© 2005 Microchip Technology Inc. DS00967A-page 1 AN967 INTRODUCTION Single-phase induction motors are extensively used in appliances and industrial controls. The Permanent Split Capacitor (PSC) single-phase induction motor is the simplest and most widely used motor of this type. The classification, construction and working principle of single-phase induction motors are explained in detail in the application note “AC Induction Motor Fundamentals” (AN887) available from Microchip. For VF theory and basic operations, refer to AN887 mentioned above, and “Speed Control of 3-Phase Induction Motor Using PIC18 Microcontrollers” (AN843). By design, PSC motors are unidirectional, which means they are designed to rotate in one direction. By adding either extra windings, and external relays and switches, or by using gear mechanisms, the direction of rotation can be changed. In this application note we will discuss in detail, how to control the speed of a PSC motor in both directions using a PIC16F72 microcontroller and power electronics. The PIC16F72 microcontroller was chosen because it is one of the simplest and low-cost general purpose microcontrollers Microchip has in its portfolio. Even though it does not have the PWMs in hardware to drive complementary PWM outputs with dead band inserted, all PWMs are generated in firmware using timers and output to general purpose output pins. THEORY OF OPERATION A PSC motor is usually a 2-phase asymmetrically wound motor. The main winding is designed to take the load current. The current flowing through the start winding is much less than the main winding. Therefore, the start winding will have a different electrical characteristic compared to the main winding. In order to produce the Magnetomotive Force (MMF) produced by the start winding very near to the main winding, the start winding has additional turns, higher resistance, and reduced current flowing through it. This makes the motor windings asymmetrical. The motor is energized with a single-phase AC power supply, with a capacitor connected in series with the start winding (also called an auxiliary winding) as shown in Figure 1. The value of the capacitor is chosen so that the total impedance on the auxiliary winding produces sufficient phase shift in current to generate a rotating magnetic field in the air gap. Typically, the current flowing through the start winding leads the current flowing through the main winding by 90 degrees. By adding a microcontroller-based control circuit to the motor, the voltage across the main winding and start winding can be maintained at 90 degrees to each other. The other advantage is that the starting capacitor can be removed from the circuit, thus reducing the total system cost. FIGURE 1: ELECTRICAL EQUIVELANT CIRCUIT OF A PSC MOTOR A multiphase inverter can be used to drive the motor. Two methods are discussed in the next section. Author: Padmaraja Yedamale Microchip Technology Inc. Squirrel Cage Rotor A C L N Starting Winding Main Winding VSTART VMAIN Bidirectional VF Control of Single and 3-Phase Induction Motors Using the PIC16F72