swapping rotation direction of a ôstabilized shuntö dc motor 1

[electricpete]

We replaced a “stabilized shunt” dc motor.

From what I can tell we have parallel field winding (F1 and F2) connected accross the input voltage.

We also have armature (A1 and A2) in series with Shunt/Stabilizing winding (S1 and S2). Diagram as follows

[tt]
L1========Arm===StabShunt======L2
| |
| |
=======Field ============[/tt]

We wired replacement motor same as original, started it up and it rotated backwards.

Our electricians swapped A1 and A2 to change direction of rotation. Then immediately after closing breaker but BEFORE closing contactor we got a ground on our dc bus and I was called in to take a look. I don’t think the wiring has any relationship to the ground (there is some moisture present and also one lead touching the term box cover with what appears to be a scrape in the tape at point of contact). But I want to verify the wiring. Off the top of my head I’m not comfortable swapping either of the series items individually since that disrupts their polarity relative to each other... would rather swap both series items together or else swap the field leads F1 and F2. I’ll be digging around for some more info, but I figured I’d ask here as well.

Question: Do you think it is ok to swap A1/A2 to reverse direction, or is it preferable to swap F1 and F2?


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[electricpete]

Attached is the motor nameplate.
The motor is operated from 250vdc system
Here is a link to instruction manual

http://www.homewoodsales.com/PDFs/t...1/LIFE-LINE-S-DC-Motors-Frames-180A-500AT.pdf

On page 7/12, I’m thinking I should be looking at the 4th diagram down?


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[electricpete]

.... in which case swapping the connections to the A1 and A2 leads appears exactly in accordance with the diagram (?)

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[edison123]

The shunt stab winding has the same polarity as the main shunt field is it wound on so that the flux adds up (cumulative). If you reverse the armature+shunt stab as a whole, then the stabilizing series field winding would be differentially compounded with the main shunt field and the speed stabilization is lost. Also, if the armature current was high enough (like starting inrush) then it could overpower the shunt field resulting in re-reversing the DOR.

So, you reverse only the armature leads in a shunt stab motor to change the DOR.

Muthu

http://www.edison.co.in

 

[electricpete]

So, you are in agreement with the manual and with those two agreeing with electricians I have an answer (everyone was right except me). That was what I was looking for... a quick answer.

I'll take some time to understand why later...

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[edison123]

Didn't see your last 2 posts before I posted.

Yes, fig 4 is correct for changing the direction.

Muthu

http://www.edison.co.in

 

[edison123]

And you don't swap shunt field F1 & F2 either in a shunt stab motor.

Muthu

http://www.edison.co.in

 

[electricpete]

By the way, I forgot to say.. THANKS for your timely reply Muthu.

It makes sense to me now. If I think of it as cumulative compounded, then the polarity of the shunt field compared to the series/stabilizing field must be preserved (additive). The lead swap on the armature causes reversal of rotation which reverses induced voltage... armature voltage polarity swap cancels armature lead swap and maintains armature voltage in same relationship to the other windings.

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[Skogsgurra]

A short adendum:

If you need to change direction on the fly (without reconnecting), you can put the series winding in the diagonal of a rectifier bridge. That is sometimes done in servos where currents usually are small and loss in the diodes doesn't matter much.

Gunnar Englund

http://www.gke.org

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