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Testing universal AC motor by measuring the resistance on the bars and the windings and checking the efficiency of the armature


There are three ways for testing universal AC motor using volt/ohm meter. The test will able you to know if
your AC motor is functioning properly. A universal AC motor has series of windings on each and every bar of
commutator. These windings makes a continuous loop around each of the iron stack teeth and extends to the
next bar mounted on the commutator.


testing universal AC motor

The windings should always remain insulated and extend without a break for the AC motor to function
properly. However if the motor is driven too hard or exposed to high temperature the insulation on the
windings can be damaged causing a short. Whenever there is a short either between the wires or from the
wires to the armature stack, the universal AC motor will fail.


When testing universal AC motor, you have to take the following three test.


1. Test the armature windings


If your AC motor does not have external brush holders you have to remove the armature from the motor.

Begin by adjusting the volt meter to measure the resistance of two commutator bars. Thereafter take the 180
degrees test. The resistance across the bars should be taken for the bars that are at 180 degrees across
each other.

Check the resistance of all the pairs so as to be sure that there isn抰 a short circuit in the the windings. The
resistance should be nearly equal for all the pairs that you test. When resistance varies drastically it means
that the windings are faulty, a drop means a short in the windings in the coil, and a huge spike implies that
there is a burn along the wire causing a short circuit.


2. Bar to bar resistance test


When testing universal AC motor you should conduct a resistance test between bars. The test involves
checking all the resistance of all coils in the AC motor. Use a volt/ohm meter to test the resistance of each
coil at a time. Depending on the model of the motor, you will have a resistance reading that should be same
or about the same for all the coils. A drastic drop in coil resistance means that there is a short in the windings
in the coil, and a huge spike implies that there is a burn along the wire causing a short circuit.


3. Resistance of individual commutator bar to that of AC motor iron armature stack


The last check when testing universal AC motor involve taking the resistance reading of individual AC
commutator to that of the armature stack. There should be no electrical flow or continuity between armature
shaft and the armature stack. If a continuity of electricity is established then it is assumed that the AC motor
armature is not functioning properly or is damaged.


These three checks should establish if your AC motor is functioning properly as they are credible checks
when testing universal AC motor.

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