MOTORING AND GENERATING MODE OF ... MACHINE - A COMPARATIVE EVALUATION ...

MOTORING  AND  GENERATING  MODE  OF ... MACHINE  -  A  COMPARATIVE  EVALUATION ...
MOTORING
MACHINE
AND
-
A
GENERATING
COMPARATIVE
MODE
OF
3-Φ
EVALUATION
EFFICIENCY
SMRUTI RANJAN BEHERA
SAROJ PRASAD
Department of Electrical Engineering
National Institute of Technology Rourkela
INDUCTION
FOR
ENERGY
MOTORING AND GENERATING MODE OF 3-Φ INDUCTION
MACHINE - A COMPARATIVE EVALUATION FOR ENERGY
EFFICIENCY
A Thesis submitted in partial fulfillment of the requirements for the degree of
Bachelor of Technology in “Electrical Engineering”
By
SMRUTI RANJAN BEHERA
Roll No- 107EE047
SAROJ PRASAD
Roll No-107EE044
Department of Electrical Engineering
National Institute of Technology
Rourkela Orissa
May-2011
MOTORING AND GENERATING MODE OF 3-Φ INDUCTION
MACHINE - A COMPARATIVE EVALUATION FOR ENERGY
EFFICIENCY
A Thesis submitted in partial fulfillment of the requirements for the degree of
Bachelor of Technology in “Electrical Engineering”
By
SMRUTI RANJAN BEHERA
SAROJ PRASAD
Under guidance of
Prof. B.CHITTI BABU
Department of Electrical Engineering
National Institute of Technology
Rourkela Orissa
May-2011
DEPARTMENT OF ELECTRICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
ORISSA, INDIA-769008
CERTIFICATE
This is to certify that the thesis entitled “Motoring and Generating mode of 3-Φ Induction
Machine – A Comparative Evaluation For Energy Efficiency”, submitted by Mr. Smruti
Ranjan Behera (Roll no. 107EE047) and Mr. Saroj Prasad (Roll no. 107EE044) in partial
fulfillment of the requirements for the award of Bachelor of Technology in Electrical
Engineering during session 2010-2011 at National Institute of Technology, Rourkela. A
bonafide record of research work carried out by him under my supervision and guidance.
The candidates have fulfilled all the prescribed requirements.
The Thesis which is based on candidates own work, has not submitted elsewhere for a
degree/diploma.
In my opinion, the thesis is of standard required for the award of a bachelor of technology degree
in Electrical Engineering.
Place: Rourkela
Prof. B.Chitti Babu
Assistant Professor
Dept. of Electrical Engineering
National institute of Technology
Rourkela-769008
ACKNOWLEDGEMENTS
On the submission of my Thesis report of “Motoring and Generating Mode of 3-Φ
Induction machine – A Comparative Evaluation For Energy Efficiency”, we would like
to extend our gratitude & our sincere thanks to our supervisor Prof. B.Chitti Babu, Asst.
Professor, Department of Electrical Engineering for his constant motivation and support
during the course of our work in the last one year. We truly appreciate and value his esteemed
guidance and encouragement from the beginning to the end of this thesis. His knowledge and
company at the time of crisis would be remembered lifelong.
SMRUTI RANJAN BEHERA
Roll no – 107EE047
SAROJ PRASAD
Roll no – 107EE044
Electrical Engineering
ABSTRACT
Vast use of fossil fuels is leading to energy deficiency. Hence the renewable energy sources
like wind energy and solar energy are being used. Classical asynchronous induction
generators are being used in wind energy based power generation system. In markets of
micro electric energy generation unit system, induction generators are getting popularized, as
it is cheap, robust and maintenance free. But generally induction machines are used as motor,
hence the catalogues have only information about motoring mode. This report gives a
comparative analysis between motoring and generating mode of induction machines. From
this report we conclude that the induction motors have more efficiency and less losses than
induction generators.
i
CONTENTS
Abstract
i
Contents
ii
List of symbols
v
List of figures
vi
List of tables
vii
CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION
2
1.2 MOTIVATION
3
1.3 ELECTRICAL MACHINES- AN OVER VIEW
3
1.4 THREE PHASE INDUCTION MACHINE
9
1.5 THESIS OBJECTIVES
13
1.6 ORGANIZATION OF THESIS
14
CHAPTER 2
INDUCTION MACHINE & ITS ENERGY EFFICIENCY
2.1 INTRODUCTION
16
2.2 ENERGY EFFICIENCY
16
2.3 CONCLUSION
20
ii
CHAPTER 3
ANALYSIS OF INDUCTION MACHINE DURING MOTORING MODE
3.1 INTRODUCTION
22
3.2 DC TEST
22
3.3 NO LOAD TEST
23
3.4 BLOCKED ROTOR TEST
24
3.5 PERFORMANCE ANALYSIS
26
3.6 POWER FLOW DIAGRAM
28
3.7 EFFICIENCY CALCULATION
29
CHAPTER 4
ANALYSIS OF INDUCTION MACHINE DURING GENERATING MODE
4.1 INTRODUCTION
31
4.2 MAGNETIZATION CHARACTERISTICS
31
4.3 VOLTAGE BUILD UP PROCESS
32
4.4 POWER FLOW DIAGRAM
34
4.5 EFFICIENCY CALCULATION
36
4.6 CONCLUSION
36
iii
CHAPTER 5
EXPERIMENTAL ANALYSIS AND COMPARATIVE STUDY
5.1 INTRODUCTION
37
5.2 MEASUREMENT SETUP AND EXPLANATION
37
5.3 EXPERIMENTAL RESULTS AND COMPARATIVE STUDY
40
5.4 CONCLUSION
46
CHAPTER 6
CONCLUSION AND FUTURE WORK
CONCLUSION AND FUTURE WORK
47
REFERENCES
49
APPENDIX-I
51
iv
List of Symbols
Pel = electrical power
Pc = PFe = Core loss
Pg = air gap power
Pm = mechanical power developed at rotor
Pcu = copper loss
Pstator = stator loss
Protor = Rotor loss
Psh = shaft power
r1 = stator resistance
r2 = rotor resistance
Ƞ = efficiency
Vnl = no load voltage
Inl = no load current
Pnl = no load power
Vbr = blocked rotor voltage
Ibr = blocked rotor current
Pbr = blocked rotor power
v
List of Figures
Figure
Page no
1.1 Constructional view of dc motor---------------------------------------------------------- 4
1.2 Shunt and Series wound dc motor------------------------------------------------------------ 5
1.3 AC motor------------------------------------------------------------------------------------------------------ 6
1.4 BLDC motor--------------------------------------------------------------------------------- 7
1.5 SR motor-------------------------------------------------------------------------------------- 8
1.6 Squirrel cage induction machine---------------------------------------------------------- 10
1.7 Grid connected Induction generator------------------------------------------------------- 12
1.8 Self-excited Induction generator----------------------------------------------------------- 12
2.1 Equivalent circuit of induction machine--------------------------------------------------- 17
2.2 Power-slip characteristics of induction machine----------------------------------------- 18
3.1 DC load test on 3-Φ induction motor------------------------------------------------------ 22
3.2 No load test of 3-Φ induction motor------------------------------------------------------- 23
3.3 Equivalent circuit of 3-Φ induction motor for No load test----------------------------- 24
3.4 Blocked rotor test on 3-Φ induction motor------------------------------------------------ 24
3.5 Equivalent circuit of 3-Φ induction motor for blocked rotor test---------------------- 25
3.6 Operating characteristics of an induction motor------------------------------------------ 26
3.7: Power flow diagram of a 3-Φ induction motor------------------------------------------- 28
4.1 Magnetizing characteristics of induction generator--------------------------------------- 31
4.2 Voltage build-up process in 3-Φ induction generator------------------------------------- 32
4.3 Effect of capacitance on voltage build up process in 3-Φ induction generator-------- 33
4.4 Power flow diagram of super-synchronous induction generator------------------------- 35
4.5 Power flow diagram of sub-synchronous induction generator--------------------------- 35
5.1 Experimental setup for 3-Φ induction machine testing----------------------------------- 38
vi
5.2 Three Φ Induction Machine coupled with DC motor------------------------------------- 39
5.3 Experimental setup of induction machine-------------------------------------------------- 39
5.4 Iµ vs Stator voltage for a 3-Φ induction machine------------------------------------------ 42
5.5 No load characteristics of 3-Φ induction machine---------------------------------------- 42
