Variable Speed Drives

When a motor is to be used with a variable speed drive there are various factors which must be taken into consideration before the motor is selected. A summary is listed below.

1. What type of load is the motor driving?
2. What are the minimum and maximum speeds and absorbed powers at those speeds?
3. What starting torque is required?
4. What de-rating factor is required by the drive supplier?
5. What enclosure of motor is required?

We will explain these in more detail

1/ Type Of Load

It is very important to know the type of load the motor is to drive, as in certain instances, a larger motor may have to be selected. If the load is a centrifugal pump or fan, where the torque varies in proportion to the square of the speed, a standard size motor can be used as at low speeds the required power is reduced. However, if the motor is to be run above the normal full load speed, care must be taken as the motor can only deliver the power it was designed to supply. The power of a centrifugal fan or pump varies as the cube of the speed. Therefore, if higher than synchronous speeds are likely, a larger motor may be required. If the motor is to operate a conveyor, crusher or positive displacement type pump a standard motor should not be used without further checking as the motor will have to deliver constant torque over the speed range, which will require either the motor to be force cooled or a larger frame size to be supplied. If the motor is to operate a machine tool where the power must remain constant a standard motor must not be used.

2/ Speed Range and Absorbed Power

These are very important in the selection of the motor. If the motor is used with a centrifugal fan or pump then providing it is not to run above normal full load speed a standard size motor can be used. If it is to run above normal full load speed then the rotor must be balanced to run at the required speed and also the power must be checked. As a general statement all except 2 pole motors can be run up to twice rated speed with the requirement that the rotor be balanced to run at the required speed. A 2 pole motor should not be run above 3800 rpm without a specially designed rotor. If the motor is required for constant torque output below 50% speed, then the motor will have to be provided with forced cooling or a larger motor used. When a motor is run below approximately half speed with a constant torque output, it will overheat as the cooling system will have little, if any, effect in removing the heat. We can remove the cooling fan and supply a separate cooling fan or else increase the size of the motor. As a general statement it will require the motor to be at least one frame size larger than normal. The power absorbed by the driven load is also important as it may be possible to use a standard size motor and not increase the frame or force cool. If the speed range and absorbed powers are provided a calculation can be done to select the correct motor.

3/ Starting Torque Required

When a motor is operated from a variable speed drive, its starting torque is reduced. The variable speed drive reduces the voltage at starting to reduce the current. The torque of a motor varies in proportion to the square of the voltage applied to it. The variable speed drive has a starting voltage of approximately 100 Volts and the starting torque is approximately 150% of full load. There are methods employed by drive manufactures to improve the starting torque, however, as a general statement the starting torque will be approximately 150%. If a larger torque is required we must use a larger motor or a larger drive. This will only apply where the motor is to operate in such applications as crushers, conveyors or positive displacement pumps. If the motor is to be used with this type of load, please provide the required starting torque either as a percentage of full load or else a figure in Newton Metres. If this is known a calculation can be done to select the correct motor and drive.

4/ De-rating factor of Motor

As variable speed drives do not produce a pure sinewave a de-rating factor is required to be applied to the output torque of the motor. The output of the drive produces extra heat in the motor and this requires that even at full speed the output torque of the motor be reduced. A typical de-rating factor is 0.95, however, it will vary depending on the switching device of the drive and should be obtained from the manufacturer of the drive. There are exception to the rules. Due to the method of switching and control in some makes of drive, we have proven by performance test that when the motor is operating at full speed the temperature of the motor increases by approximately 0.2°C. Thus with these drives no de-rating is required. Please advise the de-rating required.

5/ Enclosure of Motor

If the motor is TEFC then, providing all items mentioned above are taken into consideration, no further additions are required to be made beyond your special requirements. If the motor is Exd the motor must be selected correctly for the powers and speeds required. If required, for constant torque application, a force cooled motor may be supplied for both TEFC and Exd. Also it is a condition of our approval that the Exd motor must be fitted with 2 sets of thermistors. Only one set is required to be connected and energise a relay to disconnect the drive if the motor overheats. The other set can be used as a spare in case the main set malfunctions. For the smaller Exd frames this may require the motor to be supplied with loose leads in the terminal box and no terminal block. An extra price is required to be added to the standard motor price for both TEFC and Exd motors for these modifications.

Current regulations do not allow for Exe motors to be used with variable speed drives. However in the Exe standard it states that, where a motor is to be operated from a variable speed drive, the motor and drive must be tested together for the particular speed and power range required. Western Electric can provide this testing facility in Perth. We would also recommend that the motor be fitted with two sets of thermistors for protection.

We can offer DIP and Exn motors certified for variable speed drive use with the same conditions applying as for Exd.