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Projector speed control (Bolex M8) - question from a newcomer

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  • Projector speed control (Bolex M8) - question from a newcomer

    I am new to this forum filled with all kinds of experts, and delighted to be here! Hopefully I'll be making contributions in the future even though I'm starting off with a question.

    On the Bolex M8, an application I'm considering requires precise control of its motor speed. The M8 has a brushed AC motor. AC motors generally do not like pulse width modulated AC voltage. It may work for a while, but the motor may suffer from the added inductive loads, causing excessive brush wear or worse.

    In the forum archives I found a very helpful thread from 2016 where Graham Ritchie, an esteemed member, describes how he replaced the M8 rheostat speed control (varies voltage) with a PWM dimmer (varies pulse width) without issues.;f=1;t=010849

    Has this become the safe and now time-tested way of doing it? Not just for the M8 but for projectors with AC motors in general?

    Many thanks beforehand!
    Last edited by Michiel Kappeyne; March 04, 2021, 06:08 PM.

  • #2
    Just read the linked thread, it looks like solution there is only possible for DC capable motors. My chinon has a big reostat in series with motor. It's stable only if load is stable, AC motors tend to have a fixed speed, you are limiting torque with a reostat, so speed will be decreased only if there is a load (speed will be faster if there's no film). I don't think a reostat will damage your motor if you don't run it too low (it's essential to avoid the motor stop running). Maybe you can buy a cheap broken projector and take that part from it. What are your motor specs?


    • #3
      Thanks Isidro, the Bolex M8 at issue also has a big rheostat in series with the AC motor, old, corroded, and almost broken but not quite yet. After Ritchie introduced PWM he added a full-wave rectifier later, so that AC motor is indeed capable of running on DC. The M8 stator is a permanent magnet without any coil in it. Is that what makes it capable of running on DC and not minding PWM either?


      • #4
        The presence of a magnet doesn't guarantee DC operation, you may try checking first with a multimeter that DC voltage isn't very high (rectified will be higher than regular AC), I would use around 50V DC for that test (maybe you can get it from the lamp connector, my projector uses 48v lamp). Don't forget that if motor isn't designed for DC, it will look like a short circuit and will probably ruin the coil within seconds. I used to put a broken high wattage car lamp in series (usually it's broken LOW light but HIGH ligth filament is still intact), if everything is ok lamp will have neglective resistance, if there's a short circuit or very high current, lamp will shine and limit the current to motor. I have some designs with the 1 or 2 lamps in series always connected, it's better than a fuse because usually lamp doesnt blow so things will work ok after short circuit is gone.


        • #5
          Hi Isidro, it's not so much that I want to use the M8 with DC, it is whether its motor can withstand pulse width-modulated AC over the long run. In general, frequency-dependent motors, whether AC or DC, do not like it when part of the phase, AC or DC, is missing because it sets up inductive spikes at the underlying frequency, whether rectified or not. That's why even more "modern" projectors, designed at a time when PWM dimmers based on phase control were already widely available, still use(d) rheostats for speed control in most cases. The M8 was designed at a time when phase control dimmers did not yet exist, so I can't use the presence of a rheostat as indicator.

          And that's why using phase control as speed control concerns me a bit. I don't want to burn up the M8's motor or generate excessive wear from high-frequency sparking caused by the missing phase angles. So I wonder whether in general, and for the M8 in specific, members of this forum have been using PWM-based dimmers for their projectors without issues developing over time. The more modern AC projector motors are built as cheaply as possible with a finite lifetime in mind, while the M8 was designed at a time when quality and durability came first. That's where a difference could come in.

          The way around it all is to use Variable Frequency Drives or VFD for short. These rectify the AC and then recast it back to AC at a different frequency. They are very much more expensive than $5 PWM dimmers and also much harder to find for low power applications. But they do work well and are widely used in industrial settings, pumps and so on.
          Last edited by Michiel Kappeyne; March 05, 2021, 09:01 AM.


          • #6
            I see. I wouldn't risk your motor with a dimmer. Getting another reostat isn't an option?


            • #7
              Yes, that's a good option. Or fix up the original rheostats. In the M8 the wound resistor wire tends to break close to the one side of the cylinder where continuity is not needed, almost never in the middle. So the quick fix is to glue it back in place. And create a limit stop so the copper arm connected to the rheostat knob can no longer get there. Elegant it's not, but it should work.

              In my dreams I wanted to set up digital PID feedback to control speed with high accuracy--not that complex but requiring optimization--in order to avoid having to synchronize downstream. 'Looks like it will have to be continuous syncing then.

              Thank you all once more for welcoming me as a new member of this forum!


              • #8
                I'm a new member myself! if the reostat has three terminals (as a potentiometer), you could use the other end to avoid using the worn part.


                • #9
                  Hi Isidro, yes, that might be possible as well. The Bolex M8 is such a beautiful machine with very high precision yet gentle film gating and so on that I would like to keep using it. It's a classic. And good to know that, like me, you are a new member. From what I gather from your posts you appear to be engaged in film-to-video conversion so you are in the right spot here! The level of knowledge, experience, sheer ingenuity, and persistent effort I witness in this forum is simply wonderful.


                  • #10
                    Hi Isidro, no more worries about speed control on this end. 'Turns out I can run the front and back end of the Bolex M8 independently. So can can retain the incandescent light source, something I would very much like to do, as it's being cooled by the fan connected to the M8 motor. This liberates me to design the front end as I see fit.