Also, if it has a start winding with ‘centrifugal’ switch, something could be amiss with the start winding or switch….

Be good if you can to post a photo of the 'big old motor', especially the motor plate. It's almost certainly a single-phase motor, but these come in a few different configurations. One of the experts may recognise it. (Photo posting Instructions here.)

Single-phase motors all have 'start' and 'run' windings. The purpose of the start winding is to create a temporary magnetic field for the run winding to push against, and the field is usually angled electrically with a capacitor or two. Once the motor is turning, the 'start' winding might be disconnected by a centrifugal switch inside the motor (behind a removable cover on the back end) but not all motors have them. Myford motors usually do, so well worth a look.

Capacitors fail open-circuit. Not unusual and they are typically easy to replace.

So:

• If the centrifugal switch is stuck open or the contacts are filthy, the start winding gets no power and the motor buzzes
• If the capacitor fails the start winding gets no power and the motor buzzes.

As the start winding is only needed to get the motor turning when power is first applied, it should be possible to start the motor by spinning it manually. In the good old days when men were men and losing a few fingers was all in a day's work, early-design single-phase motors were started by pulling a rope wound around a drum on the shaft. Basically pull rope hard to spin the shaft and apply power. Yuk. You may be able to start the lathe by applying power and spinning the chuck by hand: if it runs, start problem as above confirmed.

If there's nothing obviously wrong with the centrifugal switch (does anyone have a photo?), I'd replace the capacitor. Nothing lost by replacing it as the first step because its fairly cheap and easy to do.

Also inspect connections & wiring carefully – not unknown for wires to come adrift from terminals or to chafe through. Make sure all is firm. Could be as easy as a loose connection.

Most serious possibility is a winding has failed, not easy or cheap to fix. If the motor has died I'd consider upgrading to a 3-phase motor and VFD. Speed control is lovely, and the motors are smooth…

Dave

Edited By SillyOldDuffer on 09/09/2020 09:15:18

Hello SOD,

Not all single phase motors have start and run windings. A capacitor run motor has two windings which are permanently energised, for example.

The OP's problem is almost certainly a sticky, or more likely dirty contacts on the internal centrifugal switch as detailed by others.

Andrew.

Dave ?

Hi Dave

ll of your suggestions for starting a motor that buzzes I would try myself and have done many times . I dare not suggest this to someone . I thing Health and Safety > Big No/

Regards Brian

Thanks for pointing it out Brian. I hoped my reference to losing fingers was sufficient clue, but yes starting motors by hand is risky. Flicking the chuck whilst taking care not to get caught is one thing, pulling on a belt is asking for a trip to Accident & Emergency.

I'm serious about H&S but come from a background where anything can be done provided it's been risk assessed carefully and mitigations applied to minimise risk of injury. Quite a few jobs are intrinsically dangerous: anything maritime, bomb disposal, firemen, police officers, carpentry, electric line repairmen, dustmen, and street cleaners. The best that can be done is to keep them as safe as they can be by constantly reviewing the risks and what's been done about them. Same is true in the workshop; engage brain first, and ask someone else what they think before trying it!

Dave

It is to be hoped that capacitor would be open circuit! If it has a low resistance, it is a faulty surely?

They are dangerous if charged to a high voltage, and are functioning so as to retain the charge!

Sometimes, a motor which buzzes, (Due to a failed capacitor, or centrifugal switch), can be made to run by starting it rotating, by hand. For years, a friend used to start his ML4 in this way.

Hopefully, a centrifugal switch with dirty contacts can be cleaned, or even replaced, and a failed capacitor can be replaced, for a modest sum.

Howard

I thought a megger was for seeing if an apprentice could be charged

Mike

The only time you would need reverse is to back up the leadscrew. Assuming you were going to do screwcutting. Otherwise the myford is no good to do any cutting in reverse because of the screw on chuck.

Steve.