# Losing voltage



## kdoyle2011 (Feb 8, 2013)

If i have 4160 volts from a transformer to a motor and on start-up i get 3600 what is the cause


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## wildleg (Apr 12, 2009)

the motor starting up ?


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## kdoyle2011 (Feb 8, 2013)

wildleg said:


> the motor starting up ?


thats what I thinking but for some reason someone thought it was something to do with the xmfr


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## Big John (May 23, 2010)

It has to do with the size of the transformer and the capacity of the primary circuit.

If voltage is sagging on the primary side, it means the whole system is undersized for the load. If voltage is sagging on only the secondary, it means only the transformer is undersized.

But to be clear, a very brief sag may not be a problem at all. A long, delayed undervoltage would definitely be an issue. But if you're starting a large motor like that across the line, you'd probably be hard pressed to engineer a system that wouldn't bog down. Your best bet in that case might be to put in a medium voltage soft start.


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## bkmichael65 (Mar 25, 2013)

kdoyle2011 said:


> thats what I thinking but for some reason someone thought it was something to do with the xmfr


Depending on the size of the transformer, you'll have a drop there, too. I'm not sure if this is correct, but I was taught 10-15% VD during motor starting is acceptable


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## kdoyle2011 (Feb 8, 2013)

It's a 8000hp motor


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## Big John (May 23, 2010)

That's a beast. What's the setup? How is it being started, what size is the transformer, how severe is the the undervoltage, is it causing problems, etc?


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## kdoyle2011 (Feb 8, 2013)

I would be lying if I told u what the whole start up process was all I know tomorrow I'm going to look at it. But I do believe it is on a soft start up system


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## dronai (Apr 11, 2011)

........


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## 3xdad (Jan 25, 2011)

kdoyle2011 said:


> It's a 8000hp motor


:blink:

Damn. Post some pictures. Thx.


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## kdoyle2011 (Feb 8, 2013)

One glass of water and a turkey baster with a 2 cup measuring cup. Don't get wet.


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## varmit (Apr 19, 2009)

If you have a load that requires an 8000 HP motor, the voltage drop on starting will often last longer than a "normal" size motor. This is due to the larger mass that the motor must accelerate to speed. 

It always felt odd to need a ladder to climb to the top of a motor where the motor base and the ladder feet were on the same elevation.


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## Chrisibew440 (Sep 13, 2013)

kdoyle2011 said:


> If i have 4160 volts from a transformer to a motor and on start-up i get 3600 what is the cause


You will always get voltage drop especially on a motor like that. There's probably two miles of windings in that thing. What's the horse by the way? Excuse me now I see its a big one.


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## Chrisibew440 (Sep 13, 2013)

Big John said:


> It has to do with the size of the transformer and the capacity of the primary circuit.
> 
> If voltage is sagging on the primary side, it means the whole system is undersized for the load. If voltage is sagging on only the secondary, it means only the transformer is undersized.
> 
> But to be clear, a very brief sag may not be a problem at all. A long, delayed undervoltage would definitely be an issue. But if you're starting a large motor like that across the line, you'd probably be hard pressed to engineer a system that wouldn't bog down. Your best bet in that case might be to put in a medium voltage soft start.


Exactly


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## kdoyle2011 (Feb 8, 2013)

Thanks everyone y'all have been very helpful


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## triden (Jun 13, 2012)

I'm just remembering the last 12,000 HP motor I saw start up. Was on a ball mill to crush ore in a copper mine. When the thing started it would vibrate the whole building and shake all the dust off the rafters. All the lights would dim and then it would hum to life. Totally amazing. They were custom wound GE motors and also on 4120 volts.


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## oldtimer (Jun 10, 2010)

triden said:


> I'm just remembering the last 12,000 HP motor I saw start up. Was on a ball mill to crush ore in a copper mine. When the thing started it would vibrate the whole building and shake all the dust off the rafters. All the lights would dim and then it would hum to life. Totally amazing. They were custom wound GE motors and also on 4120 volts.


 
4120 volts ??

Are you sure it was not 4160 volts ? 

I have never seen a 4120 volt system .

I don't doubt you , I just have not come across that before .

I used to work at an Iron Ore mine , and the working voltage was 4160 volts.


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## circuitman1 (Mar 14, 2013)

kdoyle, can you post some pics of this monster?


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## dronai (Apr 11, 2011)

3xdad said:


> :blink:
> 
> Damn. Post some pictures. Thx.





kdoyle2011 said:


> One glass of water and a turkey baster with a 2 cup measuring cup. Don't get wet.





kdoyle2011 said:


> Thanks everyone y'all have been very helpful


 
:thumbsup:


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