Why DC Series motor can not operates at no load condition

What are the types of DC Motors?

The construction of dc motor is same as dc generator.DC motor are two types Self excited motor and separately excited motor.Based on field winding connection with armature winding self excited motor are divided into three category namely dc series motor, dc shunt motor and dc compound motor.

As the name indicates, in DC series motor field winding is connected in series with the armature winding. Hence Line current(I_{L}), Field current (I_{se}=I_{f}), and armature current (Ia) all are equal

I_{L}=I_{f}=Ia---------------(i)

DC series motor

Armature current of any DC motor depends upon load. Hence if load increases armature current (Ia) also increases and vice versa. At no load condition, load on the motor is zero hence armature current(Ia) is also zero. From equation (i) (I_{f}=Ia) hence field current(I_{f}) will also=0 at no load condition.

Now as we know that flux of any DC motor is directly proportional to field current (I_{f}).We can easily understand this with the help of back emf equation of DC motor:

Basic equations

Hence based on above discussion it is advised that DC Series motor should not operate at no load condition.We can also understand above by knowing characteristics of DC series motor.

How to solve numerical on DC Series Motor? Step by step process: