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frequency and voltage in DC motor


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Hi Duratia,
Motors have inductance which limits current at high frequencies.
PWM circuits have a max slew rate which limits output current at high frequencies.
Attach your PWM circuit as a nice clear GIF or PNG file type and include part numbers for its transistors and ICs for us to see if they are fast enough to operate at 23kHz. ;D

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I use L293D H-Bridge IC. I have just re-read the datasheet, the max frequency is only 5 KHz.

"This device is suitable for use in switching applications

at frequencies up to 5 kHz."

Once, I read a book that suggest to use more than 20KHz PWM in order to no audible sound can be heard. Is it right?

PWM is generated by microcontroller (8051). LM358 is used, but its signal ends on the ADC. It won't affect the PWM, I guess.

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Usually when you use old parts, you get lousy old performance.
Some of the new PWM IC's use a 1MHz switching frequency, so that tiny or integrated-on-chip capacitors and an inductor can be used.

Some young people can hear 20kHz. I heard "ultrasonic burglar detectors" very loudly when I was young. They drove me nuts! The deaf old engineers who designed them thought they couldn't be heard.

My ability to hear the harmonics caused by distortion made me interested in electronics and ways to reduce distortion. At my last job the latest speaker came off the production line. I heard its tweeter's coil rubbing its magnet. My boss said it sounded fine. A replacement tweeter fixed it. Getting rid of my boss fixed him!
I'm an old geezer and am happy that I can still hear 14kHz. I would hate to hear everything sounding like an AM radio or telephone. Because I've been in electronics so long, my hearing has a bit of a notch at exactly 1kHz. Some musicians can't hear 440Hz anymore. :'( ;D

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  • 2 weeks later...


What is the different if I use PWM 1kHz 75% and 100 Hz 75%? Is it true that the voltage is different? Or maybe the energy supplied to motor DC is different? Any comment? Can anyone tell me the relationship?

If the PWM signal has fast rise and fall times, I think the two frequencies would operate a DC motor exactly the same.
I haven't tried it, but I think that the inductance of a DC motor would limit its current if a much higher PWM frequency is used.
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75% duty cycle is 75% duty cycle and frequency has nothing to do with what the voltage will be. The average voltage will be the duty cycle multiplied by the input voltage. Motors are wound such that they have pole pairs. I have seen up to a 12 pole DC motor. The easy way to find out how many poles your motor is to count the "strips" on the commutator ring and divide by 2 (at least I think that will tell you, not 100% sure). There is a correlation between the number of poles and what the optimum PWM frequency is. Most small motors are PM (permanent magnet) The motor controller I built for my wind tunnel uses a PM 24VDC fan, and I had the "adjust" the swithching frequency to make the fan turn at really low speeds (low duty cycle...~3%).

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