Help with lathe speed

I am in the middle of restoring my Drummond b lathe and I was wondering if you can have a speed controller on a 240volt single phase ac motor at all as I use to use a cj18a mini lathe and you could control the speed on that one

thank you in advance

You can slow down a single phase motor but in doing so will loose the torque needed. It is better to look for a 3 phase motor with 240 windings As 415v star 240 delta motor. Then use a 240v 3ph inverter . to control it. Probably going to cost you in the region of new 300+ quid. 2nd hand if you are lucky 200.

Steve.

Your Drummond's motor's speed is fixed by the 50Hz frequency of mains electricity. Whilst it is possible, it's not practical to alter the way you can with a 3-phase motor.

Your mini-lathe will have a DC motor which is why you can use a speed controller.

Be careful the top speed for your lathe with bronze bushes is below 1000 RPM and most speed controlled motors loose power at slow speed on my M Type I can screw cut at 50 RPM with back gear and pleanty of power not so on a friend's variable Chinese lathe.

David

You are right regarding the lathes top speed but torque at low speed shouldn't be a problem as the Drummond has back gear (I think) and this can be used for turning larger diameters and screw cutting etc.

If you can find a scrap yard willing to let you poke around, you may be able to rumage up a three phase motor for not too much.

There are any number of cheap inverters from China for £50-£60. I don't have any experience of these (I have one in a box waiting for the time to come available) but others here will have used them.

David.

If you go three phase make sure the nominal speed of the three phase motor is similar to that of the current one. The number of poles on a three phase motor determines the nominal speed. A scrap motor may look the part but cause problems of running too slow or too fast at nominal frequency from a VFD.

Martin C

Should you end up deciding to change to a 3 phase motor run from a VFD, then there is a choice of basic types. At 50Hz standard mains frequency, the choice is as follows:

2 pole @ 3000rpm

4 pole @ 1500rpm

6 pole @1000rpm

8 pole @ 750rpm

The price increases with the number of poles. They all can be controlled with a VFD and the torque values are good between 25 and 75 Hz, (0.5 to 1.5 times their standard speed). Lower frequencies bring a steep dip in torque.

My lathe has a Bronze front spindle bearing & is rated at around 2150 rpm from the single phase as supplied motor. A Super 7B 1977 vintage. I rarely run much above 1000 though.

Steve,

Depending on what you intend to use the lathe for, why not just use a motor and speed controller from a mini-lathe?

Probably not widely known, but split phase, cap run single phase motors can be run from VFDs. These motors are in effect a two phase motor and the generic AT2 VFDs are design to provide control for them.**LINK**