King Saud University
From the SelectedWorks of Hadeed Sher
Winter October 25, 2012
Performance of inverter fed induction motor
under open circuit DC link capacitor
Hadeed A Sher, King Saud University
Available at: http://works.bepress.com/hadeed-sher/11/
Performance of Inverter Fed Induction Motor under
Open Circuit DC link Capacitor
Hadeed Ahmed Sher, Khaled E Addoweesh, Yasin Khan
Syed Abdul Rahman Kashif
Department of Electrical Engineering
Department of Electrical Engineering
King Saud University
University of Engineering and Technology,
Riyadh,Saudi Arabia
Lahore, Pakistan
Email: hsher.khaled.yasink(@ksu.edu.sa)
Email: kashif@uet.edu.pk
Abstract-Induction motors are widely used in industry due
to their promising performance. Majority of induction motors
are used in inverter based drives in industrial setups. Efforts are
being put in the fault analysis of induction motors. Most of the
analysis available in literature discusses the failure of electronic
components like power MOSFET, optocoupler failure, failure of
pulse generation card etc. This research work is a continuation
of fault analysis studies of an inverter fed induction motor drives
under different fault conditions. This paper presents the effect of
dc link capacitor open circuit on the performance of inverter fed
induction motor. Effect on stator current, inverter output, the
speed and torque of the motor are particularly studied in this
paper. The faults in the DC link is generally not that common
but there is always a chance of capacitor failure. The simulated
results presented in this paper can augment the efforts for the
reliability enhancement of induction motor drives.
Index Terms-DC link capacitor, Inverter, Fault analysis,
Induction motor
I. INTRODUCTION
Induction motors are widely used in the industry as a basic
unit of automated systems. Due to their robust nature and so
called ease in control mechanism they are jewel in the eye for
both the researchers and industrialists. Their wide used in the
industry has forced the researchers to carry comprehensive
research studies on the issues related to their performance
under faulty conditions. These motors are mostly used in
industry with a power electronic based control systems. This
not only helps in achieving the user required mechanical
characteristics but it also conserve energy. The association
of sophisticated power electronic system needs to have a
very accurate fault tolerant system for smooth operation of
industrial work.
In literature, a lot of work has been cited by researchers in
the field of analysis and diagnosis of faults in inverter based
motor based systems [1], [2]. The faults discussed in these are
Transistor failure, DC link capacitor fault (open circuit or short
circuit), Gate drive pulse failure and Inverter leg fault. The
probability of different faults in inverter based setup is reported
in [2], [3]. They stated that in early days the semiconductor
failure was mainly due to Pulse Width Modulation (PWM)
control chips as they were not that reliable and sophisticated.
Since, the Voltage Source Inverters (VSI) required to have a
smooth dc link voltages therefore capacitors are used in the
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IEEE
dc link. Lahyani et.a!' [2] had worked on the issues that are
related to capacitor failure and deduced that the probability of
capacitor failure is almost 60 %. It is important to specify here
that the ageing of capacitor results in the increase of equivalent
series resistance (ESR). This is a big issue in switching circuits
because ESR can combine with the switching frequency and
causes self heating and indirectly leads towards the failure of
capacitor. Most of the investigators have included this fault
while penning down the possible faults that can occur in an
inverter operation [4]-[8]. However, they did not investigated
this problem systematically. For some researchers this is not
a very serious issue as most of the commercially available
electric drives are well designed to respond in faulty conditions
[4], [8]. Others [5] discussed the voltage drop in DC link but
for a short period of time. Work of B.Biswas et.al [6] is also
semiconductor oriented and the results are presented in terms
of harmonics and he did not worked on possible failures of
dc link capacitor. Peuget et.a!' [7], [9] classified the DC link
capacitor failure but they did not work towards its thorough
investigation as they stated that they are not interested in non
semiconductor issues. It can therefore be concluded that almost
all the work in the fault analysis for inverter based induction
machines is semiconductor based.
In this paper, we studied the performance of inverter fed
induction motor under open circuit DC link capacitor. This
work is a continuation of efforts in enhancing the reliability
of VSI based induction motor system. In our previous research
study we did analysis of the performance of VSI fed induction
motor under short circuit DC link as well as the effect of
back EMF on freewheeling diodes [10], [11]. Capacitor open
circuit may occur due to dry soldering or due to an error in
the system fabrication stage. The results can be incorporated
in the designing of a fault tolerant system and the optimal
protection system design.
