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Rotating a simple DC motor really slowly (1 rotation per 2 hours)? Answered

This is to rotate a bike chain on a small cog.
After doing some research, I came across a component called a Pulse Width Modulator. Would this be suitable for turning a simple DC motor at 1 rotation per 12 hours(EDIT: I meant 1 rotation per 2 hours, not 1 every 12 hours)? Or, would it only get me to a certain point before I need to use gear ratios to achieve my ideal revs?
This really is a budget project so nothing too fancy. 



2 years ago

Here we are giving all this motor wisdom to a newbie who has no inkling of best answer responsibility on his part !

Anyway a DC Gear Motor is the answer to a PWM slowpoke revolution.

No sorry you will not get a standard DC motor to move reliably at such a slow RPM without gears.

At very slow RPM the motor will either move in rather coarse steps or will just stall as the pulse driving it would be so narrow.

Unless your use is a very heavy load then you will be better off using a synchronous motor such as they use in a mains clock mechanism - Often found in electro mechanical timers. These are synced to the mains frequency so can be quite accurate and have a 12 hour output.

If your not looking for industrial strength you can easily make a lego gear box with 100s to 1 ratio.

What's the difference between synchronous motors and brushless dc motors?

Not very much really. They both have multiple poles and operate as a stepper motor. the Synchronous motor generally has a magnetic multi pole rotor cut from a sheet of steel and is run off the mains frequency - hence synchronised. The brush less motor has a set of discrete magnets round the stepper coils and is run from a much raster source. often microprocessor controlled.

Not really, a stepper has a fixed power input, the power input doesn't vary as a function of load. A BLDC motor ( and a synchronous motor) have power input that vary as a function of torque.

You need gears.

It is impossible to PWM a motor to that slow


2 years ago

A simple DC motor has a only four commutator brush bars [first pic] which cannot be used to smoothly rotate the shaft, like the 100 bar concrete cutting saw blade motor.

You would want a 32 bar DC torque motor (a somewhat expensive machine) to attempt slow rotation..

But your average DC drill motor most likely has only ten commutator bars and with the quiet nylon gears and PWM (pulse_width_modulation) speed control I doubt you could approach your single turn per two hours.. And if you could, it would burn up from internal heat because these machines are designed to use fan forced air cooling in order to be as light as possible... That means they have to turn at speed for the fan to drag cool air over the armature...

Please click the pictures to see the entire image!


2 years ago

Yeah..... no.

You will not be able to accurately achieve that with a brushed DC motor. However, there are these things called gimbal motors, they are a usually a brushless 3 phase motors but the windings are pretty high impedance and they are optimized for a reasonable amount of torque (just enough for their purpose, really), overall low-to-mid RPMs, and quick changes in speed. They are used to stabilize cameras by rotating them such a way the camera faces the same direction no matter what the gimbal/camera is mounted to. You will need a special brushless motor controller for this. A small hobby grade ESC is good.

Alternatively you could use a stepper motor. It rotates in tiny increments or steps, but they can be difficult to drive. They have a good amount of torque and, when driven correctly, can be very accurate. One of the big advantages of them is that you do not need a control loop to regulate their speed. But be careful if you are going to do positional dead-reckoning (that is, after so much time has passed, figure out exactly how many rotations the motor has done based on how many steps it did) because stepper motors can "miss" steps under some circumstances, which means that errors can integrate up over time. This is why 3D printers (which use these lots of these motors) have limit switches, so they can calibrate themselves and fix positional errors.

Lastly you could use a continuous rotation servo. Servos are really just ordinary DC motors that have gear reduction and a control loop inside them to help achieve a specific angle or rotational speed. There are several guides on taking the much more popular hobby servos and hack them to make continuous rotation servos, although these often have no control loop and are really nothing more than convenient gear-reduced motors. Servos are great when you need lots of torque and can sacrifice fast rotation and rapid changes in rotational speed. Servos are arguably the most easy to control, just give them 5V for power (or whatever the specific servo requires.) and a PWM signal to tell the servo what position to rotate to, or how fast to rotate (for the legit continuos rotation servos).

No matter what you do you will need gears however a PWM can help to get the right RPMs if the gears you have won't quite give you 1 RPM every 12 hours.

Depending on the RPMs of the motor, if you can get the gearing within a couple hundred RPM a PMW will take you the rest of the way. And that can be done with a single worm gear.