Direct Current (D.C.) motors are widely used in various industrial and commercial applications. To start and control the speed of D.C. motors, starters are employed. Starters are devices that provide the necessary starting current and control for the motor. Two common types of D.C. motors are shunt motors and compound motors, and they require different types of starters.
Shunt Motor Starter:
A shunt motor has its field winding connected in parallel with the armature winding. This design allows the motor to maintain relatively constant speed even under varying loads. The starter used for shunt motors is typically a Three-Point Starter.
A Three-Point Starter consists of three main components:
Main Contacts: These are the main switch contacts that connect the armature and field windings to the power supply.
Field Regulating Resistance: This is a variable resistance connected in series with the field winding. It allows for controlling the speed of the motor during startup by adjusting the field current.
Starting Resistance: This is a variable resistance connected in series with the armature winding. It limits the initial armature current to prevent excessive current draw during startup.
The operation of the Three-Point Starter involves moving a handle through different positions: Off, Running, and Starting. As the handle is moved, the field winding and armature winding are gradually connected to the power supply, and the starting resistance is progressively reduced to allow a controlled startup.
Compound Motor Starter:
A compound motor combines features of both shunt and series-wound motors. It has both a shunt field winding and a series field winding. This type of motor offers better torque characteristics and can handle varying loads effectively. The starter used for compound motors is typically a Four-Point Starter.
A Four-Point Starter is similar to a Three-Point Starter, but it includes an additional contact for the series field winding. The main components are:
Main Contacts: These connect the armature and both the shunt and series field windings to the power supply.
Field Regulating Resistance: A variable resistance in series with the shunt field winding for speed control.
Starting Resistance: A variable resistance in series with the armature winding for startup.
Series Field Switch: This switch connects or disconnects the series field winding, allowing the motor to be operated in either a cumulatively compounded mode (both shunt and series fields contribute) or differentially compounded mode (shunt and series fields oppose each other).
The operation of the Four-Point Starter is similar to the Three-Point Starter, but it also involves engaging the series field switch as needed to control the motor's performance characteristics.
Both types of starters and motors allow for controlled startup, speed regulation, and varying load handling, making them suitable for a wide range of applications. Proper selection and adjustment of starters are crucial for the safe and efficient operation of D.C. motors in various industrial settings.