Passive Filter Topologies for Motor Protection

Passive filters are sometimes used at the output of a drive connected to an electric motor in order to mitigate the negative effects of fast rise times and high peak voltages due to ringing at the motor terminals. These low pass filters are commonly needed when the motor leads are long, or a wide band-gap drive is used. Many different passive filter topologies that are in use. Figure 1 shows a few of the most common types.

Passive filter topologies
Figure 1 – Several three phase passive filter topologies

LR Filter

A common filter used on motor and drive systems with long motor cables is an LR filter or load reactor. The inductance slows down the voltage rise time. The optional resistor damps the overshoot caused by the interaction of the inductance and the cable capacitance.

RC Snubber

One method common with very large motors is the RC snubber circuit, which is placed across the terminals of the motor to reduce the peak voltage from the voltage spikes. The advantage of this filter is that there is no series inductance, which works well with high power drives. However, it only reduces voltage spikes, and it cannot change the dv/dt of the waveform.

LC dv/dt Filter

Another method of slowing down the voltage rise times (lowering the dv/dt) is to employ a damped second-order LC dv/dt filter at the output of the drive. This filter can increase the rise time and damp out the overshoot. It is possible to accomplish this with a smaller value of inductance than in the other filter types.

LC Sine Wave Filter

It also possible to eliminate dv/dt and overshoot problems by employing a second-order sine wave filter at the drive output. This filter provides near sinusoidal waveforms at the motor terminals. However, it is quite a bit larger than other filters due to the larger L and C values needed to bring the filter corner frequency below the PWM frequency.

One common problem with output filters for motor drives is the large size weight, and cost of these filters. The filters also add additional losses to the system. However motor protection requirements necessitate filters in some situations, especially in systems with very long motor cables.

Head over to the Motor Voltage Simulation to see how the voltage waveforms from the motor drive affect overshoot and ringing at the motor terminals, and how filters can mitigate these challenges.