Page 227 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Types of starters: Starters used to start the DC motors are generally of three types.
• Two-point starter
• Three-point starter
• Four-point starter
Two-point starter: This contains the following components.
• The series resistor required for starting a motor.
• The contacts (brass studs) and switching arm required to include or exclude the resistor in the armature circuit.
• A spring on the handle to bring the handle to the `OFF’ position when supply fails.
• An electromagnet to hold the handle in the `ON’ position.
The two-point starter is frequently used with a DC series motor. The starting resistance, electromagnet armature
and the series field are all connected in series as shown in Fig 2.

When the arm is moved to the first contact point, the circuit is completed, and the armature begins to rotate. As
the armature speed increases, the arm is slowly moved towards the right side electromagnet, thereby the starter
resistance is reduced. When the arm is against the electromagnet, complete starter resistance is cut off from the
circuit.
Three-point starter: Fig 3 shows the internal diagram of a three(terminal) point starter connected to a DC shunt
motor. The direct current supply is connected to the starter, the motor circuit through a double pole switch and
suitable fuses. The starter has an insulated handle or knob for the operator’s use. By moving the starter handle
from the `off’ position to the first brass contact (1) of the starter, the armature is connected across the line through
the starting resistance. Note that the armature is in series with the total starting resistance. The shunt field, in
series with the holding coil, is also connected across the line. In this mode of operation, the rush of the initial

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CITS : Power - Electrician & Wireman - Lesson 30-37