Stir plate: what kind of DC motor has high torque at low speed?

[piojo]

My current stir plate is basically a computer fan. The torque isn't bad, as the fan will try to control its own speed (4-wire fan taking a PWM signal for speed), so power isn't automatically reduced when I turn it to low speed--only when it's moving too fast. There must be some kind of feedback in the fan's controller. It is stronger than the cheap stir plate I bought from taobao.

I'd like to step up this idea and make a stir plate strong enough to mix honey at 1-2 RPM. Is there any kind of motor that would be good for that, or would any motor need a gear box?

 

[GrimBeaver]

The only thing I can think of that would hit 1-2 RPMs without a gearbox is a stepper motor. That is not going to be straight forward to setup though.

 

[passedpawn]

Must have a gear box.

If I were doing this, I'd use a brushless motor with a microstepping (sinusoidal) drive. In fact, I just designed a 3-phase into a boat motor (steering) application recently, just like this. TONS of torque (you cannot come close to stopping by hand).

 

[piojo]

Okay, gears it is.

A friend just told me a decent motor probably won't have back-EMF I could use to detect rotation speed. It seems like for speed feedback/control, I'll have to use a stepper motor or something like a photodiode to detect a line painted on the shaft. Any thoughts on what's the best way to detect the speed of a modern little DC motor?

I'm sure I can control a stepper motor after I understand what signals they need. I just don't know how they work or how to look for the type I need.

 

[passedpawn]

Okay, gears it is.

A friend just told me a decent motor probably won't have back-EMF I could use to detect rotation speed. It seems like for speed feedback/control, I'll have to use a stepper motor or something like a photodiode to detect a line painted on the shaft. Any thoughts on what's the best way to detect the speed of a modern little DC motor?

You must have speed to get back-EMF. When the motor is stopped (stalled), there is no BEMF. If you need feedback for commutation, you'll want some magnetic sensors (hall effect) or optical (as you suggest) or similar. For very application I just did, I had continuous position feedback by using a custom magnetic ring on the rotor, and mounting a magnetic position sensor right above it.

 

[passedpawn]

You can also just run it open-loop. With enough current, it'd turn fine, but you'd have no idea if it stalled / skipped a pole.

 

[piojo]

Precision at low speed is pretty important. A stir bar at the bottom of a carboy will fly around if it gets to be going too fast.

And I don't need speed control when it's stalled. If stalled, the motor needs full power. But I think I can't use back EMF feedback anyway, because I couldn't detect it during PWM, especially at low power where it's switched off for most of the duty cycle.

I will read about the Hall Effect for motor control, and how to control stepper motors. Thanks!