Simple Motor Control

How nice a slow rate? If your PWM circuit goes high impedance when it's off, and if the motor's not going too slowly you can sense the voltage during the off time -- it should be roughly proportional to motor speed.

During the off period of the pulse, measure the back EMF generated by the motor. You may have to filter out armature noise. That voltage is proportional to the motor speed. It is acting as a generator during the off period.

Assume this is a brush motor (seems likely). That armature "noise" is from brushes making and breaking. I would rather use that as a speedometer than back EMF.

That's a definite thought. However I have built analog systems based on using full wave rectified and unfiltered AC through SCRs to run model trains, and have achieved extremely slow speeds with back EMF. I did that about 25 years ago, so the details have slipped my mind, but it was cheap. A scope solidly synced to the line was handy.

I don't know, but I have a guess that if the load varies significantly, low speed control with back EMF may be lacking.

Finding more may be difficult, because fancy controls are generally not used for brush motors. I have seen brush motors used as servo motors, but the position (velocity) feedback was an encoder. Older analog control schemes put a small generator inline with the motor, to get a voltage proportional to speed that did not vary with the motor load. The motor is a reactive device moving or not.

It would be nice to learn a little more from the OP. Is this a brush motor? What is driven?

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Ick. I wouldn't -- at least not if I could measure the back EMF during high-impedance intervals.

If you measure the back EMF when the driver is high impedance and any electrical transients have settled you'll be using the motor as a generator and getting a pretty good indication of motor speed.

You're probably thinking of trying to monitor the motor current when you're driving, and deducing the back EMF -- that's difficult.

Using an encoder or a tach is best, but sometimes you just don't have the room in your system for the extra sensor. If you can put one in, however, that's great.

Yes, more info would be nice.

Thanks...

its actually a brushless single phase 42 volt motor from an old printer I had laying around. From what I understand, arent brush motors generally used for heavy load applications requiring 3 phases?

In any case, I dont think the pwm output goes hi-z when its off, however I did find that microchip makes a 2 chip solution, the PIC SERVO SC. Only needs serial to control. Anyone use this?

Brush motors have several attributes. They tend to be cheap. They can run fast. Most appliances use brush motors - drills, leaf blowers, kitchen mixers. They cause sparks, and are not good at fine positioning. They are easy to drive, requiring DC, and they go backwards if you reverse the polarity. The armature is powered, and the field is permanent magnets. They do not require 3 phase.

Brushless motors have permanent magnets for the armature, and the field is powered. Brushless motors are 3 phase, and they run without sparks, and they are good at fine positioning. They are also much more complicated to drive (commutate) than brush motors.

There are some brushless DC motors with driving circuitry built in. These have 2 wires coming out, and can be difficult to identify as such.

I do not know what a single phase brushless motor is (other than AC motor). If you have an AC motor, it is probably the synchronous variety, if it came out of a printer, although I bet it is a BLDC motor with integrated driver.

I've seen references to 2-phase brushless motors, and it's certainly possible, although unless you have a convenient mid-point voltage you use just as many H-bridges driving a 2-phase brushless motor as you do a

But that's just trivia.

A "brushless single phase" motor doesn't make sense, at least in the common usage of "brushless DC" motor where the motor is really an AC synchronous machine with the field provided by a permanent magnet.

As Bryan Hackney said, a BLDC motor doesn't have just two wires sticking out of it unless it has a built-in driver. Its either an AC machine, in which case it will really only work at it's design frequency, it really is a brushed DC motor, or it is a BLDC motor with a built-in driver.

Almost everything you've been told up to this point pertains to brushed DC motors -- if you really don't have a brushed DC motor then you need to flush most of it down the toilet.

Right now, what you really need to do is identify what kind of motor you have.

Tim Wescott wrote: [...]

A bipolar stepper motor is a 2 phase motor. I guess a unipolar one could be considered 2 phase also.