Why DC Series Motor has High Starting Torque?

DC Series Motors are widely used for the purposes requiring high starting torque like in Hoist, Crane, Electric Traction etc. In this post we will discuss the reason behind the high starting torque of DC Series Motor.

For any DC motor, torque is directly proportional to the flux and armature current i.e.

Ƭ = KaØIa  ……………….(1)


Ka = Constant

Ø = Flux

Ia = Armature current.

In case of DC Series Motors, the flux produced is directly proportional to the armature current. Hence torque is directly proportional to the square of armature current. Therefore we can write as

Ø = CIa, where C is some constant.

Therefore, from equation (1), torque in DC Series Motor

Ƭ = K(Ia)2  where K = KaxC  ………..(2)

As we know that the maximum permissible current at starting of DC Motor is limited to about 1.5 times that of rated current, thus loads requiring high starting torque such as Cranes, Hoists and Traction type loads, Series Motors are most suited machines.

Thus from equation (2), we can draw the Torque-Current Characteristics of DC Series Motor as shown in figure below.

It is clear from the above characteristics of DC Series Motor that, higher the armature or load current, higher is the torque till the magnetic flux causes saturation. Once saturation occurs, the flux will no longer increase in proportion to the increment in current and thus limits the increase in torque.

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