The amplifier for AC three-phase motors includes a pulse-width modulation circuit for voltage, current, and frequency control. Figure 11-81 shows an example of this type of amplifier. From the diagram you can see that this circuit is designed specifically for a three-phase trapezoidal motor. The transistors in the amplifier are connected in an H-bridge configuration. The motor windings are connected as a three-phase wye with no external wires connected to the wye point. This type of motor is also called a star connection when it is used with brushless AC servomotors.
The drive logic and PWM switching controller is shown in the diagram as a block that is identified as a logic and PWM circuit. This block shows six arrows pointing away from it and pointing to the transistors. These arrows represent the six circuits for the base of each of the six transistors. The block below the PWM circuit represents the current-sensing part of the amplifier. This section of the amplifier uses a recirculating chopper system to control the current in a manner that is similar to the chopper circuit in the DC amplifier. The signals for this section of the amplifier come from the voltage that is developed across the series resistors connected between the transistor section and the motors. As you know, the amount of current flowing to the motor will determine the amount of voltage drop across these resistors.