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Comparative Performance study of a
self-excited three phase induction
generator with a three phase
synchronous generator
Geno Peter, Samat bin Iderus
University College of Technology Sarawak, Malaysia
[email protected], [email protected]
Abstract
An induction generator works on the principle of induction to
produce power. Induction generators or asynchronous generator
operates by turning the rotors mechanically much faster than
its synchronous speed. A normal Induction motor can be used as
a generator, without any internal modifications. This paper
presents a comparative performance study of a self-excited three
phase induction generator and a synchronous generator. The
rotor of induction generator was driven by using an induction
motor, which acts as prime mover. Three phase delta connected
capacitors was connected along the stator winding of the
induction generator. The output voltage, output current, speed
and frequency was recorded under no load and load conditions.
Similarly the rotor of the synchronous generator was driven by
using an induction motor, which acts as prime mover. The field
winding was excited using DC supply. The output voltage,
output current, speed and frequency was recorded under no load
and load conditions. A comparative study was made between
both the generators.
Index Terms— induction generator ∙synchronous generator ∙
capacitors ∙motor∙ prime mover
1. INTRODUCTION
International Journal of Pure and Applied MathematicsVolume 118 No. 20 2018, 2511-2522ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu
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Squirrel cage induction motors is used to generate power from different
renewable energy sources like wind, small scale hydro plants, biomass and
biogas. The three phase AC generators are classified into two types as
induction generator and synchronous generator. Synchronous generator have a
good voltage characteristics compared to induction generator. Induction
generator has its own advantage like maintenance free operation as no carbon
brushes is present, simple in design and inexpensive. In the case of brushless
permanent magnet synchronous generator, the excitation field is created using
the permanent magnet in the rotor. The major disadvantage of these types of
generator is the field flux cannot be controlled, the cost of the magnet and an
accidental increase in speed will make the permanent magnet to lose its
property ie., magnetism. In this paper, the performance of the induction
generator and synchronous generator is studied. The no load characteristics
and the load characteristics of both the generators are investigated by means
of simple analysis and experiments.
2. CONNECTION DIAGRAM OF A 3 PHASE INDUCTION GENERATOR
The Figure 1. , shows a three phase Induction motor being used as prime
mover.
Figure 1. Connection diagram of 3 phase Induction Generator
A three phase induction generator of rating 1.3 HP, 415V, 2.3A, 2880 rpm is
used for experimental purpose. The prime mover is of rating 1.3 HP, 415V,
2.3A, 2880 rpm. The rotor of the induction generator and the rotor of the prime
mover is coupled together. Delta connected capacitors, is being connected
across the stator windings of the induction generator to provide excitation [1].
3. DESIGN OF DELTA CONNECTED CAPACITORS
The rating of the induction generator is three phase , 1.3 HP, 2880 rpm, 415 V,
the full-load current of the motor is 2.3 A and the full-load power factor is 0.8.
Required capacitance per phase if capacitors are connected in delta:
International Journal of Pure and Applied Mathematics Special Issue
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Apparent power S = √3 E I = 1.73 × 415 × 2.3 = 1653 VA
Active power P = S cos θ = 1653 × 0.8 = 1323 W
Reactive power Q = 990 VAR
For an induction motor to run as an asynchronous generator or induction
generator, the capacitor bank connected must supply minimum of 990 / 3
phases = 330 VAR per phase. Voltage per capacitor is 415 V because capacitors
are connected in delta [2].
Capacitive current Ic = Q/E = 330/415 = 0.8 A
Capacitive reactance per phase Xc = E/Ic = 518 Ω
Minimum capacitance per phase:
C= 1 / (2 x 3.141 x 50 x 518) = 7 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
50 Hz:
4. EXPERIMENTAL SETUP ON AN INDUCTION GENERATOR
Figure 2. No load /Load Test on Induction Generator
The induction generator is first tested on no load and then on load. As seen in
the above figure, the induction motor is used as a prime mover, and delta
connected capacitors are connected across the stator winding of the induction
generator. The prime mover is excited and the voltage is increased steadily [4].
The output voltage generated, the magnetizing current (no load current), the
load current, the speed and the frequency was measured and plotted as a
graph.
5. RESULTS OF NO LOAD TEST ON INDUCTION GENERATOR
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For convenience, the graphical representation for ‗B‘ Phase is shown.
Figure 3. No Load test on Induction Generator - Output voltage vs No load
Current
It is seen from Figure 3, that the induction generator output voltage becomes
constant once it‘s driven above its synchronous speed. The average no load
current measured at rated voltage is approximately 1 Ampere. [3] The
induction generator operates at low power factor because of the presences of
the air gap between the stator and the rotor.
Figure 4. No Load test on Induction Generator – Frequency vs Speed
The speed of the prime mover was increased slowly until the synchronous
speed is achieved for the induction generator. It is seen from Figure 4, the
frequency is approximately constant with a small variation around the 50 Hz
value (about 0.45 Hz at 0.350 KW).
6. LOAD TEST ON INDUCTION GENERATOR
International Journal of Pure and Applied Mathematics Special Issue
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The induction generator was loaded up to the rated current of 2.3 Amperes
using a motor load gradually and the respective voltage and load current was
plotted as a graph. From Figure 5, it is confirmed that the voltage regulation is
good, due to the effect of the delta connected capacitors.
