Bearing Fluting Specialists Stop the Attack at the source
 How to troubleshoot a Variable Speed Drive and Motor Circuit.

How to troubleshoot a Variable Speed Drive and Motor Circuit.

Friday, August 12, 2022

Have you a Drive that’s tripped and you don’t know if it’s the Motor?

A variable speed drive (Variable frequency Drive) is often built with current transformers monitoring each phase for current balance or over current and will trip on predefined limits.

What is the cause of the VSD (VFD) trip?

This can be a Motor defect or a Drive defect.

How do you identify the VSD trip cause?


1. Check the DC bus voltage either by measurement or within the drive menu error page. A under voltage on the DC bus can be supply voltage or more common the Capacitors are degraded.

See 3Phi Blog for Capacitor testing

2. Check the current in each Phase if you are able to run the Drive.

Current imbalance should be within 10% across the three phases.

Checking In Rush current will amplify any defect, so check steady state and In Rush current.

Note: A current clamp with a low pass filter is needed if measuring a VSD (VFD). Standard current clamps

have a very narrow frequency measuring range and are subject Drive emissions (EMF Noise). 

3. Disconnect the Supply leads to the Motor and conduct a Motor Test.

Test for

a. Resistance with Milli Ohm or Micro Ohm meter with a 5% limit on imbalance

b. Impedance with MotorGenie or LCR meter. The three phases should be balanced with ideally 3% (10% if motor defect expected)

c. Conduct a winding testing with a Surge Test or MotorGenie.


If the DC Bus voltage is within specification (V), and the Current Measurements are imbalanced, there are two possible causes (Motor Winding Load or Drive Defect)

To be able to distinguish between the two possible causes, look at the Impedance Measurements.

The current a motor will draw is determined by its Impedance in Ohms. Ohms law

V=I * Z (Z impedance Ohms)

If the Motor test shows no winding or Impedance Imbalance then your Drive is very likely the cause.

The Motor Test results will identify if the Motor has a defect

No comments yet