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Discussion Starter · #1 ·
Hi everyone,
I got a call today to check out a problem at the local golf course. The irrigation pump was working this morning, then the guys shut it off to go for coffee. After coffee they went to switch it back on....nothing.

I got there and tested the voltage at the disconnect. It was 470v across L1-L2. 470v from L1-Gnd. 2v from L2-Gnd. When I turned on the disconnect and checked the voltage at the contactor and disconnect and my voltage was 25v L1-L2. 470v L1-gnd. 470v L2-gnd. I turned the disconnect off.
I was checking the fuses and then I checked the voltage again and now I was reading 240v from L1-L2 for some weird reason. So I took the golf cart to the step up 240v-->480v transformer to check the voltages and I have 484v from L1-L2.
I went back to the pump disconnect and still only 240V.
I don't know if the transformer is messed or why my voltages are changing.

I hope I explained this right.
Hope someone can help!
The motor is a 30hp 480volt 1phase beast.
 

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Do you think you may have been looking at this backwards? The disconnect you checked at first was line side of a 480 to 240 volt step down transformer feeding a 240 volt motor? What does the motor nameplate read?
 

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Off the top of my head I'd think you have a high resistance ground fault INSIDE of the disconnect mechanism itself, likely on the L2 pole.

For the benefit of others, people make custom 480V single phase motors all the time for rural farm applications, because the PoCos don't want to run 3 wires or even 2 out to a farm that doesn't use a lot of power all of the time (no revenue for them). So they feed them with a Single Wire Earth Return (SWER) system that uses a single 12.7kV line, and they hook up a transformer that is 12.7kV Line to Ground on the primary, 480V line to line on the secondary.
 

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Off the top of my head I'd think you have a high resistance ground fault INSIDE of the disconnect mechanism itself, likely on the L2 pole.

For the benefit of others, people make custom 480V single phase motors all the time for rural farm applications, because the PoCos don't want to run 3 wires or even 2 out to a farm that doesn't use a lot of power all of the time (no revenue for them). So they feed them with a Single Wire Earth Return (SWER) system that uses a single 12.7kV line, and they hook up a transformer that is 12.7kV Line to Ground on the primary, 480V line to line on the secondary.
I didn't say they don't exist, but I've never seen one. In this case though, he says they're stepping up to 480. It's just unusual to me. But I've only seen a little bit of what's out there.
 

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... In this case though, he says they're stepping up to 480. ...
So I took the golf cart to the step up 240v-->480v transformer to check the voltages and I have 484v from L1-L2.
You're right, I missed that, but also referring to the LOAD side of a step-up transformers as "L1 and L2" is confusing to me. I would have said H1 and H2 (or as labeled)

Very odd that they would do that, then go have a custom 480V 1 phase motor made. Trying to get around a voltage drop issue maybe by boosting the voltage to 480 thus reducing the current? I know that golf course people can get really picky about locating electrical equipment.
 

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Unless this is hacked together (and because it's a golf-course water feature, there's about a 99% chance it is), it's possible your L-G measurements are fine because one leg of your 480V transformer should be grounded. You need to confirm if this is actually the case.

That said, I think you have a broken feeder conductor.

  • It would provide a solid ground on one leg and explain the 0V to ground, and still allow 480V L-L.
  • With the disconnect on, if the resistance of the earth was in series with the load, you'd have low current flow, and low voltage drop, and it would explain the 25V L-L. But you shouldn't have 470 L-G on both legs.
  • If you do have a ground fault with a changing impedance, it would also explain why your L-L voltage kept changing.
But there's other stuff there that's a little out-of-whack, too, like the voltage drop at no load. These voltages are a little suspect, I'm guessing your using a high impedance meter? Get a wiggy and take your measurements again.
 

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Discussion Starter · #16 ·
Broken feeder conductor

I just wanted to say thanks to everyone for your help and you guys guessed right it is a broken feeder conductor. Now we need to find someone with that fancy locating tool to tell me where the break is.

I snapped a pic of the motor nameplate: but I don't know how to attach it without getting a "too many character error".
here is the info:
Manufacturer: Written-Pole
synchronous ac motor
30hp
480v
1 phase

 

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The fancy locating tool you are likely thinking of is called a TDR, Time Domain Reflectometer.

Written pole... those are really odd ducks.
 

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Discussion Starter · #20 ·
The Secondary is bonded, Thanks!

I am using a fluke multimeter. I've never heard of a wiggy until yesterday.
My measurements were a little weird. At one point yesterday I had 480v-gnd on L1. And 480 L1-L2. Which makes sense that L2 was the culprit. When I left yesterday, at the tranny I had 480v on H1-H2. 240v from H1-gnd and 240v from H2-gnd. At the line side of the disconnect I had 240v Across L1-L2, 240v from L1-gnd, 2.5v from L2-gnd.
Today my numbers were the same. So I disconnected the H1 and H2 from the tranny and had my apprentice hold L1 and L2 together at the disconnect and I checked for continuity from the transformer and there was nothing. Then he held L1 to the white wire of the underground triplex (ground) and at the other end I checked for continuity and I had it. Then we tried L2 to the white ground and nothing!
So i'm pretty confident that L2 is busted.

You guys are the best!

Now I just need to get my hands on that TDR!
 
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