DC Link and Dynamic Performance Features
of PWM IGBT Current Source Converter Induction Machine Drives
with Respect to Industrial Requirements
Friedrich W. Fuchs, Alfons Kloenne*
Institute of Power Electronics and Electrical Drives; *now: Bosch GmbH
Christian-Albrechts-University of Kiel
Kiel, Germany
fwf@tf.uni-kiel.de
Abstract— When rating the IGBT PWM current
source induction machine drive in relation to industrial requirements, there are some important performance features to be studied. These are especially the dynamic control performance and the size
and influence of the inductance in the dc link. These
subjects and their interference are evaluated here,
giving a basis for rating. With a high pulse frequency of the converters, with respect to the current
ripple, only a low inductivity in the dc link is necessary, leading to low weight and volume and improving the ideal basic dynamic performance, too.
A high dynamic performance, comparable to PWM
voltage source induction machine drives, is reached
when, beneath a dc link inductance with suitable
low inductivity, selected control methods as field
oriented control and, as chosen here, optimal state
control, are applied. The basic performance is analysed and the the results are partially verified by
measurements at a 7.5 kW test drive.
Keywords: variable speed drives; current source
converter; current source inverter; induction machine;
industrial requirements; dc link inductance; dynamic
performance
I.
INTRODUCTION
Nowadays converter fed electrical ac drives are
used in industry in numerous applications and in high
numbers. Basic type used in the power range from less
than 1 kW to up to 2000 kW and more is the induction
motor fed via PWM voltage source converter (PWM
VSC IM) [1]. This drive fulfills today nearly all requirements given from industrial applications and so
sets up the standard for drives. Nevertheless, research
on this type of drive still continues.
Besides this type of drive with voltage source converter there is the in some way dual type with induction
machine fed via PWM current source converter (PWM
CSC IM). This PWM CSC IM drive, however, today is
used mainly only for special applications, preferably in
the high power range [2] or for retrofitting existing
fixed speed machines. There are possible new applications being investigated in the medium power range as
there are high speed drives and feeding of superconducting magnetic energy storage systems, where only
the rectifier is needed. The CSC is in competition with
the widely spread PWM VSC IM drive and as a matter
of fact has gained only a small application range. Nevertheless, regarding and investigating the performance
of the PWM CSC IM drive, especially with IGBT, and
in the medium power range, some remarkable features
can be found. These show, that this drive is well applicable, moreover there are only limited technical disadvantages but also advantages compared to the PWM
IGBT VSC IM drive. Some of these performance features are presented here.
Industrial applications set up the economical and
technical requirements for electrical drives. Though
depending on the individual application, in general the
performance requirements for drives can be defined for
a certain group of applications, as for example for
general industrial application. There, some most important technical performance requirements can be
found. The basic requirement is to enable the motion
function of the drive required from the process driven,
in order to enable a required dynamic and stationary
control of the torque or of the speed of the machine.
Another requirement is a high efficiency, which means
low losses in the drive, as this leads to low operation
costs. Moreover, the drive has to withstand the environmental conditions. The electromagnetic interference
of the drive has to be in the standardised limits to correspond to the laws and make an operation of other
electronic systems possible. To avoid destruction, a
suitable protection system has to be installed. An economical reliability and lifetime of the drive are required, in order to minimise the costs, too. Often a
limited weight as well as a limited volume of the drive
are further important requirements.
In this article, for rating the PWM IGBT CSC IM
drive two important performance features, with respect
to industrial requirements, are selected, both influencing each other. These are the dynamic control performance and the properties and influence of the dc link
inductance. These subjects will be evaluated, giving a
basic aspect for rating. Nevertheless, analyses in this
field are well known, for example from [3, 4]. Here, a
special field of analysis is chosen, the focus is put on
the interference of dc link inductivity, current ripple
and control dynamics and on an advanced control sys-
Ld
uG1
~
LL
ASM
~
~
CL
CM
Figure 1: PWM current source converter induction machine drive system, power section [5]
tem. The results of the analysis are partially verified by
measurements at a 7.5 kW test drive.
