EMC and VFDs


VFDs and EMC

Understanding Conducted EMC reduction

The PWM waveform at the output of the VFD exists on virtually all VFDs sold in New Zealand. Induced noise voltage on the frame of the motor is a direct result of the switched waveform and the capacitance within the motor.
The best means of minimising the stray currents to flow in other circuits and earth paths, is to minimise the noise voltage on the motor frame.
At high frequencies, the current flows on the surface of the conductor. Providing a very low impedance earth return from the motor frame to the VFD frame will reduce the voltage on the isolated motor frame by the effect of the voltage divider.
Using a cable screen as an earth return path provides a surface area that is much larger than the surface area of a standard circular conductor.
Laboratory tests indicate that the voltage on the frame of the motor can be reduced by a factor of 20 to 50. One test carried out by UL laboratories showed a noise voltage drop from 69 volts to 1.4 volts with a 5 meter length of cable when an unscreened cable was replaced by an equivalent correctly terminated screened cable.

Many field problems, where stray voltages in the order of 50 to 100 volts have been measured, have been corrected by the fitting of a screened cable with the screen correctly terminated at each end with stray voltages commonly less than 2 volts after correction at the same point of measurement.

The effectiveness of the screen as a low impedance earth return is inversely proportional to the frequency of the noise current. Lower frequencies are not attenuated to the same degree as higher frequencies when a screened cable is employed.

Another solution, is to completely isolate the motor and the cables from all other conductive paths in the environment. This is not a practical option in most installations and is of academic interest only.

The only other way to overcome this problem, is to eliminate all high frequency components from the output voltage of the VFD. This can be done, but is expensive and has a major impact on operating efficiency. Filtering on the output of the VFD can result in the bandwidth of the noise being compressed into the lower frequencies where the screen is less effective, but the existing standards are met. In this situation, because the screen is less effective, the stray voltages can be higher and harder to eliminate than the unfiltered VFDs where the potential divider using the screen is much more effective.
Sine wave filters are available for use with VFDs, but the commonly used filters are designed to produce a sinewave voltage between phases, but not relative to earth. The voltage waveform can have a very high common mode PWM component that still causes high stray voltages and currents.  The dv/dt of the common mode voltage is greatly reduced by the filter, reducing the effectiveness of the decoupling within the drive and forcing much of the return path current to flow via the incoming phases. Noise voltages in excess of 100V have been superimposed on the phase to earth voltages causing lamp failures, switchmode supply failures and many other issues. To be effective, a sinewave filter would need to filter relative to earth as well as between phases.