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Author Topic: RCA Soundhead Motor Speed? Pete Lawrence Expert Film Handler Posts: 192 From: Middleburg, PA Registered: Aug 1999 posted 09-28-2003 11:21 AM                     This is in a different thread because it's really a different subject, but it relates to one started Chris Seo, "Crompton-Greaves (RCA soundhead) motor failure" Chris lists a motor speed 1440 RPM on a RCA 1040 sound head. The book shows 1765 RPM for a MI-1040 at 60Hz. The book also shows 1480 RPM @ 50 Hz for an MI-9031 and 1763 RPM @ 60 Hz for a MI-9030. All non-synchronous induction motors "slip". Slip is the difference between the actual RPM at a given load, 1/4 HP in this case, and the synchronous speed for the same number of poles. Most common four pole induction motors are rated at 1725 RPM @60 Hz. At no load the slip is only one or two RPM. This increases as the load increases and the motor slows down. Four pole synchronous speed is 1800 RPM for reference. If I attach a motor rated at 1725 RPM to my RCA sound head (or 1440 RPM in Chris' case) the film speed will be wrong. I'm pretty sure Chris quoted the 50 Hz speed for his motor, as they will also run 50 Hz. But it's still wrong according to the books and will result in the wrong film speed. Anyone like to explain what's going on here? Why did RCA chose 1765 RPM? How many of you replace R&M 1765 RPM motors with an off the shelf 1725 RPM motor? I've seen it done.  |  IP: Logged Darryl Spicer Film God Posts: 3250 From: Lexington, KY, USA Registered: Dec 2000 posted 09-28-2003 11:29 AM                    Actually it is the projector head that runs at 1440 R.P.M. not nessesarily the motor. Remember if you multiply 24 frames by 60 seconds it will tell you that the projector turns at 1440 R.P.M. If the motor is direct drove gears no belts then it could very well be rated at the 1440 R.P.M. speed. If it is belt then the sice of the drive gear will determin the speed of the projector head.  |  IP: Logged Christopher Seo Jedi Master Film Handler Posts: 530 From: Los Angeles, CA Registered: Jun 99 posted 09-28-2003 06:54 PM                     You are indeed correct, Pete... when I examined the motor nameplate again I saw that someone had scratched the '5' in '50 Hz' to make it look like '60 Hz'. This at least makes more sense since our other motor, a slightly newer Crompton-Greaves, is also rated at 1440 RPM @ 50 Hz. So at 60 Hz, would the RPM theoretically be (60/50)*1440 = 1728, or ~1725 as you say? Still, these were "Manufactured for Photophone Ltd." in beautiful Bombay, India.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-28-2003 08:23 PM                     Compton was originally a british motor builder that went with westrex to india I gather they had no relation though to compton organs  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 09-29-2003 10:18 AM                     The motor drives the soundhead gearbox, the soundhead sprockets are (as I recall) 16 tooth so they should run at 360RPM. The projector drive steps this up to whatever the projector wants, commonly the intermittent drive is 1:1 so the projector drive shaft is 1440RPM. There were originally 2 gearsets for the soundhead, 50Hz and 60Hz, as the motors used were 1725/1437RPM running about 2 AC cycles/turn. Now you can get sync gears for most soundheads for 1800/1500 RPM motors as well. The drive from the soundhead to projector (shaft, gears, belt...) provides the proper projector shaft speed. There's not a huge difference between a 60 and 50Hz motor, many will work on either although with reduced torque and/or increased heat at the "wrong" frequency. The manufacturer may have just changed the 5 to a 6 so that someone finicky wouldn't get upset, they presumable would have the correct gears installed for the AC power where the system was to be installed...? The non-synchronous motors will not give exactly 24FPS operation - pretty close though. Nobody should ever notice the slight speed error. Synchronous motors have become normal on new systems because so many multiplexes run in interlock regularly, speed differences will screw up an interlock operation rather badly.  |  IP: Logged Pete Lawrence Expert Film Handler Posts: 192 From: Middleburg, PA Registered: Aug 1999 posted 09-29-2003 02:57 PM                     I agree basically with what Dave says. My main question is what should the motor speed really be rated at. What started this whole thing was I picked up an old MI-1040 head for parts. Mounted on it was a 1/4 hp GE replacement motor rated at 1725 R.P.M. I realized there's something wrong there. That's where we disagree. This whole discussion really applies only to RCA sound heads. The RCA MI-1040 manual (from the manuals section here) states: quote: The A.C. motor is of the split phase starting, induction run type and runs at a constant speed of 1765 R.P.M. It is rated at 1/4 h.p. If you work backward from a desired (and correct) 360 R.P.M. sound head sprocket rotation, a 49 tooth sprocket shaft gear with a 10 tooth spiral gear (both numbers are from the book) directly coupled to the motor shaft gives (49/10) * 360 = 1764 R.P.M. for the motor speed. Acceptably close to the 1765 R.P.M. quoted above, but far from the 1725 R.P.M. that both Chris and Dave have mentioned. That's a speed difference of about 2.2%. My tin ear couldn't hear the difference, but I know a couple musicians (symphony orchestra types) that would go ballistic if they ever heard their music played back that far off. I have a MI-1040 running with an Simplex E7 and a MI-9030 that's almost finished it's rebuild under a Simplex 35. Both have their original R & M motors. I'll set up a tach pickup with a counter and take some measurements just for the fun of it.  |  IP: Logged

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