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  • Bidirectional VF Control of Single and 3-Phase Induction Motors

  • Created: Sep 03, 2014

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Summary

In terms of appliances and industrial controls, the use of single-phase induction motor is widely utilized and the most common type of induction motor is the Permanent Split Capacitor (PSC) single-phase induction motor. PSC motors natural operation is unidirectional; this means that they rotate in single direction only. The only way to change the rotation of the PSC is to add either extra windings, relays, switches or using gear mechanisms. In this application note, the use of PIC16F72 microcontroller and power electronics is highlighted in order to control the speed in both directions of a PSC induction motor.


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 characteristics 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. 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.


Microcontroller-based control for a PSC motor makes the system easy to implement and have control over the motor in two directions. Implementing the algorithm using a 3-phase inverter bridge gives flexibility and efficiency of control. In addition, a 3-phase induction motor can be controlled using the same hardware by making minor modifications to the firmware. The PIC16F72 is a popular low cost general-purpose microcontroller algorithm. To a great extent, PIC18FXX31 devices can provide flexibility in implementing motor control algorithms.