Dear all,
Some times I come about a three phase induction motor without terminal labled like u1 u2 v1 v2 and w1 and w2. The only tools that would often avail will be multimeter. So it would be possible to determine each coils terminal pairs no further detail. So what worst consequences happen if I arbitrary assign one pair of coil u1 and u2 and another v1 v2 and the last w1 and w2?

Is there any possibility that by wiring in the traditional start delta contactor starter that in the star cycle it will rotate one direction and it rotate in opposite direction when it changes to delta?

 

Aside from getting the wrong winding assumed to be the start and burning out the motor, the other unfortunate possibility could be reversing the direction when switching from start to run. That might not be a destructive error if it were done with no load in a low power (reduced voltage) setup. So yes, that is a possible result. At least it looks that way to me.
But if one also has a phase rotation sensor then it should be possible to get the rotation correct the first time..
Lucky for me I have not faced that problem so far.

 

Aside from getting the wrong winding assumed to be the start and burning out the motor,

I don't think that three phase induction motors have start winding.

 

OK, not a start winding, but when the connection arrangement changes from star to delta, that series to parallel shift in some two speed start sequences.
I am thinking about some of the large motors that used a rather complex change over scheme for the start up sequence.

 

With multimeter you can find the 3 phase windings U-V-W but not determine coil polarity 1-2. One way to determine polarity is to run the motor and measure phase currents. If high and unsymmetrical, one coil is out of phase. Find it by swapping wires from one coil at a time until you measure lower and symmetrical currents.
With a traditional star-delta starter, the motor will run in the same direction when switching to delta, as the phase sequence on the supply does not change.

 

With multimeter you can find the 3 phase windings U-V-W but not determine coil polarity 1-2. One way to determine polarity is to run the motor and measure phase currents. If high and unsymmetrical, one coil is out of phase. Find it by swapping wires from one coil at a time until you measure lower and symmetrical currents.
With a traditional star-delta starter, the motor will run in the same direction when switching to delta, as the phase sequence on the supply does not change.

What I understood from you is:- the worst scenario would be unsymetric, high current. There is no way that the motor direction would change after the starter finish star connection and begin delta.

 

OK, not a start winding, but when the connection arrangement changes from star to delta, that series to parallel shift in some two speed start sequences.
I am thinking about some of the large motors that used a rather complex change over scheme for the start up sequence.

I see, but I am considering the common single speed three phase induction motor

 

when the connection arrangement changes from star to delta, that series to parallel shift in some two speed start sequences.

Isn't starting from Delta to Star, since Delta has lower line-to-line voltage to reduce the starting current?

 

What I understood from you is:- the worst scenario would be unsymetric, high current. There is no way that the motor direction would change after the starter finish star connection and begin delta.

Traditional YD-starter - no change.

 

I see, but I am considering the common single speed three phase induction motor

A basic star-delta motor IS single speed, just voltage optional.
Different In run, Star for high voltage, Delta for Low'
Or if single High voltage with soft start, run up in star, switch over to Delta.

 

I don't think that three phase induction motors have start winding.

They may have. They might have split windings where a partial winding is powered for start, then the other brought in for run. I’ve seen this on some European equipment.

 

Post #6 is correct.
Remove the load from the motor.
Run it in star ( disconnect the delta timer coil winding so it stays in star) and check each winding connection current. They should be very similar. If one is high, swap its connections. (Obviously, switch it off first!) .
Check again. If it's now OK. Good.
If not, the wires going to the motor windings from the panel are not connected where you thought they were. Get a meter and check them again.
Do the star current check again.
When OK, if the motor rotation is correct, put the timer back in circuit.
You're done.

 

Post #6 is correct.
Remove the load from the motor.
Run it in star ( disconnect the delta timer coil winding so it stays in star) and check each winding connection current. They should be very similar. If one is high, swap its connections. (Obviously, switch it off first!) .
Check again. If it's now OK. Good.
If not, the wires going to the motor windings from the panel are not connected where you thought they were. Get a meter and check them again.
Do the star current check again.
When OK, if the motor rotation is correct, put the timer back in circuit.
You're done.

Yes, I understood that but I would be happy if I got more confirmation that beside unbalanced current there would be no difference in the star and delta phase motor rotation direction.

 

If it a replacement motor on an existing panel and it didn't do it before, it won't do it after.
If it's a new panel, it might if it's not wired correctly.

 

If it a replacement motor on an existing panel and it didn't do it before, it won't do it after.

It is the case of new motor without labels u1 u2 etc and the panel is to be buit

 

Entirely possible then.....

 

Entirely possible then.....

Let me explain my question again:
Is there a possibility of say when the star begin, it rotate clockwise and when it change to delta the rotation could be counter clockwise? Conside the traditional star delta circuit is okay and the motor three winding identified except that u1 and u2 not identified also for v and w

 

Yes. If the panel is wired incorrectly.
If you check the motor as previously explained in star, there will be no problem associated with the motor.
The panel wiring is a different matter.

 

Yes. If the panel is wired incorrectly.
If you check the motor as previously explained in star, there will be no problem associated with the motor.
The panel wiring is a different matter.

I didn't understand

 

If you require to find out the equivalent polarities between 3ph motor windings, there is a simple test using a suitable LV DC source, a PB & DC meter.

https://www.allaboutcircuits.com/te...t/chpt-10/three-phase-y-delta-configurations/