Single phase motor hum no start

[disco stu]

The single phase lathe motor doesn't like to start all the time. It will hum at what I assume is 50hz. If I turn the spindle a bit then try again it will usually go, but can need another try many times. If I leave it on and humming and I give the spindle a turn it will fire up, but that's not something I'm keen on doing as I want to stay away from the chuck and similar when they're spinning.

Doing a bit of reading I gather it could be the starting cap or the starting switch. I just wasn't sure if they're easy to replace or the switch to clean up.

Motor is a British Thompson Houston 1/3 HP

I had pondered getting a DC motor with speed control, which might be easier than changing belts. Not sure if this is a good time to look into that change or not though.

Would appreciate any advice on what to do about this

Cheers, Stu

[kafie1980]

So what is it? A capacitor start motor or a centrifugal start switch motor?

Usually the contacts on the centrifugal switch just get dirty and need a quick clean. The only issue is the motor needs to be pulled apart and the internal wiring insulation is usually deteriorating, so you need to be gentle.

If its a capacitor start then you will see a big capacitor secured to the motor and its a matter of getting a replacement capacitor with the same capacitance. Usually this is a quick swap if you can read the markings on the capacitor.

[disco stu]

Thanks kafie. From what you say I gather they either have capacitor or a start switch, not both. Something I read indicated they had both on there. I didn't think I had seen start switches on most AC motors I have had apart. But, AC is not my area, its behaviour confuses me if I'm honest. DC I'm very comfortable with and understand, but AC has all this weird inductance and capacitance going on causing all sorts of funny things to happen.

I did pull the cover off the wiring box on the motor after posting here and didn't see a cap in there. I thought if it had capacitor it would be there. Would that indicate start switch is likely?

[kafie1980]

If you found no capacitor in the wiring box then it’s a centrifugal switch start motor.

Centrifugal switch motors are simple in their design and operation.

The centrifugal switch is mounted onto the motor shaft and the wiring is connected to the start windings.

There are often a set of weights on the centrifugal switch.

The centrifugal switch completes the connection to the start windings on startup which forces the motor shaft to rotate in the desired direction.

Once the motor reaches its running speed, the weights on the centrifugal switch will open up the switch and break the connection to the start windings and only the run windings remain energized.

Once you cut power to the motor the motor slows down to a point where the weights can no longer keep the centrifugal switch open and eventually the switch closes. At this point you will hear a loud click sound.

It’s simple, don’t worry.

You just need to clean the contact points on the centrifugal switch at this stage. This is the plan.

While you are at it take a few photos of the windings and label the wires.

[disco stu]

Awesome, thanks for the help.

And in typical fashion opening up the wiring box seems to have somehow miraculously fixed it.... For the moment at least. It's starting every time now, where before it might have started on its own once every 3 times or more. I'll still get in and clean it out

[kafie1980]

Seems like you have some loose connections in that wiring terminal and just need a fresh wire strip/twist and securing back to the terminal block.

[disco stu]

I'll give that a go. Thanks

[BaronJ]

Hi Stu, Kafie,

Generally a capacitor start and start n run motors have the capacitors mounted on the outside of the casing, so easy to identify.

The centrifugal switch ones have the switch mounted on the motor shaft inside the bell end casings. Often its just a matter of pulling the end casing off and cleaning the switch contacts. No need to touch anything else in there ! Do not stretch or damage the springs in any way. Besides being vital to the operation of the motor, the springs control the speed that the motor has to get to before the contacts open.

[disco stu]

Pulled it apart and cleaned up the centrifugal switch. Also changed the wiring from the switch to the motor as it was black and not looking even close to pristine, plus the motor connections were just the stripped wire wrapped around the terminal and not nut tightened down on it. Got decent terminals on good wire now. Starts up nicely now, and seems quieter for some reason, plus there is now no heavy clunk on starting up