DC motors are widely used in the industry because of their low cost, less complex control structure and wide range of speed and torque. With the increasing use of power semiconductor units, the speed control of the DC motor is becoming increasingly sophisticated and accurate. A TRIAC is a bidirectional thyristor with three terminals. Its three terminals are designated as MT1 (main terminal 1), MT2 and gate by G. Triac can be lit on each half cycle of the voltage applied by applying a positive or negative voltage to the gate with respect to terminal 1. To successfully activate a TRIAC, the gate current must come from the main side of terminal 2 of the circuit.
The three speed control methods are:
• Controlled by armature voltage
• Armature resistance control
• Controlled flow
For the control of armature voltage, the output delivered by the motor (speed) decreases with the decrease in applied voltage. The formula for the control of the armature voltage is as follows.
Advantages
• In the early days, the DC motor speed was controlled using a resistor that resulted in power loss, but the introduction of TRIAC eliminated the power loss
• The armature voltage control method provides good speed regulation and high efficiency
• Gives a large speed range with any desired number of speed points.
Disadvantages
• Today, the introduction of IGBT made DC motor speed control more feasible
• The switching circuit used in the circuit makes the speed control less accurate.