one direction makes the drive topology and control algorithm relatively easy. The task is to generate a variable voltage and frequency power supply from a fixed voltage and frequency power supply (such as a wall-outlet power supply). The figure on page 85 shows the block diagram representation of this drive topology, with the three basic building sections discussed earlier. Motor windings are connected to the center of each half bridge on the output-inverter section. Many motors available off the shelf have both the main and start windings connected together with a capacitor connected in series with the start winding. With this configuration, the motor may have only two protruding wires (M1 and M2).
The MCU shown in the block diagram has a Power Control PWM (PCPWM) module, which is capable of outputting up to three pairs of PWMs with deadband in between the pairs. Deadband is essential in an induction motor control application to avoid cross conduction of the dc bus through the power switches when one turns OFF and the other turns ON. The diagnostic circuit may include motor current monitoring, dc-bus voltage monitoring, and temperature monitoring on the heat sink connected to the power switches and the motor.
|Block diagram representation of the drive topology with the three basic building sections. With this configuration, the motor may have only two protruding wires (M1 and M2). The MCU shown has a PWM module that is capable of outputting up to three pairs of PWMs with deadband between the pairs.|
|Bidirectional control using an H-bridge.|
Most PSC motors are designed to run in one direction. However, many applications call for bidirectional motor rotation. Historically, gear mechanisms or external relays and switches were used to achieve bidirectional rotation. When mechanical gears are used, the motor shaft runs in one direction, and the gears for forward and reverse engage and disengage according to the direction required. Using relays and switches, the polarity of the starting winding is electrically reversed based on the direction required.
Unfortunately, all of these components increase the cost of the system for basic ON and OFF control in two directions.
In this section, we will discuss two methods of bidirectional speed control for PSC motors using a microcontroller-based drive. The drive topologies discussed here produce effective voltages, which drive the main winding and start winding at 90-degree phase shifts to each other. This enables the system designer to remove the capacitor, which is in series with start winding, from the circuit permanently-thereby reducing the total system cost.
Option #2: H-Bridge Inverter
This method has a voltage doubler on the input side; on the output side an H-bridge or two-phase inverter is used (see figure above). One end of the main and start windings are connected to each half bridge; the other ends are connected together at the neutral point of the ac power supply, which also serves as the center point for the voltage doubler.
The control circuit requires four PWMs with two complementary pairs and sufficient deadband between the complementary outputs. PWM0-PWM1 and PWM2-PWM3 are the PWM