Testing a DC (direct current) motor may take more than just a simple volt ohmmeter. Sometimes the problem with the DC motor may won't be apparent until it is under operation. In cases such, a meg ohm test or megger may be required. The megger tester uses a high voltage, generally in the 1000 to 10,000 volt range to identify insulation leakage in motor coils and adjacent wiring. By following a basic set-up you can test a DC motor using this high voltage testing device.
Remove all electrical power from the DC motor. You will also want to remove all electrical terminations from the power source that feeds the motor. Some DC motor controllers can either give false readings or become damaged by the reversal of current flow into the control mechanism of the high testing voltage.
Set the DC motor on an insulated pad or piece of wood so the frame is not in contact with any metal surface. If there is a short in the motor wiring and the frame of the motor is in contact with a conductive surface, the escaping voltage can travel to the nearest ground path. This pathway may include yourself touching the metal table or motor frame itself.
Connect one lead of the DC motor to the megger probe and the other lead from the megger to the frame or metal case of the DC motor. Press the power button on the megger or turn the hand crank. The battery unit will take a few seconds to analyse the voltage travel and soon a reading should appear on the LCD screen. The higher the meg ohm reading the better insulated the wiring. The hand crank type of megger will be instant. The further the needle on the meter is deflected, the least amount of meg ohms is in the windings or wiring. In other words, on a hand crank type megger, the further the needle is deflected to the right, the worse the resistance is in the DC motor.
Release the push button on the megger or stop the cranking of the meter. Rotate the end shaft approximately 30 degrees and test the motor again. Perform this procedure a number of times as most DC motors use a carbon brush and copper commutator to conduct the electrical power to the armature. This method will ensure that the separate windings of the armature are being tested.
A meg ohm reading of 999 meg ohms or greater means an almost perfect insulation of the windings and wiring. Readings of less than 1 meg ohm should be considered as a problem or serious resistance breakdown of the wiring or motor windings.
As described in the opening paragraph some test meggers can generate very high voltage. In some hand crank models that voltage can range in the 100,000-volt range. Care and caution should be exercised whenever using high voltage test equipment.