Module 6. D.C. machines

Lesson 25


25.1  Characteristics of   D.C. Motor

The performance of a d.c. motor is expressed using different graphs and curves known as characteristic curves. Operating characteristics are given by the following curve.

1.      Torque - Armature current (Ta - Ia) characteristic also known as electrical characteristic.

2.      Speed – Armature current (N -Ia) characteristics.

3.      Speed – Torque (N - Ta) characteristics also known as the mechanical characteristic

As per syllabus we will be looking into speed torque (N - Ta) characteristics.

25.2  Speed Torque (N - Ta) Characteristics

25.2.1  Series motor

Fig. 25.1 Speed torque curve for D.C. series motor

·         From the plot it is evident that the series motor exhibit high torque at low speed and vice versa.

·         Series motor must never be started at no load. Under no load conditions the value of the armature is very small.

We know,

Ia                                                                                                                                                                                                                          ……… (1)

                                                                                                                                                                                                                                                             ……… (2)


From relation (1) if armature current (Ia) is small, flux would be small. If flux () has a very small value. The motor speed (N) will become very high to cause changes to the motor part. Therefore series motors are used in place where “no load” condition doesn’t occur.

25.2.2  Shunt motors

Fig. 25.2 Speed torque curve for D.C. shunt motor

From the plot we can see as the load (Torque) on the shunt motor increase there is slight fall in the motor speed. Due to insignificant redirection in the speed, shunt motor are considered to be constant speed motor. Therefore shunt motor will approximately be operated at same speed under no load and full load conditions.

26.3.3     Compound motors  Differential – compound motors

Fig. 25.3 Speed torque curve for differential compound motor


Differential compound motor are rarely used due to its poor Torque –speed characteristics. The machine can be operated at constant speed on medium load. But at very high load / torque the speed is very high to damage the motor.  Cumulative compound motor

Fig. 25.4 Speed torque curve for cumulative compound motor

·         In cumulative compound motor speed falls considerably at heavy load / torque.

·         When the load / torque is low the motor runs at the safe speed.

·         Motor shows the characteristics in between that of series and shunt motor.

To compare performance of different motors, the speed – torque characteristics is shown in same plot.

Fig. 25.5 Speed torque curve for different D.C. motors