5.6 Power factor characteristics of a 3-Φ induction machine--------------------------------- 43
5.7 Stator losses vs output power of a 3-Φ induction machine------------------------------- 43
5.8 Rotor losses vs output power of a 3-Φ induction machine------------------------------- 44
5.9: Efficiency vs output power of a 3-Φ induction machine--------------------------------- 44
5.10 Efficiency vs Stator current of a 3-Φ induction machine-------------------------------- 45
LIST OF TABLES:
TABLE 1: No Load Test of Induction Motor
TABLE 2: Load test on Induction Generator
TABLE 3: No Load Test on Induction Generator
TABLE 4: Load Test on Induction Motor
vii
CHAPTER
1
Introduction
1
1.1 INTRODUCTION:
The conventional energy sources are limited and have pollution to environment as more
attention and interest have been paid on the utilization of renewable energy sources such as wind
energy, fuel cells and solar energy. The increasing importance of fuel saving leads to the
decentralization of power generation and increasing use of non-conventional energy sources such
as wind energy, bio-gas, solar and hydro potential, etc. So the market of small or micro energy
generating units is becoming increasingly important[1]. For these applications classical
asynchronous machine with cage rotor is a better option. The grid connected induction
generators are being considered as an alternative choice to the well-developed synchronous
generators because of their lower unit cost, inherent ruggedness, operational and maintenance
simplicity and ability to generate power at varying speed. The induction generator’s ability to
generate power at varying speed facilitates its application. Irrespective of the chosen mechanism
for alternative energy source, the efficiency of the system is important. Based on efficiency,
classical asynchronous machines have some disadvantages with respect to synchronous
generators. Induction machines have better position in market due to their lower price. In
induction machines no rotor excitation, voltage regulation equipment or slip rings are required
which results lower maintenance cost.
In this report the energy efficiency of induction machines are analyzed. Induction machines are
mainly used as motor, however they can be used generator when rotor is rotated with speed more
than synchronous speed. The power flow of induction machines is studied in motoring and
generating mode. This is necessary to explain the operational difference and efficiency of
motoring and generating mode of induction machines. Other aspects that can influence the
efficiency are studied.
In market catalogues data such as power rating, power factor and efficiency are only provided
for motoring mode of induction machines. This report will help to compare these parameters for
generating and motoring mode. These values can differ significantly for motor and generator
mode depending upon the size and efficiency class of induction machine.
2
1.2 MOTIVATION OF WORK:
The conventional energy sources are limited and have pollution to environment as more
attention and interest have been paid on the utilization of renewable energy sources such as wind
energy, fuel cells and solar energy. In Europe and countries like DENMARK and INDIA, wind
farms are mostly used to generate some part of electricity. So asynchronous machines like
induction generators are being used[2]. But information about energy efficiency of induction
machines is available for motoring mode mostly. Hence this report gives a comparative study
between motoring and generating mode of induction machines.
1.3 ELECTRICAL MACHINES – AN OVERVIEW:
From the past days, human beings have been using the energies for improving their living
conditions. Among those energies electrical energy hold the top in because electrical energy is
adaptable to all human needs in an easy way due to some advantages. It is pollution free, easily
controlled and economic and can be used in efficient way. Electrical machines are divided onto
dc machines and ac machines. In this chapter we discussed about different types of electrical
motors, 3-Φ induction machines (their classification, construction and principle of operation).
1.3.1 – DC MOTORS:
Direct current motors use a dc supply for their operation. It converts dc power to mechanical
power. They are mostly used in steel mills, mines. The construction of a dc motor is given
below:
It has three main components:
1. Field system: It consists of yoke, field windings, field poles. The DC motor field poles
are stationary and an armature that turns on bearings in the space between the field poles.
A simple DC motor has two field poles: a north pole and a south pole. The magnetic lines
of force extend across the opening between the poles from north to south. For larger or
more complex motors there are one or more electromagnets. These electromagnets
receive electricity from an outside power source and serve as the field structure. The
main work of these components is to produce and carry the working flux. The yoke is
laminated to reduce the eddy current loss.
3
FIGURE 1.1: Constructional view of dc motor
2. Armature: It has armature core, armature windings. Armature core houses the armature
coils and provides low reluctance path to magnetic flux. When current goes through the
armature, it becomes an electromagnet. The armature, cylindrical in shape, is linked to a
drive shaft in order to drive the load. For the case of a small DC motor, the armature
rotates in the magnetic field established by the poles, until the north and south poles of
the magnets change location with respect to the armature. Once this happens, the current
is reversed to switch the south and north poles of the armature.
4
3. Commutator: This component is found mainly in DC machines. Its purpose is to
overturn the direction of the electric current in the armature i.e. changing ac current into
dc current and vice-versa. The commutator also aids in the transmission of current
between the armature and the power source.
The operation of DC motor is based on the principle that when a current carrying conductor is
placed in a magnetic field, the conductor experiences a mechanical force whose direction is
given by Fleming’s left hand rule. DC motors have usually been applied in two types of
application. One of these categories is when the power source is itself DC. This is why motors in
automobiles are all DC, from the motors that drive fans for engine cooling and passenger
compartment ventilation to the engine starter motor. A second reason for using DC motors is that
their torque-speed characteristic has, historically, been easier to tailor than that of all AC motor
categories. This is why most traction and servo motors have been DC machines.
Types of DC Motors:
1. Shunt-wound motor: In shunt wound motor, field winding is connected in parallel with
the armature such that the field winding current and armature current are not same.
2. Series-wound motor: In series wound motor, field winding is connected in series with
the armature such that the series field winding carries the armature current.
3. Compound-wound motor: Compound wound motor has two field windings; one
connected in parallel with the armature and the other in series with it.
FIGURE 1.2: Shunt and Series wound dc motor
5
1.3.2 AC MOTORS:
AC Motors use an electric current which reverses its direction at regular intervals. An AC
motor has two basic electrical parts: a stator and a rotor. The stator is in the stationary electrical
component. The rotor is the rotating electrical component, which rotates the motor shaft. AC
motor is shown in the figure.
FIGURE 1.3 AC motor
There are two types of AC motors:
•
Synchronous motor
•
Asynchronous motor or induction motor
. The synchronous motors are doubly excited electrical machines as they require both dc and ac
