What is DC Series Motor? Working, Diagram, Characteristics & Applications

Connection diagram of DC Series Motor

Fig. 1: DC Series Motor.

In the DC series motor, the armature and field windings are connected in series with each other as shown in Fig. 1. The resistance of the series field winding (R_s) is much smaller as compared to that of the armature resistance (R_a).

Voltage and Current relations in DC Series Motor

The total current I supplied by the supply voltage (see Figure 1) is same as the field current I_s and armature current I_a.

\[I={{I}_{s}}={{I}_{a}}\]

The total supply voltage V is given by,

V = Volatge across feild + Volatge across field

\[={{I}_{a}}{{R}_{a}}+({{E}_{b}}+{{I}_{a}}{{R}_{a}}+{{V}_{brush}})\]

But,

\[{{I}_{s}}={{I}_{a}}\]

\[V={{E}_{b}}+{{I}_{a}}{{R}_{a}}+{{I}_{a}}{{R}_{s}}+\text{ Brush drop}\]

Neglecting the brush drop we get,

\[V={{E}_{b}}+{{I}_{a}}({{R}_{a}}+{{R}_{s}})\]

The flux produced is proportional to the field current. But in series motor, the field current is same as armature, the armature current.

\[\phi \propto {{I}_{a}}\]

Since the armature current flows through the series field winding, the flux is proportional to the field current as well.

\[\phi \propto {{I}_{s}}\]

The armature current I_a and hence the field current I_s will be dependent on the load. With increase in load I_a and I_s will Increase. Hence in a de series motor the flux ϕ does not remain constant. So the DC series motor is not a constant flux motor.

Characteristics of DC Series Motor

Torque – Armature Current Characteristics of DC series motor

Fig. 2: DC Series Motor Torque – Armature Current Characteristics.

For the DC series motor, the torque produced is given by,

\[{{T}_{a}}\propto \phi {{I}_{a}}\]

But,

\[\phi \propto {{I}_{a}}\]

∴

\[{{T}_{a}}\propto I_{a}^{2}\]

Where, T_a represents the gross torque produced by the motor.

As the torque is proportional to square of armature current, the starting torque of de series motor is much higher than that of de shunt motor. The torque armature current characteristics of a DC series motor is as shown in Fig. 2. The characteristics of T_a versus I_a can be divided into two parts :

1. In part-I, the torque is proportional to the square of armature current. (upto point S in Fig. 2). Hence torque increases exponentially with increase in armature current.
2. But at point S, the saturation of the field electromagnet may take place. It means that there is no change in flux even when there is a change in field current. Hence in part II of the characteristics in Fig. 2, the torque produced is directly proportional to the armature current and not proportion to its square. Hence the torque increases linearly. The dotted characteristics of Fig. 2 shows the variation of shaft torque T_sh with the armature current. Note that,

\[{{T}_{sh}}={{T}_{a}}-{{I}_{f}}\]

T_sh = 0 at no load i.e. when I_a = I_a0 and the curve of T_sh can be obtained by subtracting the constant lost torque from the gross torque T_a.

Speed – Armature Current Characteristics of DC series motor

Fig. 3: DC Series Motor Speed – Armature Current Characteristics.

The speed-armature current characteristics of a DC series motor is shown in Fig. 3. We know that,

\[N\propto \frac{{{E}_{b}}}{\phi }\]

Substituting the value of E_b as E_b = V – I_a (R_a + R_s) and ϕ ∝ I_a we get,

\[N\propto \frac{V-{{I}_{a}}\left( {{R}_{a}}+{{R}_{s}} \right)}{K{{I}_{a}}}\]

Due to the small values of R_a and R_s E_b ≈ V which is constant,

\[N\propto \frac{1}{{{T}_{a}}}\]

Hence the speed decreases with increase in I_a. as shown in Fig. 1.

Speed – Torque Characteristic of DC series motor

Fig. 4: DC Series Motor Speed – Torque Characteristics.

The speed-torque characteristics of a DC series motor is as shown in Fig. 4.

We know that,

\[T\propto I_{a}^{2}\text{    and    }N\propto \frac{1}{{{I}_{a}}}\]

\[{{I}_{a}}\propto \sqrt{T}\text{   and   }N\propto \frac{1}{\sqrt{T}}\]

This shows that the speed decreases increase in the value of torque (i.e. with increase in load). Note that the nature of speed-torque characteristics is same as that of speed-armature current characteristics. DC series motors are preferred for the traction applications because of their capability to generate a high starting torque.

Why is DC Series Motor Never Started on No Load?

For a dc series motor ϕ ∝ I_a. At no load I_a is small, hence also is small. But motor speed is inversely proportional to ϕ.

\[\text{N}\propto \frac{1}{\phi }\]

Hence at no load when is small and the motor speed can be excessively high. This can damage the motor mechanically. In order to avoid this, we should never start the dc series motor on no load.

DC Series Motor Applications

Series motor characteristics indicate that it develops very high starting torque and it has adjustable varying speed. Therefore, for any load which needs high starting torque, series motor is the only suitable dc motor. Series motors are very widely used for the following applications :

1. Electric trains,
2. Diesel electric locomotives,
3. Cranes,
4. Hoists,
5. Trolley cars and trolley buses,
6. Rapid-transit systems,
7. Conveyers etc.

Summary of DC Series Motor