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ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962
Volume-6, Issue-1, January-February-2016
International Journal of Engineering and Management Research
Page Number: 30-34
Performance Characteristics of Induction Motor under a Balanced Load
by Star- Delta Starting
Oti Stephen Ejiofor1, Akpama E.J.2, Nnadi Damian Benneth3
Department of Electrical Engineering, University of Nigeria, Nsukka, Enugu State, NIGERIA
2
Department of Elect./Electronic Engineering, Cross River State University of Technology, Calabar, NIGERIA
1,3
ABSTRACT
The purpose of this work is to investigate an
induction motor model using its physical parameters so as to
examine the characteristic performances: phase currents
curves, motor speed curve, electromagnetic torque curve and
the torque-speed curve of the induction motor using
SIMULINK - a tool box extension of the MATLAB program.
This performances are to be correlated to the Star-Delta
starting Scheme so as to observe the associated response.
Keywords---- Induction, Performance, Star-Delta
I.
INTRODUCTION
Induction (Asynchronous) machines are known to
be superior to their DC counterparts and most widely used
in industries because of their ruggedness, robustness,
reliability, low cost, output power per weight, high
efficiency and good self-starting capability [1]. The main
aspect which distinguishes induction motor from
synchronous motors is that induction motors are capable of
producing torque at any speed below synchronous speed
[2]. The three-phase induction motors, which are widely
used in industrial and commercial applications, are capable
of producing torque at any speed below synchronous
speed. In [3], Nyein Nyein Soe carried out a work on the
dynamic simulation of small power induction motor based
on mathematical modeling. The dynamic simulation is one
of the key steps in the validation of the design process of
the motor drive systems and it is needed for eliminating
inadvertent design mistakes and the resulting error in the
prototype construction and testing. Nyein’s paper
demonstrates the simulation of steady-state performance of
induction motor by Matlab program using a 3-Ф, 3hp
induction motor which was modelled and simulated with
30
Simulink model. A work on the simulation of an induction
machine performance under an unbalanced source voltage
conditions was also presented in [4] where they compared
the performance of the machine under balanced and
unbalanced conditions. Munira Batool [5] also looked at
the mathematical modelling and speed-torque analysis of
3-Ф induction motor using Matlab/Simulink. In his work,
he presented the speed-torque characteristics of an
induction motor which were calculated on the basis of a
mathematical model. The approach is in compliance with
the IEEE standard test procedure for polyphase induction
rotors and generators.
Induction motor tests using
Matlab/Simulink and their integration into electric
machinery were as well carried out in [6, 7]. His work
describes Matlab/Simulink implementation of three phase
induction motor tests, namely; DC, No load and blockedrotor test performed to identify equivalent circuit
parameters. These simulation models are developed to
support and enhance electric machinery.
II.
PRINCIPLE OF OPERATION OF
INDUCTION MOTOR
The equivalent circuit of the induction motor is
obtained by combining the stator figure (1) and rotor figure
(2) equivalent circuits as shown in the figure (3) that
follows, to form the equivalent identical to that of a two
winding transformer.
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determined through a series of calculations. Performing
these calculations can help the designer provide a motor
that is best suited to a particular application. The
performance of the three phase induction motor from the
parameters of its equivalent circuit is evaluated in the usual
way. From the complete equivalent circuit, the mechanical
torque developed is given by;
Tmech
1
3V 2
= ∗
= R2' ÷ S
2
2
'
'
ω s R1 + R2 ÷ S + X 1 + X 2
[ (
)] (
)
(1)
At low values of slip:
Tmech ≈
1
ωs
∗
3V12
∗S
R2'
(2)
At high values of slip:
Tmech ≈
1
ωs
∗
(X
3V12
1
1 + X2
)
2
=
R21
S
(3)
The maximum pullout or breakdown torque developed by
the motor is:
Tmax =
1
*
2ωs R +
1
[R
2
1
3V12
(
+ X 1 + X 2'
)]
2
(4)
The maximum torque is independent of rotor resistance,
but the value of the rotor resistance determines the speed at
which the maximum torque is developed.
III.
The equivalent circuit of a three-phase induction
motor as shown in the figure (3) above consists of the
fixed stator, a three-phase winding supplied from the threephase mains and a turning rotor. There is no electrical
connection between the stator and the rotor. The currents
in the rotor are induced via the air gap from the stator side.
