Film-Tech Forum ARCHIVE: Platter Drive Motor Diode

This is topic Platter Drive Motor Diode in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Michael West (Member # 428) on 02-10-2003, 11:44 AM:

i have had some platter drive motors repaired on our strong international platter system (ap 3 auto programming platter), generally they just cleaned the armatures and replaced bearings and brushes, but they came back with poor results so the motor repair shop noticed they have diodes and suggested we bypass these. so my question is what are these diodes in for? the older units did not have diodes.

Posted by Aaron Sisemore (Member # 145) on 02-10-2003, 12:02 PM:

Someone correct me if I am wrong, but the reason for the diodes in the Potts/Strong/CFS/ORC 'vacuum cleaner' motors is because the motors are DC-operated, and will not function on AC.

Another theory that dawned on me just now is that the diode might be there to prevent the motor from becoming a 'generator', the diode would block the DC current coming from it.

-Aaron

Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 02:12 PM:

DO NOT remove or bypass the diodes! The motors would be very sluggish, and will not have any torque to speak of if you did.

Posted by Michael West (Member # 428) on 02-10-2003, 04:08 PM:

Aaron as far as i know it is ac and Paul, we bypassed a diode on just one motor (experiment) and it has never worked better. i will refrain from any other (experiments) as i await more responses.

Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 05:46 PM:

Michael, that is interesting. I have no explanaion for that. The thing I know is that if, the doide shorted out (the same as it being bypassed) the platter would grossly under-feed. Try that on payout....and see what happens.

It'll probably work on take-up, but it would not surprise me that if you have a fast starting projector, the elevator might hit bottom before the inertia on the take-up platter is overcome.

But since you raised my eyebrow on this, I'll proof it for you on my CFS machines in a few days. Meanwhile, I still stand firm on my orginal recommendation.

Posted by Pat Moore (Member # 380) on 02-10-2003, 08:48 PM:

I'm with Paul on this. The diode is required to provide a half-wave pulse to improve starting torque on that motor. I'd be really surprised if performance improved without it.

Pat

Posted by Daryl C. W. O'Shea (Member # 1303) on 02-10-2003, 08:53 PM:

The only way that I can see removing the diode not adversely affecting operation is if the deck is timed really 'fast'.

Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 09:42 PM:

Yes...and that in itself can cause some very nasty problems.

Posted by Gordon McLeod (Member # 33) on 02-10-2003, 09:58 PM:

Also remeber that the take up and feed out speed are all based on the same variac output as a base so timing will be a mess without the diode

Posted by Michael West (Member # 428) on 02-11-2003, 10:40 AM:

well, it looks as if we will be removing the diodes anyways - and yes our platters timings are "all over the map", so it concerns me, i have mentioned this and was more or less ignored but at least i tried. the method used for timing platters here is to replace the drive motor prior to show and adjust as the movie plays!!! it is good to know the information you have all sent for future reference, much thanks.

Posted by Gordon McLeod (Member # 33) on 02-12-2003, 11:06 AM:

A word of caution if you are part of a circuit before you modify anything get permision from the service engineer of your theatre

Posted by Paul G. Thompson (Member # 655) on 02-12-2003, 01:14 PM:

Not only what Gordon said, but keep in mind you could "over-voltage" the components which could shorten the component's life, especially the electronics in the pay-out heads.

Posted by Michael West (Member # 428) on 02-12-2003, 04:08 PM:

Gordon i guess the projectionists in charge have told me that our theatre tech. had recommended that we do this!? Paul, the projectionists in charge told me that economy is much more important so to burn a motor out ma [Confused]

kes much more sense!!!!? gee, i feel so ignorant.

Posted by Gordon McLeod (Member # 33) on 02-12-2003, 04:32 PM:

I would be concerned about the variac being damaged as the current load will be higher on ac than 1/2 wave dc

Posted by Paul G. Thompson (Member # 655) on 02-12-2003, 04:48 PM:

Michael, if it were me, I would just flat not do it. There is absolutely nothing to gain, but the chance of damaging the equipment is very high. Gordon is correct with his assessment.

Spend a buck to save 2 cents. That's what is going to happen.

Posted by Michael West (Member # 428) on 02-12-2003, 05:49 PM:

Gordon yes that is a relevant concern especially when the diodes have eventually been removed from all three platter feed motors.
Paul, your information has convinced me "i will not have anything to do with this" but as for the other guys - well that is up to them. i will document this forum topic for my own protection, have to say this info is of great value for me i do value this web site. i am often seen at this multiplex as over protective and somewhat old fashioned and therefore "not with the team", so i have very little influence but to keep out of enevitable disasters.

Posted by Dave Macaulay (Member # 813) on 02-12-2003, 08:57 PM:

If the motor came back with "poor results" then they screwed up the motor or it was beyond repair. These motors may not be what I would choose if I was designing a platter, but there are thousands of them working every day - with the diodes.
The diodes are not the problem. It may be a 10 cent part but the designer wouldn't have wasted that dime - it's obviously required for some reason.

Your description of timing the decks indicates that the tech is not the crunchiest pickle in the jar.

Posted by Paul G. Thompson (Member # 655) on 02-12-2003, 09:22 PM:

Dave, right on! [thumbsup]

I would like to believe their tech is not totally familiar with the theory of operation of the machine.

