A DC series motor is a type of direct current (DC) motor in which the field winding is connected in series with the armature winding. Because of this series connection, the same current flows through both the armature and the field winding. DC series motors are known for producing very high starting torque, which makes them suitable for applications that require heavy starting loads.
Table of Contents
Construction of DC Series Motor
The construction of a DC series motor is similar to other DC motors and consists of the following main parts:
Stator (Field System)
The stator contains the series field winding, which is connected in series with the armature circuit. Since the full armature current flows through this winding, it is made of thick conductors with fewer turns.
Rotor (Armature)
The rotor carries the armature winding, where the electromagnetic torque is produced.
Commutator
The commutator converts the alternating current in the armature conductors into direct current
at the terminals.
Brushes
Carbon brushes are used to transfer current between the
external circuit and the rotating commutator.
Circuit Diagram
In a DC series motor, the field winding, armature winding, and supply voltage are connected in series.
Thus,
- Field current = Armature current
\[
I_a = I_f
\]
where
- \(I_a\) = armature current
- \(I_f\) = field current
Back EMF in DC Series Motor
When the motor rotates, it generates a back electromotive force (EMF) which opposes
the applied voltage.
The back EMF is given by
\[
E_b = V – I_a(R_a + R_{se})
\]
where
- \(E_b\) = back EMF
- \(V\) = supply voltage
- \(R_a\) = armature resistance
- \(R_{se}\) = series field resistance
Torque Equation
The torque developed in a DC motor is proportional to the product of
flux and armature current.
\[
T \propto \phi I_a
\]
In a DC series motor, the flux \((\phi)\) is proportional to the armature current.
\[
\phi \propto I_a
\]
Thus,
\[
T \propto I_a^2
\]
Therefore, the torque increases very rapidly with current, resulting in
high starting torque.
Speed of DC Series Motor
The speed equation of a DC motor is
\[
N \propto \frac{E_b}{\phi}
\]
In a series motor:
- At light load, current is small → flux is small → speed becomes very high.
- At heavy load, current increases → flux increases → speed reduces.
Thus, the speed of a DC series motor varies widely with load.
Important Characteristics
Torque–Current Characteristic
Torque increases approximately as the square of armature current at low currents.
Speed–Current Characteristic
Speed decreases as load current increases.
Speed–Torque Characteristic
Speed decreases rapidly with increasing torque.
Advantages of DC Series Motor
- Very high starting torque
- Simple construction
- Suitable for heavy load starting conditions
- Good acceleration
Disadvantages
- Speed varies greatly with load
- Dangerous to operate without load
- Poor speed regulation
Applications
Due to their high starting torque, DC series motors are widely used in:
- Electric traction systems
- Electric trains
- Cranes
- Hoists
- Elevators
- Rolling mills