I have a question regarding protecting a variable frequency drive (VFD). I have read that the important part to protect of the variable frequency drive is the active front end (prior to the IGBT's). I also am under the impression that the VFDs themselves protect the motors from faults. So my questions are as follows:
1. Do you need an overload to protect the motor? And if so do you put in on the PWM output side?
2. How should I protect the variable frequency drive and Motor?
VFD itself has many protection functions by working on variation of controlling voltage & current through open/close IGBT and detecting CT/ VT. Depend on size of VFD we can see on panel input/ output, DC voltage; input/ output current or can measure by tool; The basic function is overload protection by both setting normal 110% and trip 150% mode.
The variable frequency drive (VFD) can make the thermal protection. However you can also use a digital relay to make it. In that case the relay will be connected downstream the VFD. Such arrangement is used when one drive will control several motors. The only tricky point is that the frequency tracking in the relay shall use the current, whereas the usual technology is to use the voltage. But there exist relays using the current measurements for frequency tracking.
For the second question you need an upstream protection, which can be ultra-rapid fuses or circuit breaker with protection relay. Eventually, a good solution for LV VFD would be to add a line-choke upstream the VFD, this will reduce the current transients seen by the input of the VFD. In MV VFD there are usually input transformers for the VFD which realizes this function.