Dv/dt can be a problem when motors are connected to motor drives. Figure 1 shows a typical electric motor and motor drive (also called a VFD). A cable connects the drive to the motor. In some situations, a filter is installed at the output of the drive.
VFDs for electric motors use pulse width modulation (PWM) to synthesize electrical waveforms of various frequencies to control the motor. These voltage waveforms look almost like square waves. When zooming in on the leading edge of the voltage pulse, the rise time of the pulse becomes apparent. The change in voltage ‘dv’ and the change in time ‘dt’ can be obtained to determine the dv/dt. Figure 2 shows a schematic of a voltage waveform from a VFD and a zoomed in view of the leading edge.
High dv/dt due to short rise times may lead to voltage spikes and ringing on the leading edge of the voltage pulse. This can also be seen in figure 2 with the peak of the voltage spike labeled as Vpeak. This ringing is a source of high-frequency EMI and the voltage spikes may damage the motor windings if the peak voltage is too high.
Mitigation of dv/dt
If the dv/dt or the peak voltage is too high for the application, some of the mitigation methods listed below may be useful:
- Use a filter to lower the dv/dt and damp the ringing to lower the peak voltage from the voltage spike.
- Modify the drive to turn on the semiconductor devices more slowly in order to lower the dv/dt.
- Specify a motor that is tolerant of the dv/dt and peak voltage from the drive.
Want to see these principles in action? Check out the Motor Voltage Waveform Simulation WebApp to see how different filters can change the waveforms from the motor drive.