Vector Control of an Induction Motor Fed by a PWM Inverter with Output LC Filter
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030 1 Vector Control of an Induction Motor Fed by a PWM Inverter with Output LC Filter Janne Salomäki and Jorma Luomi Abstract — This paper introduces a control method for an induction motor that is supplied by a PWM voltage source inverter through an LC filter. A fullorder observer is used to estimate the system states, and no additional voltage or current measurements are needed for the vector control of the motor. Simulation and experimental results are presented confirming the functionality of the proposed control method. Index Terms — Vector control, induction motor, LC filter, observer. I. INTRODUCTION T HE voltage generated by a PWM frequency converter consists of sharpedged voltage pulses. Sudden alteration of the voltage causes unwanted effects such as bearing currents and high voltage stresses in motor insulations. The oscillation at the switching frequency causes additional losses and acoustic noise. These phenomena can be eliminated by adding an LC filter to the output of the PWM inverter. In addition, the EMI shielding of the motor cable may be avoided if the output voltage of the inverter is nearly sinusoidal. The control of an induction motor becomes more difficult if an LC filter is used. Usually, a very simple scalar control method (voltsperhertz control) is chosen. Although better control performance is needed in many cases, only few publications deal with the vector control of an induction motor fed via an LC filter. A deadbeat controller has been used to control the inductor current and the capacitor voltage [1], the highpass filtered stator voltage has been used to correct the voltage reference [2], and a multiloop feedback controller has been proposed [3]. In these methods, extra current or voltage measurements are needed in addition to the phase current and dc voltage measurements usual in a frequency converter. A challenge for the motor drive control design is to keep the number of measurements low in order to obtain cost savings and reliability improvements. In this paper, a method is presented for the vector control of an induction motor fed by an inverter with an output LC filter. A cascade control method is used to control the inverter current, the stator voltage, the stator current and the rotor speed. The system states are estimated by a fullorder observer. II. THEORY The principle of the control system is shown in Fig. 1. The inverter output voltage u A is filtered by an LC filter, and the induction motor (IM) is...
030
1
Abstract — This paper introduces a control method for an in
duction motor that is supplied by a PWM voltage source inverter
through an LC filter. A fullorder observer is used to estimate the
system states, and no additional voltage or current measurements
are needed for the vector control of the motor. Simulation and
experimental results are presented confirming the functionality of
the proposed control method.
Index Terms — Vector control, induction motor, LC filter, ob
server.
I. INTRODUCTION
HE voltage generated by a PWM frequency converter con
sists of sharpedged voltage pulses. Sudden alteration of
the voltage causes unwanted effects such as bearing currents
and high voltage stresses in motor insulations. The oscillation
at the switching frequency causes additional losses and acous
tic noise. These phenomena can be eliminated by adding an
LC filter to the output of the PWM inverter. In addition, the
EMI shielding of the motor cable may be avoided if the output
voltage of the inverter is nearly sinusoidal.
The control of an induction motor becomes more difficult if
an LC filter is used. Usually, a very simple scalar control
method (voltsperhertz control) is chosen. Although better
control performance is needed in many cases, only few publi
cations deal with the vector control of an induction motor fed
via an LC filter. A deadbeat controller has been used to control
the inductor current and the capacitor voltage [1], the high
pass filtered stator voltage has been used to correct the voltage
reference [2], and a multiloop feedback controller has been
proposed [3]. In these methods, extra current or voltage meas
urements are needed in addition to the phase current and dc
voltage measurements usual in a frequency converter. A chal
lenge for the motor drive control design is to keep the number
of measurements low in order to obtain cost savings and reli
ability improvements.
In this paper, a method is presented for the vector control of
an induction motor fed by an inverter with an output LC filter.
A cascade control method is used to control the inverter cur
rent, the stator voltage, the stator current and the rotor speed.
The system states are estimated by a fullorder observer.
The authors are with Helsinki University of Technology, Power
Electronics Laboratory, P.O. Box 3000, FIN02015 HUT, Finland (email:
janne.salomaki@hut.fi).
II. THEORY
The principle of the control system is shown in Fig. 1. The
inverter output voltage
A
u
is filtered by an LC filter, and the
induction motor (IM) is fed by the filtered voltage
s
u
. The
inverter current
A
i , the electrical angular speed
m
ω
of the
rotor, and the dc link voltage
dc
u
are the only measured
quantities, whereas the stator voltage
s
u
and current
s
i
of the
motor are estimated by an observer (the estimated quantities
being marked by ‘^’). The system is controlled by nested con
trol loops in the rotor flux reference frame.
A. Filter and Motor Models
In a reference frame rotating at angular speed
s
ω
, the equa
tions for the LC filter are
)(
1
j
d
d
sA
f
A
f
Lf
A
s
A
uu
L
i
L
R
i
t
i
−+−−=
ω
(1)
)(
1
j
d
d
sA
f
s
s
s
ii
C
u
t
u
−+−=
ω
(2)
where
f
L is the inductance and
Lf
R the series resistance of
the inductor, and
f
C is the capacitance of the filter.
The motor control is based on the inverse
Γ model of the
induction motor in the rotor flux reference frame. The stator
and rotor voltage equations in this reference frame are
s
s
s
s
s
s
t
iRu
ψω
ψ
j
d
d
++=
(3)
R
ms
R
R
R
t
iR
ψωω
ψ
)(j
d
d
0 −++=
(4)
PI
P
PI
PI
Observer
IM
+
−
refm,
ω
m
ω
s
i
ˆ
s
u
ˆ
refs
u
,
refs
i
,
refA
i
,
s
u
f
C
f
L
s
i
dc
u
A,ref
u
A
i
+
−
+
−
+
−
Fig. 1. Principle of the control system.
Vector Control of an Induction Motor Fed by a
PWM Inverter with Output LC Filter
Janne Salomäki and Jorma Luomi
T
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Vector Control of an Induction Motor Fed by a PWM Inverter with Output LC Filter

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