[sdiy] OT: home organ Leslie hacking

[Scott Gravenhorst]

<rkmoore at memphis.edu> wrote:
>I think that you can just treat it as a speaker.  Match your amp and 
>speaker impedances and make sure that you're not connected to the 
>organ amp if you are connected to an external one.  Also, the speakers 
>that I ahve seen in most home organs were pretty cheesy, so you may 
>want to replace it with a higher quality speaker that will mount up to 
>the baffle.  
>
>Don't try to get too creative with speed control.  The leslie in my 
>home organ is a little single phase induction motor.  I've seen people 
>put rheostats in series with the motor to vary the voltage in an 
>attempt to vary the speed.  This will damage the motor.  These motors 
>are only meant to run at speeds related to the frequency of the AC 
>source.  If you have an inverter for the motor then you can shange the 
>motor speed.  I have added a switch to turn the motor on and off 
>because in my organ the leslie rotates constantly otherwise.

Years ago, I built an induction motor speed control.  It operated a
phonograph motor.  The only way to get this working correctly is to vary
the frequency of the input power to the motor.  There are (at least) two
general ways to accomplish this, a variable frequency "push pull"
inverter which uses a center tapped transformer and two power
transistors (or SCRs if you want to go that route) OR it can be done
single ended with one power transistor and no transformer.  The second
method works for motors that expect a light load.  Heavier loads will
require the first method.  Note that there are some limitations with
this, one being that the motor's winding and core are optimized for 60
Hertz, so there's a frequency range limitation.  Push it too far outside
this range (which I cannot begin to guess at) and the motor will heat up
beyond it's tolerance.


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