Looking for motor recommendations for Leslie speaker cab

Hey, I have two Leslie style cabs from an old Baldwin organ that I would like to retrofit with variable speed control. My minimum viable product would be a knob on the cabinet that I could turn to adjust the motor speed, but eventually I would like to have an electronic controller controlling the motor speeds, receiving speed change messages from a guitar pedal or a laptop.

So first things first; I am a pretty big noob, but I have an Arduino, and I’ve messed around with changing RGB LED strips with it. I also am pretty handy with soldering and running wires and wood working. I have done some basic research, and I am at a loss for what would be a good motor for this project.

These speaker cabs are from the early 60’s. There is a 120v AC motor currently installed that runs at a fixed speed when you plug it into the mains. It’s not the quietest, and I assume it’s not worth messing with; I’m better off putting in a DC power supply with a modern DC motor.

There is a belt driven flywheel already installed. I don’t know exactly what the reduction is, but I am guessing it’s about 20/1. I’ll check it and update when I get the chance. With this flywheel, I am guessing I don’t need a lot of torque, and it helps get away from gears in the motor.

Ideal motor attributes:
Slow - In a perfect dream world, I would like this motor to turn at 100rpm up to 10000rpm. With the 20x reduction, this would give me a final speed of 5rpm to 500rpm. If I had to compromise here, I would be happy with 500rpm up to 5000rpm, giving me final speeds of 25rpm up to 250rpm.
Quiet - the quieter the better.

So, does anyone have any slam dunk recommendations for a motor and controller to make this work? Cheers!

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Does the AC motor currently installed have a “data plate”? If so, can you post a picture?

There are definitely motors that can do the trick, but we don’t know how much work the trick is. :relaxed:

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This is fascinating. I had no idea there was such a thing.

Motors are shaded pole motors.
image
These motors do not typically drive heavy loads

*Moreover, they are compatible with TRIAC -based *variable-speed controls, which often are used with fans. They are built in power sizes up to about 1⁄4 horsepower (190 W) output. Above 1⁄3 horsepower (250 W), they are not common, and for larger motors, other designs offer better characteristics. A main disadvantage is their low efficiency around 25 %. A major advantage is that the motor’s stall current is only slightly higher than the running current, so there is low risk of severe over-heating or tripping the circuit protection if the motor is stalled for some reason.

AC Speed Control

I would try to control the existing motor’s speed using a ceiling fan control. If this works you may not need any mechanical changes. It also may help to understand what speed ranges and control you want if you convert to a DC motor. These style motors are also much quieter than DC motors.
A control like this one: https://amzn.to/3cb1SSO, these can also be purchased at big box stores.

DC Speed Control

I would think most any DC motor driven by a PWM control will also work. You will have to come up with a coupler and mount that is compatible.
Guessing this would be plenty of torque: https://amzn.to/2PhRMqk

You can buy PWM controllers like this one that also comes with a display; https://amzn.to/3saLF5I
The exact motor and controller you chose will depend on max-min speeds and the needed torque.
You can buy or build PWM controllers using discrete components or a microprocessor. I would just buy a motor controller from amazon.

As an Amazon Affiliate, I earn from qualifying purchases.

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Thanks for the info. I was thinking I needed a full-on variac to control the DC motor, but I guess a fan controller should be able to do the trick.

By the way, the high-end Leslie’s with the rotating horns are very valuable as they are. They generally had speed controls built into them. This is a genuine OEM Leslie design, but it is a low end version with just a spinning metal plate to deflect the sound from a 12" speaker.

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Hey, yes, I wanted to post pictures with my original post, but I had a fussing baby to attend to.

!

On second inspection, I think the reduction is probably a lot more than 20x, but I don’t have time to check it at the moment. There’s a piece of tape on the motor that says 115v, 60 cycle, and two part numbers.
The 512 number is an internal model number for Baldwin. I will Google it.
The second number is possibly a date code or something. I don’t think they give much information.

Oh, it’s only rotating a swashplate? A stepper motor with a trinamic controller in silent mode and a driver would probably do fine. Alternatively a servomotor, which are now substantially cheaper than they used to be…

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IMO steppers and servos are a overkill :grin: There are no positional requirements and the needed torque is low. The gear motor I pointed to is more than enough.

