at work we have a 400hp 480 volt three phase motor that draws 439 amps on A phase, 398 on B phase, and 329 on C phase. the primary voltage is 14400 stepped down through a Y Y connected set of 500 kva cans we have replaced all the connections. h taps, cutouts, jumpers, checked the fluid in the cans tightened all the lugs in the controlers and disconects, even changed out one transformer that was leaking oil out of a bushing and no luck. we took a buckett truck down the line to the substation and no loose or bad conections. any suggestions?
three phase motor unbalance on amps
- Thread starter cmtskx500
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Open Neutral
Senior Member
- Location
- Inside the Beltway
- Occupation
- Engineer
Mark the phases [ABC] and the motor leads [123].
A1
B2
C3
Measure the currents.
Now rotate the connections:
A2
B3
C1
and measure again. What happened?
ptonsparky
Tom
- Occupation
- EC - retired
and if your current is still a bit wide do it again. Choose the combination with the smallest imbalance. Take your time, don't exceed the max starts per hour for that motor.
Works especially well with open deltas.
While you have it off, meg it. Check voltage before, during and after start.
Works especially well with open deltas.
While you have it off, meg it. Check voltage before, during and after start.
rcwilson
Senior Member
- Location
- Redmond, WA
What are your voltage readings phase-phase and phase to ground with motor off and motor running? Measure at the motor starter.
Look at the voltage drop or difference voltages when the motor is running for for evidence of a high resistance joint. If you have already checked the connections, maybe it is in the starter contacts. If it is safe to do so, measure the millivolt voltage drop across the starter contacts or do an infared scan to see if there is any excessive heating.
Maybe the three individual transformers in the Y-Y bank are not set on the same tap. A minor difference in voltage could cause the 15% difference in currents you are seeing.
Look at the voltage drop or difference voltages when the motor is running for for evidence of a high resistance joint. If you have already checked the connections, maybe it is in the starter contacts. If it is safe to do so, measure the millivolt voltage drop across the starter contacts or do an infared scan to see if there is any excessive heating.
Maybe the three individual transformers in the Y-Y bank are not set on the same tap. A minor difference in voltage could cause the 15% difference in currents you are seeing.
broadgage
Senior Member
- Location
- London, England
I would suspect slightly unbalanced line voltage.
Unbalanced currents are not normally a problem on lightly loaded motors, provided that the greatest current is still within the nameplate rating.
If however the current on any phase exceeds the nameplate rating, then overheating and reduced life are likely.
A less likely cause is a fault in the motor, this may be determined by swapping connections as suggested above.
Different tappings selected on transformers, different size transformers on each phase, different sized primary or secondary wires, or different makes of transformer can all give slightly unbalanced voltages.
You may be able to to partialy correct an unbalanced supply voltage if the facility contains any large single phase loads such as step down transformers for 120 volt lighting and small appliances.
Some or all such loads should be transfered to the highest voltage phase, in order to increase voltage drop on the higher voltage phase, and reduce the load and therefore the voltage drop on the less loaded phase.
Unbalanced currents are not normally a problem on lightly loaded motors, provided that the greatest current is still within the nameplate rating.
If however the current on any phase exceeds the nameplate rating, then overheating and reduced life are likely.
A less likely cause is a fault in the motor, this may be determined by swapping connections as suggested above.
Different tappings selected on transformers, different size transformers on each phase, different sized primary or secondary wires, or different makes of transformer can all give slightly unbalanced voltages.
You may be able to to partialy correct an unbalanced supply voltage if the facility contains any large single phase loads such as step down transformers for 120 volt lighting and small appliances.
Some or all such loads should be transfered to the highest voltage phase, in order to increase voltage drop on the higher voltage phase, and reduce the load and therefore the voltage drop on the less loaded phase.
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