# control power transformer



## Zog (Apr 15, 2009)

sparky970 said:


> What would you expect to see for resistance from h1 to h2 on a 480v to 120v cpt. I have one that I measured at 54 ohms. Seems a little high.


Depends on the CPT ratings.


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## sparky970 (Mar 19, 2008)

I don't have it with me, but it's an old Cuter Hammer C340CG. I think that's the number. It's a .105kva.


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## Zog (Apr 15, 2009)

sparky970 said:


> I don't have it with me, but it's an old Cuter Hammer C340CG. I think that's the number. It's a .105kva.


Sounds reasonable, depending on what you used to take the reading.


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## John Valdes (May 17, 2007)

Ohms law? You have a nameplate? Right?


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## JRaef (Mar 23, 2009)

sparky970 said:


> What would you expect to see for resistance from h1 to h2 on a 480v to 120v cpt. I have one that I measured at 54 ohms. Seems a little high.


Just out of curiosity, why do you care?


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## Zog (Apr 15, 2009)

JRaef said:


> Just out of curiosity, why do you care?


 I was going to ask that too.


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## Tom45acp (Sep 6, 2011)

John Valdes said:


> Ohms law? You have a nameplate? Right?


Ohms Law? You don't need no stinking ohms law,especially since it does not take into account inductive reactance.


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## JRaef (Mar 23, 2009)

Zog said:


> I was going to ask that too.


But you didn't... :boxing:


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## sparky970 (Mar 19, 2008)

I had a trouble call. 480v motor, about 160v to ground on A phase. Pulled bucket, checked bus voltage. Meggered breaker, starter, and motor. Everything was fine. I had disconnected the load side of the breaker and I put the bucket back in and all the voltages were fine. Didn't see any bad connections either. I took the starter apart today and found a tiny piece of plastic but couldn't see where it came from. Contacts weren't pitted, but there were a few spots on the plastic near the contacts where it looked like it may have gotten hot


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## Splash (Apr 11, 2008)

Tom45acp said:


> Ohms Law? You don't need no stinking ohms law,especially since it does not take into account inductive reactance.


You don't need to worry about inductive resistance. Ohms law applies to the primary and secondary side independently. All you really care though is whether one side is not shorted or open.


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## JRaef (Mar 23, 2009)

sparky970 said:


> I had a trouble call. 480v motor, about 160v to ground on A phase. Pulled bucket, checked bus voltage. Meggered breaker, starter, and motor. Everything was fine. I had disconnected the load side of the breaker and I put the bucket back in and all the voltages were fine. Didn't see any bad connections either. I took the starter apart today and found a tiny piece of plastic but couldn't see where it came from. Contacts weren't pitted, but there were a few spots on the plastic near the contacts where it looked like it may have gotten hot


I don't know what that would have to do with the resistance of the CPT. Sounds to me like a motor or motor lead problem. Did you megger the motor and leads?

If you are not responsible for the motors (in some facilities that's a Millwright's job), then you did what you needed to do, you elimiated the other possibilities. I wouldn't waste any more time on the CPT. If you get the right voltage out of it, it's fine.


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## sparky970 (Mar 19, 2008)

JRaef said:


> I don't know what that would have to do with the resistance of the CPT. Sounds to me like a motor or motor lead problem. Did you megger the motor and leads?
> 
> If you are not responsible for the motors (in some facilities that's a Millwright's job), then you did what you needed to do, you elimiated the other possibilities. I wouldn't waste any more time on the CPT. If you get the right voltage out of it, it's fine.


I did megger the motor and leads. I had just done a simple resistance check on the cpt. Couldn't find any issues with anything else.


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