Unbalanced Amp Reading

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starsandstripes

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I have a new motor installation; 20 hp, 460v 3ph, nameplate 24 amps for a belt drive air handler. S.F 1.15. Two legs read 26 amps while running, but the third reads 21 amps. Loosening the belts did not drop the voltage and the driven shaft bearings are good. Voltage between all legs is around 478v; each leg to ground is 277v.
What are the causes for unbalanced amp readings and is this enough to cause overheating? I know it is under the service factor amps, but I still don't like it. Any comments would be welcome.
Thanks.
 
You could try swapping conductors to see whether the imbalance follows the change.

Are you taking the readings where you are sure the motor itself is the only load?
 
another suggestion -
turn off power to starter

manually press in starter armature so contacts make

measure resistance across each contact pole

easier to do this if you have one person holding in starter armature and the other taking measurement
 
RB:

have you taken fop (Fall of Potential) readings from the line of the MTR starter to the load of the MTR starter
Click to expand...

This is a better method to obtain contact resistance, particular if the load is balanced.

Another method is to utilize a DLRO (digital low resistance ohm meter)/micro-ohm meter.
 
My guess for the current imbalance would be either the motor itself, less likely, or voltage imbalance. The following article states, "The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance."

http://www.usmotors.com/products/ProFacts/1-127htm.htm

It may be that someone else's meter would give quite different voltage readings from what you've found,

((26-21)/26) * 100 = 19 % imbalance
 
It is a new Baldor motor, a-b 480v, b-c 477v, a-c 477v; readings did not change on line or load side of starter motor running or not. Fluke T-5 1000 has not been calibrated. Another T-5 gave similar readings but half an amp lower, voltage the same. Simpson 260 showed 0 ohms resistance on the highest scale across starter contacts but I changed them any way. I swapped the T leads and the lowest amp reading did not follow that wire. Now my readings are 25.5, 26.7, 23.4. Ave. 25.2-23.4=1.8 or around 7%. Is this correct or should I be taking the difference between the highest and the lowest readings as a percentage of the average?
I think I will check the filters and dampers to bring all three readings down; the nameplate amperage is 24. Thanks for your input.
 
Resistance readings (across contacts) with a fluke or Simpson multimeter IMO are pretty useless, Please try utilizing FOP to perform this as stated above. Also did you check that link on voltage imbalance resulting in current imbalance?
 
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