JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
V/F METHOD FOR INDUCTION MOTOR DRIVE
SPEED CONTROL USING MATLAB SIMULATION
1
MR. H. M. KARKAR, 2 DR. S. N. PANDYA
1M.E.
[Power System] Student, Department Of Electrical Engineering, L.D. College Of
Engineering, Ahmedabad
2Associate Professor, Department Of Electrical Engineering, L.D. College Of
Engineering, Ahmedabad
hmkarkar@gmail.com, saunipandya@gmail.com
ABSTRACT: To analyze the steady state equivalent circuit model of an induction motor to establish the
equation that justify the use of a constant V/f speed control of open loop. The main objective is to maintain the
speed at fixed reference. To investigate the control schemes for controlling induction machine, particularly the
scalar control method and to evaluate their performance under variable input-output conditions. The merits and
limits are reviewed based on a survey of the related paper
Keywords— Frequency, Induction motor, Parameter variation, Speed control, Scalar control method,
Voltage.
I: INTRODUCTION
It is very important to control the speed of induction
motors in industrial and engineering applications.
Most of the drives for industrial processes and
domestic appliance have been designed to operate at
essentially constant speed, mainly because of the
ready availability of economical induction motor
operating on the available constant frequency ac
power supply. In mechanical system, it is well known
that a variable speed drive provides improved
performance and energy efficiency. However, until
recently, the provision of a continuously variable
speed has been considered too expensive for all but
special applications for which the compromise of
constant speed was not acceptable e.g. elevators, mill
drives, machine tools.
Speed control techniques of induction motors can be
broadly classified into two types scalar control and
vector control. Scalar control involves controlling the
magnitude of voltage or frequency of the induction
motor The V/f control method is, in principle a
control method for keeping the air-gap flux constant
by controlling stator voltage V and f so that the ratio
V/f is kept constant.
In industrial applications where wide range and
smooth speed control has to be provided such case
D.C. drives are preferred in many cases For these
reasons preference is often given to adjustable speed
induction motor drives as the induction motor is
cheaper, easy in construction and more economical to
operate. But the plain induction motor is essentially a
constant speed machine, as its stable operation is
restricted within a small range of speed.
II: SPEED CONTROL OF INDUCTION MOTOR AND
EQUIVALENT CIRCUIT
Having known the Torque-speed characteristic of the
motor, its speed can be controlled in following ways:
a) Changing the number of poles
b) Variation of Motor Resistance
c) Variable-Voltage, Constant-Frequency Operation
d) Variable-Frequency Operation
e) Constant Volts/Hertz Operation
To maintain torque capability of the motor close to
the rated torque at any frequency, the air gap flux, φ ag
is maintained constant. Any reduction in the supply
frequency without changing the supple voltage will
increase the air gap flux and the motor may go to
saturation. This will increase the magnetizing current,
distored the line current and voltage, increase the
core loss and copper loss, and it makes the system
noisy.
The air gap voltage is related to φag and the frequency
f as,
Input voltage,
or,
where is a constant.
Fig.1 Equivalent circuit of the induction motor
To study the behavior of the induction motor at
various operating conditions, it is convenient to an
equivalent circuit of the motor under sinusoidal
steady state operating conditions.
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For a balanced 3 phase system, equivalent circuit for
any one phase will suffice. From the per-phase
equivalent circuit of the induction motor, the current
drawn by the circuit is,
Maximum torque,
=
The air-gap power is given as,
2
Mechanical output power is given as
Hence, the mechanical torque is given as,
(I)
Plotting the torque against slip or speed gives us
the torque-speed characteristic of the motor.
For positive values of slip, the torque-speed curve
has a peak. This is the maximum torque produced by
the motor and is called the breakdown torque or the
stalling torque.
From equation (i) we observe that the torque is
proportional to the square of applied voltage. Figure
2 shows the variation of torque-speed curves with
changing applied voltage.
III: CONSTANT VOLT/HERTZ OPERATION
If the air gap flux of the machine is kept constant
(like a dc shunt motor) in the constant torque region.
The torque sensitivity per ampere of stator current is
high, permitting fast transient response of the drive
with stator current control. In variable frequency,
variable- voltage operation of a drive system, the
machine usually has low slip characteristics (i.e., low
rotor resistance), giving high efficiency. In spite of
the low inherent starting torque for base frequency
operation, the machine can always be started at
maximum torque. The absence of a high in-rush
starting current in a direct-start drive reduces stress
and therefore improves the effective life of the
machine.
=
At higher frequency
neglecting ,
and therefore,
=
=
Fig.2 Torque-speed curve for variable voltage
Its value can be calculated by differentiating
the torque expression with respect to slip and then
setting it to zero to get ŝ, the slip at the maximum
torque.
