How to

How to test a 3 phase motor by using a megger


In an electrical device, the quality of insulation is an important factor. There will be changes in the insulation property of electrical devices according to certain factors like environmental conditions, temperature, time, etc. So we need to check the insulation resistance quality at regular intervals so that we can avoid electric shock. A megger is used to check the insulation resistance of the electrical equipment. So we can use a megger to test a motor, generator winding, cable, etc. A megger tests a wire by applying a high voltage DC to it for a certain time period so that the insulation quality can be determined. A megger is comprised of a DC generator and an Ohm meter.

How to use a megger to test a three-phase motor


Before we use a megger for testing we must check certain factors

  • The leads in the megger must be checked for any faults. and the leads must have electrical continuity.
  • To check the electrical continuity of a megger we must connect both the leads together while the megger is ON
  • Check the dress wiring of the device which is to be measured
  • Make sure that all the connections are complete before testing.

Check the three-phase motor is connected to delta or not, if so remove the links.

Winding resistance test of the motor

The first step would be checking the winding resistance. So like the multimeter a megger would also have a rotary selector so turn it to Ohms and after that check, the leads and this will be checking electrical continuity. Connect the probe to the motor to check the winding resistance from phase to phase terminal. So we must check the two ends of the first, second, and third winding, in order to do this connect the probe to the first winding and check its two ends with the other probe and do the same for second and third winding. While doing this we must switch on the megger testing. During this testing all the windings must show high and almost equal value then the motor is ok.

Insulation resistance test of the motor

In order to do this testing change, the megger setting to voltage and the voltage level must be set greater than 250V. Connect one probe of the megger to the earth connection in the motor. So insulation resistance to earth can be checked then connect the second probe to the first winding and check the value, then do it for the second and third, and if all the values are almost the same then its ok. Then check the insulation resistance between the windings.

While checking the insulation resistance of the motor the values will be almost the same for the three windings if so the motor is ok and if the probes are connected to the same winding then the meter will show zero.

So if all the readings are almost the same and high for the insulation and winding test of the motor then the motor is ok.


post-graduate in Electronics & communication.

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