Engineering, 2011, 3, 525-531
doi:10.4236/eng.2011.35061 Published Online May 2011 (http://www.scirp.org/journal/eng)
Modular Approach for Investigation of th e Dynamic
Behavior of Three-Phase Induction Machine at
Load Variation
Mazouz Salahat, Omar Barbarawe, Mohammad AbuZalata, Shebel Asad
Faculty of Engineering Technology, Al-Balqa’ Applied University, Amman, Jordan
Received March 9, 2011; revised March 28, 2011; accepted April 8, 2011
Abstract
In This paper, a modular approach for investigation of the dynamic behavior of three phase induction motor
is developed and described in details. This model has been built up, systematically, by means of basic func-
tion blocks found with MATLAB/SIMULINK. This model is described in similar but modular approach as
in electrical machines theory. The motor model includes multi-level blocks solving equations for each motor
part or component. This approach enables the researcher to calculate or investigate any motor variables;
voltage, current, flux, speed and torque. This model could also be used for a wide range of horse power
needed in scientific research and numerical applications. A q-d axis based model is proposed to analyze the
transient performance of three-phase squirrel cage induction motor using stationary reference frame. Con-
structional details of various sub-models for the induction motor are given and their implementation in
SIMULINK is outlined. Direct-online starting under different load conditions of a 3 hp induction motor (as
case study) is also studied. The motor stator voltage, the stator and rotor currents, the developed torque and
rotor speed are, numerically, calculated and plotted for different operating conditions.
Keywords: Induction Motor, Modeling, Stationary Reference Frame, MATLAB/SIMULINK
1. Introduction
The induction machine is the most used machine in a
wide variety of industrial applications as a mean of con-
verting electric power to mechanical work due to its ro-
bustness, reliability, low cost, high efficiency and good
self-starting capability [1-3]. Induction machine is con-
sidered as nonlinear dynamic system suffering from dif-
ferent levels of complicities and difficulties when oper-
ating under different input parameters and load variation.
The induction motor, particularly, with a squirrel cage
rotor, is the most widely used source of mechanical
power fed from an AC power system [4, 5]. Its low sen-
sitivity to disturbances during operation makes the
squirrel cage motor the first choice when selecting a
motor for a particular application.
For electric drive studies, usually the steady-state and
transient state are very important to be taken into account.
The recent improvements in power semiconductors de-
vices and fast digital signal processing hardware have
further accelerated this progress [3].
SIMULINK is chosen as a numerical environment to
justify our proposed modular approach, that is able to
predict such these phenomena, and this is what we are in
charge to initiate in the present work with respect to
various previous works [3-8]. It’s a very powerful and
easy tool for simulation instead of compilation of pro-
gram code. The simulation model is built up systemati-
cally by means of basic function blocks.
For the selected motor rating, given in section 4, the
motor draws large currents and oscillatory torques during
startup and other severe motor operation modes. The
numerical results with MATLAB/SIMULINK are all
given in section 4.
In the following model and equations, the subscript
“s” indicates stator quantities, and the subscript “r” indi-
cates rotor quantities; the subscript “q” indicates the
quadrature axis quantities and the subscript “d” indicates
the direct axis quantities; Vm is the motor voltage peak
value; vrms is the motor voltage root square value; Vs and
Vr are the stator and rotor voltage vectors; Vqs, Vds are q-
and d-axis stator voltages; Vqr, Vdr are q- and d-axis rotor

