To control the effects of the reflected wave created by IGBTs in certain applications, a dV/dT filter will need to be applied. The design of a dV/dT filter is based on a number of factors, including IGBT switching time, load ampacity, cable length, and cable diameter. It is always best to use a dV/dT filter with an ampacity matched to the motor, and cable sizes (AWG) that are appropriate for the distance between the motor and the VFD while resisting the urge to oversize the cable. When an oversized cable must be used, upsizing the filter is an option if the lugs are not large enough for the cable. However, there is a risk when upsizing the filter too much; this is because the performance of the filter is reduced when the amperage of the filter is too large for the load. In these situations, it is advisable to add a terminal block at the dV/dT filter to step the cable down to the filter lug size. When applying a dV/dT filter, always remember to reduce the VFD carrier frequency to 2–4 kHz. Taking these factors into account will assist in the performance of the dV/dT filter in the application and the protection of the motor from dangerous reflected wave voltages up to 1000 feet from the VFD.

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