INDUCTION GENERATOR
CALCULATIONS
(1) Self-excited Induction generator
Capacitance required to obtain;
1) Rated voltage of 240V at the given speed.
C = 71.5μF
2) Rated voltage of 240V at the rated frequency of 50 Hz.
| Ic| = | VCω |
Here as | Ic| = | Im |,
when V =240V, f = 50 Hz from the graph we can obtain I m = 2.4 A
Also ω = 2πf =2π*50 =314.159 rads-1
2. 4
Then, C = 240∗314 .159
= 31.83 μF
(2) Grid Connected Induction Generator
Efficiency
Input Power = Ґ*ω
Output Power ×100 %
Efficiency = InputPower
Sample Calculation
ω = 3090 rpm Wattmeter Reading = 0.56 kW Ґ= 3.3Nm
3090 * 2* π
Input Power = 3.3 Nm * ( 60 ) rad/s
= 1067.626 W
560×100 %
Efficiency = 1067 .626
= 52.45%
Power Factor
Re alPower
Power Factor = ApperantPower
Apparent Power = VI
Sample Calculation
V = 230V I =2.1A Reading = 0.56 kW
560
Power Factor = 230∗2. 1
= 1.16
Speed (rpm) Efficiency Power Factor
3016 0 0
3045 38.19 0.76
3070 47.70 1.11
3090 52.45 1.16
1. Self-excited Induction generator
Part (a) – No Load Characteristics for varying capacitance and constant prime mover speed
Line Voltage vs. Magnetizing Current
Mag. Line
Current (A) Voltage (V)
0 1.6
0 1.6
3.2 280
3.5 290
Line Voltage vs. Capacitance
Capacitance (µF) Line Voltage(V)
0 1.6
50 1.6
75 280
80 290
Part (b) – No Load Characteristics for varying prime mover speed and constant capacitance
Voltage vs. Speed
Speed (rpm) Voltage (V)
2224 210
2260 220
2290 230
2326 240
2360 250
2404 260
Frequency vs. Speed
Speed (rpm) Frequency (Hz)
2224 36
2260 36
2290 37
2326 37
2360 38
2404 39
Magnetizing Current vs. Speed
Speed (rpm) Mag. Current (A)
2224 2.1
2260 2.3
2290 2.4
2326 2.5
2360 2.7
2404 2.9
Part (c) – Performance of the loaded generator with constant speed
Voltage vs. Load Current
Load Current (A) Voltage (V)
0 285
0.5 275
1 270
1.4 260
1.8 250
Frequency vs. Load Current
Load Current (A) Frequency (Hz)
0 40
0.5 40
1 40
1.4 40
1.8 40
Generator Current vs. Load Current
Load Current (A) Generator Current (A)
0 3.3
0.5 3.2
1 3.1
1.4 3.2
1.8 3.3
Part (d) – Performance of the loaded generator without speed regulation
Voltage vs. Load Current
Load Current (A) Voltage (V)
0 285
0.5 270
0.9 255
1.3 240
1.6 224
Frequency vs. Load Current
Part (c) - Loaded Generator with constant Part (d) - Loaded Generator without
speed speed control
Load Current(A) Frequency (Hz) Load Current (A) Frequency (Hz)
0 40 0 40
0.5 40 0.5 40
1 40 0.9 40
1.4 40 1.3 38
1.8 40 1.6 38
Frequency (Hz)
10
15
20
25
30
35
40
45
0
5
0
Load
0.5
Frequency vs Load Current
2. Grid Connected Induction Generator
Power Output (kW)
Power Output vs. Speed
0.28
0.46
0.56
0
Speed (rpm)
3016
3045
3070
3090
1 1.4 1.8
Loaded Generator with Constant Speed
urrent (A)
Loaded Generator without Speed Regulation
Line Current vs. Speed
Speed (rpm) Line Current (A)
3016 1.5
3045 1.6
3070 1.8
3090 2.1
Efficiency vs. Speed
Speed (rpm) Efficiency
3016 0
3045 38.19
3070 47.70
3090 52.45
Power Factor vs. Speed
Speed (rpm) Power Factor
3016 0
3045 0.76
3070 1.11
3090 1.16
DISCUSSION
An induction generator is a type of electrical generator that is mechanically and electrically
similar to a polyphase induction motor. Induction generators produce electrical power when their
shaft is rotated faster than the synchronous frequency of the equivalent induction motor. 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 not self-exciting, meaning they require an external supply to
produce a rotating magnetic flux. The external supply can be supplied from the electrical grid or
from the generator itself, once it starts producing power. The rotating magnetic flux from the
stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower
than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned
faster, it acts like a generator, producing power at the synchronous frequency.
