How we check a three phase motor winding?

Question

How we check a three phase motor winding?

Answer 1

There are 2 tests to be carried out

Continuity of the windings of which there are 3 (Will it Work)

Insulation Resistance (Is it safe)

Each winding is exactly the same length so using a multimeter the resistance measured should be generally the same within about 5%.

If a cable is connected then measure across the cable ends between  Phase 1 and Phase 2 Then between Phase 1 and Phase 3 and finally between Phase 2 and Phase 3 if the windings are OK the readings will be the same. Note it does not matter if the motor is Delta or Star (WYE) connected.

To test the integrity of the insulation use an insulation tester set (in UK) at 500 V DC measure between one phase cable and earth(GND) if its star or delta connected we will test all 3 coils at the same time. The results should be high Megohms.

If the motor is being tested at the terminal block and there are no links we will have 6 terminals Line up the terminals like a 6 on a dice three on the left and three on the right.

Using multimeter test between bottom left side terminal and middle right side terminal Record reading if no reading test between bottom left and top right one of these combinations should give a reading as we are trying 2 find 3 sets of pairs. The winding ends are diagonally opposite

Next left middle to top right record reading if no reading then left middle to bottom right

Last test top left to middle right if no reading top left to bottom right.

You will see the ends of each winding are diagonally opposite this allows easy linking across for Star(wye) or Delta connections. The resistance of each winding should be the same

If a reading is higher than others then it indicates a problem with that winding often they will be open circuit.

Variations of a few ohms probably means loose connections check and re-test

Now Insulation test 500 VDC test left bottom terminal to earth(gnd) then middle left to Earth then top left to earth This tests each coil to earth one at a time. Now test insulation between coils Bottom left to middle left, bottom left to top left and finally middle left to top left. All insulation readings should be very high in UK minimum acceptable value 1 Megohm practically above 10 Megohm is preferred.  

  

Answer 2

Best test is a megger test (insulation resistance). A low voltage motor should have resistance higher than 2MΩ when tested at twice the operating voltage.

Ideally you should test phase to ground and phase to phase but phase to phase is only possible if all of the winding ends are brought out. If you only have 3 wires then the link (wye or delta) is made inside and you can only test phase to earth.

You can do a rough winding resistance test to see all 3 winding have the same resistance but this only tells you if its bad if they are different, it doesn't confirm its good because its AC.

There are some meters you can buy that give phase relationship and all kinds of good stuff but are too expensive for the most part and actually the best test for a motor is to run it. Disconnect the load and if it runs, its most likely good. Of course there are exceptions but rare.

You can PI tests on high voltage motors in addition to the other tests.

Answer 3

use a megger