Posted by Michael West (Member # 428) on 02-12-2003, 11:31 PM:

no i won't blame the tech. as we only have one for the entire maritimes and he is very good but over worked as we all are.
it is very easy to time the strong platters the manual makes it very clear but no one (i am the exception) seem to see this as being terribly important and what started this was actually when i took and bad motor apart, cleaned and changed bearings and brushes gave it back to them with unsuccessful results so then we sent them to the motor shop and so it goes, i think our problems could be resolved with new motor bearings/brushes cleaning and obviously platter timing - now if only sombody at work would take me seriously hmm. i don't think i would have the time to sneak in and do it myself...

Posted by System Notices (Member # 2357) on 07-22-2008, 10:43 AM:

It has been 1986 days since the last post.

Posted by Richard May (Member # 2627) on 07-22-2008, 10:43 AM:

I remembered this thread and thought I would reply. I was on the phone with someone from Design and Engineering yesterday. They have been bypassing the diode for a year or so now on the black motors. The funny thing about it is they are unaware of any problems when putting one of the newer black motors on an older strong platter with the grey motors. If I'm not wrong, Strong sent out a memo on this years ago. All the shafts had to be the same size. Well the 3 theaters that have the black motors on them do not take up fast enough on start up. The yo yo drops. I have tried timing the platters with no good results. The newer motors don't seem to have enough power for some reason. It is very easy to stop the platter from spinning with very little resistance. They seem to be puzzled by this. I know that I have heard this in other theaters too.

Posted by Jim Cassedy (Member # 4115) on 07-23-2008, 12:42 PM:

quote: Richard May
The newer motors don't seem to have enough power for some reason. It is very easy to stop the platter from spinning with very little resistance.

I recently had what I at first thought was a motor torque problem at one of the theaters I'm responsible for.

Platter tables did not seem to have enough torque and would bog-down under long (heavy) prints to the point where sometimes the yo-yo thingy would drop and shut everything down.

I started checking motors and platter control circuits, etc- - but the problem turned out to be nothing more than old grease gumming up the platter turn-table bearings.

I took the platters apart, cleaned out the old grease and properly re-lubricated everything and have had no problems since.

Just for good measure, I ordered a couple of spare sets of platter bearings, but I haven't had to use them.

Posted by Noel Mackisoc (Member # 4852) on 07-23-2008, 02:30 PM:

Be assured, the diode is necessary as Pat Moore states. Rarely will performance improve by shunting it; more often it deteriorates.

Probably, Strong replaced the motor itself with a new unit (cheaper than actually replacing parts). Trouble is, the new motors, although very much like the old (grey) ones, do not have the same low-speed torque characteristics and will appear to run much slower, especially when mixed with other--worn--motors.

The same obtains if you install a new (black) replacement motor.

You can get 'round this by replacing all 3 motors with rebuilts/new and re-time the platter afterward.

Posted by Richard May (Member # 2627) on 07-23-2008, 03:48 PM:

Why would the company that makes these motors be bypassing the diode on purpose then? This is what they told me over the phone the other day. They said it is not needed. This is coming directly from them.

Jim, thanks for the idea but I compared the newer black motor with the older grey one and there is a huge difference.

Posted by Randy Bowden (Member # 587) on 07-23-2008, 05:49 PM:

The slower speed problem seems to be rather typical of the new motors. We do motor re-cores here at ACE. That involves getting a new ametek motor. I have done the diode bypass experiment in my shop and I did see a very noticeable speed increase with the motor but so far no trials with that in the field. With older platters a good practice is to just re-core all 3 motors.

Posted by Louis Bornwasser (Member # 3063) on 07-23-2008, 06:49 PM:

The newer motors will gain speed if properly "run in." Not likely to happen on the platter, but seems to happen at moderate speed on my work bench. (less than 1 hour) Louis

Posted by Steve Guttag (Member # 268) on 07-23-2008, 09:06 PM:

What Louis said.

Posted by Richard May (Member # 2627) on 07-23-2008, 09:21 PM:

I will try it.

Posted by Richard May (Member # 2627) on 05-25-2009, 05:28 PM:

Just got a new motor from Design and Engineering. Along with the motor was an instruction sheet specifically saying to bypass the diode to increase motor speed. Between the size of the hub and the diode, no wonder it sucks timing these things. Nobody knows the correct way.

Posted by Louis Bornwasser (Member # 3063) on 05-25-2009, 08:04 PM:

Later, then the motor speeds up and you reinsert the diode. Louis

Posted by Richard May (Member # 2627) on 05-25-2009, 08:46 PM:

So can you just hook up the make up table and run the motor for an hour at almost high speed? Is that enough to break it in?

Posted by Dave Macaulay (Member # 813) on 05-25-2009, 09:24 PM:

You have to seat the new brush on the commutator, the new brush has "teeth" on it that wear down relatively quickly so the brush end curve fits the commutator shaft properly. If it doesn't, you have a small contact area and poor performance.
On the actual platter, the motor never gets close to full power, the variac might get to 50% line voltage maximum - the geometry of the arm crank dictates this. Using the makeup table you can get 100%. The break-in routine is printed on the brush box if you are using the Grainger brushes - 1/2 hour at 50% voltage and 1/2 hour at 75% voltage.

Posted by Richard May (Member # 2627) on 05-25-2009, 09:40 PM:

I have a few theaters that have the newer black motors to be put in. I will check the speed before I break them in and then check it again after.