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vb Instrument: Induction Motor Current Analysis


Can the vb instrument be used to diagnose rotor bar/end ring damage in induction motors?


Yes. You can determine when these problems are developing by analyzing the current drawn by the motor.

When you have broken bars in an induction motor this increases the thermal load, therefore the temperature of the stator will increase. If you continue running the motor it can burn out. If you stop the motor and try to start again normally the stator may burn out even sooner.

An end ring fault is not quite as critical because there is an end ring through two bars and the motor gives the same nominal power out. Even so, end rings are normally quite large and, because of centrifugal forces, a crack in the end ring may cause rotor imbalance. Forces are higher in two pole motors than four pole motors and in the worst case scenario the end ring can touch the stator winding and break the stator.

With either of these faults you should rewind the stator and repair the rotor before failure occurs.


Note: If you have a vbOnline device or vb3000 instrument that uses firmware version 4.30 and higher you can measure the current directly: please ignore any subsequent references to using a current clamp.

• Take a current clamp that outputs no more than 1 V peak-peak given the amps that you know you will be measuring. Attach the clamp to the channel 1 accelerometer input on the instrument.
• From the Set Parameters Menu of the vb instrument, set up a measurement with the following parameters:

• Use the instructions for your current clamp to attach it to one of the supply lines of the motor.

Observe all warnings and precautions in the current clamp documentation.

• Make sure the motor is at maximum rated load possible - absolute minimum of 50%, preferably 70% or higher.
• Take a measurement with the instrument either in Measure Mode or by recording it and transferring to Ascent/MAS VB.

Note: More information on measuring current with the vb instrument can found in the FAQ article on Current Readings with the vb Instrument

Performing the Diagnosis

• Look at the dB difference (adB) between the main peak at 50 or 60 Hz and the first lower sideband (to the left).

• If the dB difference is:


In some cases peaks to the side of the line frequency will come from other sources. To confirm your diagnosis you can calculate the exact distance from the main peak to the sideband using this formula:

sideband spacing = (synchronous speed - motor speed) * number of poles

synchronous speed is the speed in Hz of the AC power supply (usually 50 or 60 Hz) divided by half the number of poles. It is the speed that the motor would run at if there was absolutely no load/slip. It will appear as the largest peak in the current spectrum and you should use the actual measured value if possible.

motor speed is the speed of the motor in Hz

number of poles is the number of poles in the motor, usually 2, 4 or 6

To confirm your diagnosis make sure that the sideband peak you are looking at is spaced at the correct distance from the main (line frequency) peak.