The content of this investigation is structured in that
way, that in section II the PWM CSC IM drive system
is introduced. Section III covers the design of the dc
link with respect to current ripple and to basic control
dynamics, limited by the power circuit. In section IV
the control structure is presented and the dynamic control performance is analysed. Section V contains the
conclusion.
II.
III.
DESIGN OF THE DC LINK
A. Limiting the Current Ripple
For steady state behaviour, the mean value of the
mains and machine side dc link voltages have to be
equal when neglecting losses. The inductance in the dc
link has to smooth the dc link current to the determined
ripple value to render regular drive operation. The
difference of the alternating part of the line side ud,L,ac
and machine side ud,M,ac dc link voltage affects the
current pulsations. The inductivity Ld has to be designed according to the maximum voltage versus time
integral [6] divided by the tolerable dc current ripple
∆id:
1
Ld min =
∫ u d , L , ac − u d , M , ac dt max .
∆i d
PWM CSC IM DRIVE SYTEM
The PWM CSC IM drive system, see fig. 1, consists of a mains side rectifier and a machine side
inverter, both equipped with IGBTs and series diodes
[5]. The diodes prevent back voltage on the IGBTs.
Because of the use of IGBTs a high PWM switching
frequency of for example 3 kHz or more, comparable
to that of voltage source PWM converters, can be applied. Interphase capacitors have to be inserted at the
line and machine side to enable a switched rectifier
respectively inverter input current. Rectifier and
inverter are connected via a direct current link by
means of th dc link reactor. In recent years, several
investigations on this PWM IGBT CSC IM drives have
been carried out and published in the literature [3, 4, 5].
(
)
(1)
The maximum of the integral is reached, when both
components, on line and on machine side, have their
maximum. For a PWM converter there is a maximum
with half of the triangle voltage amplitude for half of
the pulse period (at a modulation of 0,5). This leads to
an inducitivity [3] of:
L d min = 2 ⋅
UL
6
.
⋅
4 f p ⋅ ∆I d max
8
7
6
5
4
L+dmin 3
2
+
dmax
−I
= 0.3; 0.2; 0.1; 0.05
1
0 0
10
10
fp
1
10
2
+
Figure 2: Necessary inductivity in the dc link of a PWM CSC versus pulse frequency (normalised)
(2)
The factor 2 represents that both, line and machine
side, voltages have their influence. The necessary inductivity becomes lower with higher pulse frequency
according to the smaller voltage pulses switched to the
inductance. Figure 2 shows the dependency of the
inductivity from the pulse frequency and the allowed
current ripple.
Here and in the following diagrams for normalised
quantities are given to be independent on the power of
the system. The quantities are related to their nominal
values, normalised quantities marked by a “+”. The
normalisation factors are given in the appendix.
Fig. 2 shows the advantage of IGBT PWM CSCs
with high pulse frequencies. For high power GTO
PWM CSCs with low pulse frequency of for example
500 Hz (fp+= 10) a high inductivity is necessary. An
IGBT PWM CSC with standard 3000 Hz (fp+= 60)
pulse frequency makes an inductivity possible, that is
six times lower, for 10.000 Hz (fp+= 200) it can be 20
times lower. From the diagram can be concluded, that a
high pulse frequency has to be recommended.
Corresponding to the following equations from [7]:
W Ld =
1
⋅ Ld ⋅ I d2
2
3/4
V Ld ~ m Ld ~ W Ld
(3)
UL
6
.
⋅
48 f L ⋅ ∆I d max
30
tresp,id
Ud,M+ = 0.9
20
[ms]
10
0
0
Ud,M+ = 0.1
0.2
(5)
The prefactor in this equation is 12 times lower than
that of the IGBT PWM CSC in eq. (2), for 20 % current
ripple the necessary inductivity is 1.6. So, both types
need the same dc link inductivity for a pulse frequency
of 600 Hz (12⋅50 Hz). For higher pulse frequencies the
inductivity for IGBT PWM CSC is lower than that of
the line commutated CSC.
