DC MOTOR
Starting of DC Motors:
DC motors require a high starting torque to overcome the inertia of the rotor and
start rotating. This is achieved by applying a high voltage to the motor at start-up,
which produces a large starting current. However, this high starting current can
cause damage to the motor and the supply system.
Two types of starters are used for DC motors:
1. Three Point Starter
2. Four Point Starter
1) Three Point Starter:
The three-point starter is the simplest type of starter used for DC motors. It consists
of three terminals, namely L1, L2, and L3. The L1 terminal is connected to the
positive supply, while the L3 terminal is connected to the armature. The L2 terminal
is connected to the field winding.
During starting, the start button is pressed, which closes the contacts of the starter.
This connects the armature and the field winding to the supply. The starting current
flows through the field winding, producing the magnetic field, and the armature
winding, producing the torque required to start the motor. When the motor reaches
its rated speed, the start button is released, and the starter contacts open.
2) Four Point Starter:
The four-point starter is similar to the three-point starter, but it includes an
additional terminal, called the "interlocked neutral" terminal. When the starter
contacts are open, the interlocked neutral terminal is connected to the armature
terminal, and the field winding is disconnected from the supply.
When the start button is pressed, the starter contacts close, and the field winding is
connected to the supply. The armature is also connected to the supply, but through
a resistive branch. This limits the starting current and reduces the stress on the
motor and the supply system.
Losses and Efficiency:
DC motors are not 100% efficient, as they suffer from various losses during operation.
These losses are as follows:
Copper losses: These losses occur due to the resistance of the armature and
field windings. They are proportional to the square of the current flowing
through the winding and can be calculated as:
Pc = I^2R
Starting of DC Motors:
DC motors require a high starting torque to overcome the inertia of the rotor and
start rotating. This is achieved by applying a high voltage to the motor at start-up,
which produces a large starting current. However, this high starting current can
cause damage to the motor and the supply system.
Two types of starters are used for DC motors:
1. Three Point Starter
2. Four Point Starter
1) Three Point Starter:
The three-point starter is the simplest type of starter used for DC motors. It consists
of three terminals, namely L1, L2, and L3. The L1 terminal is connected to the
positive supply, while the L3 terminal is connected to the armature. The L2 terminal
is connected to the field winding.
During starting, the start button is pressed, which closes the contacts of the starter.
This connects the armature and the field winding to the supply. The starting current
flows through the field winding, producing the magnetic field, and the armature
winding, producing the torque required to start the motor. When the motor reaches
its rated speed, the start button is released, and the starter contacts open.
2) Four Point Starter:
The four-point starter is similar to the three-point starter, but it includes an
additional terminal, called the "interlocked neutral" terminal. When the starter
contacts are open, the interlocked neutral terminal is connected to the armature
terminal, and the field winding is disconnected from the supply.
When the start button is pressed, the starter contacts close, and the field winding is
connected to the supply. The armature is also connected to the supply, but through
a resistive branch. This limits the starting current and reduces the stress on the
motor and the supply system.
Losses and Efficiency:
DC motors are not 100% efficient, as they suffer from various losses during operation.
These losses are as follows:
Copper losses: These losses occur due to the resistance of the armature and
field windings. They are proportional to the square of the current flowing
through the winding and can be calculated as:
Pc = I^2R
