Leslie speed control idea?

[JKowalski]

Alright - this is obviously kind of off-topic for a stompbox forum (well, it is an effect!). I am working on a DIY leslie made out of a shaded pole ceiling fan motor. I am trying to figure out the best way to control the speed of the motor.

The triac PWM controller method is one method - however, fan motors take a long time to slow down and a shorter time to speed up. I am not knowledgeable with motors to a great extent, I can't seem to find any good reading online. But I came up with an idea and was wanting it to be analyzed by someone who might be able tell me if it would work or drastically fail.

This is my idea: Have the motor switch between directions (switching hot/neutral) in accordance with a duty cycle adjustable square wave. So - the on time of the square wave corresponds to the motor spinning clockwise, and the off time to counterclockwise. Altering the proportions between each state would control the average amount of force the motor pushes in one direction. If the clockwise is on longer then the counterclockwise, the average momentum of the motor would be in the clockwise. If it is equal, the motor has zero average speed increase. Hopefully, this system will let me "brake" the motor.

SO - I would have a maximum speed, by pushing all the way up on the pedal (controlling the duty cycle) or all the way down. Then I could slow it down by switching to some position towards the other side, the magnitude of the braking force determined by the distance away from the side you went to at first. If you go past the middle, eventually the motor would stop completely and start going the other way. If you land the pedal before going through the middle travel, then the motor just slows down but doesn't stop.


So what do you think? It seems like this plan would allow me to have a fully controllable motor speed with pretty precise control over it's actions, and would alleviate the problem of variances in start up speed and slow down speed.



If this plan falls through, I suppose I can just stick with either triac control or capacitor phase lagging (like they typically do in ceiling fans with 3-4 preset speeds, works wonderfully with a wide range but cant use smooth control, need switches).


ALSO: Anyone have any good electronic motor reading material they can suggest? 

[Joe Hart]

I'm not that smart, but since no one has replied yet I will offer my thought. Isn't this a recipe for a burned out motor? It seems that running a motor at the speeds needed for a Leslie (or any rotating speaker), then suddenly ramming it into reverse (even if you intend to do it slowly, some day it may be "rammed" into reverse) would cause serious stress on the motor. But maybe I'm wrong. I am in the middle of a rotating speaker cab project, so I would like to see what you come up with. Thanks.
-Joe Hart

[Sir H C]

Part of the love of the Leslie sound is the slow speed up/down.  The Maestro Phaser went to lengths to copy this aspect.  I have seen Leslie users keep flipping back and forth to get a middle speed.  Embrace that facet of the motor.

[JKowalski]

I'm not that smart, but since no one has replied yet I will offer my thought. Isn't this a recipe for a burned out motor? It seems that running a motor at the speeds needed for a Leslie (or any rotating speaker), then suddenly ramming it into reverse (even if you intend to do it slowly, some day it may be "rammed" into reverse) would cause serious stress on the motor. But maybe I'm wrong. I am in the middle of a rotating speaker cab project, so I would like to see what you come up with. Thanks.
-Joe Hart

That's exactly what I am worried about. I don't know enough about the workings of this motor to be sure of that. I have a couple fan motors I salvaged for free, and can find a couple more, so perhaps I should build a prototype and test it for a while? (If nobody has an answer, of course) I was planning probably to do the switching at 30hz, that way it is still too fast to be noticably jerky in speed, is too low to be a major problem in audio hum, and leaves the waveform as undistorted as possible.

So at 30Hz, that would allow a full cycle in one direction and a full cycle in the other at a 50/50 setting.



Also - I know for a fact that some ceiling fans are designed to be reversed. This one had a switch only accessible from the top of the fan, but if you recall remote control fans typically have a reverse control. Obviously, you don't press that button at 30 Hz, but it does give some hope.

Part of the love of the Leslie sound is the slow speed up/down.  The Maestro Phaser went to lengths to copy this aspect.  I have seen Leslie users keep flipping back and forth to get a middle speed.  Embrace that facet of the motor.

I should say - I am in no way going for a "traditional" leslie sound. I should say that I am simply creating a "rotating speaker assembly". I am just going to adjust my design until I like the sound that it gives me, not until it replicates a leslie. The adjustable speed feature is something that truly appeals to me. Also, if you notice, with this design you can do the ramping up and down as in a traditional leslie, but you can control how fast it will ramp and where it will end up.

[Sir H C]

Biggest problem I see with the idea is that every time you reverse the motor, all that stored energy will want to go somewhere during the break-before-make on the switches and could well smoke the switches very quickly.