Chapter 6

How Three-Phase Inverters Work

To control the phases during six-step commutation, a three-phase inverter is used to direct the DC power to the three phases, switching between positive (red) and negative (blue) current. To supply positive current to one of the phases, the switch connected to that phase at the high side needs to be turned on, while for negative current the low side switch needs to be on.

Doing this in the pattern described above while the rotor is at an angle between 60 and 120 degrees to the stator magnetic field, the three-phase inverter keeps the motor rotating at a constant speed. Varying motor speed can be achieved by adjusting the applied voltage. Another way of controlling the motor speed without varying the source voltage is pulse-width modulation, or PWM.