# Motor Amperage Indifference



## alpha3236 (May 30, 2010)

I work on a couple of similar installations in my town (250hp submersibles). If you are confident that the problem is not in the starter, you have 2 choices: 1) Get permission from the city (in writing if you can) and set up your imbalance parameters, then run it till it fails. Or 2) pull it.
2 years ago when the well outfit was pulling one of ours they dropped it to the bottom of a 600" hole and the cable gat all wadde up around the 10" galvanized pipe in the bottom of the hole. Had people on water rationing all summer till they finally got it fixed.


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## Mike in Canada (Jun 27, 2010)

When the pump was pulled did you try meggering the cables without them being connected to the peckerhead of the motor? Did you megger the motor disconnected? It's nice to know for sure that the problem isn't in the motor.


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## micromind (Aug 11, 2007)

In my experience, submersible pump motors are notorious for current imbalance. 

I just posted this in another thread, but It applies here too. 

Try rolling the phases. I know it sounds silly, but often it'll result in better current balance. 

If you're using brown for A, orange for B, and yellow for C, try putting brown on B, orange on C, and yellow on A. The rotation is the same, but the phase angle could possibly be more suitable to the motor in question. Go the other way too, put brown on C, orange on A, and yellow on B. 

Since most soft-starts have by-passes, I'd do a FOP (Fall Of Potential) test across the soft start, as well as the breaker that feeds it. 

It's easy, you need the pump running, and you measure voltage across each pole. Line of A to load of A, line of B to load of B, same with C. You're not looking for a specific number here, but difference between phases. 

The readings will likely be less than 1 volt, if they're all close to each other, the device under test is good. If two phases read say, 300mV and the other one reads 1200mV, you've found a problem. 

Rob

P.S. the results of the megger test worry me a bit. When meggering a motor, all 3 lines should read the same. This is because each line is basically shorted to the other two through the windings. When meggering anything involving electronic controls, always disconnect the electronic stuff first. And make sure each of the lines is isolated when testing.


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## Big John (May 23, 2010)

micromind said:


> ...P.S. the results of the megger test worry me a bit. When meggering a motor, all 3 lines should read the same. This is because each line is basically shorted to the other two through the windings....


 Agreed. I'd repeat the megger test. For all intensive purposes a connected motor winding is just one conductor. There's no good reason that different phases should have substantially different insulation resistance values.

-John


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## CHECKtheE-STOP (Jan 17, 2011)

Did you check the resistance between the three motor leads? Their resistances should be very close to each other. I have seen motors get single phased and still run but they usually end up with an unbalanced load due to damage cause to the windings.


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## DRofElectricity (Apr 10, 2010)

Micromind, we tested voltage at every spot around the soft start and motor saver and all are the same. We clocked all the leads or rolled it as you said and still no change. Yes, we megged the motor with the leads off the terminals so there was no connection with the soft start and the contactor. Single phasing does kinda make me wonder but we have a pump saver on it that gives us readouts on all faults in the past and nothing has showed up and even if it did the pump saver kicks out the motor. But our city just put in a new well (which is having problems too, ran for 6 hours and a bearing went out and destroyed the motor and impeller) so soon as that goes on line it will take care of the part of town that this bad pump is running. We decided that we will run the motor till it pewks.


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## JRaef (Mar 23, 2009)

Depending on the brand of Soft Starter, many of them must have a minimum value of current in order to satisfy the Phase Loss / Imbalance protection circuit, typically around 20-30% of the _*starter rating*_. If the pump motor is unloaded because there is no flow, that will often be the case. Because a soft starter is a voltage control device, the only way they can accurately detect phase loss or imbalance is by monitoring current. So if the current in ANY PHASE is lower than the threshold after a predetermined wait state (typically 3 seconds), the starter assumes you have a phase loss or severe imbalance. The problem is, IF ALL 3 PHASES are below the threshold, it still satisfies the trip condition.

Some starters, such as Motortronics (and by default Toshiba who brand labels it) allow you to defeat this feature if you are using a soft starter that is over sized compared to the motor.


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