AC Generators and Transformers

AC Generators and Transformers

Physics 9

Simple A.C. Generator (Alternator)

Physics 9

A.C. Generator

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.

Frequency

The frequency of an a.c. is the number of complete cycles it makes each second and is measured in hertz (Hz), i.e. 1 cycle per second = 1 Hz. If the coil rotates twice per second, the a.c. has frequency 2Hz. The mains supply is a.c. of frequency 50Hz.

Practical generators

Physics 9

Power stations

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.

Bicycles

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

Transformers

Physics 9

Mutual induction

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

Transformer equation

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).

Step-up transformer

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

Stepdown transformer

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

Transformer equation 

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)

Energy losses in a transformer

• a) Resistance of windings

• b) Eddy currents

• c) Leakage of field lines

Transmission of electrical power

Physics 9

Thank You

Physics 9