II. PROBLEM DESCRIPTION
For a three phase VSI it is a general practice to use a DC link
capacitor for smoothing the voltage. This capacitor also acts as
a voltage damper and provides a sort of rid through capability
for any sag or swell in the input voltages. This capacitor is
fixed in shunt with the rectifier and the inverter therefore,
any fault in it can affect the performance of both inverter
651
and the rectifier. In case of open circuit of DC link capacitor,
the inverter gets dc input voltage with ripples, the frequency
of which is dependent on the type of input. For example if
the three phase VSI is fed via a single phase rectifier then
the ripples at the input of inverter will be double the source
frequency. However, if a three phase supply is rectified for
obtaining voltages at DC bus then the ripple frequency will
be six times the source frequency. Therefore in this paper we
have bifurcated this issue for analysis on single phase AC
supply and three phase AC supply. This paper discusses the
analysis of capacitor open circuit fault on the performance of
inverter.
VqS
I
�
] [
- 2
3
CO (e - 120°)
S
Sin(e - 120°)
0.5
Case
Sine
0.5
COS(e + 120°)
Sin(e + 120°)
0.5
][
vas
Vbs
Vcs
(I)
where, VOSs is a zero sequence component that may be
present. Solving eq.l gives
VqsS
=
Vds8
':Vas - �Vbs - �Vcs
3
=
3
3
V3
V3
Vqs
= Vqs' cosOc
- Vds' sinOe
(4)
Vds
= Vqss SinOe + VdsS cosOc
(5)
The flux linkage equations on the basis of DQ transformations
are given below as described in Krause model [12]. Based
on Fig. 1, the transient model of electric machine in terms of
voltage and current can be written as below [12], [13].
Vqs
Vds
[ 1
Vqr
=
I
idr
.-
(We-Wr) If/qr
We If/qS
Vds
�
Fig. 1.
]
Lm
If/dS =Fd/Wb
I
If/dr=Fd/Wb
/
Vdr
I
DQ equivalent model of Induction Motor. [12]
Where, S is a Laplace operator and as in our case for a
squirrel cage induction motor the two values i.e. Vqr and Vdr
will be zero [12], [13].
From eq. 6 above, the speed Wr is related to the torque and
cannot be considered as a fixed entity. The torque is given as
[13]
c = TL + J
T
dw =
dt
= TL + ':J
p
dwr
dt
(7)
Where,
(2)
This stationary frame of reference is then converted to rotating
two phase frame of reference. This rotating reference frame
rotates at a synchronous speed We w.r.t. d s -q s and the angle
8e= wet. The realization of this synchronously rotating refer­
ence frame is given below [13]
r
Vqr
(a)
•
•
(3)
-�Vbs+ �Vcs
If/qr=Fq/%
/
idS
---
Induction motor is represented by the equivalent models as
shown in Fig. I [12], [13]. The conventional per phase equiv­
alent circuit is sustainable only in the analysis of induction
motor in steady state. For analysis in transient state the three
phase induction motor stationary reference frame (AS_BS and
GS ) is converted into two phase stationary reference frame
(dS_qS) which is then transformed into the rotating two phase
e e
frame of reference (d _q ). Here the superscripts 's' refers to
stationary reference frame and 'e' for rotating reference frame.
The mathematical model of induction motor is presented here
since the motor used in our simulation setup is tested against a
d-q modeling arena. Axis transformation from the three phase
stationary axis AS_Bs_Gs to qS_dS is given below [13]:
Vq S
S
Vds8
Vas'
Lm
If/qs=Fq/%
III. MATHEMATICAL MODELING OF INDUCTION MOTOR
[
(we-wr) If/dr
We If/dS
•
TL represents the load torque
Wm the mechanical speed
and J is the rotor inertia
The value of torque derived after keeping in mind the inter­
action of air flux gap and the rotor m.mJ. relating the d-q
components of variables can be expressed as following [13].
(8)
In terms of de-qe components, the eq.(8) can be expressed as
(9)
The developed torque in terms of inductance and the current
values with d-q phases can be rewritten as
(10)
(6)
Vdr
Rs + SLs
-weLs
-(We - wr)Lm
weLs
Rs + SLs
SLm
SLm
-weLm
-(Wc - wr)Lr
wcLm)
SLm
Rr+ SLr
Eqs. (6),(7)and(1O) above represents as a whole the complete
dynamic behavior of induction motor which is obviously a non
linear model [13].
652
Fig. 2.