I would try the triac ceiling fan controller first. There are no mechanical changes required with this approach.

From what I read the sweet spot is 40-400RPM.

Does the unit have a brake? A brake may be a reason to use a DC gear motor which would stop pretty fast.

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Thanks Don,

It doesn’t have a brake, and although I hadn’t thought of it before, being able to stop the rotation fairly quickly would be a cool feature. The main reason for wanting to do a DC motor was so I could control it with an Arduino or something, sending changes from a stomp box or from a computer. That’s just on the roadmap though, for now I was hoping to just get a DC motor going with a rotary control.

There is a cool effect with the speed of the rotation slowing down from innertia when you shut off power, but that could be mimmicked in software.

Yeah, I’ve never heard the term swashplate before, but I like it.

Part of the reason why the Leslie’s with the rotating horns are so popular is because the horns spin at a different rate from the drum, adding to the complexity of the sound. The single speed rotary speakers are generally seen as a cheap immitation.

I rescued these from going to the landfill, but I figured since there’s two I could split the signal and send a high-pass to one spinning at a faster speed and a low pass to the other spinning at a lower speed and approximate the effect.

I was thinking about precise control of rate from arduino — this might be musically interesting.

One could even make it MIDI-controlled in that case. :relaxed:

I would expect that PWM speed control of a DC motor would be sufficient for an application like this. The PWM can be software or hardware generated.

Either the TRIAC control of the AC motor or the PWM control of the DC Motor can be software generated and controlled by a foot pedal.

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My comment is only tangentially related to this post, but I used to jam with a keyboard player who had an old Leslie speaker on his Hammond B3. It made a bit of a scraping noise (his was used and pretty old) but it still sounded amazing. Virtually guaranteed that you’ve heard it before if you were ever into 50s or 60s (or 70s?) rock. I think Greg Rolle (from Santata–think woodstock) used this. Come to think of it, I think REO Speedwagon used it on Roll with the Changes (among other songs).

Related:
http://theatreorgans.com/grounds/docs/history.html

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Every deep purple song, every doors song, every three dog night song, etc. Haha. If there’s a Hammond organ on the track, it almost certainly is being played through a Leslie. John Lord of Deep Purple actually took the signal from his Hammonds and would plug it into a Marshall amp to get thick overdrive sounds, and then would plug the Marshall into his Leslie cab to get the chorus/tremolo effect.

In fact, lots of guitar tones were had from Leslie’s too. Jimi played Little Wing through a Leslie.

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Hey, sorry for not checking in for w while, I got busy with a few things. Do you have a specific recommendation for a stepper motor and trinamic controller? Are there geared stepper motors? What is a good power rating?

I apologize for being vague. It’s not really a swashplate; I was using it just to identify the part by analogy. A real swashplate is part of a mechanism to turn rotary motion into linear motion. It’s an oblique plate (as here, why I used it) but also a cam rides on it so that shaft rotation is translated into linear motion parallel to the axis of shaft rotation.

I think that @donkjr was right. I was thinking of using arduino timers for a consistent STEP pulse, and was thinking of the trinamics generally because they have a silent mode, but I think the plate might possibly end up acoustically coupled, which would be terrible. Steppers move slower, and “silent mode” on the trinamics only works at low speed, so you might reasily run into a speed limit. So while there are geared stepper motors (that move even slower), I’d echo @donkjr 's suggestion of PWM control of a DC motor. (Or AC, but if you specifically want arduino control, say to add MIDI control, DC will probably be easier.) My other suggestion was a servomotor, here intending by that usage a DC motor with position feedback so that you can calibrate speed in software; so basically Don’s suggestion adding the ability to calibrate in software. But because the load shouldn’t change and the DC voltage should be stable I would expect you to be able to calibrate PWM to speed and it will be just fine without position feedback. So I was suggesting overkill.

I’m a programmer who dabbles in electronics, Don’s the one who actually knows what he’s doing with electronic design. :relaxed:

I would suggest a PWM frequency of at least 100kHz to avoid the human hearing frequencies (20kHz is typical, but there are people who hear frequencies significantly higher). You’ll set the duty cycle to control the speed.

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