3 phase compressor

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JdoubleU

Senior Member
What is an acceptable current unballance on a three phase compressor? I have a fuse that keeps on blowing but when I replace the fuse and put a clamp meter on it it reads fine. One phase though is about 5 amps more than the others.
 
augie47

augie47

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State Electrical Inspector (Retired)
the devil will be in the details.... 5 amps on a 200 amp compressor is insignificant, on a 10 amp, different story :)
What is the compressor FLA and what size fuse are you using ?
 
Electric-Light

Electric-Light

Senior Member
Jakewhis said:
What is an acceptable current unballance on a three phase compressor? I have a fuse that keeps on blowing but when I replace the fuse and put a clamp meter on it it reads fine. One phase though is about 5 amps more than the others.
Click to expand...

Acceptable unbalance is 5% for motors.

Details below.

Are you running this off of a true 3 phase service or an idler RPC? Current unbalance is greater if you're using an idler because of the way it works.

If it's running off of a 3ph utility, one technique used is to rotate the wiring

If it's wired:
_A_
B.C

Try
_B_
C.A

Then

_C_
A.B

Use the configuration that yields the least current unbalance

Measure current at all three legs and write them all down, then take the average.

say 24, 27, 29

(24+27+29)/3 = 26.7
26.7 = mean

Find the one that's furthest from average, so
26.7-24 = 2.7
or...
24-26.7 = |2.7|

Take the difference over the mean (if the highest deviation is higher than average, then use the absolute value)

2.7/26.7 =0.101 = 10.1% current inbalance.

If you tried all three configurations and can't make it be below 5%, you need to address it upstream.
 
ptonsparky

ptonsparky

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Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
The formula I have for imbalance indicates anything over 2% should be investigated. Check your voltage as well. Rotate the leads and compare imbalance at each position. If the problem rotates with each change it is the motor. If it is stays on phase ___ it is the source.

Check for a poor connection at the fuse holder or close to it. As stated 5 amps difference is relative to the load.
 
iwire

iwire

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Location
Massachusetts
Electric-Light said:
Acceptable unbalance is 5% for motors.
Click to expand...

No, not even close to 5% more like less than 1% before you should start derating the HP of the motor.

At 5% imbalance the motor should be derated by 0.76 which is significant.

http://www.engineeringtoolbox.com/electrical-motor-voltage-imbalance-d_648.html

http://progress-energy.com/custserv...ntations/MotorProtection_VoltageUnbalance.pdf

http://www.phaseconverterinfo.com/phaseconverter_voltagebalance.htm


http://www1.eere.energy.gov/industr...minate_voltage_unbalanced_motor_systemts7.pdf
 
I

IslandSparky

Member
Do you have any accessories that are single-phase? Dryers, controls, etc. Barring that, for that small of a compressor you clearly have a problem. Have you looked at the current and voltage phasors?
 
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