Electromagnets and Transformers

The simplest alternating current (a.c.) generator consists of a rectangular coil between the poles of a C-shaped magnet.

The ends of the coil are joined to two slip rings on the axle and against which carbon brushes press.

In power station alternators the electromagnets rotate (the rotor, Figure 43.7a) while the coils and their iron core are at rest (the stator, Figure 43.7b). The large p.d.s and currents (e.g. 25 kV at several thousand amps) induced in the stator are led away through stationary cables, otherwise they would quickly destroy the slip rings by sparking.

The rotor of a bicycle generator is a permanent magnet and the voltage is induced in the coil, which is at rest

When the current in a coil is switched on or off or changed, a voltage is induced in a neighbouring coil. The effect, called mutual induction, is an example of electromagnetic induction and can be shown with the arrangement of Figure

A transformer transforms (changes) an alternating voltage from one value to another of greater or smaller value. It has a primary coil and a secondary coil wound on a complete soft iron core, either one on top of the other (Figure 44.3a) or on separate limbs of the core (Figure 44.3b).

A step-up transformer has more turns on the secondary than the primary and Vs is greater than Vp

In a stepdown transformer there are fewer turns on the secondary than the primary and Vs is less than Vp

V_s : secondary voltage (Volt)

V_p : primary voltage (Volt)

N_s : secondary turns

N_p : primary turns

I_s : secondary current (Ampere)

I_p : primary current (Ampere)