Protecting Your Motor From PWM Spikes

Variable Frequency Drives can have a dramatic impact on efficiency and the cost of operating a motor or pump. However, the addition of a VFD can have side-effects that need to be addressed. One of these side effects comes into play when the distance from the VFD to the motor is 50 feet or longer. Another side effect is the development of Common Mode voltage.

The reason for the >50 feet distance side-effect is that the output of a VFD is never a true sine wave. Instead, the output is a PWM (pulse-width modulated) signal. Depending on how much power is needed, the VFD provides “slices” of voltage of varying widths. A motor that is designed to handle this PWM output will actually see these slices in the same way that a sine wave would appear. The problem arises when these slices travel over a long cable distance; the longer the cable distance, the more likely that each slice will reflected and multiplied on top of itself. There have been reported instances where a 480VAC PWM slice has been multiplied to over 2100 VAC. These higher voltage spikes cause damage to the cable and the motor because they are significantly higher than what the motor/pump is designed to handle. Generally, VFD and motor manufacturers recommend a reactor for lead lengths from 50 feet to 300 feet, a dV/dT filter for lead lengths from 300 feet to 1000 feet, and a sine wave filter for lengths from 1000 feet to 15000 feet. These are general recommendations and you may be able to refine this based on a specific recommendation from your VFD manufacturer or based on the needs of your application.

The other side effect, Common Mode, is not based solely on the lead length. Normally, a 3-phase sine wave that is perfectly balanced has a unique, symmetrical feature that we don’t often think about. With a balanced 3-phase sine wave, if you add the positive and negative voltage of the 3 phases together an any point in the sine wave, the sum of the three will be Zero. This is all due to the phase angle and symmetry of the sine wave and is what an across-the-line started motor would see in a perfect world. When the VFD and it’s PWM output is introduced, the symmetry of the sine waves is removed completely. With a PWM output, the slices or spikes rarely sum up to Zero. This difference is what is called Common Mode. Common mode damages the motor bearings. There are various methods to reduce or mitigate Common Mode. Our vendor, MTE Corporation, offers a dV/dT filter that reduces 50% of the Common Mode voltage and a Sine Wave filter that reduces 90% of Common Mode.