M. SALAHAT ET AL. 526
voltages; va, vb and vc are the phase voltages for the a, b,
and c phases; Ψqs and Ψds are the q- and d-axis stator flux
linkages; Ψqr and Ψdr are the q- and d-axis rotor flux
linkages; Ψqm and Ψdm are the q- and d-axis magnetizing
flux linkages; is and ir are the stator and rotor currents; iqs
and ids are the q- and d-axis stator currents; iqr and idr are
the q- and d-axis rotor currents; Rs and Rr are the stator
and rotor winding resistances; Ls, Lr and Lm are the stator,
rotor, and mutual inductances, respectively; xls and xlr are
the stator and rotor leakage reactances; xm is the magnet-
izing reactance; ωb is the base speed at nominal ratings;
ωr is the rotor speed; f is the rated nominal frequency; p
is the number of poles pairs; Sb is the rated base power in
kilowatts; H is the inertia constant; J is the moment of
inertia; Tem is the torque developed by motor; Tload is the
load torque on motor shaft.
2. Induction Motor Mathematical Model
Classical techniques are used to establish the voltage and
torque equations for a symmetrical induction machine
expressed in terms of machine variables. The machine
voltage equations are written in the arbitrary reference
frame. To eliminate the time-varying inductances, the
equations are frequently transformed to q-d quantities.
The equations then expressed in d-q reference frame by
appropriate assignment of the reference frame speed in
the arbitrary reference frame voltage equations.
In this work, a stationary reference frame has been
used, which has the advantage of eliminating some terms
from the voltage equations. MATLAB/SIMULINK solu-
tions are used to illustrate dynamic performance of typi-
cal induction machines and to depict variables in station-
ary reference frame during free acceleration. The mate-
rial presented in this paper forms the basis for solution of
more advanced problems. In particular, these basic con-
cepts are fundamental to the analysis of induction ma-
chines in most drive systems and variable frequency
drive applications.
The simulation of the induction motor is conveniently
accomplished by solving for the flux linkages per second
in terms of the voltages applied to the machine and ma-
chine parameters. The derivatives of the stator flux link-
ages are given by (1) to (2):

d
d
qs s
bqsqm qs
ls
R
V
tx


 




(1)

d
d
ds s
bdsdm ds
ls
R
V
tx


 




(2)
The stator currents (in the stationary reference frame)
can then be found using (3) to (4):

1
qsqs qm
ls
ix  

(3)

1
dsds dm
ls
ix  

(4)
Likewise, the derivatives of the rotor flux linkages
(per second) are given by (5) to (6):

qr rr
bdr qmqr
blr
dR
dt x

 








 (5)

qr rr
bqr dmdr
blr
dR
dt x

 








 (6)
The rotor currents (in the stationary reference frame)
can then be found using (7) to (8):

1
qrqr qm
lr
ix  

(7)

1
drdr dm
lr
ix  

(8)
The mutual flux linkages are given by, (9) to (10):
*qs qr
qm lm
ls lr
X
x
x







(9)
*ds dr
dm lm
ls lr
X
x
x








(10)
where;
111
1;2π;
2π; and 2π;
lmls ls
ls lrm
lrlr mm
X
xf
xxx
xfLxfL

 


 
L

Equations (1) to (10) provide electrical quantities. The
induction motor is an electromechanical device, so the
model also requires expressions for the electromagnetic
torque and the speed of the machine. Equation (11) ex-
presses the electromagnetic torque in terms of the flux
linkages, and (12) determines the rotational speed from
the machine torque, load torque, and moment of inertia.

emT dsqsqsds
TK ii   (11)
where 3 and 2πf
4
Tb
b
p
K





;


d1
d2
rb
em load
TT
tH




(12)
where 2
bb
b
J
HpS




;
The model for the induction motor requires voltages as

M. SALAHAT ET AL.527
inputs. Thus the power supply block consists of a three-
phase source that provides a balanced set of three-phase
voltages. In stationary reference frame the following set
of (13) can be used:



cos
cos2 π3
cos2 π3,
am b
bm b
cm b
vV t
vV t
vV t




 
 
(13)
where rms 2π, and V23;
bm
fV   
Since the inputs and outputs to the induction motor
model are phase voltages and currents, they must be
transformed to the stationary reference frame. Thus, (14)
and (15) are used to transform the phase voltages to the
stationary reference frame, and q-d stationary reference
frame currents to phase currents, respectively.
 
211
;
32 3
qsab cdsc b
vvvvv v

 



v (14)


