Electrical Test and Electric Motors

The basic principle of motor

The basic working of a motor is based on the fact that when ‘a current carrying conductor is placed in a magnetic field, it experiences a force’.

If you take a simple DC motor, it has a current-carrying coil supported in between two permanent magnets (opposite pole facing) so that the coil can rotate freely inside. When the coil ends are connected to a DC source then the current will flow through it and it behaves like a bar magnet, as shown in figure below. As the current starts flowing, the magnetic flux lines of the coil will interact with the flux lines of the permanent magnet.

This will cause a movement of the coil (figures (a), (b), (c), (d)) due to the force of attraction and repulsion between two fields. The coil will rotate until it achieves the 180° position, because now the opposite poles will be in front of each other (Figure (e)) and the force of attraction or repulsion will not exist.

The role of the commutator: The commutator brushes just reverse the polarity of DC supply connected to the coil. This will cause a change in the direction of the current of the magnetic field and start rotating the coil by another 180° (figure (f)).

The brushes will move on like this to achieve continuous coil rotation of the motor.

Similarly, the AC motor also functions on the above principle; except here, the commutator contacts remain stationary, because AC current direction continually changes during each half-cycle (every 180°).

Basic principle of generator

In principle, an AC generator’s construction is similar to the construction of the motor.

Instead of putting current in, current is taken out from the coil in an alternator.

A mechanical prime mover rotates the coil in between the poles of a permanent magnet and an AC potential is induced in the coil. To further define: if an AC current will make a coil turn, then turning the coil will create an AC current.

As per Faraday’s law, when a wire is moved in to cut across magnetic field lines, a force is exerted on the charge (electrons) in the wire by trying to move them along the wire. This is how current will start flowing if a complete circuit is provided to it. The magnetic field is provided not by magnets, but by field coils.

The coil in which the voltage is induced is called armature winding, while the coil that provides the magnetic field is called field winding.

In high-voltage generators, it is not good practice to have armatures rotating because current-collecting brushes of high ratings are required. Rather, the armature is kept stationary and the field is kept rotating.

Alternators of low capacity use a permanent magnet as a field, while in high-capacity alternators field winding supply is derived from the exciter assembly. An exciter assembly is a small alternator connected on the same shaft.