There are
many types of 3 phase motors but by far the most common is the induction motor.
It is quite useful to be able to test them for serviceability.
Before
carrying out electrical tests it is a good idea to ensure that the rotor turns
freely. This may involve disconnecting any mechanical loads. The rotor should
rotate easily and you should not be able to hear any rumbling from the motor
bearings.
Next, if the
motor has a fan on the outside of it, check that it is clear of any debris
which may have been sucked in to it. Also check that any air vents into the
motor are not blocked.
Generally,
if the motor windings are burnt out there will be an unmistakable smell of
burnt varnish. However, it is still a good idea to test the windings as the
smell could be from the motor being overloaded.
Three phase
motors are made up of three separate windings – in the terminal box there will
be six terminals as each motor winding will have two ends. The ends of the
motor windings will usually be identified as W1, W2; U1, U2; or V1, V2.
The first
part of the test is carried out using a low resistance ohm meter. Test each
winding end to end (W1 to W2, U1 to U2 and V1 to V2). The resistance of each winding
should be approximately the same and the resistance value will depend on the
size of the motor.
If the
resistance values are different, then the motor will not be electrically
balanced and it should be sent for rewinding. If resistance values are the
same, then the next test is carried out using an insulation resistance tester.
Join W1 and
W2 together, U1 and U2 together and V1 and V2 together. Carry out an insulation
resistance test between the joined ends, i.e. W to U then W to V and then
between U and V. Then repeat the test between joined ends and the case, or the
earthing terminal of the motor (these tests can be in any order to suit you).
Providing
the insulation resistance is 2MΩ or greater then the motor is fine. If the insulation
resistance is above 0.5MΩ this could be due to dampness and it is often a good
idea to run the motor for a while before carrying out the insulation test again
as the motor may dry out with use.
To reconnect
the motor windings in star, join W2, U2 and V2 together and connect the 3 phase
motor supply to W1, U1 and V1. If the motor rotates in the wrong direction,
swap two of the phases of the motor supply.
To reconnect
the motor windings in delta, join W1 to U2, U1 to V2 and V1 to W2 and then
connect the 3 phase motor supply one to each of the joined ends. If the motor
rotates in the wrong direction, swap two phases of the motor supply.
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