operation, then the selection of a lower base speed motor will provide additional torque.
For a given application, the variable frequency drive costs should be related to the selected motor. The key factor is the required torque at the continuous operating speed. Set up a table showing the combination of different motors, variable frequency drives and gearbox selections that will provide the required torque. The results of this table will yield the most effective, lowest cost operating system.


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