Hence, the maximum torques will remain constant
if the speed is controlled by supply frequency
variation with the ratio Vs/f kept constant
But at small frequency, the equivalent
leakage reactance
, becomes comparable to or
even less than the stator resistance rs which causes
the maximum torque to decrease. Hence, to keep the
maximum torque unchanged and, thereby,
maintaining the motor overload torque capacity
sufficiently high at low frequencies, it is desirable the
voltage less than the frequency in that range.
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IV: SCALAR CONTROL
Slip at maximum torque,
Maximum torque,
Above equation shows that the maximum torque is
independent of frequency and hence remains the
same for each E/f and the maximum torque occurs at
a speed lower than the synchronous speed for each
combination of E and f. However, we get a slightly
different set of curves for constant V/f, so for fixed
V, E changes with operating slip and the maximum
torque is reduced, as shown in figure 4.
Only magnitudes of the input variable-frequency and
voltage are controlled, this is known as “Scalar
Control”. In such controls, very little knowledge of
the motor is required for frequency control. Hence, a
control of this type offers low cost and is an easy to
implement solution. Thus, this control is widely
used. A disadvantage of such a control is that the
torque developed is load dependent as it is not
controlled directly. Also, the transient response of
such a control is not fast due to the predefined
switching pattern of the inverter.
Scalar control, as the name indicates, is due
to magnitude variation of the control variable only,
and disregards the coupling effect in the machine.
The voltage of a machine can be controlled to control
the flux, and frequency or slip can be controlled to
torque. However flux and torque are also functions of
frequency and voltage respectively.
The most popularly used scalar control
methodology is Volts/Hertz method of Open Loop
Constant V/f speed control.
OPEN LOOP V/F CONTROL
The open loop V/f control of an induction motor is far
the most popular method of speed control because of
its simplicity. This types of method are widely used
in industry. Traditionally induction motor have been
used with open loop 60Hz power suppply. For
variable speed application, both voltage and
frequency are changed proportionately so that the air
gap flux
remains constant. If ψm is
Fig.3 Torque-speed curve for variable voltage
Fig 4 Torque-speed curves for constant V/f
allowed to vary, it may either cause saturation or vary
the torque sensitively and in this control no feedback
signals are needed. Figure 5 shows the block
diagram of block diagram of Open Loop Constant
V/f speed control.
Fig 5 Block diagram of Open Loop Constant V/f
speed control
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The frequency ωe* is the primary control variable
which considered as constant one. The phase
voltage Vs* command is directly generated from the
frequency command. Variable frequency speed
control of the drive with the fully open loop reduced
power consumption. As the frequency becomes
small at low speed, the stator resistance tends to
observe major amount of stator voltage thus
weakening flux. The ωe* signal is integrated to
generate the angle θe*, and the corresponding
sinusoidal phase voltage (va*, vb* and vc* singal) are
generated by expression as shown in figure 5.
Fig 7 Simulation of open loop v/f speed control using
six step VSI (Voltage Source Inverter)
Fig 6 Torque -speed curve showing effect of
frequency variation. Supply voltage and load torque
changes
The drive’s steady-state performance on a torquespeed plane with a fan or pump-type load
is shown in figure 6. The points 1, 2,
3, etc. indicated that as the frequency is gradually
increased, the speed is also increases almost
proportionally. The speed drop from ωr to ωr' if
initial operating point is 3 and load torque is
increased
for same frequency. Now the operation
is at point a in another torque-speed curve. If ac line
voltage reduces, that lowers the machine termianl
voltage. Coresponding to point b the speed will also
drop.
The six-step generator illustrated in the figure 8
contains six comparators to produce the six-step
switching waveforms. Some supplementary logic
enables a speed reversal by inverting two phases.
Speed set point is used only to determine the motor
voltage and frequency applied by the six step
inverter in order to maintain the (V/F) ratio (or the
motor flux) constant from 0 to the nominal speed.
Above nominal speed, the motor operates in the
flux weakening mode; that is, the voltage is
maintained constant at its nominal value while the
frequency is increased proportionally to the speed
set point.
When reversing speed, a short delay is required at
the zero speed crossing so that air gap flux decays
to zero.
V: SIMULATION OF OPEN LOOP CONSTANT
V/F SPEED
The simulation is carried out for 3hp, 220 V squirrel
cage induction motor drive using six step voltage
source inverter (VSI) is shown in figure 7.
Simulation of open loop v/f speed control is built
from seven main blocks. The induction motor,
three-phase inverter, three-phase thyristor rectifier,
bridge firing unit, DC bus voltage regulator, sixstep generator and DC bus voltage filter are
provided.
Fig 8 Control circuit of six step VSI (Voltage Source
Inverter)
This topic confirms that this presented method is
possible to be implemented. The simulations were
carried out at different frequency using
Matlab/Simulink as shown in following Figure.