Stator and rotor are made of highly magnetizable core
sheet providing low eddy current and hysteresis losses.
The magnetic field generated in the stator induces an emf
in the rotor bars. In turn, a current is produced in the rotor
bars and shorting ring and another magnetic field is
induced in the rotor with an opposite polarity of that in the
stator. The magnetic field, revolving in the stator, will then
produce the torque which will “pull” on the field in the
rotor and establish rotor rotation. In the design of the
induction motor, operational characteristics can be
31
BALANCE AND UNBALANCED
LOAD STATES
The analysis of an induction machine is always carried out
with the assumption that there is symmetry. That is, the
source voltages in the three phases are balanced and the
single phase loads connected to the system are also
balanced. But in practice, there is however, a possibility on
accident short circuit’s between coils, that the three phase
winding may not remain symmetrical.
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Also unbalanced phase voltages do exist due to the
presence of unbalanced load on the system are also
balanced. Voltage unbalance comes in diverse ways; single
phase under-voltage, two phase under-voltage unbalance,
three phase under-voltage unbalance, single phase overvoltage unbalance, two phase over-voltage unbalance,
three over-voltage unbalance, unequal single phase angle
displacement, and unequal two-phase-angle displacement.
For a balanced, pure sinusoidal three phase
supply, the sum of the three phase voltages is zero; as a
result the zero sequence voltage will be zero. In order to
analyze 3 − φ induction machine, it is usually assumed
that the source voltage is a balanced three phase network
as shown in figure 4(a). The three-phase winding of an
induction machine is usually symmetrical as a result of
proper design and construction. In the case of the
unbalanced state, there is however, a possibility on account
of accidental short circuits between coils etc., that the
three-phase winding may not remain symmetrical in which
case an unbalanced system of figure 4(b) is obtained.
IV.
EQUIVALENT CIRCUIT OF
INDUCTION MOTOR USED
Mathematical model of an induction motor is
usually done in the arbitrary rotating reference frame, from
which other reference frames are realized. The two
commonly used reference frame are the stationary
reference frame and the synchronously rotating reference.
,
The former is realized by substituting the variable
, where
and the later is realized by substituting
is the synchronous speed of the motor in electrical
radians per second. Details of the derivation of the
conventional synchronous motor can be found in [8], and
the equivalent circuit is shown below:
Figure 5: q-axis Equivalent in the Arbitrary Reference
frame
32
Figure 6: d-axis Equivalent in the Arbitrary Reference
frame
Figure 7: Zero Sequence Equivalent Circuit
V.
MOTOR PARAMETER
The three phase induction motor parameters used
for this study are presented in the table below. It is a 4pole, 60-Hz, 3-phase induction motor, and its reactance
parameters are expressed in ohms as shown in the table
below.
Table1. Induction motor Parameters used
Parameter
Value
Rated Power
500 hp
Rated phase voltage
230V
Rated Speed
1773rpm
Rated Torque
1980Nm
Nominal current
93.6A
Stator resistance per phase, r s
0.262 ohms
Stator leakage reactance per phase, 1.206 ohms
X ls
Magnetizing reactance, X m
54.02 ohms
Rotor leakage reactance referred to 1.206 ohms
the stator, X' lr
Rotor resistance referred to the 0.187 ohms
stator, r' r
Moment of Inertia (J)
11.06 Kg.m2
VI.
MATLAB SIMULATION OF STARDELTA STARTING
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In the course of this work, this major simulation
on Star-delta starting was carried out by starting the motor
in star and afterwards it is changed
ptheta_e
1
s
pFds
1
s
pFqs
1
s
pFdr
1
s
pFqr
1
s
theta_e
Fds
Fqs
Fdr
Clock
[ib]
Fqr
[wr]
pwr
2*pi*60
we
fcn
Constant
Vab
ia
[ia]
1
s
[ic]
[iar]
[ia]
y
[ibr]
vao
timer
Va
Vbc
ib
[ib]
ic
[ic]
Te
[Te]
iar
[iar]
ibr
[ibr]
icr
[icr]
Switch
vbo
timer2
Switch2
Figure 10: Rotor currents in the three phases for the stardelta starter
[wr]
Switch1
[Tl]
[icr]
[Te]
vco
timer1
Vb
Vca
To Workspace
[Tl]
Tl
wr
Step
theta_r
Vc
1
s
Embedded
MATLAB Function
Figure 8: SIMULINK model for Star-Delta Starter.
over to delta. The changeover to delta occurred 1.5
seconds after start-up and it was loaded with the rated load
of 1980Nm after 3 seconds. The SIMULINK model is
shown in figure 8 below:
VII.
