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Single phase electric motor testing

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Hi All

I have a single phase electric motor which I am testing as I suspect one of the windings has gone (exhausted all other possibilities). It got a start and run capacitor, a centrifugal switch, and a thermal overload switch. Its from a 3hp air compressor. It starts up and run for about 3-4 seconds pretty slowly and then blows the fuse in the power plug (UK plugs have 13a fuses in them). I've tested the amperage draw with a clamp ammeter and it seems to remain at around 45A whilst it runs for these 3-4 seconds. Normally, when I test with a clamp ammeter on these things they drop from 45A to about 9A within a second or so as the start winding is deactivated by the centrifugal switch. I suspected the centrifugal switch was not working so I uncoupled the piston assembly from the motor and spun it up with a pneumatic impact driver and watched the meter go from 001Ω to Open Circuit - so the centrifugal switch is definitely working.

If you see my attachments below I've done some testing on the windings with my multimeter, set to Resistance Ω (Auto Range) setting.

From what I understand the blue wires are the start winding and the red is the run winding. Haven't a clue what the black wire is though, as all literature I've read on these types of motors never show a 'common' wire? Would love to know what it is and how its integrated into the windings.

Anyhow, here are my test results:

R1 to R2 = 001.6Ω
B1 to B2 = 000.0Ω

Bk to R1 = 001.7Ω
Bk to R2 - 001.1Ω

Bk to B1 = Open Circuit
Bk to B2 = Open Circuit

R1 - B1 = Open Circuit
R1 - B2 = Open Circuit
B1 - R2 = Open Circuit
B2 - R2 = Open Circuit

Any help much appreciated - I don't know how to determine whats wrong from the readings.
F.


 

MaxHeadRoom78

Well-Known Member
What I suspect you have is the low resistance run winding R1,R2, and you may have a split start run winding that the cap and centrifugal SW uses one of the sections, the other is in circuit all the time as a start/run winding.
The odd readings on the start winding (0) may indicate a shorted winding.
May require a strip down to confirm dark/burnt/ shorted winding section.
Max.
 
What I suspect you have is the low resistance run winding R1,R2, and you may have a split start run winding that the cap and centrifugal SW uses one of the sections, the other is in circuit all the time as a start/run winding.
The odd readings on the start winding (0) may indicate a shorted winding.
May require a strip down to confirm dark/burnt/ shorted winding section.
Max.
Thanks for your input Max.
There is nothing obviously burned out, I've a look around inside already.
I do have another identical compressor with motor, and that one works perfectly - so I decided to take the readings on the known good motor, and I've posted them below - does this perhaps prove anything? Only two of the readings were different, BK - R1 and Bk to R2

 

MaxHeadRoom78

Well-Known Member
Aha, The capacitors on Chinese motors are Notorious for failure.
Often within a year of operation.
If you replace, use AC motor rated versions and NOT Chinese origin.
Max.
 
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