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how to know pwm frequency for 12v,300rpm dc motor speed control by using l293d,

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NRollick

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I would be using at89s52 micro-controller to drive 12V,300RPM BLDC Motor at any variable speed from 100rpm to 300rpm by pwm method.I would be using L293D to drive the motor.what must be the frequency that i must give pwm from the microcontroller to L293D.
 

Did you read the data sheet? It suggests maximum switching frequency of 5 kHz. Lower frequency gives probably lower switching loss but higher current ripple and more sound. Try range between 2 and 5 kHz.
 
1>actually i m thinking to take 1khz for pwm frequency
2>now to generate a dutycycle for a given rpm,,,do i have to measure the back emf for given rpm (considering tachometer as a load ) E(BACK EMF) PROPORTIONAL TO N(SPEED) ...OR directly considering V proportional to N(Speed of motor) and thus calculate average voltage of supply ,,,WHICH WAY IS BETTER.
 

Dependent in your Speed accuracy requierements. Speed controller with tachometer feedback is better. Constant voltage gives torque dependent speed drop. Can be compensated with positive current feedback.
 
yes...i have used IR sensor by giving feedback the set rpm which is then displayed to the lcd module . tachometer is used to test the rpm speed and the feedback rpm by ir sensor which is handled by counter program in the at89s52.
hence a standard rpm of 50,150,200,250,300...is contolled at 1khz pwm duty cycle .altough feedback speed is not 100 percent constant...but it is deviating ...suppose for 50 rpm duty cycle is showing 48,60...and these fluctuation is there.....and some of it getting drop if tachometer is used....but it is working fine
 

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