Induction motor + VFD ?

[greybeard]

I'd like to run a fractional hp, single phase induction motor to drive a small model makers lathe.
Can I use a vfd to give me speed control ?

If the answer is yes, has anyone built one, as the cost of a new one is several times more than the cost of my lathe and motor.

John

[fkaCarel]

No, phase converters output 3-phase and are to be used with 3-phase motors. However a phase converter for this size can be fed with single phase. A new fractional hp motor will set you back 50-100 euro.

[Geof]

Originally Posted by greybeard ....to drive a small model makers lathe.....speed control ? John

A small model makers lathe is going to be a small motor. Buy a 1/2" capacity variable speed reversing hand drill. Take the chuck off and figure some ingenious way to attach a pulley and mount the drill in place of your lathe motor. Figure out another ingenious way of using a fairly coarse pitch bolt to depress the trigger on the drill and plug the drill cord into a socket with a switch. The bolt on the trigger presets your speed, the reversing switch gives you forward and reverse and the switch gives you on/off.

[greybeard]

fka - are phase converters the same as vfd's then ? I didn't think so.
Geof - neat, thanks for the idea.
John

[fkaCarel]

Greybeard, my mistake, I meant vfd. The rest still applies. It's just confusing how many flavours of electricity is used on this planet.

Take in consideration that a drilling machine produces a considerable greater amount of noise than a vfd'd lathe.

[fyffe555]

Originally Posted by greybeard are phase converters the same as vfd's then ? I didn't think so.John

VFD and Phase converter are different animals - almost... VFD changes the A/C frequency to control speed and can be used on Single or Three phase. Commercially the requirement is usually to drive a 3 phase motor from single phase so the 'VFD' actually contains both a VFD AND a phase converter. There are not many pure three or single phase vfd's around for us bottom feeders, those VFD's that are are usually single phase in and three phase out.

To take geof's idea further, the usual solution to speed control on small machines is a DC motor and a speed controller... have any unused treadmills lying around?

Edited; can't type today....

[Geof]

Originally Posted by fyffe555 ....To take geof's idea further, the usual solution to speed control on small machines is a DC motor and a speed controller... have any unused treadmills lying around? .....

If my idea is not the cheapest solution (excluding all the time spent on ingenuity ) I'll buy greybeard a couple of beers next time I am in England.

[greybeard]

Ok guys, Geof wins on pints ...

One thing though, and perhaps I should have spelt everything out, I did say a single phase motor, so three phase is not in the frame. And I should have emphasised that the reason for choosing the induction motor out of the scrap bin was that it is virtualy silent compared with any brushed motor I have.

John

[handlewanker]

Hi greybeard, be carefull using a VFD on a small (1/4hp) induction motor. As you appear to be into model engineering and are using a table top type lathe, then the "want" from your end is a speed range from about 50 rpm for cast iron etc and 2000 rpm for small drilling and polishing.
The problem is, induction motors run slow get HOT, and as you will be simulating a lathe in back-gear then the torque low down will only be achieved by pumping the amps into the coils in a virtual dead stall situation.
The electric drill method will work but the whining of the motor will drive you nuts. If this is an option then try a vacuum cleaner motor, or angle grinder, modified, with a light dimmer switch rated at about 350 - 500 watts.
Top speed of the vacuum motor is about 10,000 rpm so if you belt this down by vee pullies to 2000 rpm input then the motor will still get some cooling running slow.
The best option for your case is to use the induction motor "as is", and arrange a variable VEE pulley drive to get speed change.
They're not hard to make, and if you use standard vee belts, just use two variable pulleys in compound arrangement. This way you still get "dial up" speed control on the move and plenty of low down torque.
Ian.

[Al_The_Man]

Originally Posted by greybeard t, I did say a single phase motor, so three phase is not in the frame. And I should have emphasised that the reason for choosing the induction motor out of the scrap bin was that it is virtualy silent compared with any brushed motor I have.

John, Going back to your original question, although there are VFD's out there for 1ph motors they are not very successfull unless the motor is Capacitor start-capacitor run, and even then they tend to drop out of run or will stall at low to medium rpm, especially with a load.
The brushed motors you mention must be Universal type, as a true DC permanent magnet motor is very quiet, apart from a bit of brush noise which should be negligable.
Al.

[greybeard]

Ian - thanks for reminding me about variable vee pulleys. I'd forgotten those as a possibility.
Al - yes, the concensus seems to be that low speed end of the control is the problem area. I think I'd best stick with a mechanical solution.

How about a Sturmey-Archer 3 speed hub as a gear box ????
Change the sprocket to a vee pulley ?

John

[handlewanker]

Hi greybeard, "sturmey Archer" now that is cutting edge technology. They're not the easiest thing to mount up and don't give much speed range. Have you thought about a motorcycle gearbox. The four speed one is really compact and gives a speed range from about 1:1 to 1:4.
I would strip the guts out and reassemble in a made up casing.
Ian.