Electrical machines

1. LECTRURE :VICTOR WABWIRE
ELECTRICAL POWER MACHINES
AND DRIVES

2. Electrical machines are a form of converters that transform
energy from mechanical to electrical or vice versa.
The basic operation of these machines are similar, though they
slightly differ when looking at the construction.
We are mostly going to look at the transformers, DC and AC
motors.
It’s the norm to have the power plants located far away from
the point of use.
See the diagram below!

4. P power out
V voltage
I current
J current density
R resistance
Epislon coefficient of resistivity
𝑷𝒍 power loss through the
transmission line L
The equation given shows that
the ratio of power L to Power
out is directly proportional to
the ratio of Length L and the
transmitted Voltage

5. How is transmission voltages achieved
A transformer is one of the most common
devices found in electrical system that links the
circuits which are operating at different voltages
.These are commonly used in applications
where there is a need of AC voltage conversion
from one voltage level to another.
When the primary winding is energized with
alternating voltage source, an alternating
magnetic flux or field will be produced in the
transformer core. This magnetic flux amplitude
depends on the applied voltage magnitude,
frequency of the supply and the number of turns
on the primary side.

6. This flux circulates through the core and hence
links with the secondary winding. Based on the
principle of electromagnetic induction, this
magnetic linking induces a voltage in the
secondary winding. This is called as mutual
induction between two circuits. The secondary
voltage depends on the number of turns on the
secondary as well as magnetic flux and
frequency.
MMF is the magneto motive force, set up when
a current flows through the windings and
consequently induces a current into the
transformer core

7. Bio-Savart’s law
H is the magnetic field intensity
vector while B is the magnetic
flux density vector

9. The structure of a magnetic circuit consists of a regular core and a winding.
Now when this winding is excited there will be flux line inside the core.
Since the relative permeability of the core is normally much higher than air
or free space almost all the flux lines will be confined inside the core. Also
since the total amount of flux crossing the core at
any position of the core is fixed the flux density along this flux line is
constant, and hence the H over this path is constant.

10. So, in a magnetic circuit finding out the value of H and hence the value
of B given by B=μo. μr.H is relatively simple. This of course makes the
assumption that the leakage flux outside the core is almost negligible.
Now this gives you a method of relating the flux with the MMF in I.
The corresponding equivalent electrical circuit is
a voltage source E sending a current
through a resistance R; that is why this kind of a
structure with a core with a winding on it is
referred to as a magnetic circuit because of its
similarity with an electrical circuit. In practice
though this magnetic circuit is somewhat more
complicated than a electrical circuit. In a
electrical circuit the resistance is normally
constant; however, in almost all practical
magnetic
circuits the core material is made up of
ferromagnetic material.

11. This is due to the special constructional feature of this
magnetic material which are made up
of very tiny molecular level magnetic dipoles; however,
almost all ferromagnetic material
exhibits these characteristics which is called the B−H
characteristics, and that B−H
characteristics has a prominent saturation phenomenon
where the relationship between B and
H is nonlinear, and after a critical value of B further
increase in H does not really increase B
to a very great extent. Therefore, for ferromagnetic
material the practical operating point that
realizes the true potential of the material is somewhere
in this junction in this region where
the B−H curves starts bending. This is called the knee
point of operation.
B-H CHARACTERISTICS

12. WHY!!!
Because beyond this point even if you put a large MMF that is H there will not be
much
increase in the magnetic flux density or the total magnetic flux circulated;
obviously, in this
region the relative permeability of the material which is μr= B/μo H does not
remain constant;
therefore, reluctance of the magnetic core which is given by R= L/μo μr A does
not remain
constant for all values of B.

13. A magnetic core need not always be made of
a single material. In many cases a magnetic
core
in addition to the ferromagnetic core
material we will also possibly include an air
gap. Now
the relative permeability of the air gap and
the magnetic core are very different

14. dphi/dt multiplied by the number of turns in the
coil N, and the polarity of the induced voltage
will be such that it will try to oppose the very
cause which it is due
So, by Faraday’s law the induced voltage e will be such that
magnitude |e| and the direction of the polarity of the induced
voltage will be such that it will tend to oppose the very cause
which it is due; what is the
cause that the changing of the flux. Now let us see at t=0, the
flux was 0, and as time
progresses t becomes positive; this flux tries to increase.

15. Faraday’s law states that a current will be
induced in a conductor which is exposed to a
changing magnetic field. Lenz’s law of
electromagnetic induction states that the
direction of this induced current will be such
that the magnetic field created by the
induced current opposes the initial changing
magnetic field which produced it. The
direction of this current flow can be
determined using Fleming’s right-hand rule.