Testing of Motor Winding & Core...

What is your purpose for wanting to test the winding and core? If you want to know if a motor is good that is already in service and it Meggers ok and has continuity, try to start it. If it kicks the breaker indicating a ground, isolate to ensure the associated wiring is good. If so, the motor is bad. If the overload is kicking, the mechanical load is too great, the motor has a bad bearing, or a single phase condition exists. Test instruments only go so far. Sometimes a motor that reads ok is actually bad because in motion it acts differently than when at rest.

If you are in the motor repair or manufacture business, there are way to test the core. This requires some equipment designed specifically for that purpose.

Winding resistance, No load & full load currents, temperature raise, etc are few more tests.

Check the physical place of the poles, while applying a small constant current through the windings. Measure the L (inductance) of each winding, they should be close to each other.

Dear Amith,

You may carryout test in " OFF- LOAD" and " ON- LOAD" conditions.

" OFF- LOAD" condition test:

1) Insulation Resistance test.

2) Measurement of Inductance of all three coils(Line to line or Line to neutral, if neutral point is accessible ) and comparison of the values; if there is a % variaion of more than 5% then further test to be carried out to check whether there is inter turn short in coil.

3) Measurement of Resistance all three coils(Line to line or Line to neutral, if neutral point is accessible ) and comparison of the values; if there is a % variaion of more than 5% then further test to be carried out to check whether there is any looseness in connections of coils

3) Full flux loop test to determine whether there is any HOT-SPOT in the core due to damage of inter lamination insulating vernish( core plate vernish)

4) Grauler test to determine whether there is any crack in rotor bars.

5)P.I. (Polarisation Index ) value test. Measurement of I.R. value for 60 secs and 600 secs. For a healthy motor the P.I. value should be more than 2.0

ON- LINE test:

1) Run the motor at no-load and rated load and measure the three line currents and observe the balance value of the currents( applied voltage must be balance).Also measure the power consume during on-load run and load run.If there is more than 5% out of balance currents, close inspection should be carried to find out the reason(s).Measurement of no- load and rated load Power Factor also to be done.

2)Temperature rise in the coils/windings and bearing housings may be measured after running of the motor in full load condition for four hours. Also please observe if there is any abnormal sounds are generating during running condition.

Regards,

Manindra.

Apart from the tests indicated by various members, you may like to include core tightness test (knife test), stator wedge tightness test, stator winding overhang inspection for looseness. These all are off line tests.