0.4
i+d,step
0.6
0.8
1
0
1.5
1
0.5
2
Ld +
Figure 3: Step response performance of the dc link
current with ideal rectifier voltage control without
dynamic inverter influence (normalised quantities)
control performance is given by a linear current rise,
depending on the dc link inductivity Ld. The response
time tresp on a reference current step id,step thus can be
calculated to:
(4)
a low magnetic energy WLd, proportional to the inductivity Ld, leads to a lower weight mLd and lower volume
VLd. This is especially advantageous for IGBT PWM
CSCs compared to line commutated CSCs. So, small
inductances can be made possible for PWM CSC with
high pulse frequency by means of proper design in the
dc link.
The advantage of the IGBT PWM CSC against the
former line commutated CSC can also be defined. Their
dc voltage consists of time sections of the sinusoidal
line or machine voltage. The maximum voltage at the
inductance arises for a control angle of 90° with a voltage area of half of the triangle voltage amplitude decreasing during the half of one sixth of the mains period
down to zero [6]. This leads to a dc link inductivity for
line commutated inverters of:
L d min = 2 ⋅
40
t resp =
Ld ⋅ i d ,step
u d ,L − u d ,M
.
(6)
It leads to a dc link current control performance as
shown in figure 3. The step response time increases
with higher dc link inductivity and higher machine side
dc link voltage. Table I gives some response times for
this system. The real control times of the whole system
will be longer because of the delay of the real control
on the line side and machine side.
TABLE I
Step response times for ideal control
ud,M
0,1 ud,M,mean
0,3 ud,M,mean
0,9 ud,M,mean
tid, resp
tid, resp
i+ d,step = 1 i+ d,step = 0.33
Ld+=1
Ld+=1
1.9 ms
0.6 ms
2.5 ms
0.8 ms
17.4 ms
5.8 ms
tid, resp
i+ d,step = 1
Ld+=0.2
0.4 ms
0.5 ms
3.6 ms
Thus, as a basic action, for the PWM CSC IM drive
a dc link inductivity with a low value has to be chosen,
not only for low weight and cost, but also to allow fast
current control. Nevertheless, a minimum value has to
be kept to limit the dc link current ripple.
IV. DYNAMIC CONTROL PERFORMANCE
B. DC Link Basic Control Dynamics
Assuming an ideal fast control of the line side dc link
voltage ud,L at the rectifier, taking into account the
influence of the machine side dc link voltage ud,M, the
A. PWM IGBT CSI IM Drive Control
Research results have been published on control of
CSC IM drives as there are applications of standard
control methods as field oriented control [8, 9] and,
iLq,ref
pre-control
id,ref
id
~iSq,ref
tref
nref
n
n control
ΨR,ref
iLd,ref
~
id control
mInv,
Field
oriented
control
iSb
mRec, ,ref
Optimal
state feedback
control
mRec,☺,ref
,ref
~
mInv,☺,ref
iSa
M
3~
Figure 4: Control structure of the PWM CSC IM drive
moreover, of applying modern control methods to CSC
IM drives, for example state feedback control on the
machine side and on the mains side [10, 11]. The dynamic performance can be optimised in this way. The
rectifier and inverter PWM control, vector control or
similar procedures, have been investigated in many
works. The resonant network on mains and machine
side has to be included in the control design. The system design, here the inductance in the dc link, has a
great influence on the dynamic performance. Some
research work has also deepenend this field, some results are given in [12]. So, reports on the dynamic performance of the system components can be found, but
there are not many results on the complete PWM CSC
IM drive system. Some examples can be found, stating
minimum 100 % torque step response times of about 10
ms [13] or presenting a step response up to 50 % of the
nominal torque generating current of 2 ms [10], but
only in simulation.
In the CSC IM drive, see figure 1, the line side rectifier has to control the dc link current via the mains
side dc link voltage. Disturbance value is the machine
side dc link voltage, produced by the induction machine via the inverter. The control performance is basically influenced by the dc link inductivity. The machine side inverter has to control the current in the
motor to adjust the motor torque to the reference value.