1
;3
2
13
2
asqs bsqsds
csqs ds
iiii i
iii

 
;
(15)
3. Simulink Implementation
The induction motor block is a compound block that
contains multi-level sub-blocks, and will be described in
this paragraph. That has the advantage of keeping the
amount of blocks to a reasonable number at any given
level of the model. The complete simulation system of
the induction motor shown in Figure 1 includes the in-
duction motor model and a power supply model. A
separate block is included in the simulation model to
implement motor winding variables, calculations of mo-
tor constants and coefficients, variable voltage parame-
ters, and load block. This approach is used to provide an
easy way to simulate different operational parameters
and conditions. The model can be very useful.
Figure 1. Full model of an induction machine in stationary
reference frame.
The motor parameters block, shown in Figure 2, is
suitable to simulate different motors dynamic behavior
by inserting there parameters only. The voltage block
allows simulation of motor behavior supplied with dif-
ferent voltages and methods of control.
The suitable choice of constants allows constant power,
constant torque, horsepower squared, and horsepower
cubed loads.
Compound blocks can be used to allow multiple levels
in a model. Any motor variable can be obtained and ana-
lyzed by selecting the appropriate block and level [9].
This block also contains constants for the parameters of
the machine, which can be changed by the user to repre-
sent other machines.
The power supply model, seen in Figure 3, includes
two sub-blocks to implement the three-phase voltage
generator based on (13) and its conversion to q-d sta-
tionary frame based on (14).
The motor model includes two sub-models: electrical
model, representing equations from (1) to (10), and me-
chanical sub-model representing (11) and (12).
Figure 2. Motor parameters blocks.
Figure3. Power supply block sub-models.

M. SALAHAT ET AL. 528
The electrical sub-model includes separate blocks for
motor stator, rotor and magnetizing circuits, as shown in
Figure 5.
This approach provides means for studying and ana-
lyzing different machine variables and circuits, and solves
equations given in teaching electrical machine courses in
universities [10,11].
The magnetizing circuit which provides an easy way
to study and investigate magnetizing flux and current is
also included as subsystem in the magnetizing model in
Figure 5.
It also enables the researcher to solve equations for
stator fluxes and currents in q-d stationary frame.
The q-d to abc block (the right one in Figure 6) is an-
other transformation. In this case the stator q-d currents
in the stationary frame are transformed to phase currents
(ia,b,c).
In the same way as for the stator circuit, the rotor cir-
cuit can be simulated and rotor quantities which not ac-
cessible for measurements in a squirrel cage motor cam
be calculated and investigated.
Figure 7 represents a model for the motor mechanical
part. This block contains the code for the motor devel-
oped electromagnetic torque, as in (11), and determines
the speed and position of the rotor as a function of time,
as described in (12). To widen the benefit of this model,
in the code used, the motor developed torque and rotor
speed are represented in per unit. This approach helps to
investigate different induction motors with different val-
ues of rated torque and speed. In this case, it is required
to enter the motor parameters only in the related block,
and the model will calculate all the coefficients required
to solve the system equations and all the motor voltages,
currents, fluxes, torque and speed can be obtained from
this model.
4. Investigation of Dynamic Behavior
By starting from zero speed and applying the voltages,
the acceleration of the induction machine can be ob-
tained either with or without a load on the shaft.
Figure 8 shows the stator phase voltage, the stator
current, the rotor speed and the developed torque of a
220 volt, four-pole, a three-horsepower induction motor
started with no-load. It is interesting to note that this
motor has a rated torque of about 12 N.M.
In Figure 9 the torque is shown as a function of speed;
however it could also be plotted as a function of time, as
in Figure 1. Most textbooks and manufacturers’ manuals
show the steady-state torque-speed characteristic, but the
reality is the motor is subjected to pulsating torques dur-
ing the startup, as shown in Figure 9. The oscillating
torques, however, reach a peak of about 132 N.M. (T/Tb
= 11) at time equal to 0.0107 sec.
Figure 11 shows the free acceleration. Since friction
and windage losses were not included, the motor reaches
synchronous speed. Doing that, it was found that the in-
vestigated motor took approximately 0.32 second to ac-
celerate to the synchronous speed.
Figure 12 shows the stator phase currents as a func-
tion of time during the free acceleration of the three-
horsepower motor. Their frequency is essentially con-
stant at 60 Hz, but the amplitude is much larger than
rated current until the machine reaches breakdown torque.
Once the machine reaches synchronous speed, the motor
draws only a small current to provide the excitation and
the losses of the stator and rotor windings.
It would be difficult to measure the rotor currents in a
squirrel-cage induction motor, but they are obtainable
Figure 4. Induction motor block.
Figure 5. Model of the e lectrical part of an induction motor.
Figure 6. Induction motor stator mode l.

M. SALAHAT ET AL. 529
Figure 7. Model of mechanical part of an induction motor.
Figure 10. Motor acceleration.
Figure 8. Motor transient state Characteristics.
Figure 11. Motor de velope d torque.
Figure 9. Speed-torque characteristic.

