# Single & Three Phase Motor Loads on Three Phase Circuit



## josefbartos (Sep 4, 2010)

NEC Motor Article 430 does not address three phase and single phase motors on a three phase circuit/feeder. The single phase motor can cause some voltage and current imbalances that can potentially damage the three phase motor.
Internet link:
http://www.engineeringtoolbox.com/electrical-motor-voltage-imbalance-d_648.html
Indicates the following:
Voltage imbalance between phases should not exceed 1%
Voltage Imbalance % Derating Factor
0 1.00
0.5 1.00
1.5 0.97
2.0 0.95
2.5 0.93
3.0 0.89
3.5 0.85
4.0 0.82
4.5 0.78
5.0 0.76
Please, does anyone have more information about this topic?


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## brian john (Mar 11, 2007)

A voltage imbalance by a single phase motor would depend on quite a few items.

Capacity of the utility transformer and distribution system.

System voltage.

Size of the single phase motor anything above 5 hp is rare.


I did a PQ study for a high school that had voltage imbalance issues and MAY still have the information, If I locate it I'll post the information.

But I am unsure how to utilize the derating table you posted.

Seems it should say 1% imbalance-derate by X%.


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## wildleg (Apr 12, 2009)

I would ask the editor of the site you got that table from for information (his email link is posted on that same page)

If I am reading that table right, a 100 HP motor on 480 would only be good for 75 HP if one of the phases was 24 volts off (like, say, 477,453,477 ?). That might be a ball park guess but my gut tells me it's off.


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## micromind (Aug 11, 2007)

Any voltage imbalance to a 3 phase motor will cause an increase in current. This is because when a higher voltage appears on one of the phases it causes a slight increase in speed, and thus higher current. When a lower voltage appears, this causes a slight decrease in speed, and the motor will actually become a generator for a very short time. This will also cause an increase in current, both when it shoves current back down the line as well as when it has to accelerate during the next phase voltage peak. 

The table in the first post is correct. 

Using Wildlegs example, a 24 volt imbalance on a 480 system is roughly 5%. Using the table, a 100 HP motor operating at a 5% imbalance could be expected to produce 76 HP. 

In reality though, it could very likely continue to produce its full 100 HP, because most motors have a service factor of more than 1.0

Service factor is the absolute maximum HP a motor can produce when it's being operated at exactly nameplate voltage (measured at its terminals), perfect phase balance, and 40˚C still air. In the example above, assuming a service factor of 1.15, the 100 HP motor is actually a 115 HP. It's rated at 100 HP because perfect conditions do not exist in real life. 

Rob


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## brian john (Mar 11, 2007)

wildleg said:


> I would ask the editor of the site you got that table from for information (his email link is posted on that same page)
> 
> If I am reading that table right, a 100 HP motor on 480 would only be good for 75 HP if one of the phases was 24 volts off (like, say, 477,453,477 ?). That might be a ball park guess but my gut tells me it's off.


I'd bet that is close to right as noted by Micro


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