Hello, today I connected a six wire 3 phase induction motor to an invertor at work (delta conn-invertor output 3 phase 240v) and it ran o.k. The issue is the cable markings U1,U2 etc had fell off. I identified each pair of windings and made the delta connection not thinking about the winding polarity. I obviously got away with it but I have since read if you don't match all the windings polarities you can burn the motor out or it can be noisey. Is this the case and I would appreciate a reason why, never really thought about it before because all the motors I have installed have been labelled. If it is critical how do I determine the correct polarity? Thanks

 

Attached:
Manuf such as Megger make meters for this application.

1. Question: How to find out Motor Terminal?
Answer:
i) With a multimeter or continuity tester, you can segregate three sets of windings.
ii) Now, assume one set of windings to be, say, winding "U" and also, designate its terminals as assumed "U1 & U2".
iii) Take a 9V Cell; Connect + ve of Battery to the assumed "U1" preferably thro a push-switch and - ve to the assumed "U2".
iv) Connect a zero centred galvanometer, to another set of winding, say winding "V" such that the assumed "V1" is connected to the + ve terminal of the galvanometer and the assumed "V2" to the - ve terminal of the meter.
v) Now, when the switch is pressed, the zero centred galvanometer would deflect in the positive direction, if the assumed "U1, U2, V1 & V2" are correct.
vi) Else, if the deflection is on the negative side in the galvanometer, change your assumptions for either one of the winding (That is may be the assumed U1 can be U2 and the assumed U2 is U1 . Or keep the assumptions for U winding as it is and reverse it for the V winding (that is assumed V1 is V2 and assumed V2 is V1).
vii) Repeat this test for applying voltage to other windings and confirm your terminal assumptions.

 

Thank you Max

 

Absolutely yes.

 

The reason the winding polarity is critical is that the windings must have the proper phase to generate a rotating magnetic field to drive the rotor in a given direction. If one of the windings has the wrong polarity then that winding will be trying to rotate the field in the wrong direction. This causes poor motor torque and generates high winding currents.