5. energy conversion
Week 11
5.1 Electro-mechanical energy conversion
Energy is converted to electrical form because of the advantages listed in the introductory
part of the note. It is seldom available or used in electrical form, but converted into electrical
form at the input to a system and back to non-electrical form at the output of a system. A
typical example is the processing of energy from and hydro generating plant. It is converted
into electrical form at the power plant. Transmitted through transmission lines and
distribution lines, and coverted to mechanical energy in an electric motor are the point use. A
second example is in the conversion of the energy in sound pressure waves, and the
transmission in electrical form from the taker to the listener in a telephone system. Few more
energy conversion principles will be mentioned.
5.1.1 Major energy coversion principles
Energy conversion between electrical and non- electrical forms includes
5.2
(i)
Electrochemical eg battery
(ii)
Electrothermal eg. Thermocouple
(iii)
Photo electrical eg photo cell
Energy coversion
Theoretically, only a sourceless current is needed to develop a mechanical force
magnetically. But in a machine the production of force is hardly enough: something must
move in order to do useful work done demands a corresponding energy supply form
somewhere.
In a device energized only by a permanent magnet, the only energy source is the magnet
itself. If the displacable part of the machine moves under force and does work, this can only
be at the expense of the field energy of the permanent magnet, which must decrease. Such as
5. energy conversion
Week 11
arrangement, has obvious limitations. It may also be inconvenient a permanent magnet”
lifting magnet, for example would not e capable of releasing its load. Where the magnetic
affected by the movement is produced by a current circuit, changes of field energy have to
be supplied electrically from a source. This implies the appearance in the circuit of an
electromotive force e, which, with the current I, represents the delivery or absorption by the
source of energy at the rate ei consider the elementary system of fig 2.1 A sources of voltage
is connected to a device (e.g a secondary battery or a machine) in which the energyconversion process results in the appearance of an e.f.m.
The effective resistance of the circuit is represented by R. if
current flows into the circuit
form the positive terminal of the source, and the input power p = Vi Rl + el, has the
direction as shown at (a). However¸ if e >, the current reverses and we can now call it –e. the
power input from the now p = v (-i) = Rl2 + e (-l), which is negative, i.e it is an output from
the device into the source, as at (b). to illustrate this simple but fundamental point, suppose
that v = 10v d.c nad R = 1-2. then if e = 8vd.c.
The current o = v-e
R
=
10-8
=2A
1
And the source provides an input power p = 10 x 2 = 20w The converting device accepts 8
x 2 W as a motor And power loss due to plissipation
= 1 x RT2 = 4W Conversely, if e
12V, The current again is 10 -12 -2A (I.e reversed) The device produces 12 x 2 = 24 W as
a generator of which RT2 22 x 1 = 4W is dissipated in R, and 10 x 20W is delivery as an
output to the source. In the case of the electromagnetic machine, the relationship between
the emf and the magnetic field is obtained from the faraday induction law (which had been
mentioned in 1.1)
5. energy conversion
5.3
Week 11
Linked energy systems
An electromechanical machine forms a coverting link between an electrical energy system (
such as a main power –supply network) and a mechanical one (such as a prime- mover or a
train). In action a machine is not an isolated things, but has a behavious strongly influenced
by its terminal systems. A relay, for instance, will be affected if its operating battery
becomes discharged; a loudspeaker will behave very differently it enclosed in an evacuated
vessels with the air loading thus removed; a hydro electric generator, suddenly shortcircuited, will react severely on the turbine and pipe-line.
A machine can, of course, be studies initially in
isolation, but the engineering interest
begins in fact when the complete linked system is considered. Again, the steady-state
behavious is informative up to a point, but operation in responses to change – i.e, the
transient responses is fro move important and fundamental.
Fig 2.1 Electro–mechanical linked energy system
System analysis can be complicated. Fig 2.2 shows diagrammatically a typical electric
supply system feeding a mechanical load through an electromechanical machine. In some
cases we might simplify the analysis by assuming, say, that the terminal voltage and
frequency of the machine were constant. This is good enough if the machine is a small
contactor but if it is a 25MW motor the effects of its behaviour reach for back through even
an extensive supply system. Methods are available for evaluating such a complex for any
5. energy conversion
Week 11
given stimulus, such as the occurrence of a transmission- line fault or starting of a large
motor.
5.4 Energy storage
We now consider how a flux is established and energy is stored in simple toroidal magnetic
Circuit of cross sectional area A, path length L, and of material of constant permeability u,
The flux is to be established by a current i, in a uniformly wound coil of N-turns. In order
To concentrate on energy storage we neglect the coil resistance. With i initially zero, let a
Voltage V, be applied to the coil terminals, what happen thereafter depends on Faraday’s
Law of electromagnetic induction.