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Author Topic: Problem with Century Starting Too Fast Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 02-16-2009 07:30 PM                        Century once (1976) shipped a 1/6 hp motor. . . .didn't need a start kit (1725 rpm) In fact, if the intermittent had an o-ring on the star, sometimes it wouldn't start at all. Removed the o-ring. Louis  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 02-16-2009 07:46 PM                     Louis, Weren't the Black finned Bodine Sync motors 1/6th HP? Seems to me they were... they also all burn't up eventually, all good Bodines do!  |  IP: Logged Andy Muirhead Master Film Handler Posts: 323 From: Galashiels, Scotland Registered: Dec 2000 posted 02-16-2009 08:02 PM                     quote: Tony Bandiera Jr Now a variac or the old style resistance dimmer would probably work well. We installed variacs on our Century's and Monees, they work well.  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 02-16-2009 08:26 PM                     Andy which Variac are you using?  |  IP: Logged Andy Muirhead Master Film Handler Posts: 323 From: Galashiels, Scotland Registered: Dec 2000 posted 02-17-2009 07:47 PM                     I don't recall at the moment, and i'm at home, however next time I get a chance I'll check and post.  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 02-17-2009 08:01 PM                        What type(s) of AC motor(s) can be safely controlled with a variac? I have two "Superior Electric Powerstat type 146" variacs that came out of an old theatre that were used for lighting control. Can these be used to provide variable-speed capability for some types of projector motors without destroying the motors? I really should learn more about AC motors.  |  IP: Logged Damien Taylor Master Film Handler Posts: 493 From: Perth, Western Australia Registered: Apr 2007 posted 02-17-2009 09:00 PM                     Sort of on topic, but i'm replacing a dying start cap on an RCA 1050/1040 motor, does anyone know the details of the cap, its sealed in a metal tube.  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 03-13-2009 03:55 PM                     The easiest solution seems to be adding mylar leader to every reel which I've done for years now. The problem is that I just can't stop a reel and restart it with non-mylar film. It's also the cheapest fix.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 03-13-2009 05:05 PM                        Scott, A variac or triac controller (light dimmer-style) can't control an AC motor's speed with any precision. To properly control a projector motor's speed you would need a variable frequency drive. AC induction motors (the type with the odd number of RPM, not 1800 but 1725) rely on some slip under load to run. Slip accounts for the difference in the synchronous speed of a motor (a 4 pole 60 Hz motor has a synchronous speed of 1800 RPM) and the actual running speed. The greater the load is in proportion to the maximum torque, the greater the slip. This is the principle that household fans use to control their speed. But this is not a very accurate way to control motor speed, as you can see by how fans speed up when you block their airflow and reduce the load on the motor. Ceiling fans actually have an autoformer winding (much like a variac) built into their motor windings for speed adjustment, and ceiling fan controllers use triacs, much like light dimmers. These both work by varying the fan's supply voltage and thus current, and thus torque, and thus speed. However, in a projection application, we don't want to approach the point where the load torque is so high that the motor speed varies significantly. If this happens then the projector speed will start to vary significantly with input voltage sags. A soft-start circuit limits the current the motor draws when it's starting to limit the starting torque. There are both solid-state soft starters that use triacs and those that use a relay to switch a resistance or an autoformer into the circuit during starting, bypassing it once the motor reaches rated speed. If you don't need the motor's full rated torque (like discussed above) you can leave the autoformer or triac in the circuit and the motor will run at full speed, but it will start slower owing to the reduced supply voltage.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 03-13-2009 11:24 PM                     Any 120v "variac" type variable autotransformer rated for over 5 amps would work to reduce starting torque but you'll also lose synchronous locking torque. The motor may run slow or get into an odd oscillating speedup/slowdown action that you can hear as wow in analog sound - of course you can just increase voltage once up to speed but you have to remember to turn it down again for the next start. The series resistor works nicely with slip motors (1740 rpm) and slightly less well with sync motors but it's better than a variac I think. The voltage drop is proportional to current, so starting torque is reduced more than running torque becuse the starting current is usually at least twice the running current. Sync motors have a weird torque/current relationship though: quite line current high with no load, lowering as load increases, until some point where increasing load increases current again. With slip motors the current just increases with torque load as one might expect. For the resistor get a wattage size rated a bit higher than the power dissipated at constant running speed, the higher power dissipation on startup won't damage it since it's only a second or two. Remember that resistor power ratings are for "free air" mounting, don't put one inside a small electrical box and expect it to last long. The old Century resistor units are in perforated metal cases to allow cooling air to circulate.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 03-14-2009 10:07 AM                     Sounds like the answer for Scott is to hook up his variac and twiddle it to find an appropriate starting torque, then measure the voltage drop (or read it off the variac dial). Knowing the desired voltage, divide by the startup current to get the desired series resistance. Of course, there's still some guesswork since you need to figure out the starting current. You can also then calculate the running voltage drop with that resistance and set the variac to that voltage and confirm there is sufficient running torque. (Of course, having an appropriate variable power resistor would work too, but I suspect there isn't one handy.) --jhawk  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 04-18-2009 06:49 AM                     UPDATE: Problem solved. I first tried the small Century provided square box with the resistor that is placed in line with the hot lead but discovered since my motor was the "1800 RPM" version...it wouldn't work. I did find a time-delay/resistor box with a 1-5 second timer that works perfectly. Thanks for everyone's comments and ideas.  |  IP: Logged

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