View Full Version : Problems with a 480V three phase rotary air compressor

Eric MFG
07-01-2012, 10:08 PM
Hi Guys,

I’m having some problems with a 480V three phase rotary air compressor, the compressor keeps blowing a single time delayed fuse. I was performing a quick and dirty ohm test on the motor not sure what the coil windings resistance should be. Basically I tested each leg to earth ground my readings were, L1 and L2 were 1.5 ohms and L3 was 65 ohms. Does anyone know where I could find motor technical specifications?

I know the resistance will be relative to horse power ratting voltage exedra.

Also if a motor is rated 40 amps 480 volts does that mean 40 amps is the total draw of all three legs combined? So would the 40 amps be divided in to each leg? Knowing voltage and current draw per leg I could figure out the resistance with ohms law or somthing close.

Thanks Eric,

07-01-2012, 11:37 PM
You cannot get a meaningful resistance measurement between phases of a 3ph induction motor, the winding resistance will be very low, if you are getting any kind of resistance to ground then the motor has a problem, it should be very high or open circuit?

07-03-2012, 01:08 PM
I'd expect each leg to run 40 amps.

07-03-2012, 01:16 PM
And generally this is FLA , Full Load Amps so off load would be considerably less.
The difference between each phase should be no more than a couple of %.

07-09-2012, 08:21 PM
Sounds like you have to replace that motor.

Something I found to help you determine if a motor is ok or what is wrong with it.

How to check the Windings of a 3-Phase AC motor with an Ohmmeter (http://pcbheaven.com/userpages/check_the_windings_of_a_3phase_ac_motor/?topic=worklog&p=0)

I havn't found a good referance for the actual values you should measure. I know it is partly because what is acceptable can vary a bit for different motors and motor types.

I think you can usually use any isolated coil should read 0.1-9 ohms
Any isolated coil to ground or another coil 500,000 to 1,000,000+ ohms

07-09-2012, 09:26 PM
On a three phrase motor usually there is a motor nameplate. That should give you some basic electrical info. What is the rating of the time delay fuse that keeps tripping? Also normally each phrase should be fused. Normally those fuses (3 ea) should have the same rating. FanFan