How can I rotate a small 240VAC induction motor with 12VDC?

Question

I have an induction(?) motor that might be 30 years old. It's principal dimension is ~50mm so fairly small. It's actually the fan motor from a fan heater. It has one winding and runs off 240VAC with a DC resistance of 375Ω. There is just one simple stator winding. It's just like:-

I also have a 12VDC source of sufficient capacity. How can I get the motor to rotate using the 12V direct source? All I want is rotation. It doesn't have to do any useful work just go round. And the speed is irrelevant, any constant rate is fine. Beyond any rhyme or reason I'm assessing the feasibility of using it as an on /off indicator in a Steampunk project. Steampunk gives us licence to do stupid stuff.

Initial thoughts: some form of siney wavey generator + 12V audio amplifier chip like a LM384 (5 Watts)? Horrible impedance mismatch but might it rotate? Or is this just too stupid? Can feasibility be assessed without building it?

I though that running AC synchronous motor with DC might help but that motor is properly voltage matched and seems to be performing work.

Answer 1

You could use a 60 Hz oscillator feeding an audio power amp, followed by a 240 Volt->12 Volt transformer used "backwards" as a step-up transformer. (I once built something like this at work to power a three-phase 240 V 300 Hz motor)

Answer 2

If you want to operate the motor without an electronic circuit, you can build an electromechanical inverter like this:

It requires three parts, an auto radio vibrator, a six-volt battery and a transformer. I don't know the operating frequency the vibrator, but you can probably find that if you search. It is probably not very high.

Answer 3

An AC motor is wound to rotate at a specific speed/Hz with a specific minimum current to sufficiently magnetize the rotor. In general, there is a linear relationship between volts and Hz that will cause the motor to spin with the same current. If you double the frequency, you must double the voltage to get the same current. If you halve the frequency, then you halve the voltage to get the same current. This beautiful simplicity is ruined by one thing... resistance. At a certain lower frequency, the resistance will prevent enough current to flow for the core to magnetize properly.

The relationship goes like this: Close to DC frequency, we have a maximum of 12V/375 Ohm = ~32mA available.

So, basically, if the motor will spin with significantly less than 32mA current at 240VAC there's a very good chance you can get it to spin from 12V. The probability of success increases the smaller the required current draw is in comparison to 32mA.

To make it work, however, you would need to lower the frequency of the power going to the motor. At 12V, you would need to have around 3 to 6 Hz to make it work. The 12VAC (chopped) would spin the motor very slowly ... like 20x or 10x slower than when plugged into the wall.

In order to figure out if your motor even has a chance... all you need is an AC amp-meter, and a way to plug your motor into 240VAC (use a clothes dryer outlet, or you can try 120V, it might still work.) If it spins at less than 32mA... you've got an excellent chance. You might want to add teflon oil to the bearings to improve your chances. ;)

Otherwise, the transformer (doorbell transformer) idea is necessary to step up your 12V to something that can overcome the DC resistance of the AC motor.

It might also be possible, if the current is small enough, to use a capacitor transistor circuit to generate pulsed DC at higher than 12V from your 12V supply. These kinds of circuits are actually pretty easy and cheap to build.