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I am using L298 motor driver to drive motors (12v , 900mA),I am using PWM to control motor speed but the problem is
1.When I set PWM frequency to 100 Hz motor work with jerks no noise at all.
2.When I set PWM frequency to 500 Hz motor work with little jerks and little noise.
3.When I set PWM frequency to 1000 Hz motor work with noise and no jerks at all.
4.When I set PWM frequency to 20 kHz motor dosen't work at all.
what should I do?
As long as your PWM frequency is in the audible range (50 - 15000 Hz), there will be some audible noise. It doesn't mean something is wrong or about to break, simply vibrations at those frequencies propagate through air and can be heard.
If you absolutely want to get rid of the noise, you'll need a frequency of 20 kHz or more. L298 has a maximum commutation frequency of 25 kHz, so you should be able to get it working at 20 kHz (at least at 40-60% duty cycle) if your PWM waveforms respect that limit. Perhaps if you display your PWM signals with an oscilloscope, you'll realize why it didn't work.
