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Author
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Topic: Convert Fixed Speed Projector to Variable Speed?
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Joseph Collura
Junior
Posts: 10
From: Friendswood, TX, USA
Registered: Feb 2014
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posted February 12, 2014 01:52 PM
Hello all,
I piggy-backed a similar post to this one on another thread, so please excuse the double post. I'm converting some reels of super 8 film to digital and the B&H 357Z projector that I'm using has a fixed frame rate @ 18fps and doesn't match that of digital camcorders, which I believe is 29.97fps creating "flicker" in the digitized image. From what I've read in order to remove the "flicker" the frame rates have to be adjusted to match. I have also read that reducing the frame rate to 15fps will also stop the "flicker". In order to test whether or not that is valid I set up my homemade telecine box and while running the projector, pinched the ratchet sprocket between my thumb and forefinger, slowing the projector and the "flicker" disappeared.
So, I'm hoping there is someone here with knowledge of the motors used in Bell and Howell projectors and about whether the 466A motor, or any other rheostat controlled B&H motor, is of the same or similar design to the 357Z and can therefore also be controlled by a rheostat?
I thought of trying a Speed Control designed for a woodworking router. I realize it will also dim the light, the way its designed to plug into the controller but I'm thinking for transferring to digital via a camcorder that dimming the light some might not be a bad idea.
If it can't be done electronically, cheaply and reliably, I'll reduce the speed by changing the drive pulleys.
Thanks,
JC
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Martin Jones
Phenomenal Film Handler
Posts: 1269
From: Thetford , Norfolk,England
Registered: May 2008
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posted February 14, 2014 03:18 AM
That's more like it!
Your motor is a "Shaded Pole Induction Motor", similar, but not the same, to those fitted to many Eumig projectors.
It can only be varied over a wide range of speeds by supplying it with a variable frequency supply instead of the fixed mains frequency. Not really economical since such a supply would cost you more than a more amenable projector (in fact several!).
Speaking now from my experience with Eumig motors, it is possible to vary the speed DOWNWARDS from the set mains speed by using a Thyristor type controller such as the one I use...
http://www.quasarelectronics.co.uk/Item/cebek-r-8-230v-ac-motor-speed-controller-module-375-watt
With this one, I set my projector to about 17.5 fps, and I can control smoothly and stably down to about 15 fps, enough for transfer at 16.67 fps.How that particular device would perform on your motor depends on the motor: I reached my conclusions by experiment.
If you wanted to use "Rheostat" control of a projector you would need one with a "DC Magnetic Motor". With these, speed is varied over a wide range by varying input voltage or by using a PWM Controller
--------------------
Retired TV Service Engineer
Ongoing interest in Telecine....
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