supply. The rotor rotates at synchronous speed. These are for delivering mechanical power and
improving system power factor. The synchronous motors have no self starting torque so external
means used to start it.
The induction motors require no dc excitation, hence called singly excited ac machines. They
have self starting torque. Speed control of induction motors is possible.
6
1.3.3 SPECIAL MACHINES:
1. BLDC Motor
2. PMSM Motor
3. SR Motor
1. BLDC Motor: BLDC Motor means brushless DC motors which are electronically
commutated motors. They are synchronous electric motor. BLDC Motor is shown in the
figure below:
FIGURE 1.4 BLDC motor
Brushless motor are the smallest available motors for a given power rating. The brushless motor
has highest efficiency for an industrial application. The stator winding also creates a rotating
field which creates a rotating torque by pulling the permanent magnet rotor. This permits the
rotor to develop a smooth torque regardless the speed.
PMSM Motor: PMSM Motor is permanent magnet synchronous motor which is also known as
brushless ac electric motor. In this rotor magnetic field is supplied by the permanent magnets
rather than by electromagnets.
7
SR Motor: SR Motor is switched reluctance motor which is a synchronous machine and runs by
reluctance torque. SRM is a form of stepper motor which uses few poles. Because of its simple
structure, SRM has the lowest construction cost. It has its applications in mining areas and where
the rotor must be held for longer period of time[25]. SR Motor is shown in figure:
FIGURE 1.5 SR motor
8
1.4 THREE PHASE INDUCTION MACHINE:
Construction- The main parts of a three phase Induction motor are stator and rotor.
Stator- It consists of a steel frame which encloses a hollow, cylindrical core made up of thin
laminations of silicon steel. Slots are provided on the inner periphery of the laminations. In the
stator slots the insulated conductors are placed and connected to form a balanced 3-phase star or
delta connected circuit. Numbers of poles are provided according to the requirement of speed in
stator winding.
Rotor- The rotor is mounted on a shaft which is a hollow laminated core having slots on its outer
periphery. The two types of windings are done on the rotor slots –Squirrel cage type and Wound
type.
(i)
Squirrel cage rotor: It consists of a laminated cylindrical core having parallel slots
on its outer periphery. Each copper or aluminum bar is placed in each slot and all
these bars are joined at each end by metal rings. This forms a permanently shortcircuited winding. The entire construction resembles a squirrel cage and hence the
name is given. The current induced in the rotor circuit by transformer action from the
stator.
(ii)
Wound rotor: It consists of a laminated cylindrical core and carries a three phase
winding. The rotor winding is generally star connected and uniformly distributed in
the slots. The open ends of the rotor winding are joined to three insulated slip rings
mounted with brushes on each slip ring. These brushes are connected to a three phase
star connected rheostat. These external resistances are connected to give large starting
torque at starting.
Principle: Three phase balanced power supply is given to the stator winding of a three phase
induction motor by which three phase currents flow in the stator winding which produces a
rotating magnetic field .This rotating flux wave cuts the stationary rotor conductors and e.m.f.s
are induced in it . As the rotor circuit is short circuited, these induced e.m.f.s give rise to current
in the rotor conductors. The interaction of these rotor currents with rotating flux wave produces
9
torque in the rotor of a three phase induction motor and as a result rotor begins to rotate. The
figure shows the three phase induction motor.
FIGURE 1.6 Squirrel cage induction machine
1.4.1 INDUCTION GENERATOR:
The construction of induction generator is same as of induction motor. In case of induction
generator the rotor speed is advanced with respect to stator magnetic field rotation. The rotor is
being driven at a speed more than synchronously rotating magnetic field for prime mover speed
above synchronous speed. Rotating flux cut the rotor conductors in a direction opposite to that
during motoring mode. So rotor generated emf, rotor current and hence its stator components
change their signs. When the speed during induction generator operation is not synchronous then
it is called an asynchronous generator.
10
Classification of Induction Generators:
For hydro and wind power plants, the induction machine has great advantages because of
its easy operation as either a motor or generator. It has different application in different
areas.
Induction generators can be classified on the basis of excitement process as
Grid connected induction generator
Self-excited induction generator
Further induction generators are classified on the basis of rotor construction as
Wound rotor induction generator
Squirrel cage induction generator
Depending upon the prime movers used and their locations, generating schemes can
be broadly classified as under
Constant speed constant frequency [CSCF]
Variable speed constant frequency [VSCF]
Variable speed variable frequency [VSVF]
Grid Connected Induction generator:
For excitement process the grid connected induction generator takes its reactive power from the
grid supply where generator is driven by a prime mover above its synchronous speed and hence
in case of grid connected induction generator the slip is negative.
11
FIGURE 1.7 Grid connected Induction generator
Self-excited Induction Generator:
Self-excited induction generator (SEIG) means cage rotor induction machines with shunt
capacitors connected at their terminals for self-excitation. The shunt capacitors may be constant
or may be varied through power electronics. A capacitor bank supply the reactive power to the
induction generator for self excitement process and as well as to the load.
FIGURE 1.8 Self excited Induction generator
12
Constant speed constant frequency [CSCF] :
In Constant speed constant frequency scheme, by continuously adjusting the blade pitch and/or
generator characteristics the prime mover speed is made constant. At a slip of 1% to 5% above
the synchronous speed, an induction generator can operate on an infinite bus bar. Induction
generators have many advantages like they are easier to operate, control, maintain, and do not
have any synchronization problems.
Variable speed constant frequency [VSCF]:
The variable-speed operation of wind electric system yields higher output for both low and high
wind speeds. This results in higher annual energy yields per rated installed capacity. Both
horizontal and vertical axis wind turbines exhibit this gain under variable-speed operation.
Variable-Speed Variable Frequency [VSVF]:
The performance of synchronous generators can be affected with variable prime mover speed.
For variable speed corresponding to the changing derived speed, SEIG can be conveniently used
for resistive heating loads, which are essentially frequency insensitive. This scheme is gaining
importance for stand-alone wind power applications
1.5 THESIS OBJECTIVE:
Induction generators are mostly used now-a-days in wind based power generation system and
market of micro electric energy generation unit system. But in market induction machines are
only available as induction motors. The catalogues only tell about rating, efficiency of induction
motor. Hence there is no information about efficiency, looses of the induction generators.
The objective of this thesis is to calculate the losses, efficiency of induction machine for both
motoring mode and generating mode. A comparative analysis between the motoring and
generating mode will be done. So the generating mode ratings can be calculated from the
