AC motor speed control

[Bob Minchin]

Single phase induction motors are designed to run at full voltage and constant speed. Causing the speed to reduce by any means. changing voltage, running on part cycles or reducing the frequency will all cause the starter winding to kick back in (usually with a centrifugal switch) which will cause problems.

For a hacksaw, the best option is to seek out a 6 pole motor which will run around 950 rpm and reduce the speed with a belt drive. A four pole (1425rpm in uk) will be Ok with a smaller motor pulley but the 2 pole (2850 in Uk) will be just too fast for the application.

The very successful Kennedy saws use a flat belt drive giving good grip on a small diameter crowned motor pulley and a flywheel around 6-9" according to model to drive the crank. Modern source for flat belt drive is to use a toothed belt running on its back. My kennedy model 90 has been running like that for several years perfectly.

[Dave Plowman (News)]

Ok smart arse, give some examples of a universal motor designed for continuous operation.

Really? I can think of some very powerful induction motors. As regards speed, you use a gearbox. Just as you do with a universal motor.

Can you give some direct examples of the relative costs? With power tools where mostly intermittent use gives a choice of either, the more expensive variety usually has an induction motor. My guess is for a given (small) size like say 1HP, a universal type is cheaper to make.

[The Natural Philosopher]

Give me an example of ANY electric motor designed for continuous operation?

[Cursitor Doom]

Well, it seems I've got a half horse 230V single phase 2850RPM induction motor and will have to make the best of it. Since electronic speed control seems to be out of the question, I'll just have to fabricate a gear train to get the reduction necessary. Two strokes of the blade per second ought to be about right so the maths is simple enough (damn sight simpler than cobbling up the gear train but that's life). Fortunately I have a lathe and a mill so things could be worse. :-/

[Dave Plowman (News)]

Showing your stupidity again.

[Bob Minchin]

2850 might a bit fast for simple, open gears. Perhaps OK for a worm drive. Big snag with gears on a reciprocating mechanism is that the last gear in the train will wear asymmetrically. Belts avoid this and should be easier. There are a few kits around which might same a whole load of design time. Blackgates used to do one, maybe Hemingway too.

[harryagain]

Ummmm - if it's an induction motor, which you would be able to see by the plate on the motor showing an 'rpm', you could destroy the controller and possibly the motor. Induction motors are mains synchronous and don't like 'dropping out' of that with overload or low voltage.

Induction motors do not run at synchronous speed. The rpm given is the no load speed. It drops slightly as the motor is loaded up. Reducing the voltage to an induction motor will slow it down up to a point but you must de-rate the output power of it because the iron losses increase unduely causing overheating. But only around 10% speed reduction is advisable. So probably no good to you. The traditional way is by variable sized "cone" pullys".

The proper way to proceed is with a proper motor speed control which rectifies the ACand then puts out a variable frequency.

[harryagain]

If it's an induction motor. Assuming you are in the UK the speed will be either just under 3000rpm , just under 1500rpm or just under 1000rpm. This is because we have 50 Hz /3000 cycles per minute mains supply. You are very inlikely to come across anything else.

99% of motors are induction motors because they are cheap to make.

[harryagain]

Some induction motors have brushes. There are wound rotor induction motors, The brushes are connected to external resistors. It's to do with improving the starting torque not speed control. Haven't seen one for years though. Commonly used on large conveyors etc. In days of yore.

There are also Schrage motors which are true variable speed AC motors that can run at super and sub synchronous speeds Haven't seen one of these for years either.

[Dave Plowman (News)]

Are you sure about that?

In the average house, most will have more universal motors than induction, I'd guess. Mainly because they are easier to control the speed of.

Different matter in a pro workshop or factory.

[harryagain]

Any fan motor Virtually all pumps.

[harryagain]

You can't vary the speed of and induction motor by varying the voltage. You can vary it by varying the frequency. You can buy a motor speed control that does this. But be sure it is a frequency changer.

They are quite common on new cheap lathes for example, saves having a gearbox.

[Cursitor Doom]

I'm suitably impressed by Bob's suggestion of a wormdrive. A simple and elegant solution to the problem - at least until the next fault-finder throws another spanner in the works!

[Bob Minchin]

This is bollocks Harry. You should know better A single phase motor running on reduced frequency will switch in the start winding. The start winding and capacitor are designed for 50hz.

You can only change the speed of a 3 phase motor with a variable frequency supply.

[Bob Minchin]

But the worm wheel will wear asymmetrically due to the load of a saw.

Gear trains should be designed so that the load wear is distributed evenly which is why most gear ratios are non integral.

[Cursitor Doom]

But how would you do screwcutting and whatnot if you can't change the speed of the saddle independently of the chuck? I can't see any way of avoiding a gearbox (or a set of changewheels) in such circumstances.

[Bob Minchin]

Sorry CD - this is just more Harry bollocks!

Cheap lathes have brush motor with drill speed type pulse width modulation speed controllers. They do away with having geared heads in this was but still have standard changewheels for screwcutting.

There is quite a lot of work going on under the amateur electronic leadscrew project where the leadscrew is driven by a stepper motor but electronically synchronised with the lathe mandrel for a near infinite range of screw cutting pitches. proper CNC lathes have nearly always done it this way but I digress!

[The Other Mike]

1) AGR gas circulator

[Dave Plowman (News)]

They haven't been around long enough for NP's idea of continuous. ;-)

[Dave Plowman (News)]

That they are cheaper to make than a similar power universal - or the 99% bit?

Even with a fan oven, extractor fan and microwave, there are more universal motors in my kitchen.

But so much is these days. I

It may matter how you classify the various motors in a PC and accessories, etc.