Simulation setup
IV. SIMULATION SETUP
V. RESULTS
Figure 2 shows the simulation arrangement for investi­
gation of the open circuit capacitor problem. SIMULINK
environment is considered for carrying out simulations. In this
setup the input is shown as a three phase AC source. The
same subsystem is used for the single phase AC source by
disconnecting other phases with the rectifier. The fault in the
DC link capacitor is realized by an ideal switch Sl, that is
connected in series with the DC link capacitor. The switch Sl
acts as a short circuit when ON and completely cuts off the
connection of DC link capacitor when OFF. A step change is
applied to the system by turning off the switch Sl at time t=3
sec. The parameters of induction motor used in this study are
as follows
The induction motor is connected with single and three
phase supply where the input line voltage is 381 Vp (for 220
V) with 50Hz frequency. The generated ripple frequency is
100 and 300 Hz respectively for single and three phase input.
For the circuit diagram (Fig. 2) two comprehensive analysis
studies of induction motor behavior are carried out
•
•
•
•
Rated
Rated
Rated
Rated
•
•
Figure 3 shows the system without any fault and shows very
satisfactory results in terms of electrical parameters of inverter,
thus validating our model.
Power = 5 hp
Voltage = 460 V
Frequency = 60 Hz
RPM =1750 rpm
MOSFETs are used as power electronic switches in the in­
verter. The control method is based on Sinusoidal Pulse Width
Modulation (SPWM) with following parameters.
•
•
•
•
Without creating a fault
By creating a fault at DC link capacitor
Carrier frequency = 2050 Hz
Control frequency = 50 Hz
Amplitude modulation ratio = 0.8
Frequency modulation ratio = 41
Fig. 3.
Inverter voltages and stator currents without fault
A. Single Phase Input
The motor is mechanically loaded in terms of applied
torque. A step torque of 3.5 N-m is applied to the motor for
single and 8 N-m is applied for three phase input respectively
at time t=1.5 sec.
In some applications single phase supply is used as an
input to the rectifier. In this case the DC link voltage is
less than the nominal voltage for three phase input. The
difference that forced us to simulate it separately is the visible
change in ripple frequency. For a single phase supply the
ripple frequency is only 100 Hz which means that for sure
653
a large value capacitor is required for smoothing of DC link
voltage. Figure. 4 shows the effect of capacitor fault on DC
link voltages. The ripples are very high and about to touch
the zero level. At time t=3 sec fault is created that results
in the destruction of motor parameters as shown in Fig.5.
High spikes are visible in stator current and voltage waveform,
moreover, it is studied that the speed of the motor begins to
decelerate showing clearly that the motor is no more able to
withstand the applied torque. The electromagnetic torque also
shows disturbance after the capacitor gets open circuited.
Fig. 5. Motor parameters with DC link capacitor open circuit with single
phase supply
B. Three Phase Input
Mostly the inverter fed induction motor are fed via off
line power supply that is three phase in nature. The input
considered for this simulation as stated above is 312 Vp with
50 Hz frequency. The generated ripple frequency is 300 Hz
which is 6 times the input frequency. For a three phase rectifier
the average output voltage is given as
Vdc
=
1.654
x
Vm
2.97
2.98
2.99
Fig. 6.
3.01
'.02
3.03
•. "
3."
DC link voltages for three phase supply
(11)
Where, Vm is the peak phase voltage. The high ripple fre­
quency is an advantage of using three phase systems and as
shown in Fig. 6 the dc link voltages contains ripple but they
does not go to zero. This definite advantage is very much
visible on the motor performance as depicted in Fig.7. The
disturbance in stator current is very minor and the motor runs
but due to applied torque and the ripples in the inverter output
voltage its speed is decreased. The electromagnetic torque
also shows the change after the load torque and the fault are
applied.
Fig. 7.
Motor parameters with DC link capacitor open circuit with three
phase supply
REFERENCES
VI. CONCLUSION
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This paper mainly highlights the following aspects
•
•
Possible reasons of DC link capacitor open circuit.
Investigated the change in electrical and mechanical parameters of a machine drive system.
The failure of DC link capacitor could be a serious issue if
motor is running with single phase supply. It is very much clear
that the use of three phase supply for an inverter has a good
ride through capability under DC link capacitor failure. This
also means that greater the ripple frequency is better will be
the response of inverter to dc link capacitor failure. The study
of changes in system parameters forms the bottom line for the
designing and implementation of a fault tolerant system.
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