DIY Leslie Speaker Speed Control

[JKowalski]

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 short 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?

[Enzo]

SOme thoughts. Ceiling fans are ponderous to slow down in part because of the tremendous momentum stored in the moving fan, and the free spinning bearings. Take the blades off and see if it slows faster.

Did you google "electric motor theory" maybe? Or motor basics or electric motor basics? Plenty of books at the library - yes libraries still exist.

I am no motor expert, but your idea doesn't sound right to me. AC power is already reversing hot and neutral 60 times a second. Reversing the wires won't reverse a motor. if it would, you could make your alarm clock run backwards by turning the plug over in the wall outlet. In motors with separate field and armature windings you could switch polarity on one, because it is the relative field in each that reacts to the other. In an induction motor - a brushless motor - I think to reverse it, you'd have to physically turn the field piece over.

One way to brake an AC motor is to put DC on its coil. I believe Hammond did this on some models.

Have you explored how real Leslies are built? In the typical leslie, there are two drive asemblies - one for the woofer and one for the rotating horn. Part of the Leslie sound is that the woofer is more massive and has more inertia/momentum than the horn, so the two parts ramp up and down speed at different rates. Each drive assembly is two motors - a fast motor and a slow motor. The fast one needs to be more powerful than the slow. The controls switch between them. It is more complex than that even in that the slow motor shaft bears on an idler that turns the fast motor shaft.

The traditional Leslie is fast/slow, with the transition being mechanical. COntinuously variable would not be traditional. it may be highly desirable and tradition be damned, but the original Leslies didn;t have it.

[JKowalski]

- Well, I know for a fact that this motor can reverse like I said - these ceiling fans have a "reverse" switch on them that lets you blow the air in different directions. I looked at the wiring - it just reverses the direction the AC goes through the motor.

- I will look into the DC braking system. Thanks. The benefit with my proposed sytem, however, is that in addition to "braking", you can set the speed constant by not going past the center position. That is pretty important to me.

- 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.

- I already am a little bit into this project already, it's not just in the design stage. I have removed the fan motor from the fan, and creating a baffle for the lower motor. I have been experimenting with the sounds of the speaker by placing the assembly underneath an amplifier facing the ground - I am liking it, so far. Right now I am both planning the cabinet construction and motor control.

-

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

I kind of meant books when I said that ; ) I'll probably take a trip down to the library in a day or two.

[Enzo]

You may find actual books scanned and up online, saving you some $$$. Especially at some colleges. In fact a local college library may have more than the public library.

If your motor is reversable, then so be it.