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DC motor regulation Answered

I am planning to build a small gearbox that depends on a steady rpm from a 3V dc motor. I understand that the output rpms will change in correlation to the fluctuation of input power. My question is, if I used a regulator to regulate the input voltage from 5V to 3V, will that keep the motor rpms steady?



1 year ago

Flea power hobby motors are notoriously unsteady RPM (Run_Pattern_Messy) even in a closed loop because the commutator is usually only 3 or 4 bars with wires for brushes and cannot respond properly to power changes, just saying...

I could be off because there are very steady low voltage motors.

See how slow and steady this tiny Red Lego motor turns...

Jack A Lopez

1 year ago

The input DC voltage to the motor is only part of the problem.

The angular speed (rpms) at which the motor turns, also depends strongly on mechanical loading. By that I mean, the resistance torque being offered by whatever you have the motor connected to, it is going to slow the shaft speed, independent of whatever voltage is being placed across the motor.

A better way to build a shaft that turns at constant angular speed, is to use a sensor, to measure the shaft's angular speed, and then use signal from the sensor to drive a feedback loop.

I.e. if the sensor says the speed is a little bit below the target, then give the motor a little more juice (meaning voltage, or time-averaged voltage if using PWM). Conversely, if the sensor says speed is a little above the target value, then give the motor a little less juice.

By the way, I have actually built the thing I am describing. I never got around to writing it up as an instructable, but I upped some pictures of it, to a discussion on the Answers forum, here.


That was about two years ago, I guess. Had to use a search engine to remember where I put that comment.

Do a Ctrl-F, and look for the word "SG3524", the particular PWM IC I used, if you want to quickly jump to the comment I wrote.

I don't dare ask if anyone wants an instructable on that topic (how to build a phonograph from an old cordless drill), but I am kind of wondering:

Is there enough detail in that brief comment, for you to understand what I built there, like how it works? Does what I wrote there make sense?

Tman179Jack A Lopez

Reply 1 year ago

I didn't think about how the load will affect the rpms, thanks. Do you know of a way to build a simple angular speed regulator? I am not very inclined on a circuitry level so I am looking for the easiest way to this. And I think that title makes plenty of sense so long that the reader knows what a phonograph is, but it makes perfect sense.

Jack A LopezTman179

Reply 1 year ago

Well, it depends on what the mechanical load is. If the load offers a mostly constant resistance torque, then a motor supplied with a constant voltage (or constant time averaged voltage if using PWM) will give mostly constant speed.

Including a big heavy flywheel, with lots of rotational inertia, can help smooth out small disturbances that otherwise cause the speed to change, momentary slow down, or momentary speed up, for a system with little rotational inertia.

Actually, on the subject of phonograph turntables, I have not taken one apart recently, but I think the last time I did, I noticed the mechanism driving the turntable included a big heavy flywheel. So there probably something to this idea of including a flywheel.

On the subject of "speed control", a lot of electric machines, in particular power tools, that call themselves "variable speed", are not using a closed-loop speed regulator, like I described previously. They are just using a kind of voltage regulator. Well usually it is PWM, which is essentially regulated, time-averaged, voltage.

And that kind of control (open-loop control) is good enough for a cordless drill.

Actually, I probably should have mentioned this first, but your topic included the word "regulation", and you asked will a constant voltage keep a motor, and a gearbox ( and I don't know what else) turning at constant speed.

I think I maybe explained the most complicated way first, but the reason why I thought to explain it this way is because you were asking for a regulator. In my mind, a regulator is necessarily a closed-loop kind of control; i.e. a speed regulator will necessarily have a speed sensor, and feedback from this.

Regarding this jargon of open-loop control versus closed-loop control, if you do not already grok the difference between these, the first section of the Wikipedia article titled, "Control system", explains the difference between open-loop and closed loop,


using an example of a building's heating system with a boiler controlled by a timer. It's on some of the time, off some of the time. Essentially this is the same thing as PWM, and it is open-loop.

The closed-loop is what you get when you introduce a temperature sensor, and use feedback from that sensor to decide when the boiler turns on, and when it turns off, also called a, "thermostat".

The wiki article for "Control system" might lead you to other places too. For example there are purely mechanical speed regulators, like that "centrifugal governor" in the first picture, and I think those are as just as complicated as electronic regulators, but maybe the mechanical version will be more meaningful to someone, "not inclined on circuitry".