SIMULATION RESULTS OF STARDELTA STARTING
Figure 9: Stator currents in the three phases for the stardelta starter
33
Figure 11: Plot of electromagnetic torque for the star-delta
starter
Figure 12: Plot of Load Torque for the star-delta starter
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not accelerate insulation deterioration or shorten motor
life span.
IX.
Figure 13: Plot of rotor speed (star-delta starter)
VIII.
DISCUSSION OF THE
SIMULATION RESULTS
Figure 9 which was started with a star-delta
starter has much reduced starting phase current amplitude
of about 800A. If it was a DOL scheme it would have
given about twice that quantity which makes this method
a safer one.
Also, figure 10 shows the rotor currents for the
star-delta starter. The rotor starting current is about the
same magnitude with that of the stator current because of
transformer action and also because it is referred value to
the stator. After the start-up, the frequency of the rotor
current becomes relatively low because the speed of the
motor is now close to the synchronous speed. At the time
interval when the motor is moving at synchronous speed
it can be seen that the rotor currents are all equal to zero.
Figure 11 shows the electromagnetic torque for
the star-delta starter. The motor shows a pulsating torque
during start-up, but on the average, this torque is positive
and helps to overcome the inertia torque. When the load
is applied, as observed in figure 12, the motor reacts to
produce an almost steady torque after a while to
overcome the load torque.
Figure 13 shows the motor speed for star-delta
starter. Whenever the motor is loaded the speed drops and
if it is unloaded the speed increases, this is not so obvious
in these figures however, the reason for this is because
when the induction motor is loaded, its speed drops so
that more emf will be induced in the rotor, consequently
more current and torque would be produced to counter
this load.
Also from the result, it was observed that an
induction motor under balanced load (i.e) balanced
voltages will result in; decreased heating at rated
horsepower load, in which under extended operation may
34
CONCLUSION
This research result has shown that there is an
appreciable success in the performance of an induction
motor under balance load as demonstrated using star-delta
starting. The induction motor operational characteristics
for star-delta may differ with the direct online scheme only
during transients and start-up periods but remain the same
subsequently. The results prove that, the operational
performance of an induction machine can be studied using
simulated result from MATLAB® without going through
the rigorous analytical method. This is just joining the fact
that this work has demonstrated the elegance of Matlab in
the performance characteristics of an induction motor more
so with the Matlab Embedded function developed.
REFERENCES
[1] M.C. Richard, “Electrical Drives and their control”.
Oxford University Press: New York, 1995.
[2] O.I. Okoro, “Dynamic Modelling and Simulation of
Squirrel-Cage Asynchronous Machine with Non-Linear
Effects”. Journal of ASTM International, 2(6), 2005, 1-16.
[3] N.S. Nyein, “Dynamic Modelling and Simulation of
3 − φ Small Power Induction Motor”. International
World Academy of Science, Engineering and Technology,
2000, pp. 18.
[4] E.J. Akpama, O.I. Okoro., and E. Chikuni, “Simulation
of the Performance of Induction Machine under
Unbalanced Source Voltage Conditions”. Pacific Journal
of Science and Technology, 11(1), 2010, pp. 9-15.
[5] M. Batool. “Mathematical Modelling and Speed
Torque Analysis of 3 − φ Induction Motor using
MATLAB”. Second Edition, McGraw Hill, 1995, pg. 9-17.
[6] C.O. Nwankpa, “Induction Motor tests using
MATLAB”. IEEE Transactions on Education, Vol. 48,
2005, pp. 1.
[7] O.I. Okoro., ‘Dynamic and thermal modelling of
induction machine with non linear effects’, Ph.D. Thesis,
University of Kassel Press, Germany, September 2002.
[8] Chee Mun Ong. “Dynamic Simulation of Electric
Machinery using MATLAB/SIMULINK”. Upper Saddle
River, New Jersey, 1997, pp. 229.
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