D.c. machines

At the end of this chapter you should be able to:

• distinguish between the function of a motor and a generator • describe the action of a commutator • describe the construction of a d.c. machine • distinguish between wave and lap windings • understand shunt, series and compound windings of d.c. machines • understand armature reaction • calculate generated e.m.f. in an armature winding using E = 2p nZ/c • describe types of d.c. generator and their characteristics • calculate generated e.m.f. for a generator using E = V + IaRa • state typical applications of d.c. generators • list d.c. machine losses and calculate efficiency • calculate back e.m.f. for a d.c. motor using E = V − IaRa • calculate the torque of a d.c. motor using T = EIa/2πn and T = p ZIa/πc

• describe types of d.c. motor and their characteristics • state typical applications of d.c. motors • describe a d.c. motor starter • describe methods of speed control of d.c. motors • list types of enclosure for d.c. motors

When the input to an electrical machine is electrical energy (seen as applying a voltage to the electrical terminals of the machine), and the output is mechanical energy (seen as a rotating shaft), the machine is called an electric motor. Thus an electric motor converts electrical energy into mechanical energy. The principle of operation of a motor is explained in Section 8.4, page 101. When the input to an electrical machine is mechanical energy (seen as, say, a diesel motor coupled to the machine by a shaft), and the output is electrical energy (seen as a voltage appearing at the electrical terminals of the machine), the machine is called a generator. Thus, a generator converts mechanical energy to electrical energy. The principle of operation of a generator is explained in Section 9.2, page 107.