How am I getting 480v from each leg

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chris.doyle.391

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I was working on a 3 phase 277/480 full voltage motor starter with reverse, no delta. The relay won't open when power is turned on and motor keeps running. Relay is bad, but what I want to know what would cause me to get 480 per leg of the disconnect when the relays are connected and 277 when I disconnect the relay. I'm getting back fed some how. Does this make sense? I'm going to replace the starter but still want to know why I get funky voltages.


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chris.doyle.391 said:
I was working on a 3 phase 277/480 full voltage motor starter with reverse, no delta. The relay won't open when power is turned on and motor keeps running. Relay is bad, but what I want to know what would cause me to get 480 per leg of the disconnect when the relays are connected and 277 when I disconnect the relay. I'm getting back fed some how. Does this make sense? I'm going to replace the starter but still want to know why I get funky voltages.


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Go back and check things again. Turn all power off and make sure the motor starter contacts are all open. Make sure the mechanical interlocks are not stuck in one position or the other.
 
chris.doyle.391 said:
I was working on a 3 phase 277/480 full voltage motor starter with reverse, no delta. The relay won't open when power is turned on and motor keeps running. Relay is bad, but what I want to know what would cause me to get 480 per leg of the disconnect when the relays are connected and 277 when I disconnect the relay. I'm getting back fed some how. Does this make sense? I'm going to replace the starter but still want to know why I get funky voltages.


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What are you using to test your voltages with?
 
chris.doyle.391 said:
I was working on a 3 phase 277/480 full voltage motor starter with reverse, no delta. ...
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What can you tell us about the 480/277 system?
Is it grounded Wye?
Ungrounded Wye?
High Resistance Grounded Wye?

Could it be a 480 Delta system?

eit to add: And yes, as pt asked - we need to know where you are taking the measurements.

ice
 
no delta system. It's just ancient equipment. I checked voltage at the first disconnect over by the main. 280v reads at all three phases to ground. Then I check voltage at the disconnect where the crane is located 280v each phase to ground until I engage the disconnect. Once engaged I read 480 each phase. I was wondering if the motor might've acting like a transformer with the windings. It's just weird even with me reading 480 on the load side up down east and west work still. I can try a solenoid tester. This is more of a curiosity question because the equipment still works and once I replace the starter I'm sure west will finally stop when the button is depressed.


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chris.doyle.391 said:
no delta system. It's just ancient equipment. I checked voltage at the first disconnect over by the main. 280v reads at all three phases to ground. Then I check voltage at the disconnect where the crane is located 280v each phase to ground until I engage the disconnect. Once engaged I read 480 each phase. I was wondering if the motor might've acting like a transformer with the windings. It's just weird even with me reading 480 on the load side up down east and west work still. I can try a solenoid tester. This is more of a curiosity question because the equipment still works and once I replace the starter I'm sure west will finally stop when the button is depressed.


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You read 480v to ground... L1-G, L2-G, L3-G after the motor has started to run but 277 each phase to ground prior to it running?
 
You read 480v to ground... L1-G, L2-G, L3-G after the motor has started to run but 277 each phase to ground prior to it running?

Right but the motor isn't even running its once the disconnect is engaged I get those 480v readings per L1, L2, L3. It stumped me and the other electrician working on it. Only thing connected to the disconnect are the starters and they are open.


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I think I'm thinking to hard about it. It's got to be simple. This equipment is from the fiftys and hasn't run for awhile. 4ton hoist that doesn't stop.


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No the motor controls the hoist. Two different motors, up down for the hoist and back and forth for the other motor. Both connected to same disconnect


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The starter when energized is stuck closed and that's why the motor won't stop. I was just wondering why I'd be getting 480 per leg when the disconnect is engaged instead of what should be 277 per leg. It's baffling


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chris.doyle.391 said:
The starter when energized is stuck closed and that's why the motor won't stop. I was just wondering why I'd be getting 480 per leg when the disconnect is engaged instead of what should be 277 per leg. It's baffling


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That's why you need to verify your readings with a solenoid type meter. Until then, you can't rule out an erroneous reading from the DVOM, which is quite common.
 
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