Frequency Converters

I collect old cine film projectors, some of which use induction motors, ie speed controlled by mains frequency. I have one I want to make run slower. It is 110v, via a tranny from 230v mains, probably only 100 - 200 watts power (capacitor regulated) I want it to run at two thirds of its normal speed. Anyone know how? Are there pitfalls?

Toad

I don't think that you are going to get to 2/3 speed with variable frequency or any other technique that I'm aware of.

I've played around with single phase motors on variable frequency a little and I find that if the motor is running, you can often slow down maybe 10-20% but, on starting, the rotation is unpredictable and in some cases, the starting torque is so low it won't start at all.

In addition, I'm not aware of an inverter that outputs 120VAC. Don't be tempted to use a 230VAC inverter and voltage limit it to 120VAC. The output PWM pulses are high enough for forming 230VAC (around 330VDC) even if voltage limited to 120 and your vintage motor may well vent its smoke cannister and stop running forever!

I don't know if your projector is arranged in such a way that the old motor can be moved and a mechanical speed changer placed in the drive line.

Of course, you could always replace the old motor with a 90VDC motor and run it from a 120VAC supplied DC drive. These are inexpensive but you would have to study the mounting of the new motor carefully for proper fit, etc. And, it wouldn't be "original" if that is important to you!

Good luck. This sounds like a fun hobby.

HI group, a question for DickDV, could he get the needed speed control with a variac??? He would have to make sure that he just hooked up the motor to the variac, but then it should let him twiddle w/ the speed...yes, he may have to bump up the knob to get the motor running, but then it should work just fine.

David

Variac?

Depends on the type of motor. If it is a 'universal' type motor(series wound DC, can operate on both AC and DC supplies), this could be done. But universal motors are not well known for fine speed control without electronic feedback, so the projector probably dosn't have a universal motor.

My first thought was a 230V VF drive run through the transformer. However as DickDV pointed out, single phase motors are dificult to control this way and tend to stall.

Perhapse a stepper motor may do, this will give good, precise, speed control and, if it can handle the load, you could get away without any feedback at all.

Doug

About stalling:
If the main problem is to get the motor up to speed, why not run it at full frequency, until it is up and running, and then go back to 2/3 speed ? (OK, maybe the movie will look a little funny the first few seconds of the reel).

To me, this sounds like a perfect place for a non-electronic solution; either a small inline gear box, or a set of sprockets and a timing belt.

I'm sure the projector has a gear train already, perhaps it's possible to have a new primary drive gear train made in the required ratio. In the long run, it will probably be much more cost effective than any type of stable electronic control.

frequency converter

Thanks to everyone who responded to my original query. I think the consensus of the meeting is that it's not really a terribly good idea to do it by frequency variation, for reasons that seem to make sense to me.
A bit more info to flesh out my query. The motor has, like Gaul, three parts. The motor housing is an integral part of one of the main projector castings. Inside this fits the wiry bit, ie a coil thingy that is connected to the power. Inside this is a shaft with a big lump of metal on it, which is the bit that goes round and is the main drive shaft. This has no connections to power, there are no brushes. There is a condenser/capacitor connected to the coil thingy (sorry for all this highly tecnical language, not!) without which the motor will not start. The direction of rotation is set by which way round you connect the condenser.



Thanks once again for the inputs

Toad

Frequency Converter

(Addditional to last)I am assuming that an external drive to the main shaft (Plan B,which I think is possible) woukd give problems as the motor might then try to act in reverse and could be damaged. So I have moved on to Plan C. This involves making a new main shaft without the big metal lump, and driving it from the outside with a separate motor. This would leave me able to restore the machine to original if I want, which I see as important. Incidentally, the machine dates from about 1936 and is for 17.5mm sound film running at 24 frames per second. I want to run silent fil at 16 - 20 frames per sec, hence the original query.

Toad

Reply to Dave Emmerich

Sorry for the delay. Been on a much needed vacation!

The AC motor sounds like a synchronous or near-synchronous design which means that the speed is a function of frequency, not voltage. Under those conditions, a variac would be no help.

If the motor had been a universal type (it isn't because it was stated above that the rotor did not have brushes), then a variac would control speed.

Hope that is clear.

Hi DickDV, You are right, and I should have read that post a little closer!I was thinking about the blowers I use on my wood stove, which I vary with a SCR pack out of an old Syncrotron (sp ? )parts feeder...works really well, but is a little noisy due to the chopping. I did try a variac on the blower, but i'm sure that the only reason it gave me some 'speed control' was because I was getting near to doing a smoke test on the poor motor! Thanks again for the clarifcation!!!
David beerchug

Gday,
we use the small "Oriental" branded motors for all of our checkweigher conveyors.They are essentially a splitphase single phase 110v motor.The controllers for these motors are supplied with 240VAC and internally drop the voltage to 110v for the motors.On the back of the motor is a small tacho unit(very very simple)I think they just regulate the voltage to achieve the desired motor speed they are very reliable.The motors we use are around 200 watts, I think it might just work in your application if you were to retrofit the tacho unit to the back of your existing motor...
Tom

More modern projectors do tend to use tachograph controlled motors,
but I hae ma douts that you can retrofit to an induction/synchronous motor tho', as you will have gathered, electrical techie I ain't, so if anyone knows beter.... And where could I find a tacho?

Toad