Lathe Motor wont start

[Runner]

Hi Everyone,

I own a very old Southbend Lathe. After a period of non use the lathe motor wouldn't start, it only growled. This was the same for both forward and reverse directions. I decided to remove the motor and the switch assembly from the lathe so that I could investigate further the possible cause of the non starting. The forward/reverse switch is a three pole rotary switch made by GE, centre position being off, left reverse, right forward. Investigation showed that the wiring was very old and that the insulation on the wires was breaking off. The condition was so bad that in the removal wires had become detached from the motor, it may have been the reason for the non starting. The AC motor is a Pope (Australian made) HP 1/3, Single phase, 240V @ 50Hz, 1420 RPM Type SIR 6114. There are four terminals to the AC Motor. I am assuming that it is an Induction Motor, two of the terminals are for the main winding and that the other two are for two starting windings, one to produce a forward rotating magnetic field and the other to produce the reverse rotating magnetic field. One terminal being common for all three windings. Am I right in my assumption? Is the growling due to the current passing through the main winding but being an absence of current through the starting winding that the motor doesn't turn? If not what other problems with the motor that was cause this growling? The growling sometimes happened during use if I didn't push the rotary switch hard enough. I assumed that this was due to some contacts being made earlier than others in the rotary switch putting say current through the main winding but not through the starting winding. My problem is that now that I have replaced the old wiring I am unsure of the connections to the motor. Can anyone assist me in either providing connection details to the Pope Motor and or a wiring diagram that would show the three pole/three position rotary switch and motor connections. Alternatively, what diagnostic tests (say winding resistance measurements) could I perform to identify the motor connections?

I don't think that the induction motor(?) has a capacitor starter (no room) or that it has some form of isolating switch that operates after start-up, could not hear such a device operating.

I would be grateful for any assistance.

Regards,

Brian

 

[Runner]

I have just looked at Wikipedia and read that the motor direction of rotation can be changed by changing the phase of the supply to the starting winding relative to the main winding. Does this then require a separate starting winding to the main winding so that phase switching can occur?

I hope I am not confusing myself or others wishing to assist me

Brian

 

[Ken I]

If the motor has no capacitor - then it has a "start" and a "run" winding - if the motor is reversible, these will be the same.

There will be a centrifugally operated internal switch to disconnect the "start" winding once up to speed.

If it just hums - this switch is probably not making therefore the "start" winding is not engaged.

The start and run windings are swopped around to reverse the motor.

On this kind of motor you can normally hear it "labour" up to about half spped, then you hear the switch kick out and the motor spins up to full speed.
When you switch off, the motor spins down and at about quarter revs you hear the starter switch drop back in.

These switches can become stuck in the open position due to dirt or corrosion on the centrifugal bob weights, the points can become dirty and fail to connect etc. etc.

You need to dismantle the motor to check.

As a "quick and dirty" - you can switch the motor on (it hums) - start it by driving the motor with an electric drill (socket spanner on the pully nut etc.) - I've also done it using the periphery of a rubber grinding disk against the belt / pulley.

The motor should start and run normally if you can spin it fast enough - this then might shake loose the centrifugal switch without dismantling.

Hope this helps.

Regards,
Ken

 

[Runner]

Thanks Ken,

I have cracked it. Quick resistance measurements showed that the two windings are electrically isolated one from the other. The resistance measurements of the two windings showed which was the main and which was the start winding. The start winding being higher resistance than the main.

Analysis of the design of the rotary switch showed how the phase reversal of the start winding relative to the main winding is achieved. I was able from this to determine the connections to the motor. When power was applied and the switch operated the motor ran in the forward and reverse directions. You are right that it does have a centrifugal switch that can be heard to close again when the motor stops.

The problem was, from the evidence available, that a wire to the start winding became broken.

Regards,

Brian