Electric Motor speed control, how to do it?

Question

I need to find a way to control the speed of an electric motor over a wide rpm range so that it is what I want it to be, per it's input w/o feedback. This would be a small motor, like an RC car type. I want to write a number to a DAC and either use a Voltage source or a Current source as the controll, but need to be able to lock the speed over a wide rpm range with a lightly varying load, ie: load will vary with rpm, pretty linearly, and be light in comparison with the torque capability of the motor, and needs to work with many motors ie: for production in a product. Any help? thanks,, C. Speed is going to be up near 7000 rpm at the top end, aroud 200 at the bottom.

Answer 1

DC motors are one tricky mother to control. About the best way I know of (and especially over the RPM range you indicate) is to use a simple on-off switch (transistor) which is then pulse-width modulated to control the 'average' current through the motor.

As for speed control (sensing) without an external tach generator..... You'll have to 'sense' the counter-EMF generated by the motor during the time the switch is de-energized (just use a diode and a simple RC filter) and measure it to get a handle on the RPM. (Remember, a DC motor is also a DC generator ☺) The real problem will be that the output voltage (as a function of RPM) may vary somewhat between motors.

And don't even *think* about doing all of this with 'hard-wired' circuitry. My own, personal, choice for this project would be something like the Microchip 'PIC12F675' microcontroller. It even has a built in, 4 channel, 12 bit A/D converter. Best part is, it only costs about $1.25 from Digi-Key. (Damned if *I* know how they make them so inexpensive ☺) If you go to Microchips website you can download a data sheet, tons of application info, an assembler, a C compiler, and a very nice IDE (Integrated Development Environment) all for *FREE*. About the only thing that you -will- have to buy (besides a few microcontroller chips) is a programmer (for about $80 or $90 IIRC). But the chip uses a flash memory for program storage and it can be erased and re-programmed many times (I have one that's been erased and re-programmed well over 1000 times ☺) There are also several in-circuit emulators/debuggers available, but they start around $250 or so and, for a project like this, I doubt that you'll need one. Engineer to engineer, it's a very nice little chip. I've used a lot of them over the last few years.

Sounds like a fun project. Drop me a line and let me know how it goes.


Doug

Answer 2

You need to specify the type of motor you are going to use. Every different type needs a different approach to speed control. If you want to use a universal motor then continuous speed control of a universal motor running on AC is very easily accomplished using a thyristor circuit, while stepped speed control can be accomplished using multiple taps on the field coil. In the case of synchronous motors you might not be able to change the motor speed. As for connecting the wheel. You might be able to use chain drive. Sprockets on both wheel and motor shaft and link with chain. Possibly you could use a belt. You would need something like a vee pulley on each.

Answer 3

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives or inverter drives.