We have looked at how adjustable frequency drives work, and why they are so useful. We have also discussed their advantages over other speed regulation methods. However, do not assume that the AF drive is foolproof. There are a number of issues you need to consider. These are:

Heat Issues

Output Filtering

Harmonics

If these issues are ignored while planning the implementation, problems can arise in application.

As we have discussed previously, an AF drive generates a certain amount of heat. This heat generation is taken into consideration in sizing the drive’s cooling fan, particularly if the AF drive is in an enclosed space.

Likewise, you need to be aware of heat issues in the customer’s application. All the other components in the system also generate heat that must be dissipated. And the ambient temperature of the operating environment needs to be considered as well.

A motor will run at a higher temperature when operating on an adjustable frequency drive than it will operating on line current. Why is this?

Slowing the speed of the motor results in an equal slowing of the motor’s cooling fan. Heat dissipation may be hampered when running the motor below its designed speed.

As we mentioned earlier, harmonics waste power. This power is lost as heat. AF drives are affected by harmonics, so more heat is generated and must be dissipated.

For these reasons, look closely at the customer’s application to be sure that heat problems won’t cause insulation failure, or, worse, burn up the motor. You may need to recommend a larger motor and/or a larger enclosure, or a separate cooling system for the customer’s application.

When the motor is located far away from the AF drive, a protection issue arises. (This distance varies by drive model. Consult your catalog for specific information.) The long cable connecting the drive to the motor could potentially produce frequent, repetitive voltage surges. These surges could eventually cause insulation failure and destroy the motor.

When the cable length for the application exceeds the acceptable length in the product catalog, you will need to add an Output Filter (also called a dv/dt Filter) to the circuit. The output filter will catch voltage surges before they can damage the motor.

As we mentioned, harmonics are produced by all non-linear electrical loads, including AF drives. What can be done about harmonics to protect a customer’s equipment and computer systems?

A trap filter can be added to the system to catch each harmonic frequency and take it to ground. The equipment is protected from damage. The disadvantage to using this method is that the trap filter can serve as a “magnet” for all harmonics on the electrical grid. This means that your customer’s other machines – or even a neighbor’s machines – can use (and burn up) your customer’s trap filter.

A Line Reactor can be added to lessen the problem. This is a device which adds impedance to the line to keep the impedance between the minimum and maximum allowed values. Risk of equipment damage is minimized.

A “Clean Power” rectifier can be used. This is rather expensive, but where the other two methods “clean up” the harmonics, this method prevents harmonics from ever occurring.