ENERGY EFFIECIENCY EVALUATION IN 3- PHASE
INDUCTION MOTOR
Rajnish kr Bharti, Preeti, Rahul Katiyar , Raman kr Singh
Galgotias College of Engineering & Technology
Abstract
Synthetic loading has previously been used as a means of identifying the temperature rise characteristic
of electrical machines. Recently synthetic loading was proposed as a method for the evaluation of efficiency of
three-phase induction motors. The basis of the hypothesis is that since the machine is being operated at rated
rms current, rms voltage, speed and temperature, that the total losses in the machine can be identified by taking
the average of the measured power over one cycle of synthetic loading of the machine. This report presents an
analysis of synthetic loading for the evaluation of the efficiency of electrical machines. The analysis is based
on an improved induction motor model. Simulations are presented for the variation of supply voltages,
operating frequency, and changes in load values of a three- phase induction motor. The results from all
three tests are in excellent agreement and confirm synthetic loading as a means of efficiency evaluation of
three-phase induction motors.
1. Introduction
Induction motors are by far the most common
consumers of generated electricity in the
developed world today. The three-phase induction
motor has been described as the “workhorse of
industry” and
more recently “the racehorse of
industry” when used with power electronic
controllers. Electric motor is the main consumer of
electricity. In industry 70% of total energy is
consumed by motor driven system. Consequently,
even a small gain in their energy efficiency can
result in a significant reduction in electricity
consumption. Increasing awareness of the finite
global energy resources require a more efficient
approach to its use .In this context, governmental
regulations impose new standards for more
efficient energy conversion in industrial application
.on the other hand consumer demand for cheap,
energy saving product exerts more pressure on
industrial manufacturer. In the context a new
category of electric motor is being promoted on the
market. This new category, high efficiency motor
has a higher efficiency than the majority of motors
in current use. There is a wealth of information
about a motor’s performance buried in the
characteristics of electrical signals at the motor’s
terminal. It is possible to quantify many energy
saving opportunities for a given motor. The
fundamental electric characteristics include
voltage, current, frequency, for each phase. By
collecting data on these fundamental characteristics
we can maximize energy saving including:





Voltage variation
Voltage unbalance
Motor load
Total harmonic distortion
Power factor
[1] Induction motors are by far the
most common consumers of generated
electricity in the developed world today.
The three-phase induction motor has
been described as the “workhorse of
industry” and
more recently “the
racehorse of industry” when used with
power electronic controllers. The robust
nature, low maintenance and moderate cost
of these machines have led to its
prevalence. The induction motor is
produced in a very wide range of
outputs from fractional to large multi
megawatt units .
[2]The efficient use of energy has found
new prominence with environmental
agencies raising the awareness of
greenhouse gas emissions predominantly
and
finite resources as a backup
argument. Previously, it was well
understood that an efficient motor would
reduce the running costs of the machine
over its working life, typically a hundred
times greater than the purchase price of
the machine.
[3] Recently, however, lobbying has
seen government incentives and
policies introduced to encourage the
use of energy efficient equipment and
particularly energy efficient motors.
Motor manufacturers have introduced
a separate category referred to as “energy
efficient motors" .
[4] Despite the attractiveness of such
energy efficient motors they are still
only used in relatively small numbers.
It has been reported that one of the
reasons for the low acceptance of
energy efficient motors is poor information
on
these
machines.
Further
to this there are no energy efficiency
standards for motors. A motor labelled
as "high efficiency" from one supplier
might be less efficient than the standard
motor from another supplier.
[5] To compound the problem, there is
no single efficiency standard test
method for induction motors that is used
throughout the industry. And to make
matters worse, a recent article identifies
the current problems with existing methods
of measuring induction motor efficiency
and stresses the need for a more
accurate
method
of efficiency
measurement.
[6] The problem of determining the
efficiency of electric motors accurately is
very topical. Dr Auinger in his recent
article states that one of the main aims of
the current revision of the IEC 61972
Standard is to improve the accuracy
when determining efficiency.
