Highly unlikely to be the winding, but could be the cap. Has it always been this way? I find when I start my lathe at a high speed pulley setting it takes a few seconds to get up to speed as the oil warms up.

My mill (with a newish one horse motor) was often unable to reach top speed in highest gear until the gearbox lube was warm enough – no problem in warm weather.

Changed to a 3 phase motor with VFD and no problem since. Admittedly the motor is 1.2 horses but the difference was very clear – increase in power or not. 3 phase motors are so much better than single phase equivalents!

Clearly you have solved the problem – until next time. Good capacitors do not come cheap these days.

I've been though the"'it could be the capacitor but what it it's something else?" doubt. In the end a swap was easy and instant restoration of performance (my bandsaw).

Neil

That could explain a lot!

Jim

Physics mean it's much easier to make electric motors spin when presented with DC or 3-phase AC than it is to make a motor able to run on single phase AC.

As producing the magnetic fields needed to turn the armature from a single phase supply is tricky, single-phase motors are a complicated compromise. The complexity consists of separate Start and Run Windings, one or two high-voltage high-value capacitors, and maybe a centrifugal switch. There is a lot to go wrong!

Unfortunately, all this complexity doesn't result in a high-performance motor. Compared with 3-phase, single-phase motors vibrate, have less torque, are inefficient, and dislike starting and stopping because it stresses the components and overheats the windings. Maximum power output from the type is limited – it's unusual to find a single-phase motor above 4kW.

Single-phase motors meet an important need. They can be plugged straight into the ordinary domestic supply and do useful work. And for most domestic purposes the long list of single-phase disadvantages don't matter much in practice. Workshops are more challenging. Single-phase is fine for small machine tools like my bandsaw. Anything given time to cool down that isn't frequently restarted in bursts should be OK. But single phase is a poor choice for anything worked hard and especially unwise for anything that's also turned on/off in rapid bursts.

The motor on Windy's lathe isn't a good match to his work profile. A clutch would help, but I think it would be easier to fit a 3-phase motor and VFD. They really do make a difference!

Myford fitted single-phase motors in 1947 because they were the best choice for hobbyists and small workshops at the time. DC motors were out because AC/DC convertors were hideously expensive back then, and 3-phase ruled out because it was (and still can be) hard to get installed. Modern electronics walk all over both problems: converting single phase to either DC or 3-phase isn't difficult or unaffordium.

Difficult to advise Windy: I wouldn't replace a working single-phase motor on a lathe just for the sake of 3-phase smoothness. But faced with an unreliable single-phase motor, I think I'd bite the bullet and replace it with a VFD/3-hase combo.

Dave

I did, and it's soooooo much better, smoother quieter and change speed with the turn of a knob. I got a fair bit of the cost back by selling the single phase motor and control gear