From the graph it is seen, the induction generator was able to supply a load
current of 2.3 amperes on ‗R‘ phase beyond which the core gets saturated,
hence the voltage and current drops instantly towards zero [5].
Figure 5. Load Test on Induction Generator Voltage vs Current
The same phenomena happens when the load current reaches 1.85 amperes on
‗Y‘ phase and 2.2 amperes on ‗B‘ phase. The above is derived from the below
table 1.
Table 1. Load test results on an Induction Generator
Phase
Generated
Voltage
Load
Current
Frequency Power
Factor
R 415 2.3 49.8 0.82
Y 414 1.85 50.2 0.78
B 416 2.2 51.1 0.83
International Journal of Pure and Applied Mathematics Special Issue
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Figure 6. Load Test on Induction Generator Speed vs Frequency
The induction generator is driven above its synchronous speed. When
voltage is applied to the stator of an induction motor, it sets up a rotating
magnetic field which in turn induces current in the rotor which is proportional
to the slip speed frequency. This generates a corresponding field that pulls the
rotor with the rotating field [6].
When the shaft is driven, the slip frequency is reduced and becomes zero at
synchronous speed thus reducing the induced rotor current to zero, hence
there is no net torque. If the rotor is driven above the synchronous speed, slip
is seen in the opposite direction, generating rotor current which creates a
rotating magnetic field that pushes on the stator field rather than being pulled
by it [12]. This creates a stator voltage the pushes current out of the stator
windings against the applied voltage.
7. EXPERIMENTAL SETUP ON SYNCHRONOUS GENERATOR
The synchronous generator was tested first on no load, then on load. The
rating of the synchronous generator used is 1200VA, 415V, 2.3A, 50Hz, 2880
rpm with rotor excitation 220V. As seen in the above figure, the induction
motor is used as a prime mover. The prime mover is excited and the voltage is
increased steadily.
International Journal of Pure and Applied Mathematics Special Issue
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Figure 7. No load /Load Test on Synchronous Generator
The rotor winding of the synchronous generator is also excited with DC
voltage. The output voltage generated, the magnetizing current (no load
current), the load current, the speed and the frequency was measured,
tabulated and plotted as a graph
8. RESULTS OF NO LOAD TEST ON SYNCHRONOUS
GENERATOR
For convenience, the graphical representation for ‗B‘ Phase is shown
Figure 8. Output Voltage vs Frequency – Synchronous Generator
The output voltage becomes constant once the generator moves into the
synchronous speed. The average magnetizing current was found to be 0.95
Ampere.
International Journal of Pure and Applied Mathematics Special Issue
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Figure 9. Frequency vs Speed – Synchronous Generator
The output frequency becomes constant once the generator moves into the
synchronous speed. The frequency of the generated voltage depends upon the
number of field poles and the speed at which the field poles are rotated. One
complete cycle of voltage is generated in an armature coil when a pair of field
poles passes over the coil [7].
At no-load, the mechanical system is rotating at the no-load speed, and results
in the generation of voltages at no load frequency [11]. The speed and
frequency are related by the equation for synchronous speed:
Where Ns is the synchronous speed of the generator
F is the frequency
P is the number of poles
When the generator is loaded, power is drawn from the mechanical system and
the generator applies a torque which opposes the direction of motion of the
mechanical system. As a result, the generator tends to slow down the
mechanical system [8].
9. LOAD TEST ON SYNCHRONOUS GENERATOR
The synchronous generator was loaded up to the rated current of 2.3A and the
output voltage, frequency and speed was noted. A graph is plotted between the
output voltage vs load current and frequency vs speed.
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Figure 10. Load Test on Synchronous Generator Voltage vs Current
Table 2. Load test results on a Synchronous Generator
Phase
Generated
Voltage
Load
Current
Frequency Power
Factor
R 417 2.3 51.2 0.87
Y 415 2.0 50.0 0.83
B 419 2.35 50.9 0.89
Figure 11. Load Test on Synchronous Generator Speed vs Frequency
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The output frequency becomes constant once the synchronous generator moves
into the synchronous speed [9].
10. SYNCHRONOUS GENERATORS IN MARINE INDUSTRY
Synchronous generator is said to be the heart of the marine power station in a
ship. The synchronous generator used is of a separately excited type, hence
requires Dc power for excitation, which in turn is drawn from the batteries.
Based on the physical structure and load on the ship the power requirement
varies. Hence the related batteries requirement also increases [10]. There are
certain drawbacks like the battery consuming more space, regular
maintenance of the batteries and the generator, replacement of carbon brushes
at the right time and finally the cost of the machine.
11. CONCLUSION
The air gap voltage of induction generator is normally 100 to 105 percentage of
the terminal voltage. The induction generator worked satisfactorily when is
loaded up to the current of 2.2 A. when the induction generator is loaded above
2.2 A current, it was seen the voltage drops down towards zero. It is seen the
core of the induction motor gets saturated making the induction generator
useless when loaded above the rated current. In a three phase Induction
generator the torque is directly proportional to the square of the supply
voltage. Hence it is recommended to use an Induction generator at 80% of its
rated current, as it is maintenance free, less cost, and simple in design
(Mechanically and electrically strong).
ACKNOWLEDGMENT
This research was funded by UCTS Research Grant
(UCTS/RESEARCH/1/2017/01) of University College of Technology Sarawak,
Malaysia.
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