Problem with 3 Phase lathe

[pwpeterson]

Hi, I hope someone can help troubleshoot a problem that has arisen over the weekend with my lathe. It's an older Sheldon manual lathe with a three phase motor. For some reason the lathe will not come up to speed and power.

That is to say, I turn on the RPC and then the lathe. The RPC sounds like it's under a load, which is normal until the lathe comes up to speed, however, the lathe's motor will not start turning on its own.

I removed the belt from the motor and if I spin the motor's pulley by hand it will come up to speed but has no power as I can easily stop the motor by slipping the belt back on the pulley holding the belt lightly with my hand.

I have a 5hp RPC that's worked well for several months and doesn't seem to be the cause of the problem as I get adequate voltage to the three contacts on the lathe's junction box.

Any idea what may be causing this problem?

[Al_The_Man]

Check the resistance across all three phases of the lathe motor, normally when a 3ph motor fails it causes excessive current, but in your case, it does not sound like it if the RPC voltages are up to normal, check if one pair is high resistance.
You may have an open winding on the lathe motor.
If you have a clamp on ammeter check the 3 motor phases.
Al.

[pwpeterson]

Al. Thank you for the quick response. I'm not an electrician and want to make sure I understand what you want me to check. I am going to disconnect the three wire to the motor and check resistance across each of the motor's connection with each other, correct? 1-2, 1-3, 2-3?

Paul

[Al_The_Man]

Thats the first thing I would do.
Al.

[pwpeterson]

Ok, Al. Here are the results...

There are nine wires in the motor's junction box. Three are twisted together and the other six are divided in groups of two each with a power wire. I'll arbitrarily name these connections T1, T2 and T3.

T1-T2 no leakage
T2-T3 no leakage
T3-T1 major leakage.

Am I suppose to turn the motor while I do this? I didn't.

Paul

[Al_The_Man]

Not sure what you mean by 'leakage' but the resistance (use an ohmmeter) measured should be the same between any of the three combinations you show, i.e. very low resistance.
Do not switch on when checking resistance.
If you check current with a clamp on ammeter, then the power should be applied, and all three phase conductors should be close in current reading .
Al.

[pwpeterson]

I apologize for the confusion...

T1-T2 high resistance
T2-T3 high resistance
T3-T1 no resistance

pwp

[Al_The_Man]

No where near what it should be, I can't see them all being open, there should be a couple of ohms at most across any phase pair.
Check the connection of the three that are twisted together, this is the common point of all the coils.
Al.

[pwpeterson]

Al, I checked the motor's three wire group and put a wire nut on them. (It was black taped.)

I did some more testing this morning. Please disregard yesterday's results as I took a short cut and was measuring across the wrong connections.

At the motor:
T1-T2 no resistance (The ohm tester's needle pegs far right.)
T2-T3 no resistance (The ohm tester's needle pegs far right.)
T3-T1 no resistance (The ohm tester's needle pegs far right.)

There is voltage on all three lines from the RPC at the 709 automatic starter but only T1-T2 has voltage at the motor. I suspect the 709 has failed. I have another, slightly different 709(AAA) here and will try to connect it next weekend.

thx

pwp

[Al_The_Man]

The right description of the reading you get (sounds like you have a moving coil meter).
Is very low resistance, not No Resistance. They appear OK, on the surface.
Is this 709 an automatic starter of some kind?
Obviously you need the RPC output to all three motor phases, so it sounds like it is up-stream of the motor.
Al.

[pwpeterson]

Yep, The 709 is an automatic starter. I know how to connect the Lines in and the terminals out but not sure where the wires from the start, stop and jog buttons go.

pwp

[pwpeterson]

Al Can you tell me where the three wires from the lathe's start switch go on this 709 AAA automatic starter?[IMG]file://temp/709aaa.jpg[/IMG]