[sdiy] OT: home organ Leslie hacking

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at blazenet.net> wrote:
>On Thursday 13 January 2005 11:02 am, Scott Gravenhorst wrote:
>> <rkmoore at memphis.edu> wrote:
>
><...>
>
>> >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.
>
>This is interesting,  do you have details of this online anywhere?

I do now (c: 

I assume you are talking about the single ended method since you can find all
manner of push-pull inverters all over.

http://home1.gte.net/res0658s/SingleEndedInverterInductionMotorControl.gif

It won't be there forever, but it's shows the basic idea.  The power transistor
should be operated in cutoff/saturation by driving with a square wave which
determines the circuit's operating frequency.  The power resistor and
electrolytic cap values should provide an RC constant at the center of the
desired frequency range - which I said is limited.  

It works as follows: When the transistor is in cutoff, current flows through the
resistor and winding which charges the cap.  When the transistor is saturated,
the charge in the cap flows in the opposite direction through the winding and
transistor to ground providing an alternating current to the winding.  Note that
there is power waste during this part of the cycle because the transistor also
conducts current through the resistor to ground.  It's not the most efficient
thing, but it does work for small AC induction motors.  I believe that for a
120VAC motor, you want a DC supply of some 165 volts (the peak value of 120 VAC
RMS), but I'm not sure, I remember using a variac to the DC supply on the test
unit.  We're talking something that I'm trying to remember from 20 or more years
ago.

>
>I remember an article some years back in Popular Electronics,  I think it was, 
>where they offered some sort of a device you could build to *slightly* vary 
>the speed of a synchronous motor.

That may have been where I got this from.

>I never considered using one of these in a Leslie speaker,  but thought it 
>would be kind of nifty to add pitch bend to a Hammond!  :-)
>
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