Trying to diagnose induction motor buzzing

[AndySomogyi]

This motor used to basically work when I got the lathe. But I cleaned every bit on this lathe, and re-wired the whole thing because wiring was all rotted.

Now the motor spins but buzz very badly.

What I’ve done:

1. completely bypassed drum switch and wired motor direct to contractor according to diagram on motor
2. Tested capacitor to ensure that it’s charging. Hooked up to ohmmeter, resistance gradually drops.
3. Checked cap with multimeter, shows it as 600 micro F.
4. disassembled motor and verified starter centrifugal switch contacts are free.
5. Centrifugal switch seems to mechanically operate, and can clearly be heard engaging when motor slows down
6. Verified motor is getting 240v
7. Each motor winding shows about 1 ohm, or 2 ohms when both winding hooked up in series
8. The motor looks essentially brand new.
9. The rags on top of the motor are just there to temporarily keep a shaft from rubbing the motor housing.

Here are a few schematics and a video of the behavior. I cleaned the lathe, maybe some gunk got in the motor, but took the motor apart and looked clean.

 

[RJSakowski]

You are wired for high voltage running and your notebook agrees with the wiring diagram. It sounds like the start winding isn't cutting out.. The diagram shows two T5's, one on each side of the capacitor/centrifugual switch. Is it possible that you have those two wired reversed?

 

[AndySomogyi]

I’ll have to double check again, but I’ve completely bypassed the barrel switch, and wired the motor direct.

I can see about putting a volt meter across the cap and make sure the centrifugal switch is actually cutting out.

 

[Ulma Doctor]

can you show a picture of the actual wiring pairs?
it sounds to me like we have wires out of order, or hooked up incorrectly.
there is a possibility of one of the windings could be shorted

 

[RJSakowski]

The sound I am hearing is what I have heard when I stall a motor and the start winding kicks in. Normally, the winding is only energized for a second or so so the increased hum isn't noticeable. With the motor unplugged and the drum switch in the reverse position, there should be no continuity between the T5 lead and the T8 lead. You will want to discharge the capacitor before testing as thee is likely some residual charge which could destroy your ohmmeter.

 

[AndySomogyi]

I’ve got a feeling the starter winding is staying active. Looking at the wiring schematic, the starter runs at 110V, it’s center tapped from the main windings.

so what if I start the motor then yank one of the leads to the starter winding, like T8 or T5?

if the motor runs normally, that would indicate there’s a problem with the starter circuit.

or maybe put a volt meter on the cap, and see if there’s still voltage with it running?

 

[AndySomogyi]

Did one more try, bypassed all the lathe wiring, hooked up direct, and still buzzing.

Hooked it up direct to the power out of the main cut off switch.

 

[markba633csi]

I guess what you could try is disconnect T8 while it's running and see if the buzz stops. Be careful of course. Maybe put a switch there temporarily to do the test, like a household light switch or similar. Then if it still buzzes you'll know it's something other than the start circuit
Mark

 

[Latinrascalrg1]

It sounds like a relay/contactor is sticking! Look carefully at the contact surface of the relay switches and centrifugal switch to see if there is any buildup or other corrosion problems.... gently clean the contacts and then retest the motor.

 

[AndySomogyi]

It sounds like a relay/contactor is sticking! Look carefully at the contact surface of the relay switches and centrifugal switch to see if there is any buildup or other corrosion problems.... gently clean the contacts and then retest the motor.

Did that, was #4 in list If things I tried. Contacts looked good, but there still might be something wrong with the starter circuit. I’ll try disconnecting the starter circuit with motor running.


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