motors Motor is an electrical device that converts electrical energy into mechanical energy.

dc motors: Fig. m11 shows schematically the features of a dc motor. The magnetic field produces a torque on the armature when there is a current in the coil. The current directions are reversed using the brush, commutator arrangement.

The torque on the armature are made constant with a set of coils instead of the single armature coil (fig. m12).

As soon as the starts rotating, a back emf will be induced in it, due to the flux it cuts. This reduces the current in the motor.

At the starting of the motor, there is no back emf because the coils are at rest. The motor draws a very large current. A protective resistance must be incorporated in series with the coil during its starting. This is removed when the motor starts running.

ac motors: The three phase synchronous motor shown in fig. m13, has a single armature coil, but three sets of field coils. The peaks in the current in the field coil occurs in phase with rotation of the armature, so that the torque is nearly constant.

The three phase induction motor, also has three sets of field coils, but the rotor is of entirely different configuration (see fig. m14). It consists of copper bars fixed on two copper rings. The varying magnetic field induces current in the bars. The bars also experience force due to interaction between its current and field. This makes the rotor to rotate.

A single phase induction motor uses a single phase ac with similar rotor.