induction motor catalogue.
13
1.6 ORGANIZATION OF THESIS:
Chapter 1: This chapter entitled as “Introduction”. It includes different types of electrical
motors, construction, working principle of 3-Φ induction motor and induction generator.
Chapter 2: This chapter entitled as “Induction machine and its energy efficiency” include the
detail information about losses and efficiency of induction machine, its equivalent circuit.
Chapter 3: This chapter entitled as “Analysis of induction machine during motoring mode”
include the induction motor performance analysis, calculation of losses and efficiency, no load
test, blocked rotor test, power flow diagram of induction motor.
Chapter 4: This chapter entitled as “Analysis of induction machine during generating mode”
include the magnetization characteristic, voltage build up in induction generator, its efficiency
and power flow diagram for different modes.
Chapter 5: This chapter entitled as “Experimental analysis and comparative study” include the
experimental set up different test, the observation tables, calculation of losses and efficiency of
induction machine for motoring and generating mode.
Chapter 6: This chapter entitled as “Conclusion and future work” include the conclusion and
future scopes of this project.
14
CHAPTER2
Induction machine and its energy
efficiency
15
2.1 INTRODUCTION:
The cost and availability of the electric energy can vary so the energy saving policies are more
important. For these reasons, electric energy consumers are interested to use apparatus with high
efficiencies. Due to the introduction of new materials like new electrical steels which have
reduced losses, electrical machines have advanced significantly in recent years and rare-earth
permanent magnet materials have provided a 'lossless' source of magnetic flux. Recent advances
in construction methods have reduced winding losses, so there is a continued trend to increase
efficiency.
Induction machines are mostly used as motors. Three phase induction motors can be found from
a few hundred watts up to several megawatts. The induction motors are characterized by data
provided by the manufacturer as rated speed, power, voltage, current and efficiency. With the
growing emphasis on energy conservation and the increasing energy prices, the efficiency value
has become very important. The efficiency values given by the manufacturer are measured or
calculated according to certain standards. The efficiency values can differ significantly for motor
and generator mode.
2.2 ENERGY EFFICIENCY:
Induction machines are two types (squirrel cage rotor & wound rotor) depending upon the type
of rotor. Squirrel cage induction machines are mostly used due to some advantages like simplest,
most rugged construction and low maintenance. Induction machine (the stator) gets supply from
a single ac source; rotor receives energy from stator by induction. Hence no separate excitation is
required for rotor and no slip rings are required creating low maintenance cost. Induction
machines can be used as both motor and generator. The efficiency is different for motoring and
generating mode.
The efficiency of a machine is the ratio of output power to input power,
16
Efficiency can be determined by direct or indirect method. In direct method mechanical power is
determined by accurate torque and speed measurement. This efficiency value depends upon
ambient and motor temperature, which is not desirable for efficiency comparison. The indirect
method is calculation of efficiency by segregation of losses, which allows correction for these
temperature values to a specified ambient and reference motor temperature. Theoretically these
losses are divided into fixed and variable losses. Fixed losses are magnetic core loss and friction
& windage loss. Variable losses include stator and rotor copper loss. Beside these conventional
losses induction machines have another power loss component called “stray load loss” caused by
non-ideal nature of practical machines.
FIGURE 2.1 Equivalent circuit of induction machine
As said above the calculation of efficiency of an induction machine in convenient method is
based on segregation of losses. The fixed losses or rotational losses (friction, windage loss and
core loss) can be calculated by practical no load test of induction machine. These losses are
assumed constant for both motoring and generating mode. The stator copper loss can also be
calculated from no load test. Rotor copper loss is equal to slip fraction of air gap power i.e.
power transferred from stator to rotor through air gap.
17
From no load test rotational loss,
where
,
are wattmeter and ammeter reading respectively in no load test of the induction
machine.
Stator copper loss =
Rotor copper loss =
Pm
=
=
=s
air gap power
Motor
region
Generator
region
ns
S=1
S= -1
S=0
Speed
FIGURE 2.2 Power-slip characteristics of induction machine
From the Fig. 2.1 we can see the mechanical power is negative for generating mode of
induction machine. The slip is negative for induction generator. At starting the starting torque is
not zero but the mechanical power developed is zero for induction machine.
18
The energy efficiency for motoring mode and generating mode are given as:
1. MOTORING MODE:
In motoring mode the rotor of induction machines revolves in direction of rotating field at speed
below the synchronous speed. Here the slip is in between 1 and 0. The efficiency of induction
motor is given by
The air gap power of induction motor is
For motoring mode, the stator current and stator voltage drop increase with load. Hence the air
gap voltage (induced emf) decreases and so the magnetizing current and core losses also
decrease. In grid connected induction motor the core losses are supplied by the grid where as the
self excited induction motor gets reactive power from a capacitor bank.
2. GENERATING MODE:
In generating mode of induction machine, the rotor revolves at speed more than synchronous
speed. The slip varies from 0 to -1 for induction generator. The efficiency of induction motor is
given by
The air gap power of induction motor is
In induction generator the air gap voltage will increase due stator voltage drop. So the
magnetizing current and core losses will increase. The effect will be more if the machine is
already in saturation and the stator resistance is not negligible. In induction generator the core
losses are supplied by mechanical driver.
19
CONCLUSION:
Efficiency in motoring and generating modes have been studied in this chapter which depend
on the iron loss, stator and rotor loss, friction and winding loss and stray load loss. By assuming
equal active electrical power, the stator loses in generator loses will be larger than in motor mode
so the efficiency in motor mode will be higher than in generator mode. In electrical machines,
efficiency gains will result from the development of new materials and construction techniques.
20
CHAPTER 3
Analysis of Induction Machine
during Motoring mode
21
3.1 INTRODUCTION:
Induction machines are mostly used in motoring mode. So detail information about
equivalent circuit, losses and efficiency are available for motoring mode. There are many
methods for calculating the efficiency of induction motor. The equivalent circuit parameters,
losses and efficiency of induction motor can be calculated from the No load test, Blocked
rotor test, DC test and Load test. The objective of this chapter is to describe the methods of
determining the equivalent circuit parameters, losses and efficiency from these tests and
discuss about induction motor performance.
3.2 DC TEST:
The dc stator resistance per phase is calculated from this test. The circuit connection is
shown in Fig
FIGURE 3.1: DC load test on 3-Φ induction motor
R=
22
3.3 NO LOAD TEST:
W1
3-Φ
A
V
IM
variac
W2
FIGURE 3.2: No load test of 3-Φ induction motor

