Nr 62
Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych
Nr 62
Politechniki Wrocławskiej
Studia i Materiały
Nr 28
2008
electrical tram drives, field weakening,
direct torque control with space vector modulation
(DTC–SVM)
Paweł WÓJCIK*, Dariusz SWIERCZYŃSKI**,
Marian P. KAZMIERKOWSKI*, Michał JANASZEK***
DIRECT TORQUE CONTROLLED PWM
INVERTER FED INDUCTION MOTOR DRIVE FOR
CITY TRANSPORTATION
In this paper an application of Direct Torque Control with Space Vector Modulation (DTC–
SVM) controlled induction motor for tram drive is presented. Thanks to its advantages like: excellent
dynamics, low torque ripples, insensitivity for motor parameters changes, constant switching and low
sampling frequency, DTC–SVM is used in various applications. In proposed case DTC–SVM is used
for tram traction drive based on PWM Voltage Sourced Inverter Fed Induction Machine. This method
was chosen after comparison with Field Oriented Control (FOC), Switching Table Direct Torque
Control (ST–DTC) and Direct Self Control (DSC). DTC–SVM combines advantages and eliminates
drawbacks commonly used methods like FOC and ST–DTC. There are no hysteresis controllers, what
gives possibility to reduce sampling and also switching frequency. It leads to reduce switching loses
(important in high power applications). Constant switching frequency is ensured by using Space Vector Modulation strategy. In DTC–SVM linear PI regulators are used. Both stator flux and electromagnetic torque are controlled directly. High dynamics is achieved and also good stationary operation
performance is kept. This advantages allow to implement DTC–SVM for traction drives.
The paper presents parallel structure of DTC–SVM. Operating ranges, including field weakening
region, are described. Some experimental results of the 75kW induction motor drive which illustrate
its performance are attached.
__________
* Warsaw University of Technology, Department of Electrical Engineering, Institute of Control and
Industrial Electronics, Koszykowa 75, 00-662 Warsaw, Poland,
wojcikpa@isep.pw.edu.pl, mpk@isep.pw.edu.pl
** Warsaw University of Life Science, Production Engineering Department, Nowoursynowska 164,
02-787 Warsaw, Poland, dariusz_swierczynski@sggw.pl
*** Electrotechnical Institute, Department of Electric Machine–Tool Drives, Pozaryskiego 28,
04-703 Warsaw, Poland, m.janaszek@iel.waw.pl
356
1. INTRODUCTION
Electrical traction drives for city transportation (tram, trolleybus, subway) have different requirements compared to conventional industrial drives. The main specifications
are as follows:
– robust and high starting torque, at zero stator frequency,
– high dynamics of torque control to reduce torque in case of slip between wheel and
rail,
– wide speed operation range (0–150 Hz) including field weakening (65–150 Hz),
– constant and low switching frequency to reduce inverter loses,
– operation at wide range of DC link voltage changes (+/–50% of the rated value),
– robust breaking in all situations including supply DC line voltage failure,
– operation without mechanical motion sensor.
Several control strategies have been used for induction motor drives: open loop V/Hz
control [4], Field Oriented Control (FOC) [1], Direct Torque Control (DTC) [2], and
Direct Self Control (DSC) [3]. The FOC method controls motor torque and flux indirectly using current loops and coordinate transformations. Therefore, FOC is more complex than DTC or DSC algorithms. However, DTC and DSC algorithms are based on
hysteresis control which requires high sampling frequency. Moreover, inverter operates
with variable switching frequency.
In this work a DTC scheme with Space Vector Modulation (referred as DTC–SVM
[5–10]) is applied for city tram induction motor drive 75 kW.
The DTC–SVM scheme combines advantages of FOC system (constant switching
frequency, SVM and PI controllers) and DTC algorithm (simple structure without current control loops and coordinate transformation, direct torque and flux control) and
recently has been successfully used in industrial drives [11].
2. DTC–SVM PRINCIPLES
DTC–SVM can be implemented in various ways [6, 7, 8, 10]. The most reliable is
parallel structure (see Fig. 1). This control scheme does not have any differentional algorithms, like in [6] or [7]. Instead of this in parallel structure of DTC–SVM reference
voltage for space vector modulator is calculated by two linear PI regulators as two adjacent stator voltage components Usx and Usy. Usx is calculated by stator flux PI controller,
whereas Usy is calculated by electromagnetic torque PI controller. Additionally, for high
speed region, where stator flux magnitude has to be lowered, in parallel DTC–SVM
structure field weakening algorithm can be applied easily.
In DTC–SVM torque and flux are controlled directly. In traction torque control loop
is used for speed regulation. Also field weakening algorithms are necessary for induc-
357
tion motor based traction drives. Therefore, presented algorithm is suitable for applications like: trams, trolleybuses, underground or fast city train.
Fig. 1. DTC–SVM parallel structure
Comparison between FOC, ST–DTC and DTC – SVM is shown in Table 1.
