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05-03-2012, 10:02 PM
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#1
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Electron Flow Consultant
Join Date: Mar 2008
Location: Longview, WA
Posts: 2,091
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control power transformer
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.
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05-03-2012, 10:04 PM
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#2
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Senior Member
Join Date: Apr 2009
Location: Charlotte, NC
Posts: 4,347
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Quote:
Originally Posted by sparky970
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.
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Depends on the CPT ratings.
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05-03-2012, 10:14 PM
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#3
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Electron Flow Consultant
Join Date: Mar 2008
Location: Longview, WA
Posts: 2,091
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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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05-04-2012, 08:55 AM
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#4
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Senior Member
Join Date: Apr 2009
Location: Charlotte, NC
Posts: 4,347
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Quote:
Originally Posted by sparky970
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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Sounds reasonable, depending on what you used to take the reading.
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05-04-2012, 12:13 PM
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#5
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Senior Member
Join Date: May 2007
Location: South Carolina
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Ohms law? You have a nameplate? Right?
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05-04-2012, 02:12 PM
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#6
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Senior Member
Join Date: Mar 2009
Location: SF Bay Area, California
Posts: 1,535
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Quote:
Originally Posted by sparky970
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.
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Just out of curiosity, why do you care?
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05-04-2012, 03:48 PM
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#7
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Senior Member
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Location: Charlotte, NC
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Quote:
Originally Posted by JRaef
Just out of curiosity, why do you care?
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I was going to ask that too.
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05-04-2012, 04:30 PM
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#8
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Senior Member
Join Date: Sep 2011
Location: Shinnston, WV
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Quote:
Originally Posted by John Valdes
Ohms law? You have a nameplate? Right?
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Ohms Law? You don't need no stinking ohms law,especially since it does not take into account inductive reactance.
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Sometimes I wonder whether the world is being run by smart people who are putting us on, or by imbeciles who really mean it.
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The Following User Says Thank You to Tom45acp For This Useful Post:
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05-04-2012, 04:32 PM
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#9
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Senior Member
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Quote:
Originally Posted by Zog
I was going to ask that too.
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But you didn't... 
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05-04-2012, 04:47 PM
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#10
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Electron Flow Consultant
Join Date: Mar 2008
Location: Longview, WA
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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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05-04-2012, 05:14 PM
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#11
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Member
Join Date: Apr 2008
Location: DeRidder, LA
Posts: 36
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Quote:
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Originally Posted by Tom45acp
Ohms Law? You don't need no stinking ohms law,especially since it does not take into account inductive reactance.
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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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05-04-2012, 05:58 PM
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#12
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Senior Member
Join Date: Mar 2009
Location: SF Bay Area, California
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Quote:
Originally Posted by sparky970
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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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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Last edited by JRaef; 05-04-2012 at 06:00 PM.
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05-04-2012, 06:49 PM
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#13
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Electron Flow Consultant
Join Date: Mar 2008
Location: Longview, WA
Posts: 2,091
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Quote:
Originally Posted by JRaef
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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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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