Interchange of main terminals and star point

[nkn5]

Hello,

We have case from construction site, where the cable is glanded incorrectly on the star point terminal box instead of main terminal box on the other side of a medium voltage star connected motor. It is difficult now to get the extra length of the cable required to re-route, we started to check the possibility of interchanging the main terminals and star point terminals.

So by this, the power connection is proposed to be made at U2, V2, W2 and star point at U1, V1, W1 (shown as connection type-2 in attached picture) instead of power connection at U1, V1, W1 and star point at U2, V2, W2 (shown as connection type-1 in attached picture). By this, I understand the direction of magnetic field in each of the winding and motor reverses its direction. So to fix the rotation, we will have to change the sequence of main power connection.(Obviously this will also be verified during the motor bump test)

Interestingly, the main terminal power box and star point terminal box are identical in size, shape and internal arrangement. This allows us to do above said modifications very easily.

I haven't see / done some thing of this kind before, and so I would like to take more opinions on this. Other than the motor's rotation direction, do you think we will have any other issues.

Thanks.

 

[ScottyUK]

It should allow the machine to run. You will need to check CT positions and possibly relocate them. Check all the clearances phase-phase and phase-earth are no smaller at the neutral ends than at the line ends, otherwise you could open the door to PD problems if operating voltage is high enough.

Will the machine manufacturer honour the warranty if you reconfigure it?

 

[Skogsgurra]

There is one additional possible complication.


If the motors are fed from VFDs, you may get a higher bearing voltage. We had a case with twin pumps where one of the pumps was connected "as usual" and the other was connected as per your type 2 connection. The "unusual" connection caused bearing damages. The other connection did not.

Frame 40 in

http://www.gke.org/presentationer/files/GE Bearings and their electric environment w O and E.pdf

shows what it can look like. The reason for this was that the "hot" end of the stator winding changed from being away from air gap to being close to it.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[nkn5]

Thanks for inputs.

@ ScottyUK: Well, I'm also concerned about the warranty issue. We are trying to contact the manufacturer. Also there are no CTs on the motor end, they are only on the switch-gear end.

@ Skogsgurra: In this case, the motors are to be started on full voltage. No VFDs. If the design of stator core is not symmetrical from either end of winding, I can understand may end up with something like your case, but I couldn't get why it has to be asymmetrical. (I dont have much knowledge on the machine design aspects)

 

[zlatkodo]

As for the winding, it is irrelevant whether the power is connected to to the beginnings or endings of phase windings.

[URL unfurl="true"]http://winding.wix.com/design[/url]

 

[Laplacian]

There is a possibility the motor manufacturer used less insulation on the neutral end windings, which may result in premature failure if operated continuously in this manner. It may also be less resistant to surges (lighting, capacitor switching, etc.). It is advised to confirm with the manufacturer if they used equivalent insulation design on both the neutral and phase ends.

 

[Skogsgurra]

nkn5
The Winding is closer to the slot opening (and rotor) in one case. That makes capacitive coupling to the rotor higher. The recordings in frame 40 show that. More coupling = more voltage across bearings.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

There is a possibility the motor manufacturer used less insulation on the neutral end windings, which may result in premature failure if operated continuously in this manner. It may also be less resistant to surges (lighting, capacitor switching, etc.). It is advised to confirm with the manufacturer if they used equivalent insulation design on both the neutral and phase ends.

Worth considering. I have heard of this for transformers ("graded insulation") but not for rotating machines. For rotating machines stators, each slot has the same dimension and coils are identical in dimension and insulation. I have seen for large rotating machines the coils are individually tested electrically before winding and the coils with the poorest results (still passing) were put at the neutral end and the coils with the best results put at the line end. So I agree with touching base with the manufacturer if it's a large machine.

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(2B)+(2B)' ?

 

[nkn5]

Hello all,

Thanks for your inputs. Those are helpful. Sorry for coming back lately. Anyway, below is the reply we received from Manufacturer.

"By making the proposed modification, the direction of rotation should remain the same, if line 1 supply is connected to U2 instead of U1 and so forth. Individual coils don´t care which end the current is coming from. Direction of rotation remains the same when the phase sequence remains the same."

However, we are taking an additional confirmation that warranty is not compromised by doing this.

 

[Masbibe]

Strange answer. For me it should rotate opposite comparing to normal connection.

Milovan Milosevic

 

[electricpete]

Reversing the polarity of each phase windings would simply shift the currents and associated fields by 180 degrees in time (relative to the power supply). It would not change the direction of rotation.

Example:
Original vector diagram has A at 0 degrees, B at 120 degrees, C at 240 degrees.
Revised vector diagram has A at 180 degrees, B at 300 degrees, C at 420=60 degrees.
This does not represent a change in rotation.

Swapping any two phase leads would change the direction of rotation.


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(2B)+(2B)' ?

 

[Masbibe]

Yes you are right my mistake. I didn't draw it on paper.

Milovan Milosevic