In induction generators magnetising flux is established by a capacitor bank connected to the
machine in case of standalone system and in case of grid connection it draws magnetising current
from the grid. It is mostly suitable for wind generating stations as in this case speed is always a
variable factor.
Slip of an induction motor is positive and lies between zero and unity, whereas when
considering an induction generator, slip of the generator is a negative value. That is, rotor speed
should be greater than synchronous speed. i.e. Ns−Nr <0
Slip of the generator should have a zero or a positive value in the no load test since there is
no active power output from the generator. However,
60f
Ns = p ;Where f = 50 Hz
Therefore Frequency f is required to be lower than 50Hz for Nr to be lower than Ns. Thus no
load test is designed to result in lower frequency than the rated frequency.
We observed a slight drop in speed when loading the induction motor which was due to
decrease in Nr. When loading the induction generator it must produce more power, which requires
changes in Nr. This causes variations in voltage and current waveforms. Generally since induction
generators cannot produce reactive power, when reactive loads are loaded changes in voltages can
be seen.
Generally Induction generators are more suitable for low power applications. This type of
generators usually are compact in size, simple, rugged, and relatively cheap, making it an ideal
candidate for grid connected mini hydro power stations in Sri Lanka. Also grid connected Induction
generators are suitable for wind power plants. Furthermore, Induction generators have the added
advantages of robust operation, less requisite of maintenance, light weight and reliable service.
Thus induction generators are mush suitable small scale power generations in Sri Lanka.
Graphs we plotted for both Self-excited and Grid connected induction generators illustrate
the following different characteristics of induction generators.
Self-excited Generator:
Part (a) – No Load Characteristics for varying capacitance and constant prime mover speed
Line Voltage Vs. Magnetizing Current
Increase in Line voltage can be seen with the increase in Magnetizing current.
Line Voltage Vs. Capacitance
Until a certain value of capacitance is reached, the Line voltage is negligible. After that the line
voltage is increasing with the capacitance.
Part (b) – No Load Characteristics for varying prime mover speed and constant capacitance
Voltage Vs. Speed
Line voltage increases linearly with increase in Speed.
Frequency Vs. Speed
An approximate linear relationship can be seen between frequency and Speed. Increase in Speed
results in increase in frequency.
Magnetizing Current V s. Speed
Linearly increasing relationship can be observed.
Part (c) – Performance of the loaded generator with constant speed
Voltage vs Load Current
A drop in Voltage can be seen with increase in load current.
Frequency Vs Load Current .
Frequency remains constant with the increase in Load current.
Generator Current Vs Load Current
We can observe with the change in Load current, approximately same level of generator current.
Part (d) – Performance of the loaded generator without speed regulation
Voltage Vs Load Current
When load current is increased a drop in voltage is observed.
Frequency Vs Load Current .
When we increase the load current slight drop in frequency is observed.
Grid connected induction generator:
Power Output vs. Speed
According to this graph we can see that the power output is increasing with the increase in speed.
Line Current vs. Speed
Line current also gradually increases with the speed.
Efficiency vs. Speed
According to this graph the power factor increases rapidly with speed at first and later on the
increase is gradual.
Power Factor vs. Speed
Power factor vs. Speed graph looks similar to the above Efficiency vs. Speed graph, with a rapid
increase at first and later with a more of a gradual increase.
