LC design consideration

[PNU7]

hi guys, i am quite new new in the LPF field.. i need help in design consideration to generate 50HZ sinewave for Inductor(MOTOR).

i am switching(CHOPPING RATE) transistor at 20KHZ, my PWM FREQUENCY is 1KHZ and my number of samples are 20... so that way my output frequency would be = 1KHZ/20= 50HZ... but i dnt know what value of L and C i must be using to generate 50HZ across INDUCTOR(MOTOR)..so i want to design the filter which will eliminate 20khz COMPONENT and generate 50HZ sinusoidal wave.

 

[crutschow]

I don't understand your description of the circuit. How does a 1kHz (modulation?) PWM frequency translate into a 50Hz output? What is "number of samples"? How do you get 1KHz/20? :?:

Normally the PWM modulation frequency would be 50Hz and you filter the 20KHz to extract the 50Hz.

 

[neddie]

His description is a bit confusing.
Maybe he's using a look-up table of 20 values.(Not a great sine wave!!) and updating the pwm value at 1mS intervals?

 

[PNU7]

actually, to maintain the V/F of motor constant there is a concept of chopping PWM with HIGHER frequency.. so i am generating PWM(digitally) @1KHZ with 20 different samples(different pulse width)..and this PWM signal contain higher 20KHZ chopping frequency.. with this type of circuit V/F control becomes easy. so now with 1khz and 20 samples output frequency will be =1khz/20=50hz..

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yes neddie you are right,, i am using look up table of 20 different values.and updating PWM at 1ms. but in the ON time of PWM , it has 20khz pulses within PWM..and that will be the transistor's switching speed./.

 

[crutschow]

You can't "eliminate" the 20kHz component but you can reduce it to any arbitrary level. What's an acceptable amount of 20kHz that you can tolerate at the motor input.