Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Wanted: Variable Speed Mirror Ball Motor
References: <[email protected]>  <[email protected]>  <[email protected]>
[email protected] (Cynical Chris) writes:

> On Thu, 21 Jun 2001 09:01:09 +0100, Tim Mitchell
>  wrote:
> 
> 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?

I have seen two kinds of electrical motors used inside mirror
ball rotator (or "mirror ball motor" as it is sometimes called).

The smallest battery powered ones use a small DC motor in them.
The speed of kind of DC motors can be controlled by varying the
voltage fed to the motor or using pulse width modulation
(somewhat like light dimmer). If you happen to have this kind
of motor you can experiment with it by wiring two wires from
a laboratory ower supply (voltage range adjustable from 0V to full 
battery voltage, usually one 1.5V battery in small rotators).
Using the voltage controller in this laboratory power supply
you can easily adjust the speed of the motor. That's how this
can be done easily. (I have tested this). You might be able to make 
this motor as dimmer speed controllable if you take on non-regulated
universal wall transformer with 1.5V output and connect that
to power the motor. This could give a limited dimmer control of
that motor speed.. (haven't tested this yet).

The other motor type which is used in mains powered mirror
ball rotators is syncronous AC motor. The characteristic
of this motor is that it tries always to run at the constant
speeds locked to the mains frequency (like one step per mains
cycle). The speed of this kind of motor can only be effectively
controlled by changing the frequency of mains voltage, which
is far from trivial. Normal light dimmers do not give an usable
control of the motor speed, because no matter how the dimmer
dims the motor tries to keep the same speed until is does
not give enough energy to turn anymore (between full on
and stop situation there can be a state where the motor
basrely moves, at that time the motor makes typically
irregular movement at reduced speed, not an usable speed control).


Here is a repost of an earlier article posted two years ago to
this newsgroup about mirror ball motors and dimmers:

Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Mirror Ball Moters and dimmers
References: <%[email protected]>
From: Tomi Holger Engdahl 
Date: 09 Jun 1999 02:01:28 +0300
Message-ID: 
Organization: Helsinki University of Technology, Finland
Lines: 50
X-Newsreader: Gnus v5.3/Emacs 19.34
"Robert Johnson"  writes:
> I want to use a standard dual D20 sensor dimmer module to control the  speed
> of a 3 rpm mirror ball motor. I called several theatre equipment dealers and
> basically got, "the motor will be OK. It's the dimmer I'd worry about." This
> led to a call to ETC tech support. There it was, "I had nothing to worry
> about with the dimmer. It was the motor be concerned about." At the going
> rate on motors I don't care; however, as I don't have a full complement of
> dimmers to begin with I am reluctant to experiment. Can anyone shed some
> light on this?

Mirror ball rotators an many similar mains powed
small rotators use synchronous motors (clock motors).
Their speed is affected by the mains frequency. 
The voltage does not affect their speed. They either
rotate if there is enough voltage or do not rotate
if the voltage is too low. In some cases you can get
a voltage somewhere in the middle and there they
barely rote (quite randomly do not rotate or rotate).
The bottom of line for this capter is that a light
dimmer can't be used to control the speed of 
typical mirror ball rotator motor.

The only thing you can do with a dimmer is to turn
the motor on and of when needed. This will generally
work with, but there are few things which can cause
problems because the motor is very indictive and
small load. Those facts cause that not all dimmers
work correctly with this kind of load. Possible
sonsequencies are damaged dimmer because of indictive
spikes (unlikely with good dimmers). Another problems
might be unreliable operation of dimmer channels 
because of of load which is highly inductive and
propably below minimum channel load rating. The worst
possible consequence of this is that the dimmer
conducts only on one half phase, which would cause
some direct current component to the output, which will
quite quicly fry the motor. The third problem might be that
the motor will run even when the channel is turned off
if the dimmer happens to pass some current to the output
even when channel is turned off (preheat current or current
leakage through simmber network fitted in parallel with triac).

This kind of problems can be easily avoided by using the
dimmer channel in swiching (non-dim) mode and putting
a resistive phantom load in parallel with the mirror ball 
motor (a normal light bulb works very well for this).

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