This test is also called Open circuit test.

The induction motor is run at no load. Friction and windage are the only load.

The motor is run at rated voltage and frequency.

Slip (snl) is very small

The per phase applied voltage Vnl, input current Inl, input power Pnl are recorded from
the voltmeter, ammeter, and wattmeters as shown in Fig.

As slip is very small,
in Fig. is very large compared to Xm. Hence resultant of
parallel branches is almost equal to jXm.

So no load reactance from the stator terminals, Xnl = x1+Xm

Stator no load impedance, Znl =

Stator no load resistance, Rnl =

Xnl =

The fixed losses or rotational losses PR consist of core loss and friction & windage
loss is calculated from the relationship
PR = m (Pnl –
)
m = no. of stator phases
r1 = stator resistance per phase

So we get no load stator reactance and rotational losses from the no load test.
23
The equivalent circuit of Induction Motor for No load test is given below:
FIGURE 3.3: Equivalent circuit of 3-Φ induction motor for No load test
3.4 BLOCKED ROTOR TEST:
FIGURE 3.4: Blocked rotor test on 3-Φ induction motor
24

Blocked rotor test is performed to determine the leakage impedance

The rotor shaft is blocked by external means.

Voltage is applied till rated current flows in the stator winding.

Per phase values of applied voltage Vbr, input current i.e. rated current Ibr and the
input power Pbr .

In this test the voltage input is very small, so the core loss.

Hence the input power Pbr is nearly equal to both stator and rotor winding copper loss.

