Variable-Speed Projector verses varying the input voltage on Fixed-Speed Projector

[Brian Fretwell]

Dave, arr, yes, found it. It has almost no resistance when I twist it to "60c/s" from "50Hz" and I can't see any difference to the film speed.

The switch itself does not alter the speed it just allows the normal speed adjuster to move more in one direction so you can use it to select other speeds.

[Alwyn Adkins]

Thanks Brian, as far as I can see, the 820 doesn't have any speed adjustment, only 18 or 24.

[David Strelitz]

Thanks Brian, as far as I can see, the 820 doesn't have any speed adjustment, only 18 or 24.

Push the 18/24 slider on the front downwards. It is NOT 18 or 24 but variable.

[Alwyn Adkins]

Got it! I didn't realise the 18/24 thingo was actually a variable slider control. With 60c/s and the slider up from the lowest position a bit, no flicker with full voltage.

Thanks Dave and Brian.

[Thomas Dafnides]

Is there any reason I could not use this to adjust the voltage for a film projector .... https://www.amazon.com/Variable-Cont...lbWF0aWM&psc=1 My concern is that it seems to say it works only with a rheostat motor.

[Stan Jelavic]

Most of the 8mm projectors use DC motors that will work with adjustable AC voltage. If your motor has brushes you should be OK. One issue is that some motors have low torque and will stall but in my experience that happens at very low speeds.

[Alwyn Adkins]

Is there any reason I could not use this to adjust the voltage for a film projector .... https://www.amazon.com/Variable-Cont...lbWF0aWM&psc=1 My concern is that it seems to say it works only with a rheostat motor.

Thomas, that is the one I used with my Eumig 820 and it controlled the Eumig's speed well (194v gave me no flicker). As was pointed out to me in this thread, I don't actually need it because the Eumig 820 can be set to run at ~16.67 fps with out it, but it certainly worked to control the projector speed. However, note my comment in post #15: it doesn't work on the ELMO ST-180; that projector maintains it's speed even as the voltage is lowered, until the voltage is very low and then it rapidly slows down and the lamp dims.

So, it will depend on your projector.

[Stan Jelavic]

Good input Alwyn. As you said some projectors use an AC synchronous motor to synch to 50 or 60 Hz an then the pulley ratio to get to 24 FPS. Those projectors are brushless. Elmo TRV8 still uses the DC motor but has a feedback cct to lock onto 24 FPS for 8mm sound and then with a switch you can change it to locked 18 FPS. Both speeds have small potentiometers to tune the lock.
But yes, anyways if the motor has brushes you can generally drive it from an adjustable AC source if the low RPM torque is high enough. This assumes that the motor has only two terminals. Some have four.

[Nantawat Kittiwarakul]

That "speed controller" is, I believe, more or less the same as most AC dimmer device. It would work by partially "chopping off" from each of AC cycle, pulse with modulation - so to speak. The frequency remains the same but with reduced peak voltage (and also rms voltage).

Therefore it works perfectly with most universal motor - it won't give a dime whether it's fed with ac or dc supply anyway.

It MAY work with dc motor found in some projector as it helps cutting down input power in the first place. As long as the torque isn't too low to move the load it should be ok. Not sure if it works with electronically - governed speed motor found in some super8 sound projector as that might upset its speed regulator circuit.

And it will definitely NOT work with projector with ac induction motor, as its speed will largely depend on ac input frequency. When used its speed would remain fairly constant but its torque will sharply reduced. So I guess that the projector's speed will remain fairly stable, then suddenly slowed down and stall.

By the way it would also mess with audio section of sound projector too. So I would avoid using with with Elmo ST-1200 projector for example...

[Thomas Dafnides]

Does anyone know what type of motor is in the B&H standard 8mm projectors of 1960-64 ? example : Vintage Mid Century Bell and Howell Model 256 Auto Load 8mm Movie Projector / Motor Comes on When Plugged In, Lamp Does Not AS IS - Etsy
Or the Bolex 18-5 standard 8mm projector?

[Shane Graber]

Does anyone know what type of motor is in the B&H standard 8mm projectors of 1960-64 ? example : Vintage Mid Century Bell and Howell Model 256 Auto Load 8mm Movie Projector / Motor Comes on When Plugged In, Lamp Does Not AS IS - Etsy
Or the Bolex 18-5 standard 8mm projector?

Are you trying to drop the voltage enough / drop the stock motor speed enough that you don't have to use a stepper to run the film through the projector? That might be a tall order given you'd need to drop the speed from ~ 20 FPS down to 0.5 - 1 FPS. I ended up using a stepper + gears on the manual film advance thumbscrew at the front of my B&H 346A projector.

[Alwyn Adkins]

Shane, only to slow from 18 to 16.67 frames per second for three-bladed shutter. At least that's what I'm using the voltage thingo for.