3 Phase Motor Troubleshooting?

randyjaco

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I just picked up an old Yates American jointer. When I cut on the motor, it buzzes and turns slowly. Being a 3 phase there are no condensers or start switches to check. The fly in the ointment is that this is a direct drive machine. The cutting head and rotor are one piece, just swapping motors is not an option. Everything turns very easily, no bad bearings or obstructions.
Any thoughts on what might be wrong?

Thanks
Randy
 
Make sure you have power on all legs of the 3 phase
 
After making sure you have proper power, with the power disconnected, ohm out the leads to each other. 1-2, 2-3, and 1-3. They should read similar and fairly low. Then ohm out each leg to the motor case and see if you read anything at all. If you read anything, that's a bum motor but it usually trips the breaker.

More than likely you are single phasing the motor by not having all three legs at the proper voltage and correct phasing.
 
Assuming you have a magnetic starter, disconnect the motor leads and check voltage at the T leads of the contactor to make sure all contacts are closing. Any chance it's wired for high voltage?
 
I just picked up an old Yates American jointer. When I cut on the motor, it buzzes and turns slowly. [three phase motor[ Everything turns very easily, no bad bearings or obstructions.

Now is the time to look for sawdust in the switch or contactor. Burnishing the contacts
will usually help. You just have to find them (all of them) first. The buzzing means (usually) a
phase has gone open, so an exploratory tightening of every electrical connection would be
in order also.
 
Hi Randy,
being a saw, i'd be suspect of the main switch.
to test, disconnect power, get an ohm meter and inspect the power cord and plug connections
take a phase to phase reading at the cord to see if there is a short
take a phase to ground reading for each phase and see if there is grounded out wiring
remove the either incoming power cord or the motor cord from the switch, so that you can test the switch.

testing the switch,
most saw switches are linear in operation. the top 2 terminals are one pole, the next 2 terminals down are pole 2, the third set of terminals are the pole 3
with the sitch on the off position test the 2 top terminal together the meter should read open, then flick the switch and retest- the meter should read low resistance
repeat for each subsequent pole, noting any variations
reinstall wiring and tighten the incoming power connections if these tests are sufficient.

a visual inspection of the motor and associated connections are important, if no obvious faults are seen you can test the 3 phase motor.

testing the motor,
test pole to pole with an ohm meter, then test the last pole to the first pole- there should be resistance measured on both poles in respect to the first.
test pole to ground of the motor frame to each pole, there should be no continuity in this check.

should there be an open in the winding of any of the 3 motor poles, the motor will need to be repaired or rewound

reconnect motor power cord to switch and tighten connections if the tests showed no faults


i wish you the very best of luck! :)
 
Last edited:
I played with it today, still no improvement.
Here's the motor plate data
Line start induction motor
Volts 220
Amps 3
HP 1
Phase 3
Cycles 60
RPM 3600
Serial 276247
Frame 512
Style C
Type CT
Temp 40
Time cont

I have 3 good legs from the wall.
I rehabbed the switch. No change. I then bypassed the switch. Still humming with slow rotation.
I only have 3 wires coming from the motor. To get to the others I am going to have to disassemble the whole cutter head. I was hoping to avoid that. Any tests that I can do with those 3 wires, prior to disassembling?

Interesting situation : at the wall I got 120 vac on each leg and 240 combining them. On the switch I got 121vac on each leg and from 260 to 290 combining the legs. How does one get more voltage at the machine than the wall?

Thanks
Randy
 
A lot of times when a 3 phase motor is single phasing (missing one leg) it will just sit there and hum, but my gut tells me you are missing a phase somewhere down the line....

Good luck,
Ted
 
Please forgive me if I’m being Captain Obvious on this... I have no idea what your level of experience is and I just thought I would offer this to help you troubleshoot your circuit. If this is basic stuff to you, maybe someone else can benefit from it.

In the attached picture, I show a partial 3-phase circuit for demonstration. The source is at top. A,B & C are terminal connections. Please note that a wire or a component (contact, connections, whatever) on the middle phase from the source is broken, so no current flow through it from the middle leg of the source to the B terminal. The L is a coil, or maybe some other component like a motor, relay, transformer, etc.. Something that current will flow through.

When testing for proper 3-phase voltage on the terminals A, B & C the measurements can be misleading if not properly applied. Measuring to ground, you will see 220v on all three points A, B & C. Now, there is a broken connection on the middle leg, but you will see a voltage there because it is being back fed through the coil from the C terminal. This could make you think that you have 220v on all three legs when, in fact, you only have 2 legs and are seeing leg C twice.

Further, measuring from A to B you will see 440v, again B is being back fed. Measuring A to C you will see 440v and this is correct because you have both of these legs. Now, checking from B to C you will see zero volts and this will show you are in fact missing the middle leg B.

I hope this makes sense... and I’m not insulting anyone’s intelligence.

Good luck,
Ted
3-phase diagram.jpg
 
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