Film-Tech Forum ARCHIVE: Problem with Century Starting Too Fast

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Posted by Don Furr (Member # 1469) on 02-15-2009, 04:31 PM:

Here's the deal. I've just installed my newer SA/RE-3 equipment and noticed that motor is starting WAY to quickly. It's a GE 1/4 HP, 1800 rpm version. It has the "deep" flywheel installed but using 6K reels it snaps the film. It's coming up to 1800 rpm PDQ.
Is there a cap kit or some other way to slow the start time? HELP!!

Posted by Scott Norwood (Member # 30) on 02-15-2009, 04:54 PM:

Are you using floating-hub takeup reels? With 5" or larger hubs? Fixed-hub 6000' takeups _will_ snap the film unless you have the takeup tension set just perfectly. You really want the 7" floating-hub reels, but those can be hard to find.

I just went through the "projector starts too fast" issue myself. I have the 1725 RPM async motors, for which there is a Century slow-start kit (resistor that goes between a start winding and the AC line). I don't know if these apply to the 1800 RPM motor, which is probably a sync motor. With the sync motor, you could use an inverter drive (TB Woods E-TRAC, or similar) to control the ramp-up.

Posted by Steve Guttag (Member # 268) on 02-15-2009, 05:21 PM:

You would want to also switch to a 3-phase motor if you went to a motor controller too.

A poor man's slow start for a sync motor is a HIGH-WATTAGE resistor and a time-delay relay...have the resistor in line just for the first couple of seconds so it is more of a two-stage start. It isn't great but I've seen it work with some success.

I much prefer inverter drive...very smooth starts.

Steve

Posted by Don Furr (Member # 1469) on 02-15-2009, 05:55 PM:

If I change this 1800 rpm motor to a 1725 non-sync will that fix the problem?

Posted by Monte L Fullmer (Member # 2797) on 02-15-2009, 06:22 PM:

I do some shift work at a drive in using a Century that unit has a 1725 rpm induction motor on it. Ramps up quite smoothly. It's the belt driven model, not the direct drive version.

True, those Sync motors, esp those Leeson models, really get the rpms up quite quickly.

-Monte

Posted by Mark Gulbrandsen (Member # 72) on 02-15-2009, 06:30 PM:

You'll also have to replace the pulleys and drive belt to go to 1725rpm. Sync is the more desriable since it gives you perfect pitch. 1/4 hp is way too large!! About 1/5th to 1/3rd HP is better for a 35mm version. You can also place a large power resistor in series with the start capacitor or start winding depending on motor style to lower the start torque.

Actually there IS a very good single phase motor controller that is British made. It will not work on capacitor start or run type motors though. Its speed is very accurate. Will get back on this tommrrow as we have them in the parts room.

Mark

Posted by Gordon McLeod (Member # 33) on 02-15-2009, 06:48 PM:

I usually just put a 1.5ohm 200watt wirewound resistor in series with the motor and leave it there
All it does is rob some of the starting torque and unless you have a low mains voltage its dropacross it while the motor is running is negligable

Posted by Mark Gulbrandsen (Member # 72) on 02-15-2009, 07:21 PM:

I agree Gord. A wall dimmer will also work just fine on split phase motors and gives you variable adjustment of starting torque. I've never tried this on a sync motor though so can't say if it'd work or not.

Mark

Posted by Don Furr (Member # 1469) on 02-15-2009, 07:40 PM:

I just inspected a pair of newer Strong 5 Star sound heads....they both have 1/4 hp 1800 rpm motors, but there are two caps on top. What purpose does the 2nd cap. serve?

Posted by Robert Minichino (Member # 3495) on 02-15-2009, 07:51 PM:

That's a capacitor-start capacitor-run motor, then. Those have high starting torque, and the run capacitor ups their efficiency.

Posted by Tony Bandiera Jr (Member # 2365) on 02-15-2009, 08:05 PM:

quote: Mark Gulbrandsen
A wall dimmer will also work just fine on split phase motors and gives you variable adjustment of starting torque. I've never tried this on a sync motor though so can't say if it'd work or not.

Mark

Um, slow your roll there Mark. [Big Grin]

A run of the mill wall dimmer is NOT rated to handle any kind of motor or inductive load, and would probably result in either the motor or the dimmer going up in smoke. Not something worth the risk, even if it might work...

Dimmers made for low voltage track lighting MIGHT work if they can handle the current rating of the motor.

Now a variac or the old style resistance dimmer would probably work well.

However. any sort of setup as above assumes that there will be someone who knows what they're doing to actually start the motor, as leaving it unattended at low enough of a setting will burn up the motor.

Better solution is to add the resistor to the start windings and leave the incoming line alone..or go to a three-phase motor and inverter.

quote: Mark Gulbrandsen
1/4 hp is way too large!! About 1/5th to 1/3rd HP is better for a 35mm version.

While I agree with this, why is virtually every projector set up from the factory with 1/4 horse motors?

Posted by Louis Bornwasser (Member # 3063) on 02-15-2009, 08:47 PM:

My Grandmother's old Maytag (with the wringer attached) had a 1/4 hp motor that is EXACTLY the same as the old Century R3 Hoover motor. "In those days" there was not the selection of models. Surely we used whatever motor was available. Louis

Posted by Phil Hill (Member # 371) on 02-15-2009, 08:57 PM:

Hot Tip, Mark: 1/3 hp is bigger than a 1/4 hp... [Razz]

Posted by Tim Reed (Member # 137) on 02-16-2009, 12:21 PM:

[rofl]

Posted by Mark Gulbrandsen (Member # 72) on 02-16-2009, 01:18 PM:

quote: Tony Bandiera Jr
Um, slow your roll there Mark.

A run of the mill wall dimmer is NOT rated to handle any kind of motor or inductive load, and would probably result in either the motor or the dimmer going up in smoke. Not something worth the risk, even if it might work...

HAve done it many times for location work. It actually works very good! The dimmer will blow before any damage could occur to the motor but a standard 15 amp wall dimmer works just fine.

quote: Phil Hill
Hot Tip, Mark: 1/3 hp is bigger than a 1/4 hp...

I'm aware of that Phil. I was just stating the range of motors I've seen on Centurys over the years.

Mark

Posted by Louis Bornwasser (Member # 3063) on 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

Posted by Mark Gulbrandsen (Member # 72) on 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!

Posted by Andy Muirhead (Member # 712) on 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.

Posted by Don Furr (Member # 1469) on 02-16-2009, 08:26 PM:

Andy which Variac are you using?

Posted by Andy Muirhead (Member # 712) on 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.

Posted by Scott Norwood (Member # 30) on 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.

Posted by Damien Taylor (Member # 4255) on 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.

Posted by Don Furr (Member # 1469) on 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.

Posted by Robert Minichino (Member # 3495) on 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.

Posted by Dave Macaulay (Member # 813) on 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.

Posted by John Hawkinson (Member # 1135) on 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

Posted by Don Furr (Member # 1469) on 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.