# A question about servo motor direction control

Before asking the question I want to mention the basic two premises I know:

1. 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.

2. 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?

• In the first sentence I wanted to say: if we want to control direction of a motor, H-bridge cannot be used for an AC motor direction control. (I'm not 100% sure whether it is correct) May 3, 2018 at 18:32
• there is an edit button just below the tags, so you can update your first sentence May 3, 2018 at 22:39

## 1 Answer

But I couldn't figure out how in this case the direction of the motor is controlled.

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

• Great, I see so it is done by swapping the windings hence reversing the flux rotation.. Can you also add how it is done for a single-phase AC motor? I know my example diagram is 3-phase but most servos work both 3 and single phase. May 3, 2018 at 18:39
• There is no generic single phase AC motor so please link to one that is capable of being reversed (not all are capable). 3ph motors are fairly generic hence my answer content. May 3, 2018 at 18:41
• Oh I see. But if they are capable, the idea is the same I guess, in a way reversing the flux rotation by swapping some terminals. I think I understand but as a final approval is this correct?: So the direction control has nothing to do with the job of the inverter and such unit(direction control) is not shown in the diagram in the question. May 3, 2018 at 18:46
• I cannot answer for a single phase motor without a specific type being proposed. You want a decent answer, yes? May 3, 2018 at 19:00
• Is H bridge used for 3 phase AC motors in practice? May 3, 2018 at 19:09