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Fig 9 Waveform of stator current, rotor speed,
electromagnetic torque at 60Hz frequency
Fig 11 Waveform of stator current, rotor speed,
electromagnetic torque at 40Hz frequency
The open loop constant V/f is simulated for 4.5s. In
figure 9, the speed set point doesn't go
instantaneously to 1800 rpm but follows the
acceleration ramp (2000 rpm/s). The motor reaches
steady state at t = 1.3 s. At t = 1.5 s, the torque is
applied to the motor's shaft step from 0 to 5 N-m.
You can observe a speed slightly. At t = 2.5 s, the
torque applied to the motor's shaft steps from 5 N.m
to 10 N.m. Again speed decrease slightly. At t = 3.5
s, the torque applied to the motor's shaft steps from
10 N.m to 15 N.m then some speed drop at this point.
At t = 4 s, the load torque is removed completely.
Figure:-12. Waveform of stator current, rotor speed,
electromagnetic torque at 30Hz frequency
From above waveform of torque and speed we find
out that when we decrease the frequency the speed is
decrease. But stator current same at all frequency so
torque is also remaining same by maintaining
constant v/f ratio.
Fig 10 Waveform of stator current, rotor speed,
electromagnetic torque at 50Hz frequency
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VI: CONCLUSION
In this work simulink model of induction machine
drives has been implemented. Unlike most other
induction machine model implements, with this
model the user has access to all the internal variables
for getting an insight into the machine operation.
The project simulation is done in “MATLAB”
software. The paper describes the design of the
control stage and presents the result obtained the
motor. This type of control is well justified in
applications requiring a constant V/f speed control
such as pumps, machine tools, mills etc. An analysis
of the steady state equivalent circuit was done in
order to establish the equations that justify the use of
the scalar control method. The open loop speed
controls of motor drive simulation results are
presented. Experimental and simulated results were
used to demonstrate the feasibility of the proposed
solution.
APPENDIX
Parameter
Motor Rating
Voltage
Stator Resistance (Rs)
Stator Inductance (Ls)
Rotor Resistance (Rs)
Rotor Inductance (Ls)
Mutual Inductance (Lm)
Frequency (f)
No. of pole (P)
Inertia (J)
Friction factor (F)
Value
2.238 KVA
220 V
0.435 ohm
0.002 H
0.816 ohm
0.002 H
69.31x10-3 H
60 Hz
4
0.089 Kg-m2
0.005 N.m.s.
[6] Rongfeng Yang, Wei Chen, Yong Yu and
Dianguo Xu, “A novel V/f control system based
on stator voltage oriented method”, International
Conference Electrical Machines and Systems,
ICEMS 2008. International Conference on 17-20
Oct. 2008 Page(s): 83-87.
[7] Mineo Tsuji, Xiaodan Zhao, He Zhang, Shin-ichi
Hamasaki and Shuo Chen,“ Steady-state and
transient characteristics of a novel V/f controlled
induction motor”, International Conference
Electrical Machines and Systems, ICEMS 2009.
International Conference on 15-18 Nov. 2009.
[8] T. Tsuji, S. Chen, and S. Hamasaki,“A novel V/f
control of induction motors for wide and precise
speed operation”, Proc. SPEEDAM2008, pp.
1130-1135, June 2008
[9] Wei Chen, Dianguo Xu, Rongfeng Yang, Yong
Yu, Zhuang Xu, “A novel stator voltage oriented
v/f control method capable of high output torque
at low speed”, Proc. PEDS2009, pp. 228-233
(2009).
[10] Janne Salomäki and Mikko Porma, “Fieldweakening method for v/f controlled hoist drive”,
IEEE International Electric Machines & Drives
Conference (IEMDC), pp 1253-1258 (2011).
.
[11] Chen Wei; Yang Rong Feng; Yu Yong; Wang
Gao Lin; Xu Dian Guo; “A novel stability
improvement method for V/F controlled
induction motor drive systems”. Electrical
Machines and Systems, 2008. ICEMS 2008.
International Conference 17-20 Oct. 2008,
Page(s): 1073-1076.
REFRENCES
[1] B.K. Bose, “Modern Power Electronics and AC
Drives”, Prentice Hall, 2002.
[2] G.R. Slemon, “ Electrical machines for variable
frequency drives”, IEEE Procceding, vol.82, no.
8, pp. 1123-1138, August 1994.
[3] G.McPherson and R. D. Laramore, “An
introduction to electrical machine and
transformers, New York, John Wiley and sons,
1990, pp. 239-302.
[4] K. L. Shi, T. F. Chan, Y. K. Wong and S. L. Ho,
“Modelling and simulation of the three-phase
induction motor simulink’, Int. J. Elect. Enging
Educ., Manchester U.P., Vol.36, pp.163-172
(1999).
[5] Ku.Trupti Deoram Tembhekar, “Improvement
and analysis of speed control of three phase
induction motor drive including two methods”,
Proc. ICETTET, pp.736-741 (2009).
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