Electric System
INTRODUCTION
Introducing the students to the electrical equipment's used in power plant. The
main electrical equipment's are
(a) Generator and generator cooling
(b) Transformers and their cooling
(c) Bus bars
(d) Exciters
(e) Reactors
(f) Circuit breakers
(g) Switch board
(h) Control room equipment
GENERATORS AND MOTORS
“Energy can neither be created nor be destroyed”. We can only change its forms, using
appropriate energy-conversion processes.
Purpose of electro-mechanical conversion device is to change the form of energy. Here, only rotary systems will be dealt with. When it is converting mechanical input
to electrical output the device is “generating”. With electrical input, when mechanical output is
obtained, the device is motoring.
Some simple aspects of an electrical machine motor / generator have to be notice at this
place.
(a) Electrical machine has a Stator, a Rotor, and an air-gap in between the Stator and Rotor. For a flux path, the magnetic circuit has these three parts in series. In general, magnetic poles are established in Stator and in Rotor.
(b) Magnetic effects of following types can be
categorized:(1) Electromagnetic: Due to currents passed
through wingdings on Stator and / or Rotor, producing certain number of poles on these
members.(2) Permanent Magnets: One side (Stator or
Rotor) can have permanent magnets.(3) Reluctance variation: Surface of Rotor
near the air-gap can be suitably shaped to
have a particular pattern of Reluctance variation so as to control the machine behavior as per requirements.
(c) Basic conditions which must be satisfied by such devices are: (1) Equal number of poles must be created on the two sides. (2) In some cases, reluctance-variation is primarily used for machine-action. The Stator
side must accommodate a winding carrying current for the electromagnetic effect, when
rotor surface is shaped so as to have the desired pattern of reluctance variation. Or,
non-cylindrical rotor cannot have the current -carrying winding for machine action.
(d) Out of stator, rotor and air-gap, maximum energy-storage at any angular position takes place in the air-gap, since its reluctance is highest out of the three members.
(e) Stored energy must depend on rotor-position and the device tends to occupy that angular
position which corresponds to maximum stored energy. If this position varies as a function
of time, the device produces continuous torque.
(f) Ideal output of a motor is a constant unidirectional torque with given currents through its
windings. In some cases, the output torque (as a compromise) is an average value of a
cyclically varying torque.
(g) Where current-switching is done for motor-control, as in modern controllers, instantaneous
effect has to be understood to conclude on any of the points mentioned above.
(h) A device can work either as a generator or as a motor, provided pertinent conditions are
satisfied for the concerned mode of operation.
Energy – Balance
(i) Electrical port (= armature terminals): receiving / delivering electrical energy.
(ii) Mechanical port (= shaft): delivering / receiving mechanical energy.
(iii) Coupling field: Magnetic field or Electric field.
An electrical motor
receives energy at the electrical port and delivers it
at the mechanical port. While an electric generator
receives the energy at the mechanical port and
delivers it at the electrical port. It is also known
that the following losses take place in such systems and are dissipated away as heat:
(i) i
2r losses
in the winding's of the machines,
(ii) friction and wind-age losses,
(iii) core-losses.
These can be either neglected or attached to electrical port, mechanical port and coupling
magnetic field respectively, for simpler analysis. With this, the simple energy balance equation can
be written as:
Change in Electrical Energy = Change in Mechanical Energy + Change in Field-Energy
dWelec = dWmech + dWfld .
MOTOR:
In a motor, we may suppose a conductor or coil to lie in a magnetic field. If current is supplied to
the coil; a mechanical force is produce and due to this force the coil will move. Immediately that
relative movement takes place between coil and
field, however, an e.m.f. is induced, in opposition
to the current.
To maintain the current and the associated
motor action, it is therefore necessary to apply
to the coil, from an external source, a voltage
sufficient to overcome the induced e.m.f. Thus the
motor requires electrical power to produce a
corresponding amount of mechanical power.
Energy – Balance
(i) Electrical port (= armature terminals): receiving / delivering electrical energy.
(ii) Mechanical port (= shaft): delivering / receiving mechanical energy.
(iii) Coupling field: Magnetic field or Electric field.
An electrical motor
receives energy at the electrical port and delivers it
at the mechanical port. While an electric generator
receives the energy at the mechanical port and
delivers it at the electrical port. It is also known
that the following losses take place in such systems and are dissipated away as heat:
(i) i
2r losses
in the winding's of the machines,
(ii) friction and wind-age losses,
(iii) core-losses.
These can be either neglected or attached to electrical port, mechanical port and coupling
magnetic field respectively, for simpler analysis. With this, the simple energy balance equation can
be written as:
Change in Electrical Energy = Change in Mechanical Energy + Change in Field-Energy
dWelec = dWmech + dWfld .
MOTOR:
In a motor, we may suppose a conductor or coil to lie in a magnetic field. If current is supplied to the coil; a mechanical force is produce and due to this force the coil will move. Immediately that relative movement takes place between coil and field, however, an e.m.f. is induced, in opposition to the current. To maintain the current and the associated motor action, it is therefore necessary to apply to the coil, from an external source, a voltage sufficient to overcome the induced e.m.f. Thus the motor requires electrical power to produce a corresponding amount of mechanical power.
Generator
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