Equivalent circuit parameters of induction motor can be calculated from no load test
and blocked rotor test results.
The equivalent circuit diagram for blocked rotor test is given below:
FIGURE 3.5: Equivalent circuit of 3-Φ induction motor for blocked rotor test
From this test:
Blocked rotor impedance, Zbr =
Blocked rotor resistance, Rbr =
Blocked rotor reactance, Xbr =
25
Xbr = x1 + x2
Generally x1 = x2 = Xbr
From the observations of no load test, blocked rotor test and DC test we get Xnl, x1, x2, r1.
Xm = Xnl – x1
r2 = (Rbr – r1)
3.5 PERFORMANCE ANALYSIS:
Induction machine is a classical asynchronous machine and mostly used due to its better
performance than other electric motors. Squirrel cage induction motors are used more due
some advantages like cheap, robust and low maintenance. Induction motor performance
depends on rotor resistance, air gap length, shape of both stator and rotor slots. The objective
of this chapter is discuss about performance and operating characteristics of induction motor
and the factors affecting them.
FIGURE 3.6: Operating characteristics of an induction motor
26
At no load, the speed of rotor is close to synchronous speed. Hence the no load slip is very
small. No load torque is very small and sufficient to overcome friction and windage loss. So
the rotor current is also small. As mechanical load are added, the load torque increases, the
rotor speed decreases and the slip increase. The decrement in rotor speed from no load to full
load is 2%-5% of rated speed.
The stator power factors is very low (nearly in range 0.1-0.3) at no load condition of
induction motor, because the current drawn by induction motor is largely magnetizing
current. As load increase, the active component of no load current increases, the power factor
angle decrease and the power factor improves. The power factor is 0.85 to 0.88 at 80% - 90%
of full load output. Power factor decreases slightly beyond this load because of the dominant
effect of stator and rotor leakage reactance drops.
The no load stator current is about 30% - 50% of rated current. As mechanical load is
added, the rotor speed decreases increasing rotor current. Therefore the counter e.m.f.
decreased allowing more stator current to flow.
The induction machine variable losses and efficiency increase with load and efficiency
becomes maximum when fixed losses and variable losses are equal. Generally maximum
efficiency occurs at 80% to 95% of rated output. Beyond this load, the variable losses
increase rapidly than output resulting decrement in efficiency.
The air gap flux remains constant for constant supply voltage in induction motors. So if air
gap length is increased, then more magnetizing current is required for constant flux. This
leads to decrease in no load and full load power factor of the induction motor. Thus the air
gap length should be kept as small as possible for better power factor of induction motor.
The slot leakage flux is directly depends upon the slot depth. So induction motors with
deeper slots have more leakage reactance, low starting torques, low maximum torque and low
slip at maximum torque.
27
3.6 POWER FLOW DIAGRAM:
Pg
Pi/p
Pm
Psh
Stator
Stator
Rotor
Rotor
Pcu
Pc
Pcu
Pc
FIGURE 3.7: Power flow diagram of a 3-Φ induction motor
Where
Pi/p = Power input
Pcu = Copper loss or I2R loss
Pc = Core loss
Pg = Air gap power
Pm = Mechanical power
Psh = Output shaft power
28
Friction
&
windage
loss
3.7 EFFIENCY CALCULATION:
Efficiency calculation of electrical machines need input power, output power and losses. For
3-Φ induction motor the input power is 3-Φ electrical power and output power is rotor shaft
power. The losses in an induction motor are two types:
i.
Fixed losses
ii.
Variables losses
Fixed losses consist of core loss and losses due to friction and windage. Normally these
losses are taken as constant for induction motor. Fixed losses (also called Rotational losses)
can be calculated from observations of no load test.
Variable losses consist of stator ohmic loss, rotor ohmic loss and stray losses. Total ohmic
loss or copper loss can be determined by performing blocked rotor test on induction motor.
The stator copper loss and rotor copper loss can be calculated directly if stator and rotor
windings ac resistances are known. Stray load loss occurs in iron as well as in conductors.
Ƞ=
Input power = Output power + losses
Mechanical power, Pm = mechanical power developed in rotor
Air gap power, Pg = power transferred from stator to rotor through air gap
Shaft power, Psh = Pm – friction and windage loss
= rotor ohmic loss + mechanical power developed in rotor
= sPg + (1 – s) Pg
Pm = (1- s) Pg
Rotor ohmic loss = Pm
Pg = Pel – stator ohmic loss – stator core loss
29
CHAPTER 4
Analysis of Induction Machine
during generating mode
30
4.1 INTRODUCTION:
Induction machines acts as a generator when the slip of machine is negative i.e. the rotor
rotates with speed above synchronous speed. When slip is negative, the rotor e.m.f., rotor
current and power becomes negative. Under such condition, the electric torque developed is
negative (opposite to prime mover) and the machine delivers power to supply mains.
Induction generator is also called asynchronous generator. The induction generators are two
types depending upon the source of magnetizing current:
i.
Self-excited induction generator
ii.
Separately-excited induction generator
4.2 MAGNETIZATION CHARACTERISTICS:
Stator
terminal
voltage,
V1
Magnetizing Current, Im
FIGURE 4.1: Magnetizing characteristics of induction generator
31
The magnetization characteristic of self excited induction generator is same as self excited
DC generator. When the stator current is zero, there is some e.m.f. in the stator terminal. This
is due to residual magnetism present in rotor. Then the curve becomes linear for some range
and then it deviates from linear relationship. When magnetic saturation occur the stator
terminal voltage does not increase with stator current.
