Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Wanted: Variable Speed Mirror Ball Motor
References: <[email protected]>  <[email protected]>  <[email protected]> 
Tim Mitchell  writes:
> >Therefore, wouldn't it be possible to create a variable speed for the
> >motor on a mirror ball?  Or would it be too complicated/expensive for
> >the limited use it would get?
> >
> Theoretically, it would be possible. I presume there is no commercial
> demand for such a product and that's why nobody sells one.

You are right. It is theoretically possible. An technically very
possible and even feasible to do that (check my other posting).
Unfortunately there seems not to be so large demand for such
product that anybody would sell such thing commercially.

Or is there ? 
How much would you be willing to pay for such special device ?
(this kind of special rotator would be quite much more expensive
than standard low-cost rotator motors used with small disco mirror 
balls).

> I think drills use a DC motor, 

Chargeable drills which run on NiCd battery pack definately use
a DC motor and puls width controlling to control the motor speed.

The drills which run on AC voltage generally use a motor
type called "universal electrical motor". That motor type
can run on both AC and DC. The speeds controlling in this
kind of drill is pulse width / mains phase controlling 
(pretty much like light  dimmer, only the phase control 
electronics different). Depending on the drill that motor
in them can run on AC or on pulsating DC (mains AC rectified
but no capacitors after rectifier).

> and most mirrorball motors are synchronous AC motors, 

You are right. All mirrorball motors which run on AC voltage
I have seen have used syncronous motors.

> so you'd have to recreate the rotator with a suitable DC motor. 

There are already such motors in the market. There are some small
plastic mirrorball motors which run on one 1.5V "D" size battery
for several hours. Those are cheap (can be bought for around $10-15)
and work nicely with small (20-30 cm diameter) mirror balls on mobile 
disco applications and such. The plastic construction might not be 
able to take the weight of the large mirror ball.

> Also if you "modify" a drill you will probably need to gear the motor
> down otherwise you may accidentally set your mirrorball to 2000rpm or
> something and pelt everyone with bits of mirror.
> 
> You would need to come up with a method of remote control as running the
> pwm'd motor drive signal down a long bit of cable would radiate
> interference everywhere and probably mess up the signal. The variable
> speed controller would need to be near the motor.

Good warnings. And remember that on mains powered drills the control
electronics are on mains potential, so the wire used between the drill
and the control potentiometer must be rated for mains voltage. 
And all parts of the system and the potentimeter to control the
drill must be well insulated and mechanically well construted
(to avoid electrocutng somebody). So there are many safety
things to consider.
  
> If you try this, let us all know what happens!

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/