This is mainly a phase control, as the current amplitude
in the machine is given according to the amplitude of
the dc link current controlled by the rectifier.
Comparing a PWM IGBT CSC IM drive, where the
rectified mains voltage being switched to the dc link
inductance controls the dc link current and the current
in the machine, with the PWM VSC IM drive, where
the constant dc link voltage is being switched by the
inverter directly to the machine, a worse dynamic behaviour of the CSC IM drive could be assumed.
B. Control Structure and Methods
For the real system, a control structure for the total
drive as shown in figure 4 [5] has been chosen. Superimposed is a speed control, which gives the torque
reference value tref. This torque is the input signal for
the field oriented control of the induction machine. The
reference active current isq,ref from this control is the
reference value for the dc link current and the input
signal of the dc link current controller, where an additional feedforward precontrol is added. The dc link
current is controlled via the line side current controller.
The machine side inverter and the machine have to
be controlled for fast torque generation. Here, the standard field oriented control for CSI fed induction machines is selected, for which very short step response
times are reported. The influence of the inverter output
capacitors has been neglected in the control structure
without relevant reduction of the dynamics [5].
The dc link current control is a standard PI control
with precontrol. The line side current control is of high
importance, because it has to work via the dc link on
the machine and affects the system dynamic in a basic
way. State feedback control has a superior performance
for this multivariable system. Here, an optimal state
feedback control is chosen, which operates according
to the optimum criterium [14]:
t
1 e
J = ∫  xT ( t ) Q x( t ) + uT ( t ) R u( t ) dt .
(7)

2 t 
0
The precondition for the general control concept is,
that the dc link current, for minimal losses in the drive,
has to be kept to a minimum. This means a high
modulation factor, preferably 1, at the machine side
converter. To achieve constant flux in the machine the
dc link current has to be controlled instantaneously as
Figure 5: Torque step response (measurement)
Without sufficient dc link current (left) and with sufficient dc link current (right); iSq; iSd; id , each 10 A/div
(400 V/7,5 kW drive; machine voltage US=0,33 USN; Ld= 10 mH; reference torque step: T = 0 to TN (I+d=0 to 1);
machine speed N=0,33 NN = 500 min-1)
torque varies. A high pulse frequency of the converter
is chosen and a low inductivity in the dc link is used, as
evaluated. A suitable control parameter tuning is applied.
C. Dynamic Performance of the Drive
All these design and control methods, including the
optimal state feedback control and optimised control
parametrisation, have been applied to the drive leading
to the control structure of figure 4. The control has
been implemented in a DSP-system for a 7.5 kW drive.
With these measures, the dynamic drive performance as shown in figure 5 has been achieved. On the
left, the drive operates with minimum dc link current
when the reference step starts, on the right the drive has
a sufficient dc link current for the reference torque. A
fast control with sufficient decoupling, avoiding greater
influence from q- to d-axis, can be seen. The response
time for a step from 0 % to 100 % of the nominal
torque generating current occurs within ca. 4 ms. This
torque step response time is very near to that of VSC
IM drives (about 2 ms [15]). The system data are according to table I, right column, third line. So, that
table gives an ideal response time of 0.5 ms. The response time from the test is about eight times higher
according to non ideal control.
The dc link current is well smoothed, the ripple is
about 15 %. Further reducing the inductivity in the dc
link could lead to even lower torque step response
times, however with a higher current ripple.
IV.
CONCLUSION
Performance features of PWM CSC IM drives have
been investigated in relation to industrial requirements.
The minimum limit for designing the inductivity in the
dc link has been stated. The inductivity can be low for
high pulse frequencies and thus its weight and volume
is substantially lower compared to that of former mains
commutated converters. The advantageous influence of
a low inductivity on the basic dynamic performance of
the drive has been evaluated. To optimise the dynamic
performance of the drive, suitable control methods
have been selected. They have been implemented and
parameterisation has been optimised as well as a proper
design of the dc link has been done. Torque response
times have been reached which are very close to those
of the voltage source induction machine drive used for
industrial applications. So, concerning losses, volume
and weight in the dc link as well as dynamic characteristics the PWM CSC IM drive investigated has a
performance according to standard industrial requirements.