[7] However, Drs Auinger and Bunzel in
their report on the draft Standard state
that the current revision
does not
provide any improvement over the
existing
IEC 60034-2 Standard.An
Australian report has suggested that the
main focus of a Bureau of Industry
Economics study is to investigate what
the Australian Government can do to
decrease the uncertainty about motor
performance and in particular efficiency .
[8]The most common currently used
method of carrying
out
full-load
performance testing of a three-phase
induction motor is to apply full-load
torque
to
the
machine's output shaft . To load a large
machine, equally large test equipment or a
duplicate machine is required. The cost
of setting up such a test facility,
maintaining the equipment, and the time
and setting up
procedures
for
mechanically
coupling
the
load
machine, may make full load tests
prohibitively expensive.
Large
vertically
mounted
machines
are
extremely difficult to test by applying a
load to the shaft because of the
difficulty in finding a suitable vertical
load. Synthetic loading offers the
advantage that the machine under test
is not coupled to a load, thereby
negating the need for a special test
bed. Also, the machine could be tested on
site and is not restricted by the axis of
mounting. The only equipment required to
do the test is an inverter rated at the same
power rating as the machine under test,
current and voltage sensors and
a
typical PC fitted with a DSP card
for measurement and control. The costs
associated with this test method would be
significantly reduced. This paper proposes
the analysis and evaluation of the synthetic
loading methods as accurate techniques
to evaluate the efficiency of a threephase induction motor. Analysis of
three
different
methods
of
synthetically loading of a three-phase
induction motor is
presented.
The
analysis is carried out using an
improved dynamic model and the
Matlab/Simulink simulation
software.
Parameters identified for an industrial
7.5kW three-phase induction motor will
be used in the model for the simulation.
[9] It is noted here that all three
synthetic loading methods have been
used by Grantham previously for
temperature rise testing . These methods
produced excellent agreement with
the standard full-load temperature rise
test method. However, this is the first time
that synthetic loading has been modelled
with the intent of determining whether
or not each of the synthetic loading
methods are suitable for the
evaluation of the efficiency of a threephase induction motor. The three main
efficiency testing standards are the
IEEE 112-1996 in the United States, the
International
Electro
technical
Commission (IEC) produced IEC-34-2
standard for use in Europe and the
Japanese Electro technical Committee
(JEC) produced standard JEC-37. The
IEEE 112 test method B is the most
rigorous and more detailed than those that
measure all the motor’s losses using a
dynamometer such as specified in the IEC
or JEC standards.A more exact efficiency
will be obtained if a direct
measurement of the losses is made,
and then efficiency calculated as input
minus losses, divided by input, instead
of calculating the ratio of input to
output.The essence of synthetic
loading is that while the motor is
running at no-load, the rotor is then
oscillated in a motoring-generating cycle
such that, on average, over one cycle of
the synthetic loading full load rms
current is drawn from the supply. The
machine under this condition will be
producing rated copper losses. If the
average applied voltage over one loading
cycle is also the rated rms voltage, then
the rated iron loss will be present. Since
the machine is also running at close to its
operating speed, then the rated friction
and windage losses will be present.
The test is also performed at rated
temperature (synthetic loading was
originally designed to produce full load
temperature rise characteristics). Each
method of synthetic loading is designed to
produce these conditions.The assumption
is that if these conditions are met, then
the losses during synthetic loading will
be the same as the losses at the
machine’s rated full load operating
condition. Therefore, if the input power
can be measured accurately during the
loading cycle, then the total full load
losses should be able to be identified and
hence, the efficiency of the motor
evaluated.
[10]The Dual Frequency (DF) method,
as the name suggests, relies upon the
input voltage being made up of two
sinusoids per phase, each with its
own
frequency and peak amplitude. The
summation of these two sinusoids per
phase is applied to the machine
undergoing synthetic loading. During
DF synthetic loading, as the motor is
running, the rotor
accelerates and
decelerates over a cycle of the beat
frequency. The beat frequency is the
difference between the two sinusoidal
frequencies applied per phase.
