Page 639 - Basic Electrical Engineering

ϕ ∝ I a

T ∝ ϕ I a

T ∝ I a 2

Since I R is very small as compared to V, V − I R ϕ V = constant, and ϕ ∝
a
a
a a
I a

Therefore, from

we can write

i.e., N × I = constant
a

Thus, the relations of N versus I and T versus I are drawn as shown in Fig.
a
a
7.21 (b). The relation between N and T is also drawn. From N versus T
characteristics it is seen that at nee, i.e., at starting T is very high. That is, a
series motor develops a very high torque at starting.

Therefore, a series motor is suitable for application as a drive motor in
electric trains, cranes, hoists, trolley bus, etc., where the drive motor should

develop very high starting torque.
From N versus I characteristic, it is observed that the motor will attain
a
dangerously high speed when I is zero. That is, at no load the speed of the
a
motor will be very high which may be dangerous. That is why a series motor
is never allowed to run on no load. A load is always connected to its shaft
before starting.

7.12.3 Characteristics of DC Compound Motors