Finding a motor

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Hi all, I’m a year 12 Design and Technology student currently aiming to create a ‘leslie speaker’- a speaker projecting into a fast spinning wooden baffle - from 40 to 500rpm- creating a cool sound, but the thing is that I have absolutely no clue of what I will specifically need for the rotary mechanism.

I have been recommended a geared 12V motor, producing about 5kg/cm of torque. - if this is appropriate, would anyone have a clue where I could find one / if it could be scrapped off a preexisting product?

I would also want a variable speed control mechanism but again, have absolutely no clue how to go about this haha.

Thank you all, any mentorship would be greatly appreciated,

Gus (:

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Hi Gus
Have a look at some of the Electronic Organs around. A lot of these are fitted with the “Leslie” speakers and you might get some ideas. I don’t know if it is just the baffle or the whole speaker that rotates but the idea is to alter the acoustic path length at a controllable rate. I think your 500RPM might be a bit high but then again I don’t know as I have not experimented with this.

I think that altering the acoustic path length would have a slight doppler effect and alter the pitch very slightly producing the sound these speakers are known for.
Cheers Bob

Could be an interesting exercise as you don’t want the mechanism making any noise.

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@Gus314059
Welcome to the forum!!

A 12V geared DC motor is probably the right sort of motor for this, and @Robert93820 is spot on that mechanical noise will matter a lot.

Before picking a motor, I’d want to know a couple of things:

  • Roughly how big is the wooden baffle?
  • About how heavy will the rotating part be?

That’ll make a huge difference to the torque you need. I’d also be a bit cautious about 500 RPM unless the whole assembly is very well balanced.

To drive the rotating part, Id consider using a belt - or friction drive system. And some thrust bearings and rotary bearings to take the weight

If you’re looking for one to buy rather than salvage, this 12V geared motor is the sort of thing I’d start from, and it’s currently in stock.

For speed control, the usual approach is a geared DC motor plus a PWM motor controller, but I’d lock down the mechanical side first.

If you can share a quick sketch or rough dimensions, people here can give you a much better recommendation.

Liam

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Hi Liam, Gus

The whole rotating bit would have to be very well balanced ti prevent any mechanical vibration translating into “noise”. I have not had any experience with these “Leslie” speaker arrangement so am not familiar with the mechanics of the rotating bit. That is why I suggest you try to have a look for yourself for ideas.

This might tend to squeal or whine a bit depending on PWM frequency and the mechanical resonance of the armature windings. This can be quite loud if the two are close.
Maybe a brushless motor and drive would be quieter and more suitable. Especially if the drive waveform is more sine wave instead of the rectangular drive of a typical PWM signal.
Cheers Bob

PS: Even something like the direct drive set up on the modern washing machine and cloths dryer might yield some ideas. I think stepper motors are utilised here. Speed and direction are easily controlled although for this application direction is not important. You are only changing the acoustic path length.
I suggest this approach as I think as soon as you introduce a gear box the potential for audible noise is increased. Dependent on gear box type of course. But any audible mechanical noise would be unacceptable In this application I think.

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Add on to above
Put “leslie speaker” into your search bar. Wikipedia article is pretty informative here

There are some videos which I have not looked at which might yield some info. Also a few sale sites. These things are not particularly cheap but in reality this is largely dependant in what you regard as “cheap”. All pretty relative.
Cheers Bob