4.3 VOLTAGE BUILDUP PROCESS:
FIGURE 4.2: Voltage build-up process in 3-Φ induction generator
When the rotor of induction generator is run, the residual magnetism present in rotor iron
creates a small e.m.f. oa across stator terminals. This voltage causes a capacitor current ob.
The flux due to current ob is added with residual flux and generates a stator terminal voltage
bc. This voltage produces current od in capacitor bank which then generates voltage de. This
cumulative process continues till the intersection point f between saturated magnetization
curve and capacitor load line. The intersection point f gives no load generated e.m.f. gf at the
magnetizing current Im1.
32
The induction generator will not build up if there is no residual flux present in rotor iron. To
overcome this problem the induction machine is run in motoring mode for some time to
create residual magnetism. This voltage build up process is similar to that of dc shunt
generator.
FIGURE 4.3: Effect of capacitance on voltage build up process in 3-Φ induction generator
The voltage build process depends upon the capacitor value. Higher the value of
capacitance, greater is the voltage build up, as shown in Fig. 4.3. If the capacitor load line
does not intersect the magnetization of induction generator, there would be no voltage build
up. In Fig.4.3 for capacitor C4, the voltage build up does not occur.
33
4.4 POWER FLOW DIAGRAM:
Induction generators can be divided into two types depending upon the speed of rotor:
i.
Super-synchronous generator
ii.
Sub-synchronous generator
The power flow diagrams of both generators are different.
4.4.1 SUPER-SYNCHRONOUS GENERATOR:
s < 0 (negative)
For generating mode,
Pm = negative
Pg = negative
Pm = (1 - s) Pg
 |Pm| > |Pg|
sPg = positive
 P2 + P2cu = positive
 As P2cu is always positive, hence P2 is also positive
P2cu = Rotor copper loss
P2 = Electrical power output from rotor terminals
P1cu = Stator copper loss
Pc = Core loss
So net electrical power output = P1 + P2
34
|Pm|
P1
sPg
Pc + P1cu
Pg
P2cu
P2
FIGURE 4.4 Power flow diagram of super-synchronous induction generator
4.4.2 SUB-SYNCHRONOUS GENERATOR:
0≤s≤1
For generating mode,
Pm = negative
Pg = negative
sPg = negative
 P2 + P2cu = negative
 P2 is negative
|Pm| < |Pg|
Net electrical power output = P1 – P2
P1
s|Pg|
P1cu + Pc
Pg
|Pm|
P2
P2cu
FIGURE 4.5 Power flow diagram of sub-synchronous induction generator
35
4.5 EFFIECIENCY CALCULATION:
For generating mode of induction machine input power is mechanical power and output
power is electrical power. As shown above the output power in super-synchronous induction
generator is greater than sub-synchronous induction generator. So efficiency is higher in case
of super-synchronous induction generator.
For generating mode the losses will be same as of induction motor i.e. fixed loss (core loss
and friction and windage loss) and variable loss (stator and rotor ohmic loss).
4.6 CONCLUSION:
So induction machines can be used as generator by increasing the rotor speed above
synchronous speed. The reactive power can be supplied from external supply or by
connecting capacitor bank (self excitation). In self excited induction generator the voltage
build up is same as the dc shunt generator. The power flow diagram and efficiency are
discussed for induction generator. The comparison between motoring mode and generating
mode characteristics and efficiency will be discussed in next chapter.
36
CHAPTER 5
Experimental Analysis and
comparative study
37
5.1 INTRODUCTION:
For induction motor and induction generator the output power is different. The losses in
both the modes will be different. In this chapter a comparative analysis between motoring
mode and generating mode of a 3-Φ induction machine is discussed by doing practical
experiment.
The determination of efficiency of an induction machines is carried out by calculating
different losses. The rotational loss or fixed loss is calculated from the no load test of the
induction machine. The ohmic losses can be calculated from blocked rotor test on induction
machine. The rotor loss is calculated from slip fraction of the power transferred from stator to
rotor through air gap i.e. air gap power. The air gap power is given by:
Pg = Pel – Pstator - Pcore ------------------- Motoring mode
Pg = Pel + Pstator + Pcore ------------------ Generating mode
In induction motor the core loss is supplied by the connected grid where as in induction
generator it is supplied by mechanical driver. In self-excited induction generator the reactive
power is supplied by the capacitor bank connected.
5.2 MEASUREMENT SETUP AND EXPLANATION:
For determining and comparing the losses and efficiency of motoring and generating mode
of induction machine, a 3-Φ squirrel cage induction machines is subjected to no load test,
blocked rotor test and load test both under motoring and generating mode. The circuit
connection for no load test and blocked rotor test is shown in Fig 5.1 on next page. In no load
test the rotor is open circuited and in blocked rotor test the rotor is blocked by means of some
external force. The ac voltmeter and ac ammeter ratings are different for these tests.
38
FIGURE 5.1: Experimental setup for 3-Φ induction machine testing
For No load test:
Voltmeter rating = 0 – 600V
Ammeter rating = 0 – 5A
For blocked rotor test:
Voltmeter rating = 0 – 100V
Ammeter rating = 0 – 30A
From load test we calculated the stator and rotor ohmic losses, power factor, output power
and efficiency for different input powers. Then from these readings we plotted power factor
characteristics, graph between output power and efficiency, output power and stator and rotor
ohmic loss, graph between stator current and efficiency using Microsoft Excel. Similarly no
load characteristic is plotted from observations of no load test.
For induction generator the rotor is rotated at speed above synchronous speed. The
