Before asking the question I want to mention the basic two premises I know:
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As far as I understand for direction control, H-bridge inverter is used only for a certain type of DC motor not for an AC motor(?). For example a permanent magnet DC motor direction can be controlled just by reversing the polarity of the supply voltage. So that means by using H-bridge one can both control the direction and the average voltage across the DC motor hence the speed.
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Reversing the polarity for an AC motor does not change its direction since there is no "polarity" in alternating current system. In an AC motor, stator windings create a rotating magnetic flux. The direction of the resultant electromagnetic flux determines the direction of the motor shaft. And the stator frequency determines the speed.
Now below is a simplified diagram for an permanent magnet rotor AC servo motor:
I can understand how the inverter stage above generates a sine wave for stator windings at a desired frequency by using PWM technique. So by this way the speed or torque can be controlled.
But I couldn't figure out how in this case the direction of the motor is controlled.
How is the direction controlled in this case? Is the rotating flux reversed? If so how is that done?
Best Answer
It's a three phase motor and as such, direction of travel of the rotor is determined by the phase sequence of the applied voltage. Consider this; to reverse a 3ph induction motor just swap two of the windings. Swap another two and you're back to the original spin direction.
Swapping the phase sequence in the H bridge/driver is really trivial: -
Picture source