I'm not missing this, I'm trying to tell you this is the problem. It's like trying to accurately control the motor speed by twisting the knob on a DC power supply, as I mentioned earlier. With the lower frequency you are dealing with something akin to inertia transfer in mechanical systems. With DC or high frequency AC it is more like trying to control the speed of a toy car by aiming a garden hose at it and adjusting the spigot - first it doesn't move, then it takes off like a shot.

I'm assuming you mean extreme low speed is the constraint and overmax impulse is the problem, I'm sure you don't think lowering the PWM rate affects the motor's top speed. The impulse is balanced against the rotor inertia. Obviously if the PWM speed is TOO low you will have an unwanted series of accelerations.

Huh. Sounds more like a classic overshoot problem to me. Very common with high frequency motor drives. First it doesn't move, then it takes off like a shot, and the poor PWM drops pulses to try and compensate.

Although basic EMF backspikes and freewheeling diodes are something any halfway decent engineer knows about, there's an element you failed to mention that really separates the men from the boys. Open up any good brush type motor made in the last ten years. See that little ring in there? The one the coils are soldered to? That's an MOV. It's in there to increase brush life. Kills the arc before it gets to the brushes.

Not that I would ever design a motor drive without backspike diodes, or trust the driver's substrate diodes to do the job.

 

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.

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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.

Dan

 

Which is completely different to the point of this thread, a fact you never made clear - and means your comments about 100KHz whatever don't apply.

Well I wouldn't say PID has to be PWM at all, but it is a closed system (obviously), regardless of it been PWM or analogue.

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I don't hope that this thread is dead.

I still don't understand how to select the "right" PWM frequency for a brushed DC motor.

I have made some measuremens and the torque is increasing with lowering the PWM frequency. And I can also use a larger range (duty cycle) at lower frequency.
At 400 Hz
The range is 30-254. (0 is equal to 0Volt, 255 is equal to 12Volt)

At 3000Hz
The range is 50-254.

At 33KHz
The range is 130-254.

The motor also runs faster at lower PWM frequencies.

So which frequency should I chose?

The lower end close to the 300-400Hz range is not good the chosen motor. The rpm is not constant. Like you can see the frequency in the turns.
All frequencies below 20kHz are not ideal eighter, do my ears... :-)
The loss of torque at high frequencies are for sure neighter a good idea.

Is profs. drives using variable PWM frequency or how do they do it?

Hope that someones can help me.

Thanks
Thomas