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What is the optimum number of pulses for a VFD?
January 21, 2008

The full text of this question, which came in as a TalkBack comment to Peter Cleaveland's Product Research on ac adjustable speed drives, in the November 2007 issue of Control Engineering, is: “…one aspect is missing that would have benefited all regarding the optimum HP at which the number of pulses should be a factor in selection of a VFD. If you have a number of back up generators, large data centers, the harmonic issues that have been pointed out by others could cause issues with the generators. Some high tech users are requiring a minimum number of pulses to make this less of an issue….”

As with most engineering decisions, the optimum number of pulses is a tradeoff between positive and negative factors. To understand the tradeoffs, we should start with what the question means by “number of pulses.”

Variable frequency drives (VFDs) control the torque and speed of an ac motor in two ways. First, they control motor speed through the fact that synchronous ac motors try to lock onto the ac input waveform. Second, they match the power delivered to the motor to the mechanical output power required from the motor.

An ac synchronous motor consists of a permanent magnet (PM) rotor having an even number of alternating north and south magnetic poles turning within an electromagnetic (EM) stator with an equal number of poles. The assembly converts electric power to mechanical power when the rotor turns at just the right rate so the polarity of each EM pole is opposite that of the PM pole approaching it and the same as that of the pole that has just passed. Since the PM poles alternate, an alternating current energizing the EM poles works perfectly as long as the timing is right.

The synchronous motor locks in at just the right speed, 


where R is the rotation speed in rpm, F is the drive frequency in Hz, and N is the number of rotor poles. It automatically adjusts the phase (angular distance by which it leads the EM magnet field) to match the mechanical torque required to maintain the rotation speed against the load. If the rotor slows, the phase changes to raise the output torque. If the rotor overspeeds, the phase changes to reduce the torque—or even act as a brake.

This is a simplified explanation that describes a motor operating on single-phase ac. Three-phase motors typically work along the same principles with obvious changes to account for multiple phases.

A VFD uses this phase locking to control the motor’s speed. In propulsion application, for example, where motor speed must continually change during operation, the VFD output frequency changes continually as well, to allow acceleration and deceleration to a desired speed.

Because the motor hardly ever operates “flat out,” the VFD also has to control the power delivered. Unless the power delivered matches the power required, proper synchronization is impossible.

A VFD powered from utility mains incorporates a linear power supply that converts ac line power to dc, a switching power supply (SPS) with variable output, and a controller to set the instantaneous output level. The trick is to pulse width modulate the SPS so that the output produces a sinusoidal ac voltage at the correct frequency.

The SPS pulses are what the question refers to. The “number of pulses” refers to the number of pulses per half cycle that the SPS uses to simulate a sinusoid at the drive’s output. The more pulses used to recreate each half cycle, the better fidelity the signal has. Since the waveform errors appear at harmonics of the sinusoid and SPS pulse repetition frequencies, they add to the output signal’s total harmonic distortion (THD)—indeed, they make up the THD—which radiates into other equipment as well as (potentially) coupling back to the utility lines.

Using more switching power supply pulses per half cycle to approximate sinusoids lowers the total harmonic distortion.

Thus, fewer pulses per half cycle increase the EMI level throughout the facility. This is why facilities managers make efforts to control it.

Since the EMI level depends on the current level generating the signals as well as the waveform, we might expect higher-horsepower motors to generate more EMI for the same sinusoid frequency and number of pulses. So one consideration is the VFD’s power output level. Another is cost, because VFDs using more pulses per half cycle cost more to buy. Since their lower THD improves motor efficiency, however, they are less expensive to operate. Adding into the mix are the availability of filters that reduce the EMI signature of any VFD.

In the end, the optimum number of VFD pulses per cycle depends on purchase price, operating cost, cost and availability of filters, and acceptable EMI levels. Motor horsepower factors in only by making the decision more critical for larger motors.

For more information about THD mitigation, visit Control Engineering [www.controleng.com] and search on “harmonics.”

Posted by Charlie Masi on January 21, 2008 | Comments (0)