N2 Electrical Trade Theory|The Easy Way! 71 4.11 Electrical tests on the stator


Tere are three electrical tests that are carried out on the stator windings of a three- phase motor. A description of these three tests follows. Reference must be made to fig. 4.9.


Insulation resistance test between windings An insulation resistance tester (Megger) is connected between terminals a1


then between a1 and c1 and finally between b1 and c1 . and b1


Readings are taken in each case (the readings will be high, usually above 500 000 ohms) and must be at least as high as that recommended by the manufacturer.


4.11.2 Insulation resistance to earth test An insulation resistance tester is connected between earth and a1


earth and b1 and finally between earth and c1 .


Readings are taken in each case (the readings will be high, usually above 500 000 ohms) and must be at least as high as that recommended by the manufacturer.


4.11.3 Short-circuit and open-circuit tests


A continuity tester such as a multimeter (AVO) or Megger with a low reading scale is connected between a1


then b1 and b2 , and finally between c1 Readings are taken in each case.


Te readings will differ from motor to motor but they must be the same in each case for the same motor. Te values will be low, in the region of less than 1 ohm or only a few ohms. Te exact readings can be obtained from the manufacturer.


Fig. 4.9 Stator terminal box and c2 . and a2 , , then between ,


A lower reading for one of the coils will indicate a short-circuit of some or all of the turns. A slightly higher reading will indicate a loose connection and a much higher reading could indicate an open-circuit with a carbon trace. A reading of infinity will indicate an open-circuit.


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