DC Motors

The construction of a d.c. motor is the same as a d.c. generator.

DC Generators converts mechanical energy to electrical energy.

DC motor converts electrical energy to mechanical energy.

The only difference is that in a generator the generated e.m.f. is greater than the terminal voltage, whereas in a motor the generated e.m.f. is less than theterminal voltage.

Back e.m.f.

When a d.c. motor rotates, an e.m.f. is induced in the armature conductors.

By Lenz’s law this induced e.m.f. E opposes the supply voltage V and is called a back e.m.f., and the supply voltage, V is given by:

Torque of a d.c.machine

Torque of a d.c.machine

Types of d.c. motorand their characteristics

(a) Shunt-wound motorIn the shunt wound motor the field winding is in parallel with the armatureacross the supply as shown

(b) Series-wound motorCharacteristics

(c) Compound-wound motor

Cumulative compound, in which the series winding is so connected that the field due to it assists that due to the shunt winding.

Differential compound, in which the series winding is so connectedthat the field due to it opposes that due to the shunt winding.

a long-shunt compound motor short-shunt compound motor.

Characteristics

The efficiency of ad.c. motor