Sensorless Motor Drive with the ST62 MCU + TRIACT. CASTAGNETINTRODUCTIONHome appliance applications are requiring more and more electronic control in order to meetnew requests and constraints of consumers.Microcontrollers have been typically limited to high end applications because theirperformance appears to be overrated when related to the functions of the application. Inreality, home appliances require microcontrollers which trade closely between the compromisebetween cost and performance.An a.c. universal motor is a cost optimized solution for home appliance applications includingfood processor and drill applications.This Application Note shows that the capabilities of simple 8-bit microcontroller allows thedesign of cost effective speed drive controller with increased functionality. When associated toa triac these microcontrollers become key components in the design of a sensorless speedcontrol.1 THE CONTROL OF THE MOTOR SPEEDAn a.c. universal motor is a brush motor with a serial excitation. Its stator windings areconnected in series with the rotor, and its flux is proportional to the motor current. The motortorque is theoretically proportional to the square of the current, so it is always positive: thespeed direction is insensitive to the current direction, and the motor can be supplied in a.c. ord.c. modes. Control of the speed is obtained by adjustment of the motor voltage. This controlis achieved by a phase angle method in a.c. mode, or by a Pulse Width Modulation method ind.c. mode.AN416 / 02,94APPLICATION NOTEFigure 1. The triac is a key device for thea.c. drive of the universal motor.Figure 2. A d.c. drive for the universalmotor: the P.W.M. chopper with IGBT.MMFor a fixed motor voltage the motor charac...
