07-08-2012, 12:47 PM
Generator & Exciter Basics
[attachment=30351]
THE ELEMENTARY AC GENERATOR
The elementary ac generator consists of a conductor (or
loop of wire) in a magnetic field (usually produced by an
electromagnet). The two ends of the loop are connected
to slip rings and they are in contact with two brushes.
When the loop rotates it cuts magnetic lines of force,
first in one direction and then the other. In the first half
turn of rotation, a positive current is produced and in
the second half of rotation produces a negative current.
This completes one cycle of ac generation.
DEVELOPMENT OF THE SINE WAVE
At the instant the loop is in the vertical position, the
loop sides are moving parallel to the field and do not cut
magnetic lines of force. In this instant, there is
no voltage induced in the loop. As the loop
rotates, sides will cut the magnetic lines of
force inducing voltage in the loop. When the loop
is in the horizontal position, maximum voltage is
induced. The rotation of the coil through 360
degrees results in an ac sine wave output
THREE PHASE VOLTAGE
Three phase voltage is developed using the same
principles as the development of single phase voltage.
Three (3) coils are required positioned 120 electrical
degrees apart. A rotating magnetic field induces
voltage in the coils which when aggregated
produce the familiar three phase voltage pattern.
INDUCTION GENERATORS
The induction generator is nothing more than an
induction motor driven above its synchronous speed by
an amount not exceeding the full load slip the unit
would have as a motor.
Assuming a full load slip of 3%, a motor with a
synchronous speed of 1200 rpm would have a full load
speed of 1164 rpm. This unit could also be driven by an
external prime mover at 1236 rpm for use as an
induction generator.
The induction generator requires one additional item
before it can produce power – it requires a source of
leading VAR’s for excitation. The VAR’s may be supplied
by capacitors (this requires complex control) or from
the utility grid.
Induction generators are inexpensive and simple
machines, however, they offer little control over their
output. The induction generator requires no separate
DC excitation, regulator controls, frequency control
or governor.
SYNCHRONOUS GENERATORS
Synchronous generators are used because they offer
precise control of voltage, frequency, VARs and WATTs.
This control is achieved through the use of voltage
regulators and governors.
A synchronous machine consists of a stationary
armature winding (stator) with many wires connected in
series or parallel to obtain the desired terminal voltage.
The armature winding is placed into a slotted laminated
steel core. A synchronous machine also consists of a
revolving DC field - the rotor.
A mutual flux developed across the air gap between the
rotor and stator causes the interaction necessary to
produce an EMF. As the magnetic flux developed by the DC
field poles crosses the air gap of the stator windings, a
sinusoidal voltage is developed at the generator output
terminals.
[attachment=30351]
THE ELEMENTARY AC GENERATOR
The elementary ac generator consists of a conductor (or
loop of wire) in a magnetic field (usually produced by an
electromagnet). The two ends of the loop are connected
to slip rings and they are in contact with two brushes.
When the loop rotates it cuts magnetic lines of force,
first in one direction and then the other. In the first half
turn of rotation, a positive current is produced and in
the second half of rotation produces a negative current.
This completes one cycle of ac generation.
DEVELOPMENT OF THE SINE WAVE
At the instant the loop is in the vertical position, the
loop sides are moving parallel to the field and do not cut
magnetic lines of force. In this instant, there is
no voltage induced in the loop. As the loop
rotates, sides will cut the magnetic lines of
force inducing voltage in the loop. When the loop
is in the horizontal position, maximum voltage is
induced. The rotation of the coil through 360
degrees results in an ac sine wave output
THREE PHASE VOLTAGE
Three phase voltage is developed using the same
principles as the development of single phase voltage.
Three (3) coils are required positioned 120 electrical
degrees apart. A rotating magnetic field induces
voltage in the coils which when aggregated
produce the familiar three phase voltage pattern.
INDUCTION GENERATORS
The induction generator is nothing more than an
induction motor driven above its synchronous speed by
an amount not exceeding the full load slip the unit
would have as a motor.
Assuming a full load slip of 3%, a motor with a
synchronous speed of 1200 rpm would have a full load
speed of 1164 rpm. This unit could also be driven by an
external prime mover at 1236 rpm for use as an
induction generator.
The induction generator requires one additional item
before it can produce power – it requires a source of
leading VAR’s for excitation. The VAR’s may be supplied
by capacitors (this requires complex control) or from
the utility grid.
Induction generators are inexpensive and simple
machines, however, they offer little control over their
output. The induction generator requires no separate
DC excitation, regulator controls, frequency control
or governor.
SYNCHRONOUS GENERATORS
Synchronous generators are used because they offer
precise control of voltage, frequency, VARs and WATTs.
This control is achieved through the use of voltage
regulators and governors.
A synchronous machine consists of a stationary
armature winding (stator) with many wires connected in
series or parallel to obtain the desired terminal voltage.
The armature winding is placed into a slotted laminated
steel core. A synchronous machine also consists of a
revolving DC field - the rotor.
A mutual flux developed across the air gap between the
rotor and stator causes the interaction necessary to
produce an EMF. As the magnetic flux developed by the DC
field poles crosses the air gap of the stator windings, a
sinusoidal voltage is developed at the generator output
terminals.