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  • Phase Angle Motor Control

  • Created: Sep 16, 2015

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Summary

The reference design as shown is a phase angle control for universal motors. This circuit is used to control the speed of a universal motor. A universal motor is a commutated series wire-wound DC motor. This circuit uses phase angle control to vary the effective motor voltage. The basic principle of the phase angle control algorithm is simple: match the firing pulse time of the triac in relation with the zero crossing of the line voltage. A phase shift of this firing pulse produces a variable output voltage on the load. In this design, the microcontroller is used to control the triac. The snubberless triac is used as the power device. This triac has high dv/dt immunity: therefore, there is no need of RC circuit around it.

The circuit configuration was chosen for two reasons. The first reason is the current capability of the pins of port A. The Kinetis L family specification has a source capability is 5mA and a sink capability of 10mA. The choice for this design is the sink mode. The second reason is determined by the triac. The snubberless triacs with high dv/dt immunity need a higher gate trigger current (IGT). The MAC4DC needs at least 25mA typically in case of negative IGT. For an operational mode with positive IGT, the gate trigger current is much higher. Our choice is to use a negative IGT. Three pins, PTC 1, 2, 3, are connected together and are powerful enough to cover the amount of current needed by the gate and to turn on the triac reliably. The phase angle control system needs to have information about the line voltage and its zero crossing points. The appropriate signal is connected to pin PTA0. The circuitry connected to pin PTA0 is composed of R7, C6 and D5. The pushbuttons SW1, SW2 and SW3 control the drive. There are external pull-up resistors because this device does not support internal pull-up resistors on each pin. The SW1 and the SW2 are used to increase or decrease the motor speed and the SW3 is used to start/stop the drive.

This reference design is very useful to applications involving universal motors. It is widely used in home appliances such as vacuum cleaners, washers, hand tools, and food processors. This design is dedicated for universal brushed motors operating from 1000RPM to 15000RPM.

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