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shehry said:i was developing a motor control using pwm. and i have a 12V 6A DC motor. could anyone direct me as to what would be the appropriate pwm frequency?
If you go too high then the inductance of the motor starts to have an effect, losing power and making the motor dissipate more heat - this isn't a good idea!. Going too low may make it jerky, so too low isn't good either.
I would suggest probably the low KHz range is a reasonable area, but you need to keep an ear open for audible effects from the motor - this is quite common in variable speed cordless drills!.
Not really, that is where it is cheapest to run them.Those frequencies are way too high. Listen to your Makita cordless drill. Hear that 50-60hz hum? That's down where you want to run these things. You get better torque, and much better speed control. I started out with the higher frequencies, too, but after a lot of different controllers in a lot of different mechs, I've learned lower frequencies work better.
Could have fooled me. I design integral horsepower DC motor controls as part of my job.I've done quite a few motor control circuits, at both high and low frequencies, I don't champion the low frequency approach lightly.
I'm talking about DC brush type motors, the type OP is probably using.
A motor control circuit will act like a buck regulator and give you DC in the windings - but before you say that's the "proper" way to control it, try controlling motor speed with a DC power supply.
Go ahead, I'll wait.
...
See the problem? There's very little adjustment between "stall" and "too fast", particularly with a light or inertial load on the motor. The High-frequency PWM behaves the same way.
Now try the same test with a frequency around 60hz for a 12V, 6A motor like OP's (or 1khz for a small toy motor) using a variable PWM. You have a wide range of control over the speed. You can make it turn very SLOWLY, if you want. Try doing that at 18khz, or with DC.
Among the worst possible situations: extreme low speeds and overmax impulse loads. While it is a simple matter to make the system regenerative, we usually do not since it would require 4 $5 power components instead of 1 (thankfully we have located less expensive parts to use now)I design motor controls as part of my job, too. I've used motor generator voltage to determine speed, motor current to sense load and stall conditions, designed dynamic and regenerative brakes, and written and tuned the associated PID loops for over twenty years now.
To drive this elegant control system, you adjust the frequency to the inertial mass of the rotor. This reduces overshoot with high torque loads at low RPM. When you get a little better with motor controls, you will discover what I'm talking about.
The other thing that you seem to be missing is that higher frequency PWM motor drives ARE running DC current in the motor.
Among the worst possible situations: extreme low speeds and overmax impulse loads.
The problem is dropping pulses at high frequencies and low duty cycles. Even when the regulator is absolutely perfect you drop pulses due to the differing rise and fall delays in the power components resulting in oscilations as the control loop compensates for the lost PWM cycles.
The motor inductance keeps the current flowing when the switch is turned off. Unless there is a high speed diode across the motor or across every element in the bridge you WILL arc over something.
As to your stall/too fast problem, it sounds like you are using an open loop PWM instead of a regulator. Ideal motor speed is directly proportional to motor voltage.
No it is still PWM. The only difference is that I am talking about closed loop PWM instead of open loop PWM.You've now totally changed the thread - we're all talking about PWM control, and you're talking about feedback control - this could explain why your point of view is totally different to everyone elses.
No it is still PWM. The only difference is that I am talking about closed loop PWM instead of open loop PWM.
BTW a PID loop IS a closed loop PWM system. What I have been saying is that you can not expect good speed control with out closing the loop and you can not expect any position control with out closing the loop on a BDC motor.