V.
APPENDIX
A. Normalszation factors
The normalised values are marked by a (+).
TABLE II
Normalisation factors
Time (sometimes
not normalised)
Frequeny
t+ = t /TLN
Line voltage
UL+= UL/ULN
DC voltage
ud+ = ud/(( 3 6 /π) ULN
Line current
IL+= IL / ILN
DC current
Id+ = Id/(π ILN/ 6 )
f+ = f /fLN
Machine voltage
UM+= UM/ULN
Machine current
IM+= IM / ILN
[4] de la Parra, I.; Gonzales, S.; Tamarit, G.: Modulation and
Control of Current Source Converters for High Dynamic Performance of Induction Motors; Euopean Conference on Power
Electronics and Applications, 1997, vol. 3, pp. 756-761.
[5] Kloenne, A.: Control and Modulation Strategies for Current
Source Converters with Induction Motors; in German; Phdthesis, University of Kiel, Germany, 2002.
B. Data of the test drive
TABLE III
Data of the test drive
[6] Fuchs, F. W.; Mueller-Hellmann, A.: Control Methods
for Reducing the Inductance in the DC Link of Current Source
Inverters; IEEE Transactions on Industry Applications, Vol. IA19, No. 5, 1983, pp. 699-707.
Line voltage (delta)
400 V
[7] Heumann, C.: Current Source Converter with Pulse Width
Modulation; in German; etz 1993, 2, pp.170-176.
Line frequency
50 Hz
Apparent power
7.5 kW
[8] Busse, A.; Holtz, J.: Multiloop Control of a Unity Power Factor
Fast Switching AC to DC Converter; IEEE Power Electronics
Specialists Conference, 1982.
Filter capacitance CF
16 µF
Filter inductance LS
1 mH
DC Link inductance Ld
10 mH
Pulse frequency fp
3.125 kHz
REFERENCES
[1] Stemmler, H.: High Power Industrial Drives; in: Power
Electronics Technology and Applications II; Editor, Lee, F.;
IEEE, New York, 1998.
[2] Zargari, N.; Xiao, Y.; Wu, B.: PEM CSI-Based Vector
Controlled Medium Voltage AC Drive with Sinusoidal Input
and Output Waveforms; Proceedings of the IEEE-IAS, 1997,
vol. 1, pp. 768-774.
[3] Kaltenbach, K.: Current Source PWM Inverter as Power
Controller for Induction Machines; in German; PhD-thesis, Berlin, 1992.
[9] Salo, M.; Tuusa, H.: A Vector Controlled Current- Source PWM
Rectifier with a Novel Damping Method; IEEE Transactions on
Power Electronics, Vol. 15, No. 3, 2000, pp. 464-470.
[10] Hintze, D.; Schröder, D.: Four Quadrant AC-Motor Drive with a
GTO Current Source Inverter with Low Harmonics and On Line
Optimized Pulse Patterns; IPEC, Tokio 1990, p. 405-412.
[11] Espinoza, J.; Joós, G.: State Variable Decoupling and Power
Flow Control in PWM Current-Source Rectifiers; IEEE Transactions on Industrial Electronics, Vol. 45, No. 1, 1998, pp. 7887.
[12] Hombu, M.; Ueda, S.; Ueda, A.: A Current Source GTO Inverter
with Sinusoidal Inputs and Outputs; IEEE Transactions on Industry Applications, Vol. IA-23, 1987, no. 2, p.247-255.
[13] Amler, G.: A PWM Current Source Inverter for high Quality
Drives; EPE-Journal, Vol.1, 1991, no. 1, p.21-32.
[14] Kloenne, A.; Fuchs, F. W.: Optimal State Feedback Control of a
Vector Controlled Current Source Rectifier; Proceedings of the
EPE conference, Graz, 2001.
[15] Aaltonen, M.; Tiitinen, P.; Laly, J.; Heikkilä, S.: Direct Torque
Control of AC Drives; ABB Technique 1995, p. 19 – 24.