comparative performance study of a self -excited three phase ...the prime mover is excited and th e...

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


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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


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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.


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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.


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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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comparative performance study of a self -excited three phase ...the prime mover is excited and th e...

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