induction machine is coupled with a DC motor. The rotor speed of induction machine is
varied by controlling the speed of DC motor. For self excitation a group of capacitors is
connected at stator side of the induction machine.
39
FIGURE 5.2: 3-Φ Induction Machine coupled with DC motor
FIGURE 5.3 Experimental setup of induction machine
40
5.3 EXPERIMENTAL RESULTS AND COMPARATIVE STUDY:
TABLE 1: No Load Test of Induction Motor
Current(Amp)
Voltage(Volt)
Power(Watt)
8.9
415
1020
8.5
400
1000
8.2
380
960
7.8
360
910
7.5
340
870
7.2
320
820
6.9
300
750
Blocked Rotor Test: 90V, 15A, 1450W
TABLE 2: Load test on Induction Generator
Current
Voltage
Power
cosΦ
Pstator
Protor
Po/p
Ƞ(%)
12A
420V
6538.42W
0.749
172.8W
226.5W
5630W
85.21
11.3A
415V
5507.02W
0.668
153.3W
137.1W
4726W
84.32
10.6A
410V
4561.66W
0.606
134.8W
82.7W
3832W
83.22
9.8A
410V
3083.01W
0.443
115.2W
38.3W
2553W
81.43
41
TABLE 3: No Load Test on Induction Generator
Current(A)
Voltage(V)
Power(W)
7
415
920
6.9
400
880
6.8
380
860
6.6
360
800
6.4
340
720
6.2
320
640
TABLE 4: Load Test on Induction Motor
Current(A) Voltage(V) Power(W) Pstator(W)
Protor(W)
cosΦ
Po/p(W)
Ƞ(%)
12
390
6430
172.8
78
0.793
5440
84.57
13
385
7355
202.8
165`
0.848
6244
85.12
14
385
8115
235.2
262.5
0.869
6937
85.48
14.5
380
8420
252.3
315
0.882
7220
85.73
15
380
8800
270
382.5
0.891
7550
85.22
42
0.6
0.58
0.56
0.54
Iµ
0.52
0.5
Motor
Generator
0.48
0.46
0.44
0.42
0.4
0.6
0.7
0.8
0.9
1
1.1
Stator voltage
FIGURE 5.4: Iµ vs Stator voltage for a 3-Φ induction machine
No-load characteristics
1.1
1.05
1
U0(p.u.)
0.95
0.9
0.85
Motor
0.8
Generator
0.75
0.7
0.65
0.6
0.4
0.42
0.44
0.46
0.48
0.5
0.52
0.54
0.56
0.58
I0(p.u.)
FIGURE 5.5: No load characteristics of 3-Φ induction machine
43
0.6
Power factor characteristics
1
0.9
0.8
cosΦ
0.7
0.6
Motor
0.5
Generator
0.4
0.3
0.2
0.1
0
0.5
0.6
0.7
0.8
0.9
1
1.1
Istator(p.u.)
FIGURE 5.6: Power factor characteristics of a 3-Φ induction machine
Stator loss
0.04
0.035
Pstator(p.u.)
0.03
0.025
0.02
Motor
Generator
0.015
0.01
0.005
0
0
0.2
0.4
0.6
0.8
1
1.2
Po/p(p.u.)
FIGURE 5.7: Stator losses vs output power of a 3-Φ induction machine
44
Rotor losses
0.06
Protor(p.u.)
0.05
0.04
Motor
0.03
Generator
0.02
0.01
0
0.3
0.4
0.5
0.6
0.7
0.8
Po/p(p.u.)
0.9
1
1.1
FIGURE 5.8: Rotor losses vs output power of a 3-Φ induction machine
100
Ƞ(%)
90
80
70
60
50
0
0.2
0.4
0.6
0.8
1
1.2
Po/p(p.u.)
FIGURE 5.9: Efficiency vs output power of a 3-Φ induction machine
45
100
Ƞ(%)
90
80
Motor
Generator
70
60
50
0.5
0.6
0.7
0.8
0.9
1
1.1
Istator(p.u.)
FIGURE 5.10: Efficiency vs Stator current of a 3-Φ induction machine
The efficiency is calculated by segregation of losses. No load test gave rotational loss or
fixed losses where as blocked rotor test gave copper losses. In this paper the losses and
efficiency are compared for motoring and generating mode.
In Fig. 5.5 No load characteristics of a 3-Φ induction machine is given for both motoring
and generating mode. From this graph we can see the rotational loss is more in case of
generating mode than that of motoring mode. This is because, with increase in stator current
the air gap flux decreases a little where as it increases with stator current in case of generator.
Hence in generating mode the core loss is more.
Fig. 5.6 shows the power factor characteristics of a 3-Φ induction machine. The induction
machine has low power factor in case of generating mode than that of motoring mode. This
shows that in generating mode, the stator current is much more reactive due to saturation
effect.
46
Fig. 5.7 shows the graph between stator loss vs output power. This show the stator losses
are higher in generating mode. This is due to voltage drop in stator and the saturation level.
This leads to larger air gap flux, higher magnetizing current and more core loss. So the stator
loss is more.
Fig. 5.8 shows the plot between rotor loss vs output power. As the stator loss and core loss
is higher, the rotor loss is more in generating mode than motoring mode due to saturation
effect. Rotor losses are determined from air gap power and slip. As we discussed above the
air gap power is more in case of generating mode.
Fig. 5.9 and fig. 5.10 show about efficiency comparison. This shows the efficiency of
induction machine is higher is motoring mode than that of generating mode. This is because
the induction generator has more losses than induction motor.
5.4 CONCLUSION:
For motoring mode the stator current increases with the load. The air gap voltage and the air
decreases a slight with load and so the core loss. But in induction generator the air gap
voltage and air gap flux increases with stator current and load. Hence the stator losses and
rotor losses in generating more are higher than the motoring mode. So the efficiency of
induction machine is higher in motoring mode than generating mode.
47
CHAPTER 6
Conclusion & Future work
48
Induction machines are used as induction machine most of the time. They can be used as
generator by rotating the rotor the above the synchronous speed. In this report, a high rated
3-Φ induction machine is compared for its motoring mode and generating mode. So for high
rated induction machines the stator and rotor losses are quite high for generating mode than
motoring mode. So the efficiency of induction machines is high in motoring mode.
But for efficiency of other class or rated induction machines are not studied. Hence the
future work of this project is to calculate the efficiencies for motoring and generating mode
for lower rated induction machines and compare them.
Hence information about generating mode can be determined from the catalogue of
induction motors. But this comparison cannot be applied to induction machine for calculating
efficiency of induction generator always.
49
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51
APPENDIX-I
3-Φ Induction Machine Details:
7.5KW
10HP
400V
50Hz
Nr = 1420 rpm
Ns = 1500 rpm
Is = 15A
Ir = 27A
52
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