This document describes a speed regulator without sensor: the speed sensing is performed indirectly by the ST6220, low-cost 8-bit microcontroller, measuring the motor current. Performance results are given, which are in line with the need of many home appliances.
The document explains principles of universal motor operation in AC and DC modes, focusing on variable speed drives. It details a sensorless regulation method where motor speed is inferred by measuring instantaneous motor current at specific times within the mains cycle, eliminating the need for expensive speed sensors like tachogenerators.
The note provides implementation details for using the ST6220 microcontroller to regulate universal motors in home appliances such as vacuum cleaners and washers. It covers circuit schematics involving capacitive supply, current sensing resistors, operational amplifiers, and triac firing delay control based on mains zero-crossing detection.
The maximum firing delay should be limited to values below 7 mS to avoid distortion when regulating very low speeds.
The current sense resistor is 0.22 ohm to keep losses as low as possible for a 500 watts motor.
The resolution is around 20 mV when the micro is supplied from 5 volts.