Very Slow Electric Motor - Chiphacker

Very Slow Electric Motor

2009-12-21T07:22:04Z

A customer has asked:

I want to slow down a small hobby sized DC motor to a user variable range that runs from slow to zero RPMs. I would simply use a wall wart for a power supply and a potentiometer to set the speed but the load on the motor might change slightly. Although the drag on the motor will be very low, if that drag does change, I would like the speed of the motor to stay fairly steady in spite of this.

A couple of people told me to use a PWM controller for this purpose because a PWM has a range of 0 to 100%. Of course this in not in RMPs. One other person said that the motor might not slow down properly because the hertz rating on the PWM could be to high to allow this or because the pulses might not have an adequate amount of strength to excite motor enough to move it at all when the motor speed is set near zero.

I thought about using a step motor so I looked a an Adafruit Motor/Stepper/Servo Shield for Arduino kit - v1.0 but I know almost nothing about this stuff so I don't know if this would be just the right thing either.

I want to turn a knob to vary the speed of a motor form a few parts of an RPM up to a "slow" speed ...say 60 RPM? ...maybe?

Oh ...comparatively inexpensive and simple to set up would be great as well !

Any thoughts?

Answer by todbot for Very Slow Electric Motor

2009-12-21T07:56:09Z

DC motors don't work well at low RPMs. They stall and have horrible torque. (i.e. they can't turn very hard) So people have created gear motors: motors with integrated gearing. The result looks like a slightly bulkier motor, but one that has low RPMs and high torque. If you were to take apart a running a gear motor, you'd see the motor part actually runs at several thousand RPMs, but it's geared down to something like 60 RPM max.

A common specialized one is the standard hobby servo, which has some additional electronic bits but is fundamentally a gear motor. Check out any place that sells motors for robotics or surplus electronics and you'll see several different gear motors to choose from.

DC gear motors are controlled just like normal DC motors, so an Arduino motor shield works just fine with them.

Answer by shutterdrone for Very Slow Electric Motor

2009-12-21T15:58:27Z

Generally speaking, a potentiometer will not be a good choice for controlling the speed of a DC motor, unless it's a very small one (think a few 100 mA draw) as the pot must be rated for the current drawn by the motor. Additionally, as you restrict current, you're also sapping power from the motor. So, at slow speeds using a current-limiting mechanism, you'll find it can only elicit a small fraction of the torque it can at high speeds.

DC Gear motors, as pointed out, are more appropriate for reducing speed. Alternatively, you can fashion your own gear chain, but it's not likely to be cost-effective. Dayton makes a well-priced range of 12V DC gear motors going as low as 0.6RPM (IIRC).

Then, if you wish to use the rated speed as the max speed, then a PWM speed controller can be quite handy. While there's nothing wrong with the adafruit motor shield for DC motor control, I do prefer an external speed controller, like the L298 Compact Driver from Solarbotics for larger DC gear motors.

Your friend is right, that each motor will have different characteristics as to the lowest PWM duty cycle it will reliably respond to. For most of my motors, it seems to limit around 25-35% duty cycle.

Yes, another excellent way to control output speed is by using a stepper. It lets you take discrete steps whensoever you choose. While a servo also lets you take discrete steps, less expensive ones tend to be limited to 1degree minimum movements, and are designed to move as quickly as possible from the current position to the defined position. A standard 200-step stepper motor, with an 8x microstepping driver, will give you effectively about 4 times the resolution, and therefor the ability to make smoother, smaller increments.

Answer by unknown (google) for Very Slow Electric Motor

2010-01-29T18:16:04Z

Stepper motor would be perfect for what it sounds like you want to do. The typical drawback of a stepper is their slow speeds. Considering however that you said you want to go from slow to slower it would work

Answer by Pashute for Very Slow Electric Motor

2010-06-09T19:48:40Z

There are "digital BLDC motors" like the ones made by ThinGap that make a motor which is both lightweight, small, and has excellent response at extremely low torque (slow, high power) as well as high speeds (RPM) without the need for any gear.