Table 1. FOC, ST–DTC and DTC – SVM comparison
FOC
Advantages
Disadvantages
Modulator
Constant switching
frequency
Unipolar inverter
output voltage
Low switching loses
Low sampling frequency
Linear PI controllers
ST–DTC
Coordinate transformation
Current control loops
Control structure depended on rotor parameters
Structure independent
on rotor parameters,
universal for IM and
PMSM
Simple implementation of sensorless operation
No coordinate transformation
No current control
loops
No modulator
Bipolar inverter output
voltage
Variable switching
frequency
High switching loses
High sampling frequency
DTC–SVM
Structure independent on rotor parameters, universal for IM
and PMSM
Simple implementation of sensorless
operation
No coordinate transformation
No current control
loops
Modulator
Constant switching
frequency
Unipolar inverter
output voltage
Low switching loses
Low sampling frequency
Linear PI controllers
358
3. OPERATION IN FIELD WEAKENING REGION
Induction machine operation range under nominal stator flux magnitude is limited.
To achieve speed higher than base speed (maximum mechanical speed which is possible
to achieve at maximum stator voltage magnitude and rated stator flux magnitude) flux
weakening algorithm has to be applied.
Tk
T
ωR
ψS
uS
p
Tk
T
ψS
p
uS
ωR
Sbase
constant electromagnetic
torque region
constant power region
Scritical
ωS
constant slip frequency
region
Fig. 2. Induction machine operation including high speed region
Tk – breakdown torque, T – maximum torque, ΨS – stator flux magnitude, p – power,
uS – stator voltage magnitude, ωR – slip angular speed
Base speed is limited by DC link voltage, modulation strategy and rated stator flux.
In constant torque region, the stator flux magnitude is constant. The simplest way of
field weakening is stator flux magnitude reduction in inverse proportion to mechanical
speed. This is known as 1/ω_r method. In flux weakening region the electromagnetic
torque capability is decreased. In constant power region maximum torque is inverse
proportional to mechanical speed. In constant slip frequency region maximum torque is
equal to the critical torque, so is inverse proportional to square of the mechanical speed.
It is shown in Fig. 2. Some results are shown in Fig. 3b.
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4. EXPERIMENTAL RESULTS FOR 75 kW INDUCTION MOTOR
Torque tracking performance is shown in Fig. 3.
a)
b)
Fig. 3. Torque tracking performance: a) operation below base speed, b) operation at weaken flux
a) From the top: reference torque 100 Nm/div, estimated torque 100 Nm/div,
mechanical speed 500 rpm/div, phase current 200 A/div
b) acceleration up to 900 rpm with 75 Nm (from the top: stator flux amplitude 0.4 Wb/div,
mechanical speed 1000 rpm/div, estimated torque 100 Nm/div, phase current 200A/div)
a)
b)
Fig. 4. Speed tracking (acceleration–steady state–speed reversal–steady state–deceleration):
a) scalar control mode, b) DTC–SVM control mode
From the top: mechanical speed 600 rpm/div, reference speed 600 rpm/div,
phase current 200 A/div, estimated torque 100 Nm/div
360
Parameters of 75 kW induction motor used in this work are presented in Table 2.
Table 2. Induction machine data
PN
IN
fN
ωN
TN
Usupply
p
75 kW
160 A
68 Hz
2000 rpm
358 Nm
3 x 380 V
2
RS
RR
LS
LR
LM
J
31 mΩ
31 mΩ
11 mH
11 mH
9 mH
15 kgm^2
Moment of inertia (J) consist not only inertia of the machine, but also additional
inertia caused by one quarter weight of the tram. There are four motors in each tram
(two motors per cart, and two carts per tram). All tests use this moment of inertia.
5. CONCLUSIONS
In this paper DTC–SVM controlled PWM inverter–fed tram drive was presented.
Induction motor chosen for the experiment is used in trams. Presented results confirm
that DTC–SVM can operate in wide speed range including field weakening region,
what is typical requirements for electrical traction drives. The drive performance in
stationary and dynamic states is satisfactory at any speed. In high speed region because of stator flux magnitude decreasing also electromagnetic torque has to be limited. Electromagnetic torque is kept lower than breakdown torque.
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361
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[11] Twerd Company, Torun, Poland (www.twerd.pl)
NAPĘD FALOWNIKOWY Z SILNIKIEM INDUKCYJNYM
STEROWANY METODĄ BEZPOŚREDNIEJ REGULACJI MOMENTU
Z MODULACJĄ WEKTOROWĄ DLA TRANSPORTU MIEJSKIEGO
W artykule przedstawiono zastosowanie metody Bezpośredniej Regulacji Momentu z Modulacją
Wektorową (ang. Direct Torque Control with Space Vector Modulation DTC–SVM) dla napędów tramwajowych. Testowano typowy silnik trakcyjny o mocy 75 kW i momencie znamionowym 260 Nm.
Przedstawione zostały wyniki badań w zarówno zakresie prędkości nominalnej, jak również w obszarze
osłabiania strumienia.
Zastosowanie metody DTC–SVM pozwala na bezpośrednią regulację momentu, redukcję tętnień prądu i momentu, a także zmniejszenie strat łączeniowych (w porównaniu z metodą bezpośredniej regulacji
momentu z tablicą łączeń ST–DTC) poprzez utrzymanie stałej częstotliwości przełączeń tranzystorów.
Ponadto, metoda DTC–SVM jest niewrażliwa na zmiany parametrów silnika, co daje jej przewagę nad
metodami zorientowanymi polowo (ang. Field Oriented Control FOC). Dzięki swym właściwościom
napęd tramwajowy stosujący metodę DTC–SVM spełnia wymagania stawiane nowoczesnym układom
trakcyjnym.