M. SALAHAT ET AL. 530
Figure 12. Motor stator current.
eferenced to the stator) from the model. Figure 13
odel can be investigated further by running a
va
(from 0.32
se
(r
shows the rotor phase currents. These are particularly
instructive, as they clearly show that the frequency of the
rotor currents changes with the speed of the machine. At
start, the rotor currents have a 60 Hz frequency, but the
frequency drops as the motor accelerates, reaching very
low frequencies as the motor nears synchronous speed.
Of course, once the motor reaches synchronous speed
there is no relative motion between the rotor squirrel-
cage bars and the rotating magnetic field. Thus the cur-
rent in the rotor bars drops to zero as shown in the Fig-
ure 13.
The m
riety of studies with it. As an example, one could vary
the load torque and observe the steady-state speed of the
machine and determine the effect on the acceleration
time. The readings could then be plotted to provide the
torque speed curve from zero to full-load as shown in
Figure 14 for the above mentioned motor with load
torque equal to nominal load (T = 12 N.M.)
Starting time increases as load increases
c with load torque T = 0 to 0.40 sec when load torque
T = Tb). The oscillating torques, however, reach a peak
of about 132 N.M. (T/Tb = 11) at t = 0.0107 sec, as in the
no-load case. Steady-state speed related to synchronous
Figure 13. Motor rotor current.
speed drops f
es increase
of
to demon-
str
5. General Conclusions
this paper, Implementation of a modular Simulink
n model built up sys-
te
rom 1 (T = 0) to 0.95 (T* = 1).
Increasing load torque on motor shaft caus
torque oscillation which will continue to rotor speed
of 0.5 - 0.6 synchronous speed. At the end of transient
state torque oscillation will be very small and decreasing
with torque increasing and finally disappear. Maximum
torque at starting obtained at the same time (not depend-
ing on load torque), but speed change behavior is differ-
ent for different loads. This maximum value of motor
torque at motor starting varies for different load very
slightly. As this maximum obtained at starting through
very small period (during 1-2 cycles), rotor speed at this
period can be considered slightly changed (constant).
These statements leads to a result: Maximum torque of
an induction motor can be obtained approximately ana-
lytically, considering rotor speed is constant.
Also, the moment of inertia can be changed
ate the effect of starting high inertia loads. As inertia
increases torque oscillations (peaks with large values)
increase at the beginning of starting process, but speed
and torque oscillations near the synchronous speed are
less (decrease at the end of starting process). Starting
period duration approximately proportional to total iner-
tia of drive, as noted in Figure 15.
In
model for induction squirrel cage motor simulation has
been introduced. This model enables the users to access
to all the internal variables for getting an insight into the
motor operation. It’s available to investigate and predict
the transient behavior of the three-phase induction motor
using stationary reference frame.
Using SIMULINK, the simulatio
matically starting from simple sub-models and blocks
for the induction motor circuits and components.
Figure 14. Motor starting under load.

M. SALAHAT ET AL. 531
Figure 15. Motor starting with high inertia load.
This model permits to calculate the voltage, current
an
he stilted expression, “One of us (R. B. G.)
th
he developed induction motor model can be used alo
iety of tutorials and assignments that could
he
. References
] R.Krishan. “Electric Motor Drives: Modelling, Analysis
d speed torque steady state characteristics of the in-
duction motor. The designed model also has the ability to
look at variables that would be difficult or impossible to
measure.
Avoid t
anks...” Instead, try “R. B. G. thanks”. Put sponsor
acknowledgements in the unnumbered footnote on the
first page.
6. Prospective
Tne,
or it can be incorporated in advanced motor drive sys-
tems. It could be very helpful in teaching and research of
electric machine drives and control. The presented model
can be adapted for large wound-rotor induction machines
and squirrel cage machines, where the voltage dips and
harmonics would be, effectively, considered and dis-
cussed.
A varbe
lpful for students and trainee in machine theory, power
system, electrical drive system and control schemes
would be available. By customizing the proposed gener-
alized model, different levels of experiments for practical
courses would be designed to assist the interested sectors
of people to understand and test the steady-state and dy-
namic behavior of actual induction motors (small, me-
dium and large horse power).
7
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