Using a variac for motor speed control

[Vintage_Man2012]

Hi everyone,

I am thinking of using a variac for speed control of a 240v Singer sewing machine motor with use for winding machines, this would replace the foot-pedal speed control as it would give greater control over the speed of the machine.

The thing is, I've heard from somewhere that using a variac for speed control with these motors can ruin it by causing it to burn out.

I'm sure I've seen other setups with this sort of thing and they must work fine. Can anybody advise on this and possibly suggest a suitable variac?

Thanks
Paul

[David G4EBT]

II can't think why a variac should cause any harm because in effect, it's just doing what a foot controller would do - varying the Voltage to the motor to slow it down or speed it up, but others more knowledgeable than me will be better able to answer that.

I wonder if you've actually tried a foot controller and found it unsatisfactory, or are just assuming that it won't be satisfactory? I say that, because the simple winder that I built about four years ago, which featured in a BVWS Bulletin and a thread on the forum, powered by a sewing machine motor, works absolutely fine with a foot controller. In fact I'd find a variac problematical because when I'm winding a coil, I've got my hands full so I find a foot controller advantageous.

You'll probably know that a foot controller is nothing more than a heavy duty wire-wound rheostat with wiper which is moved across the rheostat by movement of the spring-loaded foot pedal. I find that I can vary the speed very smoothly from just a few revs to a much higher speed than I'd want to wind at. I initially built it to re-wind a 2,000 Ohm speaker field coil - 2.3kM of fine wire, which I managed to do without any breakages in the wire.

I guess foot controllers vary from one to another, but they're designed to enable - if need be - stiches to be put into material at a very slow speed - maybe as low as one stich per second, variable at up to any speed up to full tilt.

A point worth mentioning is that foot controllers contain a capacitor across the mains to reduce interference. They go off with a spectacular bang if not replaced with an 'X' class capacitor rather as though you've stepped on a land mine, but thankfully, without such dire consequences!

Good luck with your winding efforts.

[TrevorG3VLF]

I do not think that there will be any trouble with burning out the motor.
The Variac should be big enough to provide the current without any effort. I would think that 1A should be sufficient. What does it say on the motor?

I use a Variac feeding a simple battery charger and a low voltage DC motor. I tried three motors, one was a lorry fan motor, found in a puddle but the one I use is a smaller apparently new 24V DC motor since it weighs less. It runs plenty fast enough.

The coil winder has no high voltage fed to it.

[emeritus]

The rheostat in the foot controller of older singers used stacks of carbon discs. The harder you pressed, the lower the resistance and the faster the speed. A variac should be fine.

I use a (small) Variac to control the speed of my loft based workshop ventilation (no windows), works very well with a large (about a foot across) AC Papst snail fan. I did use the 'proper' controller to start with but AM reception was obliterated. Running for 25 years now (at various speeds).

[radiotechnician]

If a Singer motor is operated with the variable resistance foot or knee
control, it essentially starts and runs from a constant current source.

But a Variac, dependent on size is a constant voltage source.

Perhaps connecting a resistance in series with the Variac output
and using a higher setting will help.

[vinrads]

Only if you don't turn the knob ! Mick

[Vintage_Man2012]

Thanks for the help everyone, the people on this forum really are a great bunch!

David, I take on board what you mentioned about having your hands free if using a foot controller, something I will definitely consider. I think I have some X2 type capacitors somewhere, I'll have to dig them out if I need one.


Cheers everyone!

Paul

[McMurdo]

Here in the potteries, some potters wheels used a variac connected to a foot pedal through a rack and pinion to control the speed. To be fair they did use DC shunt motors with a constant field supply, but that just lets them hold the speed under varying load better.

Back in that very hot summer a few years ago I brought my variac home and used it to slow my desk fan down so i could sleep with it on just ticking over as the fan was too noisy on its own speed no.1. Worked no problem.

The only issue would be, I'd say if you had a split phase AC motor with start switch, but a brushed universal sewing motor will be fine

[ricard]

I can't say I know one way or the other, but I was thinking of this: If you're running the motor slowly, and it stalls, the series resistance in the sewing machine pedal will be rather large at that point, limiting the current through the motor. Whereas with the variac, there is no current limitation other than the motor windings (and the inductance of the motor).

My point is that as long as the motor is running, the back EMF of the motor limits the current through it, but if it stalls, the current will rise quite a bit, but the series resistance of the pedal will limit that more than a variac which ideally has no current limitation at all regardless of the voltage.

Whether this is actually significant given the resistance and inductance of this size and type of motor I can't say though.

[Vintage_Man2012]

Thanks everyone for the advice, I must say this thread has generated more debate than I imagined.

After considering the comments and advice given, I have just ordered a vintage sewing machine motor/pedal so I will give that a go and see how I get on.

Write up to follow!

Thanks again everyone!

Paul

[Vintage_Man2012]

Hi everyone,

Following on from my recent post regarding using a variac and foot pedal for speed control of my coil winder, I wanted to do a little reply as I have now finally got the unit set up on the bench with a re-wired motor and a foot control.

Here are a few pictures of the Avo Douglas No. 6 automatic winding machine and Avo standard reel carrier in situ.

The winder is powered by a 1/15 hp Sew-Tric motor and the drive is connected by two polyurethane round drive belts, the motor seems to drive the machine well.

There is obviously a variable drive but as the motor is not fitted with a corresponding variable speed pulley I have it set as shown.

This machine should be fitted with a clutch pedal to allow the user to take the drive off but as I have yet to find a proper Avo clutch pedal, I've attached a 3" handle to it so the clutch can be operated by hand.

All I need to do now is get rewinding the duff transformers I have on the shelf!

Paul

[Diabolical Artificer]

Interesting device, glad you got it sorted completely different to my stepper motor based winder. How fast does it go? My winder took about 6 mins to wind a 1 1/4" wide bobbin with 32 gauge, yours looks beefier and thus faster.

Andy.