QUOTE="elecLear78, post: 1296756, member: 247756"]Thank you for the reply. I am trying to drive a motor using H bridge. Could you please clarify me that if i am giving fixed duty cycle of say some 60% of around 1 khz frequency to stator legs H1,H2,H3 with low legs being made complementary still i get 120 phase shift currents. My exact question is i want to switch the legs as per the phase currents. Since the phase currents are 120 i get sinusoidal pwm. But how should i start means which pwm should i start with? Can i start with some x duty cycle and immediately start reading the phase currents? I am sorry if i confused you.[/QUOTE]
You mention a H-Bridge and you mention PWM? Neither one of which would apply to an AC motor, let alone a 3 phase or poly phase AC motor. To reverse rotation on an AC motor two of the three phases are reversed, so for example rather than L1, L2 and L3 it would be come L1, L3 and L2 applied. Poly phase AC motor speed is generally controlled using a VFD (Variable Frequency Drive) or any of several Triac Speed Control circuits.
An H-Bridge circuit or module is designed to reverse the polarity on a DC motor to reverse direction and PWM (Pulse Width Modulation) is one method of controlling the Speed of a DC motor. You can't use an H-Bridge to control an AC motor. Additionally with any motor, AC or DC you simply can't just reverse rotation direction. The motor needs to be stopped and then rotation direction changed. When reversing a poly phase AC motor the phase currents will reverse of the reversed phases.
In the above illustration IR1, leads IR2 leads IR3. The phases are 120 degrees apart. If I reverse two phases I get this:
Note the phase rotation change.
Anyway PWM and H-Bridge are not used for any control of an AC motor.
Ron