A three-point starter is a type of control device used in direct current (DC) motors to start, stop, and control their speed. It is commonly used for controlling shunt and compound-wound DC motors, which are commonly found in various industrial and commercial applications.
The three-point starter consists of three main parts:
Main Switch: This is the main control switch that is used to connect or disconnect the motor from the power supply. It can be manually operated and is used to start or stop the motor.
Starting Resistance: The starting resistance is connected in series with the armature circuit of the motor during the starting process. It is used to limit the initial current drawn by the motor, which helps in preventing excessive current flow and possible damage to the motor and power supply components. As the motor gains speed and reaches its normal operating conditions, the starting resistance is gradually reduced.
Field Regulator: The field regulator controls the field current flowing through the field winding of the motor. By adjusting the field current, the magnetic field strength around the armature can be controlled, which in turn affects the speed and torque characteristics of the motor.
The operation of a three-point starter involves the following steps:
Starting: When the main switch is closed, the motor circuit is energized. Initially, the starting resistance is in the circuit, which limits the armature current. This allows the motor to start with a lower current, preventing damage to the motor windings.
Running: As the motor gains speed, the field regulator can be used to control the field current, thus controlling the speed and torque output of the motor. The starting resistance is gradually reduced or bypassed completely as the motor reaches its normal operating speed.
Stopping: To stop the motor, the main switch is opened, disconnecting the motor from the power supply. This halts the flow of current through the motor, causing it to come to a stop.
The three-point starter provides a controlled and gradual start to the DC motor, reducing the stress on the motor and the power supply. It also allows for speed control during operation. However, it has some limitations, such as energy losses in the starting resistance and the need for manual operation.
Modern control methods, such as electronic drives and variable frequency drives (VFDs), have largely replaced traditional three-point starters in many applications due to their improved efficiency, precise control, and automation capabilities.