Film-Tech Forum ARCHIVE: Making a Holmes 8 'soft-start'

This is topic Making a Holmes 8 'soft-start' in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=004904

Posted by Bruce McGee (Member # 131) on 05-16-2003, 10:05 PM:

All of the 3-phase remarks have got me intrigued here.

My Holmes 8 portables use a 1725 rpm AC motor. None of mine use a capacitor to start them up. One uses the old GE motor, and the other machine uses a Century motor. I really need to reduce the start-up torque as this problem is destroying the main-drive parts in both of these machines.

A friend on the net has large flywheels on his Holmes 8's. I think that this would stop the fast starts on mine, but for now, this is too expensive for me, and I no longer have access to a machine shop.

Anybody got any ideas on what I could do for these machines? I can get into the motors if I need to.

Thanks

Posted by Ken Layton (Member # 133) on 05-16-2003, 10:17 PM:

Just do what RCA did with the old 1040 soundheads: put a 40 ohm 75 watt power resistor is series with the centifrugal start switch inside the motor.

Posted by Phil Hill (Member # 371) on 05-16-2003, 10:28 PM:

Yes, that would do it. It's very common to use that method in old studio cameras that used the AC line for the motor.

You might have to adjust the resistance to make sure the motor starts and comes up to speed without stalling.

>>> Phil

Posted by Mark Gulbrandsen (Member # 72) on 05-16-2003, 11:12 PM:

Put a crank on it...then when you get within 2fps of sound speed throw the AC switch on(let go of the crank)! That way YOU control its startup and it can't talk back to you [Smile]

When you go to stop just grab onto the crank which is now whirling faster that your tired arm will go [Eek!]

. When you manage to grab the crank cut that AC switch and rev her down manually [Confused]

!

HOWEVER!!!!! Beware of those Estar film jams on startup that will very suddenly stop your twirling arm. After experiencing several of these you will become VERY paraniod of owning estar prints and you will get strange e-mails from john P.!! Inspect all your prints for a smooth painless startup [thumbsup]

!

Mark

Posted by Paul G. Thompson (Member # 655) on 05-16-2003, 11:44 PM:

Another way, although I don't necessarly agree with it is to use a 1 to 2 Ohm 200 watt wire-wound dog bone resistor in series with the motor. The resistor will drop alot of voltage on the high start-up current and soften the startup substantially. When the starting coil kicks out, the resistor will only drop 4 or 5 volts, depending on the motor.

The main reason I don't do it this way is because those resistors get too hot. If you use this method, be sure the resistor is clear of everything that could start a fire or cause heat damage to a component.

Posted by Jon Kjerem (Member # 909) on 05-17-2003, 02:03 AM:

We use three resistors, one in each of the three phases. After abt. 5 seconds the resistors are shorted and the projector gets full power. That gives a soft start and prevents heat from the resistors after startup.

Posted by Phil Hill (Member # 371) on 05-17-2003, 02:12 AM:

Jon,
At Panavision for our A/C sync-motor operated cameras, we did the same thing but only on one phase.... It worked really well.

>>> Phil

Posted by Lindsay Morris (Member # 1430) on 05-17-2003, 05:05 AM:

Bruce,

What Paul suggested was pretty well standard practice on most machines in Australia right up until the 1980's when newer stuff started to come onto the scene.
The resistor was quite a chunky brute all mounted up in a perforated metal cage with the power cable going in one end and out the other to the motor. If I remember correctly its value was around 0.5 to 1 ohms and looked like it was made out of 22gauge nichrome wire wound on a heavy porcelain base.
In use they barely ran warm and most had a slider clip which you could adjust to vary the start torque to your particular needs.
Where the motor was mounted on a plate on the pedestal the resistor was often bolted up underneath that and was out of harms way quite neatly. Others just bolted it to the pedestal central column and that also did the job nicely.
Some of the old Raycophones had it mounted on the rear of the square takeup spool box.. equally out of the way but accessable if you needed to tinker!

Lindsay

Posted by Gordon McLeod (Member # 33) on 05-17-2003, 09:15 AM:

I always put a 1.5 to 2 ohm 300watt resistor in serries with single phase motors especially if they are running large reels to slow down the start up
When the motor is running the power disipiated by the resistor is in the range of 4 watts but during startup it is very high
Also motor switchs and power relays have less contact arcing

Posted by Bruce McGee (Member # 131) on 05-17-2003, 03:51 PM:

I like all of the ideas. Especially Marks crank idea.

These are both 1/6 hp motors.

I like the big resistor ideas, too. Maybe something might be small enough to fit into the lamphouse. I've got a big fan there already.

The RCA resistor idea from Ken sounds good, too. I'll look into finding some wirewounds now.

This has been an on-going concern from the beginning. I've never had any film snap, but I am dealing with wear and tear that will ruin the machine eventually. I have 4 parts machines that have stripped worm drive shafts.

The only time either of these machines has had any problems starting was when they were in my unheated garage. I tried to start them, and the motor kept trying to build speed. The grease was cold. Since I moved them into the house, this has never been a problem. Thanks for all the assistance! [Big Grin]