Induction generator
From Wikipedia, the free encyclopedia
An induction generator or asynchronous generator is a type of alternating current (AC) electrical
generator that uses the principles of induction motors to produce power. Induction generators
operate by mechanically turning their rotors faster than synchronous speed. A regular AC
asynchronous motor usually can be used as a generator, without any internal modifications.
Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in
reducing high-pressure gas streams to lower pressure, because they can recover energy with
relatively simple controls.
An induction generator usually draws its excitation power from an electrical grid; sometimes,
however, they are self-excited by using phase-correcting capacitors. Because of this, induction
generators cannot usually "black start" a de-energized distribution system.
Contents
[hide]
1Principle of operation
o 1.1Excitation
o 1.2Active power
o 1.3Required capacitance
o 1.4Torque vs. slip
o 1.5Maximum pass-through current
2Grid and stand-alone connections
3Limitations of induction generators[1]
4Use of induction generators
5Example application
6See also
7Notes
8References
9External links
Principle of operation[edit]
An induction generator produces electrical power when its rotor is turned faster than
the synchronous speed. For a typical four-pole motor (two pairs of poles on stator) operating on a
60 Hz electrical grid, the synchronous speed is 1800 rotations per minute (rpm). The same four-pole
motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM. The motor normally
turns slightly slower than the synchronous speed; the difference between synchronous and
operating speed is called "slip" and is usually expressed as per cent of the synchronous speed. For
example, a motor operating at 1450 RPM that has a synchronous speed of 1500 RPM is running at
a slip of +3.3%.
In normal motor operation, the stator flux rotation is faster than the rotor rotation. This causes the
stator flux to induce rotor currents, which create a rotor flux with magnetic polarity opposite to stator.
In this way, the rotor is dragged along behind stator flux, with the currents in the rotor induced at the
slip frequency.
In generator operation, a prime mover (turbine or engine) drives the rotor above the synchronous
speed (negative slip). The stator flux still induces currents in the rotor, but since the opposing rotor
flux is now cutting the stator coils, an active current is produced in stator coils and the motor now
operates as a generator, sending power back to the electrical grid.
Excitation[edit]
Equivalent circuit of induction generator
An induction machine requires externally supplied armature current; it cannot start on its own as a
generator. Because the rotor field always lags behind the stator field, the induction machine always
"consumes" reactive power, regardless of whether it is operating as a generator or a motor.
A source of excitation current for magnetizing flux (reactive power) for the stator is still required, to
induce rotor current. This can be supplied from the electrical grid or, once it starts producing power,
from the generator itself.
Active power[edit]
Active power delivered to the line is proportional to slip above the synchronous speed. Full rated
power of the generator is reached at very small slip values (motor dependent, typically 3%). At
synchronous speed of 1800 rpm, generator will produce no power. When the driving speed is
increased to 1860 rpm (typical example), full output power is produced. If the prime mover is unable
to produce enough power to fully drive the generator, speed will remain somewhere between 1800
and 1860 rpm range.
Required capacitance[edit]
A capacitor bank must supply reactive power to the motor when used in stand-alone mode. The
reactive power supplied should be equal or greater than the reactive power that the machine
normally draws when operating as a motor. Terminal voltage will increase with capacitance, but is
limited by iron saturation.
Torque vs. slip[edit]
The basic fundamental of induction generators is the conversion between mechanical energy to
electrical energy. This requires an external torque applied to the rotor to turn it faster than the
synchronous speed. However, indefinitely increasing torque doesn't lead to an indefinite increase in
power generation. The rotating magnetic field torque excited from the armature works to counter the
motion of the rotor and prevent over speed because of induced motion in the opposite direction. As
the speed of the motor increases the counter torque reaches a max value of torque (breakdown
torque) that it can operate until before the operating conditions become unstable. Ideally, induction
generators work best in the stable region between the no-load condition and maximum torque
region.
Maximum pass-through current[edit]
In practice and without taking this notion into account, many users unsuccessfully apply the
principles to the actual deployment.
It's not in popular belief; that in almost every case, under the same active grid voltage, the power
that the generator produces is greater than the power it consumes when it is at the motor,fully
loaded state; its rated power. Sometimes the differences are in multiple folds. Higher the power
means higher the amperage.
For prolong operation, and implied in its guaranteed, each motor has its “maximum pass-through
current”. This amperage value; the current density; is derived from the maximum pass-through
current property of the internal copper magnet wire and the combined configuration of their
connections. Without opening up the unit to examine the internal setting of the copper wires, a
division of the wattage of its rated power by its rated voltage can give users some senses of how
much that value is.
Therefore, claims of making a unit generates more power than its rated should get a closer
examination.
Grid and stand-alone connections[edit]
Typical connections when used as a standalone generator
In induction generators, the reactive power required to establish the air gap magnetic flux is provided
by a capacitor bank connected to the machine in case of stand-alone system and in case of grid
connection it draws reactive power from the grid to maintain its air gap flux. For a grid-connected
system, frequency and voltage at the machine will be dictated by the electric grid, since it is very
small compared to the whole system. For stand-alone systems, frequency and voltage are complex
function of machine parameters, capacitance used for excitation, and load value and type.
Limitations of induction generators[1][edit]
An Induction generator cannot generate reactive voltamperes.Actually it requires voltamperes from
supply line to furnish its excitation,since it has no means for establishing air gap flux with stator open
circuited.Operation of induction generator requires synchronous machine,whether generator or
motor,on the line to supply induction generator with its needed reactive voltamperes.It is limitation of
reactive voltamperes requirements which limits the use of induction generator to few rather than
unusual applications.
Use of induction generators[edit]
Induction generators are often used in wind turbines and some micro hydro installations due to their
ability to produce useful power at varying rotor speeds. Induction generators are mechanically and
electrically simpler than other generator types. They are also more rugged, requiring no brushes
or commutators.
Induction generators are particularly suitable for wind generating stations as in this case speed is
always a variable factor. Unlike synchronous motors, induction generators are load-dependent and
cannot be used alone for grid frequency control.
Example application[edit]
As an example, consider the use of a 10 hp, 1760 r/min, 440 V, three-phase induction motor as an
asynchronous generator. The full-load current of the motor is 10 A and the full-load power factor is
0.8.
Required capacitance per phase if capacitors are connected in delta:
Apparent power S = √3 E I = 1.73 × 440 × 10 = 7612 VA
Active power P = S cos θ = 7612 × 0.8 = 6090 W
Reactive power Q = = 4567 VAR
For a machine to run as an asynchronous generator, capacitor bank must supply
minimum 4567 / 3 phases = 1523 VAR per phase. Voltage per capacitor is 440 V
because capacitors are connected in delta.
Capacitive current Ic = Q/E = 1523/440 = 3.46 A
Capacitive reactance per phase Xc = E/Ic = 127 Ω
Minimum capacitance per phase:
C = 1 / (2*π*f*Xc) = 1 / (2 * 3.141 * 60 * 127) = 21 microfarads.
If the load also absorbs reactive power, capacitor bank must be increased
in size to compensate.
Prime mover speed should be used to generate frequency of 60 Hz:
Typically, slip should be similar to full-load value when machine is running
as motor, but negative (generator operation):
if Ns = 1800, one can choose N=Ns+40 rpm
Required prime mover speed N = 1800 + 40 = 1840 rpm.
