How they work

The primary purpose of a vfd is to enable constant-speed ac induction motors to run at variable speeds. To understand how they work, it helps to recall that the speed of an ac induction motor depends on two things — the frequency of the supplied power and the number of magnetic poles per phase it has.

On 60-Hz ac power, for example, the synchronous speed of a four-pole motor is 1,800 rpm (120 x 60 cycles per second/4 poles=1,800 rpm). Unless one of these variables changes, the speed will remain constant.

That’s where the inverter comes in. It rectifies the utility’s sinusoidal ac power to variable-frequency dc current, making it possible to adjust (reduce) the speed of ac induction motors almost infinitely. Of course, there’s a little more to it.

Basically, the inverter sends the motor “pulses” of dc voltage in varied widths to mimic the increasing and decreasing amplitude of an ac sine wave. As Figure 1 (page 13) shows, the pulse at the center of each wave is the widest (longest duration), while those on either side of it are progressively narrower (shorter duration).

Because inverters modulate the width of the pulses in this way, they are sometimes called pulse-width modulation inverters (PWMs).

Figure 1 illustrates two additional points. First, the amplitude or peak voltage of each pulse is, in theory at least, about the same as the maximum amplitude of the sinusoidal voltage supplied by the utility.

Second, the polarity of the pulses changes from positive to negative during the second half of each cycle, replicating the polarity shift that occurs with ac power during each complete cycle.

Now that the supplied power is dc rather than ac, it is a simple matter to adjust the speed. All that is needed is to change the frequency (or rate) at which the pulses are delivered to the motor. Decreasing it slows the motor; increasing it (within limits) causes the motor to speed up.

Although inverter technology works well in most situations, serious problems may arise with some applications and motor-drive systems.

Problems from switching frequency, cable length

Because the inverter sends out several thousand pulses each second, the switching frequency can become a problem, especially if the cable between the drive and motor is more than 50-ft long.

In such cases, a reflected and an incident pulse can meet at the motor terminals, effectively doubling the voltage that surges into the motor winding. If that sounds like trouble to you, you’re right.

Another problem is that the amplitude of each pulse may increase as it travels down the cable toward the motor. Visualize a wave striking the beach: As it reflects back and crosses other incoming waves, it can add to their height. That is what sometimes happens as the PWM waveform speeds towards your motor.

It is not unusual to record 1,500 to 2,000 V or more at the terminals of a motor rated for 460-V operation. No rocket science needed here! Standard insulation systems are not designed to handle that kind of overvoltage. In fact, IEEE Standard 43 specifies that insulation system integrity of new 460-V motors should be tested only once at 2,000 V.

Compare that with the voltage stress that an inverter may impose several times each second throughout the life of a motor. Not a pretty picture, is it?

Fairly extensive testing (by motor manufacturers and others) suggests that cables longer than 50 ft contribute to the higher voltage spikes that cause winding failures. As a result, some motor manufacturers have upgraded the winding voltage-withstand capability of their products.

For their part, a number of drive manufacturers have improved the quality of output power sufficiently to make longer cable runs safer. Faster rise times may be desirable from a control perspective, but a slower rise time is gentler to the motor.

Use of line filters or reactors to dissipate the energy increase caused by a long cable run may also help protect the motor. If possible, though, it is best to avoid cable runs of more than 50 ft. After all, no one wants to install an expensive inverter only to have the motor fail.

Remember, too, that the relationship of cable length, motor insulation system, and inverter rise time is complex. Each variable plays a part in determining the corona inception voltage (i.e., threshold at which high voltages partially discharge or “leak,” ionizing air pockets and deteriorating insulation).

If a particular combination works well, changing any one variable can change everything.

Increased heating

Even where cable length is not a problem, motors typically run 10° to 20°C hotter on a simulated 60-Hz PWM wave form than they would on real sinusoidal power.

Excess heat, of course, is a major cause of insulation deterioration and failure. In fact, insulation life drops by half with each 10° increase above the rated temperature rise. That means the insulation system of a motor that runs 20° hotter than its rated temperature would last only one-fourth of its normal life.

Adding to the problem, inverter-driven motors generally operate at lower speeds. The fan will therefore dissipate less heat, so the motor will run hotter. And that means still more heat in the windings.

Fortunately, most inverter applications today involve motors driving fans or pumps. These are centrifugal loads, so the power required to drive them increases as the square of the speed increase.

Conversely, driving the pump impeller at a lower speed reduces power requirements significantly. That means less current, so heating due to winding resistance also decreases. It would be too easy if these factors offset each other, so figuring out the expected winding temperature is tough.

Conclusion

Inverters offer many benefits, not the least of which is the ability to directly drive variable-speed loads using ac induction motors. If you elect to install one, though, avoid long cable runs.

If that is not possible, investigate the use of line filters and reactors to reduce or eliminate harmful voltage spikes. You may also consider buying an inverter-duty replacement motor. If the motor is large and expensive, a cost-effective alternative may be to have your local service center rewind it using inverter-duty wire and special insulation. They probably will also suggest applying additional varnish treatments.

This fills spaces between wires, which increases the corona inception voltage. It also improves heat conduction from the wire to the laminations, helping to cool the motor. Both of these things will prolong the life of the motor when operating on that new inverter.

Links

• Electrical Apparatus Service Association