Motor imbalance

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JES2727

Senior Member
Location
NJ
Two 240V submersible pumps, in a septic tank, are controlled by an alternating relay. Each one is protected by a 2 pole 20 amp breaker. Pump 1 is drawing 20 amps on L1 and 11 amps on L2. Pump 2 is drawing 11 amps per leg.
What would cause such an imbalance on Pump 1?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'd look carefully at the switching arrangement of the alternating relay. It doesn't sound like the two circuits are isolated.
 

rcwilson

Senior Member
Location
Redmond, WA
Is one leg of pump 1 supplying phase-neutral 120V control power? But 9A x 120V = 840VA is a lot of control power unless there are many relays, inidcating lights or a 120V space heater.

It could be a faulted winding with a high resistance fault or a fault located near the "neutral" point of the winding where the effective winding-ground voltage is small resulting in a small fault current below the setting of the protection device. Disconnect and test the motor.
 

JES2727

Senior Member
Location
NJ
Is one leg of pump 1 supplying phase-neutral 120V control power? But 9A x 120V = 840VA is a lot of control power unless there are many relays, inidcating lights or a 120V space heater.

It could be a faulted winding with a high resistance fault or a fault located near the "neutral" point of the winding where the effective winding-ground voltage is small resulting in a small fault current below the setting of the protection device. Disconnect and test the motor.

Yes, one leg supplies the control circuit. The control circuit consists of a couple of relays, a couple of indicator lights, and some float switches. The controls are protected by a 1 amp fuse, which blew last week. This week the 20 amp breaker protecting pump 1 tripped. The relays and contactors sometimes chatter like mad when I start poking around. I'm pretty sure the alternating relay is causing the chattering, but i didn't think it would cause the amperage imbalance. I'll replace the relay and see what happens next. I surely do not want to pull this pump motor out of the septic tank.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'll replace the relay and see what happens next.
There is a lot of simple testing you can and should do before replacing the relay, like temporarily direct feeding of each motor from each circuit, and see whether any readings change.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
101110-2109 EST

JES2727:

Find an access point where you can put a clamp-on ammeter around just the two hot wires to the motor. Do this for each motor. Both should read essentially zero. If you can not get around these two wires, then measure each hot wire individually. And as you know they should be the same. The point here is that the measurements are being made where there is no other load on these hot wires except the motor. If the currents do not balance, then there is a shunt path to ground or neutral beyond this measurement point.

If you can establish that the currents to the motors are correct, then you have to look for a shunt path between this point and where you previously measured current. That shunt path will somehow get to either ground or the neutral somewhere.

Relay chattering may imply low voltage, a badly designed alternating relay, loose connections, or some other abnormality.

Do some voltage checks at various conditions. Such as start-up, use min-max hold for start-up, and maybe both instantaneous and min-max for continuous run. If you are short of meters use incandescent bulbs to look for start-up problems. Three to five volts change at 120 V should be easy to see.

.
 
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