Page 74 - Mechanical Engineers Reference Book

Electrical machines 211 5
Diode
Voltage output

Figure 2.21 Half-wave rectification circuit

A

I Voltage across R,

Figure 2.22 Full-wave rectification with a diode bridge

2.1.34 The Zener diode 2.2 Electrical machines
The diode breakdown effect is also used in a variety of circuits The function of a rotating electrical machine is to convert
to provide a stabilized reference voltage. Special diodes which
are designed to operate continuously in the reverse bias mode mechanical power into electrical power, or vice versa. The
are called ‘Zener diodes’. These diodes are manufactured with conversion from mechanical to electrical power is made with a
a range of breakdown voltages from between 3 to 20 V. Figure ‘a generator’ and the conversion of electrical to mechanical
2.23 shows a Zener diode being used in a circuit to give a power with a ‘motor’. Electrical machines may be further
stable voltage which is essentially independent of the current sub-divided into a.c. or d.c. machines. The major part of all
flowing through the device. The series resistor in the circuit is electrical energy generated in the world today is produced by a
included to limit the reverse current through the diode to a particular type of a.c. machine called an ‘alternator’. The
safe value. applications of electric motors are no less substantial and they
are used in a great variety of industrial drives. It is muaily the
mechanical features of a particular application which deter-
mines the type of electric motor to be employed, and the
torquespeed characteristics of the machine are therefore very
important.

2.2.1 The d.c. generator
All conventional electrical machines consist of a stationary
element and a rotating element which are separated by a air
voltage gap. In d.c. machines - generator or motor - the stationary
I element consists of salient ‘poles’ which are constructed as
laminated assemblies with coils wound round them to produce
Stabilized a magnetic field. The function of the laminations is to reduce
voltage
the losses incurred by eddy currents. The rotating element is
traditionally called the ‘armature’, and this consists of a series
of coils located between slots around the periphery of the
armature. The armature is a150 fabricated in laminations
which are usually keyed onto a locating shaft. A very simple
Figure 223 Zener diode as a reference voltage source form of d.c. generator is illustrited in Figure 2.24.