How to Test a Three-Phase Motor with a Megger?

In this post, we will discuss how to test a three-phase motor with a megger. We use a megger to test the insulation level of all the electrical equipment like motor, transformer, cable, insulator, etc. The value of insulation resistance is expressed in kilo-ohm, Mega-ohm.

The megger is the brand name of the insulation meter. The megger consist a DC generator & Ohm meter.

The quality of the insulation plays a very vital role in the hassle-free operation of electrical equipment. The insulation deteriorates with changes in the operating temperature, environment conditions, aging, etc. Therefore, it is a must to check the insulation quality periodically to ensure the healthiness of the electrical equipment.

Insulation resistance test of the motor

Ensure the following safety checks before performing megger test of an induction motor

  • We must first check the meggar leads for any physical damage.
  • Check the continuity of the megger lead. We can check the continuity of leads while megger is ON. The zero resistance shows the leads are not open.
  • Wiring of the test set up must be OK.
  • Check tightness of all the connections.
  • Barricade the area where megger test is being performed.

Steps of Performing Megger Test on Motor

Step-1

Disconnect the motor from the supply source, and discharge the stator by shorting all the terminals with the ground.

Step-2

Connect the megger leads between one stator terminal and motor body.

Step-3

Apply twice the motor operating voltage between one phase and the motor body. If Motor rated voltage is 440 volts, apply voltage 880 volts. We should test the older motor at less voltage( about 80% of the twice operating voltage).

megger testing of induction motor

Step-4

Set the Megger voltage as defined in step no.3 & press the test button on megger. Note down the insulation resistance value.

Step-5

If the megger reads zero, it shows that winding is earth. Note that if the Megger value is zero, it means any of the three winding of stator may be earth. The motor stator is in star or delta configuration. Therefore, there is no need to test the Megger value (IR value) at different points of the connection terminal.

Step-6

If the insulation resistance value is high. if 440 volts motor has an IR value of more than 1 Mega-ohm and above, it means motor winding insulation is good.

Step-7

After the megger test of the motor, We must connect the winding to the earth in order to discharge the build-up voltage of the winding.

This is the way we can test a three-phase motor with a megger.

Purpose of Insulation resistance Measurement

The insulation resistance or Megger value of motor winding depends on many parameters. We know that the insulation quality deteriorates with dirt, moisture, corrosion & weather condition. Therefore, we must carry out the megger test periodically.

The periodic measurement of the megger value( IR Value) is a must for enhancing the useful life of the motor. The motor megger value can be improved, if it is known before.

The motor winding can be re-varnished with suitable insulation material to increase the IR value.

In other words, the preventive maintenance of the motor and measurement of insulation resistance using megger enhance the life of the motor.

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