[11]The Sweep Frequency (SF) method
causes the input voltage to oscillate about
the supply frequency at a set rate
determined by the sweep frequency, fm,
measured in Hz. The sweep depth (how far
the frequency varies away from the main
frequency) is set by the sweep
magnitude, Sm, also in Hz. The resultant
input voltage waveform is similar to the
more familiar Frequency Modulation
(FM) in telecommunications.
[12]CSORMF is the newest method
and not easily understood. The principle
is that the rotating magnetic field in the
stator is rotating at constant speed whilst
the amplitude is modulated. The input
parameters for the voltage are the
peak
amplitude,
main
and
modulation frequency and modulation
depth (in Hz). Conceptually, this method
is similar to a transformer, where the
magnetic field is oscillating due to the
primary applied voltage which in turn
induces the voltages and currents in the
secondary or in this case, the rotor
circuit. A detailed presentation of this
method is found in this article
2. ENERGY EFFICIENT MOTOR
Introduction
Energy-efficient motors, also called
premium or high- efficiency motors, are 2
to 8% more efficient than standard motors.
Motors qualify as "energy-efficient" if
they meet or exceed the efficiency levels
listed
in
the
National
Electric
Manufacturers Association's (NEMA's)
MG1-1993 publication. Most generalpurpose motors sold after October 1997
must meet the NEMA definition,
according to federal law. However, motors
that exceed these standards are readily
available. Recommendations on efficiency
levels that exceed the standards can be
obtained for a given motor size from the
Consortium
for
Energy
Efficiency..Energy-efficient motors owe
their higher performance to key design
improvements
and
more
accurate
manufacturing tolerances. Lengthening the
core and using lower-electrical-loss steel,
thinner stator laminations, and more
copper in the windings reduce electrical
losses. Improved bearings and a smaller,
more aerodynamic cooling fan further
increase efficiency.
3. MODELLING
OF
AN
INDUCTION MOTOR
UNDERGOING SYNTHETIC
LOADING
V=400; P=4; Nr=1450; f=50;
Vp=V/sqrt(3);
R1=0.3; R2=0.25; X1=0.6;
Xm=35;
RLoss=1500; % Rotational loss
Ns=1500;
Ws=2*pi*Ns/60;
Sf=(Ns-Nr)/Ns;
X2=0.6;
Fig.3
%Using Thevenin's Equivalent Ckt Model
Zth= j*Xm*(R1+j*X1)/(R1+j*(Xm+X1));
Vth= (Vp*j*Xm)/(R1+j*(Xm+X1));
ThevninVoltage=abs(Vth)
Fig.4
SIMULATION RERSULT
Case by Case study
Case1. Variation of Frequency fraction
Fig.5
Case2.Variation of Voltage fraction
Fig1
Fig.6
Fig.2
Fig.7
Fig.11
Fig.12
Fig.8
Fig. 13
Fig.9
Fig.10
Fig.14
Case3.Variation of load fraction
Fig.15
CONCLUSION
Synthetic loading as a method of evaluating
the efficiency of electric motors has been
confirmed, using computer modeling and
simulation techniques, as accurate, and able to
identify the total losses in the machine under test.
Synthetic loading can only be analyzed using the
dynamic model. The improved model used for
the simulation in this paper includes the iron loss.
This means that the model is able to account for
all the losses in the machine, namely, copper,
iron, friction and windage losses.
The simulation technique will be used to
determine the best possible combination of input
voltage frequencies and amplitudes for a particular
method of synthetic loading and the individual
machine under test, given the machine’s
electrical
and
mechanical
parameters.
Simulation,
using
accurate
machine
parameters, will reduce test time in the practical
situation, since the correct input conditions can
be determined prior to commencement of the test.
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