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
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"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 pages at http://www.hut.fi/Misc/Electronics/