Strange Motor Amperage Readings

[wireddd]

Have a customer with a farm with numerous motors from 5hp thru 50hp for for conveyors and bin dryers etc. Customer recently changed out a 25hp motor and when done noticed that on the control panel one phase for motor was drawing half of full load while the other legs were showing full load. Upon our checking we found that all the motors were doing the same thing. (One leg at half current of full load) This is a 240 volt open delta service.
Had the power company out, they replaced one transformer, said everything was fine, all voltages between phases are normal but still the one leg of all motors is still off.

Any thoughts would be appreciated.

 

[nakulak]

what were you using to measure the amperage, and what do you mean "off" (you started out saying "half load")

 

[Rick Christopherson]

I can't quite put my finger on how this could be happening, but I feel it has something to do with the unbalanced neutral position associated with an open delta. Because you commented that the amperage was recorded by a control panel, it makes me wonder if the amperage sensors are somehow recording phase-neutral currents.

How that could be happening doesn't make much sense, but if it were happening, it would make perfect sense. If you measured the supply current through the transformer, you would find that the amperage from neutral to the stinger phase would be about half of the amperage from neutral to the other two phases.

Oh, just a thought on that note; is there any chance this person took the "wye configured" term too literal and connected the motor to the external neutral? I have run into some people that mistakenly believe that a wye configured motor refers to how it should be connected to the supply, as opposed to how it is internally configured.

Otherwise, you could have a dropped phase and the amperage you are reading is generated current, not consumed current. The more I think about it, the more likely this sounds, especially if this system has multiple motors, where each one will act as a very poor, speudo phase converter.

 

[wireddd]

There isn't any neutral at the motor JC as all the motors are three phase with a ground wire.
With everything shut off and the service main in the off position all vlotage measurements between phases and phase to ground as well as phases to neutral are all normal.
No problems with any of the lighting circuits either, just the motor loads on one phase being half of the other phases

 

[Besoeker]

There isn't any neutral at the motor JC as all the motors are three phase with a ground wire.
With everything shut off and the service main in the off position all vlotage measurements between phases and phase to ground as well as phases to neutral are all normal.
No problems with any of the lighting circuits either, just the motor loads on one phase being half of the other phases

Have you measured the voltages at the motor terminals?

 

[wireddd]

Yes all are 238 to 242 between phases under load. No other problems reported other than motor amperage being 1/2 on one phase

 

[ZElectric]

3 Phase problem

3 Phase problem

Try this. Go to the weather head and measure the voltage phase to phase and then phase to ground. If you get phase to phase 240v then check phase to ground. There are some 240v three phase services that are only 3 wires. In this case you should find one phase is grounded. Phase to ground on this leg is 0 volts, and phase to ground on the others is 240v. If you have a 4 wire service then you should have 240v phase to phase and phase to ground is 120-120 and 208v on the teaser leg(should be taped orange). If your voltages are ok then check the high side of the main breaker and then the load side of the main breaker to see if they are the same as the voltage you checked at the weather head. If all is the same then go to the motor disconnect and check the high side and load side of the disconnect. Then go to the motor and do the same. If all the voltages are the same and there are no bad breakers/fuses/connections then the next step is to look at the motor wiring diagram to see if it is connected correctly, or wired correctly. You may have to take the motor out and have someone check it in a shop.
Good Luck, and have FUN

 

[brian john]

Cut your job in half check voltage at the service and at the motor. If there is a major difference then check all conductors and distribution equipment in between.

1. Is this a 3 phase 3 wire or 3 phase 4 wire?
2. What is the phase to phase voltage and phase to ground?
3. What are the phase currents?
4. Are you holding the amp clamp at a 90 degree to the conductor with the conductors dead center in the CT?
5. We had a site where everything was perfect and the currents were like off like 8 amps (it has been 5 years). Everything check out including the compressor (the customer had carrier out), Chiller is still operating.

http://leconsystem.com/Articles/Technical paper 1.pdf