Greetings David, Bill, Mark, Dan and any other forum members interested in this subject!
I have some information on the 24 fps sync motor that I can share. I am also an owner (and operator) of the Arri 24fps sync motor, 60~ (Hertz these days), 42 volts AC, 8 volt DC for magazine torque motors. I have used this motor with the Arri 16S camera as the motor was designed.
The thing to keep in mind, when this sync motor was designed for use with the Arri 16S camera, the 16M was not on the market. The earliest listing I have for this motor is a price list from 1961.
The motor was designed for use in the blimp for the 16S, but it could be used without the blimp. I never used my 16S in a blimp, so I have no experience with that set-up.
The 24 fps sync speed was maintained by the frequency of the mains, there were no xtal controlled motors back then.
This motor can be used on the 16M camera, even though there is no 8 volt magazine torque motor, you still need the 8 volts to turn on the 42 volt AC motor and operate the clap stick feature.
To answer a question, no, no 42 volts AC went into the camera (disclaimer: as long as the motor and cables were operating properly as designed!) The power supply is more than just a step down transformer.
David, in the picture of your 16M with the sync motor attached, I noticed that the motor was sitting further away from the body. That's when I realized, you have another Arri 'accessory' between the motor and the camera. That accessory is the 'Phase Shift Attachment' that was used for filming CRT tv screens back in the day. Description.....
I have never used this accessory, so I can't say how it works. But, the main thing is, when filming CRT screens you needed a sync motor so the shutter bar didn't drift.
So the previous owner of the motor used this on either a 16S or 16M camera to record tv screens. I'm assuming that this Phase Shift Attachment came attached to the motor when you purchased the motor.
The reason I bring this up, there are 2 conditions for using the sync motor and recording screens in the old analog days. Also, Arri offered a special sync motor modified for interchangeable geared heads to change the speed from 24 fps to 30fps for this purpose (it uses the same power supply as the 24 fps motor I talked about above).
One way to film the screen was, shoot 24 fps with a shutter at 144 degrees. Or, shoot at 30 fps with a 180 degree shutter, the mirrored shutter that came with the camera.
The special sync motor came with the 24fps gear drive, but you would have to send your camera to Arri to be fitted with a 144 degree mirrored shutter.
Or, you could keep the existing shutter in your camera and get the 30fps geared head for the motor, but you would be shooting everything at 30 fps, which may not work for sound synchronization.
You take your motor off the Phase Shift Attachment by loosening the clamp on the side of the phase shift unit. Pull the motor off and see which motor you have. Unless you are shooting old CRT screens, you really don't need the Phase Shift Attachment, also, by eliminating it, it would be 1 less rubber coupling that could be slipping on the camera's drive shaft.
The speed of the motor should be marked on the end where the drive shaft sticks out....
If your motor has the 30 fps gear head, then you'll be shooting everything at 30fps. You could always have the footage transferred at 30 fps. It's too late to find any 24 fps geared head assemblies, unless the seller included it when you bought the motor.
Does your motor have a pigtail lead on it as is shown on the right in this pic? It could have a connector like shown, or it could have a cannon connector such as the one that goes into your camera's power supply port.
Description of the 24 fps sync motor from a 1960's sales brochure. That is exactly how my motor and power supply look. I do have the accessory film counter for the blimp. That accessory also has contact points on the shaft that 2 wires are plugged into so when the motor is running it activates lights on the blimp that then blink so you know the camera is running.
As it states, the motor is 3 phase 60~ (c.p.s., the 'old cycles per second' term) 42 volts.
The accessory footage counter....
A newer Arri description of the 24 fps sync motor from the mid '70s (notice the smaller sized power supply).....
The 2nd paragraph in this Arri description lists the availablity of a 117 volt sync motor. I've never seen one, and I think that was a motor that Arri USA made for US customers, it's not from the factory in Germany. However, it seems with this motor, 117 volts went to the motor on the camera, but I'm not sure if you could use the camera's on/ off switch. Not much info on that motor is around. I guess the 'shock hazards' they mention in the previous paragraph about the 42 volt motor no longer exist when using this motor.
David, if you want to know which pins on the plug connect to the motor's windings, or the the 8 volt pins, I can make a plug schematic for you. The pins are not numbered externally. Of the 6 pins, 3 of them are used for the 42 volt 3 phase AC supply, 2 pins for 8 volt DC positive, and then one for ground.
I don't have schematics for the power supply.
So, in slow motion, this is how motor and power supply work on either the 16S or 16M....
8 volts positive comes out of the power supply via a seperate wire to the sync motor, it then travels out of the motor on the pigtail that is plugged into the left pin (the positive pin) on the 16 S camera's power input. The sync motor housing blocks the neg pin so you can't plug it in incorrectly. It then goes to the on/off switch. When the switch is activated, the 8 volts go to the torque motor on the 16S camera (not needed on the M model), the automatic clapstic on the GS models, and to the electrical contact on the sync motor (yellow arrow)...........
It then travels back (on it's own wire) to the power supply that contains 2 relays and activates those relays. When the relays are activated and they close the contacts, they connect the 42 volt 3 phase A.C power that travels back on 3 wires to the windings on the motor, and the camera runs.
When the camera is shut off cutting the 8 volts to the relays in the power supply, the power supply applies DC current to the windings of the motor to stop the motor from turning. It does stop the motor very quickly.
This dynamic braking is necessary on the 16S because of the torque motor on the magazine. When the 8 volts are cut, the torque motor stops right away, no more film is wound on the take-up side. Because the 42 volt AC motor is brushless, when the power is cut, it does not stop abruptly like a DC motor does, but it coasts to a stop. When the motor is connected to the camera, that coasting could advance the film movement and draw 5 to 8 more frames of film through the camera. The torque motor is stopped, so those extra frames of film cause slack in the film on the take-up side. When the camera is next started, the torque motor takes off, and depending on the condition of the clutch, it could pull on that slack in the film hard enough to strip sprockets and cause a lost lower loop, or break the film. Then the buckle switch works. If the camera is in a blimp, this would cause downtime, and the DP retires to craft services table. The Dynamic Brake prevents this.
I don't know why Arri used 42 volts for the motor. I have not run across any literature about why the engineers decided on this voltage, other than safety reasons mentioned above.
I have used my motor and power supply, not only on mains, but on location plugged into a crystal controlled generator. No problems.
I hope this clears up some of the mysteries of this motor!