DP70 Motor Slow Start Modification

[Thomas Piccione]

Howdy everyone! I'm looking for some advice from someone who's performed this modification before or has a better understanding of AC motors than I do.

Our 2 DP-70's (To clarify, they've got 2 sperate Master motors for 24 & 30fps) start nearly instantaneously after you press motor start. There's not much appreciable ramp-up to speed, definitely under a second from button push to full speed. Obviously that's not ideal, and reading through the service manual we have (which is slightly different than the file in the warehouse), Theatre Service Instruction #8 suggests adding an adjustable resistor in-line with the motor. The part listed is a vintage component, and I *think* I found a modern replacement here. Assuming I got the resistor right, is this still the best way to do it without replacing the whole motor? Burning 50 watts off with a resistor whenever the projector is running seems like a bit of a "sloppy" method to me. I've theorized adding a relay and stealing a connection off the centrifugal switch so the resistor is only inline when the motor is starting, but that sounds like it might create issues. If there's a better way to do it, I'd love to know! I've heard of replacing start capacitors before, but I have absolutely no knowledge on that.

Thanks!

[Steve Guttag]

Are you going to run these to the point of worrying about the heat off the resistor? The resistor method (and it is typically just done on the 24fps motor) is sufficient. If you really wanted to be efficient then you can go with an inverter drive and motor...set up whatever ramp up/down you like. As for jumping out the resistor when its running...you can but the bulk of the power comes at start up...not holding it while running. It's be a while...does the motor have separate windings so the start winding is out once it is at speed? If so, the resistor is out of circuit then anyway.

A "gotcha" on that resistor (Which is typically mounted in the base near the top of the service opening...it will shock you if you touch it. There is a narrow sweet spot where it provides slow-start but doesn't bog the motor down so it can't get up to speed.

[Dave Macaulay]

The running current is much lower than the start current, you won't be dumping 50W all the time even if it is doing that during start-up. As I recall the resistor was pretty warm after a reel but not close to 50W hot. I haven't seen one with two motors, I remember just a weird double driven pulley and two belts that had some sort of mechanism to change which was the driving pulley. But we never used 30fps or even had a belt on that pulley.

[Mark Gulbrandsen]

The resistor should not get hot at all. as there is still a centrifugal start switch in these motors that takes the start capacitor out of the circuit once it reaches speed. You could also lower the value of the start capacitor as well. The idea is to limit the start up current, but still have sufficient torque to start a projector threaded with 70mm film on a 25 degree day. Or on what ever is a typical cold day in Mass.

[Thomas Piccione]

The resistor should not get hot at all. as there is still a centrifugal start switch in these motors that takes the start capacitor out of the circuit once it reaches speed. You could also lower the value of the start capacitor as well. The idea is to limit the start up current, but still have sufficient torque to start a projector threaded with 70mm film on a 25 degree day. Or on what ever is a typical cold day in Mass.

Haha, good one! Out here in Worcester we occasionally hit 0°F in the winter, though I've don't think I've seen the booth drop below 65.

As for the resistor, good to know. I read this part of our service manual which lists a running current of 4.8 amps (so, 576 watts) and 4.8 amps through a ~2 ohm resistor puts it at 50 watts of dissipation. I could also totally just be reading that line wrong. I've never measured the power so I don't know if that's accurate or not. Either way, if the resistor isn't getting all that hot, then it's all a bit academic.

Since I'd like to avoid the electrical hazard described by Steve, it's definitely going in a ventilated enclosure, though I'm not quite sure what that would be. Something made of plastic seems like a poor idea.

[Mark Gulbrandsen]

Far as I remember, even the GE motors used in the older two motor machines still have a switch inside them as well. But they require different capacitors. The last 20 or so machines we sold and installed had variable speed motors and I actually prefer those.

[Steve Guttag]

That document describes putting the resistor in line with the entire motor, without respect for which winding so, yeah it will be heating up a little. As for the shock hazard, it is only present if the cover is off the base and you stick your hands in there...there are LOTS of shock hazards in there and if the machine is running there are mechanical ones too.

[Marcel Birgelen]

Just watch the safety video first and all will be OK.

[Mark Gulbrandsen]

That document describes putting the resistor in line with the entire motor, without respect for which winding so, yeah it will be heating up a little. As for the shock hazard, it is only present if the cover is off the base and you stick your hands in there...there are LOTS of shock hazards in there and if the machine is running there are mechanical ones too.

They did not use Master Motors in Europe, so the mod would say to put it in line with the motor. But I never did because all four wires come down to the terminal blocks in the base anyway.

[Thomas Piccione]

They did not use Master Motors in Europe, so the mod would say to put it in line with the motor. But I never did because all four wires come down to the terminal blocks in the base anyway.

I'm not sure if I'm just reading this wrong, but I think the paper is listing this modification specifically for machines with Master motors and says to put it inline. If you did it some other way (it sounds like you put it inline with the starting capacitor?) I'd love to hear it!

[Mark Gulbrandsen]

I'm not sure if I'm just reading this wrong, but I think the paper is listing this modification specifically for machines with Master motors and says to put it inline. If you did it some other way (it sounds like you put it inline with the starting capacitor?) I'd love to hear it!

That is correct Thomas. We also changed to smaller value capacitors. But we did more resistor installs than capacitor changes. The two motor machines are less of a problem in this regard, because one motor also has to spin the other motor and there is always some load..

[Thomas Piccione]

That is correct Thomas. We also changed to smaller value capacitors. But we did more resistor installs than capacitor changes. The two motor machines are less of a problem in this regard, because one motor also has to spin the other motor and there is always some load..

Good to know! Do you have any recommendations for resistors? What I'm looking at right now is an adjustable 8 ohm 300 watt one. 300 watts sounds like a lot of capacity and means it's pretty massive resistor which might be fun to find space for. I know these motors will pull a lot more power when they start up, but that's also a very short time period so it's not a huge amount of energy at the end of the day. I'm really looking to see if I can get away with something more like 50 watts.

[Mark Gulbrandsen]

We used this style of resistor and mounted it usually upper left of the contactor. Am pretty sure the ones I installed were 12 ohm and then I'd set it down from there, or just leave it at 12 ohms if that was satisfactory.... These usually come with mounting clips.

[Paul Finn]

Because of the age, I'm with Mark to question the present value of the motor start capacitor. I've seen them in a pair of AAII's that were increased in value to the point of being almost a short at the 60Hz line frequency, causing the machines to jump-start.

Paul Finn