Can I convert my 8mm projector motor to variable speed?

I have a Super 8 movie projector than can show either 18 or 24 fps. I’m trying to sync it with my camcorder which can only do 60fps. I want to get the projector to show 20fps.

I can see the motor is using around 13v in 18fps and 17v in 24 fps. Is there some sort of dial I can splice in the wire to the motor so that I can vary the voltage to the motor? I’m guessing it would be a potentiometer of some sort, but I’m not sure what I would need.

In the (distant) past I would have gone to Radio Shack and asked one of the clerks, but they’ve long since been helpful for these sorts of questions.

It’s called a rheostat which just means it’s a variable resistor and the term is usually reserved for those able to handle quite a bit of power. If you motor has brushes a rheostat should do the job. The only difficulty is knowing how much power is dissipated in the rheostat at the slower speed setting. Check the motor nameplate. It should tell you how much current the motor uses at full voltage. I don’t know what that is but suppose it is 1 A. If you take this to be the current through the rheostat you will have a conservative estimate. You will have 4 V across the rheostat at the lower speed (17 - 13 = 4) multiplied by the 1 A gives a total of 4 Watts dissipated in the rheostat. You should use a rheostat with a dissipation rating of quite a bit more than that to prevent a large temperature rise in it. Say 4 times the power or so, so you will need at least a 16 W rheostat or maybe even 20 for the illustrative case.

You’re probably not going to have much luck with this. Oh, you might be able to exert some control over the motor speed using the method outlined above, however, without some kind of regulation, the speed will probably drift around a bit. What are you trying to do here? If all you want to do is transfer some 8 mm footage to video, you’ll have much better results using a professional service which has specialized equipment for this purpose. With your method, you may find aliasing effects to be very annoying in the final product unless you can get the speed to hold very steady.

Check around in your town, and I’ll bet you can find places that will convert your 8mm films to DVD or VHS for you. They have professional equipment that is designed to work around the frame rate problem, and will undoubtedly be less expense/hassle than trying to modify your projector.

Darn it, Q.E.D. you beat me to the punch again.

May?!!! If he stabilizes the projector to within 1% of 20fps, he’ll still be videotaping interframe darkness every 5 seconds. I don’t think there’s anyone who wouldn’t find that annoying.

I’ve looked around at the professional services, but it will cost upwards of $800 to convert all the film I have. I figure I have the projector, digital camcorder, and editing suite already, so I might as well give it a try myself first. If it doesn’t work, I’m only out a few bucks.
Anyway, getting back to the question at hand, I checked the motor and there doesn’t appear to be any markings on it. Can I figure out the amperage of it in some way? It’s a relatively small motor–about 2 inches long by 1.5 inches around. Could I measure the resistance of the motor and then use V=IR to get the amps?

Sounds like super-8, rather than 8mm. Super-8 commonly runs at 18 or 24 fps, while 16mm commonly runs at 16 fps.

Super-8 telecine machines, often an Elmo projector, have five-bladed shutters to prevent flicker. These generally go for about $500.

When a motor runs the armature is a coil of wire rotating in a magnetic field and so it generates a voltage that opposes the applied voltage. This is the main limitation on the current. With a large motor this back voltage is the only real limitation on the current. With a small motor the armature resistance might have quite an effect. If appears that you have a pretty small motor.
If you measure the armature resistance you know that the current can’t exceed Voltage/armature resistance and actually will be somewhat less than that. If all else fails you can size you rheostat to handle the power dissipated with that current.

If you don’t have an ammeter about the only thing you can do to find the current is get a small resistor, say 1 ohm, 2 W, and measure the voltage across it with the motot running.

As to speed control and regulation. The speed should be just about as constant with a resistor in series as without. The the motor will slow down as the resistor heats up because the resistance increases with temperature. You can adjust for that.

This assumes that all you want to do is project your already existing films at the proper frame rate.