email: swierczdiihser, edu ~ 1 , ni~k~i~,i-,er, r,w edu 171 email tb@iet auc dk. Abstract - This paper presents digital signal processor (DSP) based direct torque ...
DSP Based Direct Torque Control of Permanent Magnet Synchronous Motor (PMSM) Using Space Vector Modulation (DTC-SVM) DARIUSZ
SWIERCZWSKI I , MARIAN
P. KAZMIERKOWSKI
2,
FREDE BLAABJERG
‘Dept. of El Eng., Univ. of Warsaw, Koszykowa 75, 00-662 Warsaw, Poland, , r,w edu 171 email: swierczdiihser, edu ~ 1 ni~k~i~,i-,er, ’Dept. of El Eng., Univ of Aalborg, Pontoppidanstraede 101, DK9220 Aalborg ,Denmark, email tb@iet auc dk I
I I
Abstract - This paper presents digital signal processor (DSP) based direct torque control scheme using space vector modulation (DTC-SVM) for permanent magnet synchronous motor (PMSM) drives. The analysis of PMShl shows that the increase of electromagnetic torque is proportional to the increase of the angle between the stator and rotor flux linkages and therefore fast torque response can be obtain by increasing the rotating speed of the stator flux linkage as fast as possible. The presented control strategy DTC-SVM is implemented in software of the DSl103 board. Simulations and experimental results well demonstrate the effectiveness of the proposed control scheme. Also comparison with conventional DTC scheme are given.
l
Hysteresis Controllers
,
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,
I I
Selection Tabie
inverter
1. INTRODUCTION Peniianent magnet (PM) synchronous motors are widely used in high-performance drives such as industrial robots and machine tools thanks to their known advantages as: high power density, high-torquehnertia ratio, and free maintenance. In recent years, the magnetic and thermal capabilities of the PM have been considerably increased by employing the high-coercive PM material. Direct Torque Control (DTC) method has been first proposed for induction machines (Takahashi and Noguchi [l], Depenbrock [2]). This concept can also be applied to synchronous drives [3,4]. The DTC technique is different froin the conventional vector control method, where torque is controlled in the rotor reference frame via current control loops [5],[6]. Fig. 1 shows two system configuration for DTC controlled PMSM drive. Both systems use stator flux vector and torque estimators. The advantages of the hysteresis based DTC (Fig. la) over conventional rotor oriented control include the elimination of the dq-axes current controllers, coordinate transformation, On the other hand, among the well-known disadvantages of the hysteresis based DTC scheme are [7],[8]: variable switching frequency, violence of polarity consistency niles, current and torque distortion caused by sector changes, high sampling frequency needed for digital implementation of hysteresis comparators. All the above difficulties can be eliminated when, instead of the selection table, a voltage modulator is applied (Fig. Ib).
0-7803-7369-3/02/$17.00 0 2002 IEEE
encoder-
(a) I I I
I
Predictive Controller
1
I
I
Voltage Modulator
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sl,s,
4 4 4
PWM
S,inverter --b I
motor model PMSM encoder
(b) Fig. 1. Comparison between two schemes of DTC transistor PWM inverter fed PMSM drive. (a) Conventional scheme with hysteresis controllers and selection table. (b) Proposed scheme with predictive controller and voltage modulator.
In the proposed scheme, shown in Fig. lb, the error and reference amplitude of stator flux y,y,.e, signal e,,,c, are delivered to predictive controller, which also uses information on the amplitude and position Y , ~of the actual stator flux vector and measured current vector. The predictive controller determinates the stator voltage ] command vector in polar coordinates II’,-~/= [V,., ,v,,~?, for space vector modulator (SVM), which finally generates the pulses S,,!,S , ,S , to control the inverter.
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Above equation consist of two terms. The first is the excitation torque, which is produced by the permanent iiiagiiet flux and the second term is the reluctance torque. From equation (5) we can see that for constant stator flux amplitude and flux produced by permanent magnet, the electromagnetic torque can be changed by control of the torque angle. This is the angle between the stator and rotor flux linkage, when the stator resistance is neglected. The torque angel 6 , in tum, can be changed by changing position of the stator flux vector in respect to PM vector using the actual voltage vector supplied by PWM inverter. In the steady state, 6 is constant and corresponds to a load torque, whereas stator and rotor flux rotate at synchronous speed. In transient operation, 6 varies and the stator and rotor flux rotate at different speeds (Fig. 3).
11. MODEL OF PMSM
The vector diagram of the PMSM is shown in Fig. 2 .
d
(a)
(b)
Fig. 2. (a) Different coordinate of PMSM. (b) Flux. ciirrent and voltage vectors. (A-B-C) 3-phase coordinates. (a-P)stator oriented and (d-q) rotor oriented coordinates
The voltage and flux equations for a PMSM in the rotor oriented coordinates d-q can be expressed as: lid
f l , d
= 1; id + -- P
dt
~ Y q d -- m i s
Fig. 3. Vector diagram of illustrating torque control conditions.
and the electromagnetic torque equation
111. FLUX ESTIMATOR The block diagram of flux estimator based on the equation (2) is shown in Fig. 4 . where
p is the number of pole pairs,
i;T
is the stator
winding resistance, U,,,is the angular frequency, z/,sd, ~
1
,
~
and i,T,,i, are d , q coiiiponents of the stator winding voltage and current, y.sd, ysqare d,q components of the stator flux linkage,
L, ,Lqare d and q axis
inductances,
is the PM rotor flux linkage. Finally, the iiiotion and Yphj equation is expressed as:
Fig. 4. Cunent model for flux vector estimation.
In order to estimated the flux vector actual current components and rotor position are measured.
(4) where
IV. DIRECT TORQUE CONTROL SPACE VECTOR MODULATION (DTCSVM)
J iiionient of inertia, I I motor ~ ~ load and U' damping
constant. From the vector diagram of Fig.2b and eqs. (2) we can obtain following relation:
The block scheme of the investigated direct torque control with space vector modulation (DTC-SVM) for a voltage source PWM inverter fed PMSM is presented in Fig. 5a [9,10]. The intemal structure of the predictive torque and flux controller is shown in Fig. 5b.
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niotor
Voltage Vector Controller
(b) Fig. 5 . Direct torque coiitrol of PWM inverter-fed PMSM using space vector modulation (DTC-SVMj. (a) Block scheme, (b) Predictive controller.
Sampled torque error amplitude Y,,
Siniulation study of the hysteresis based DTC and DTC-SVM schemes was carried out for the motor and inverter parameters given in Appendix. The steady state at no load and rated torque operation are presented in Fig. 6 and Fig. 7, respectively. Note, that in spite of lower switching frequency DTC-SVM guarantee lower current and torque ripple. This is mainly because in SVM operation, contrary to hysteresis operation, the inverter switching is unipolar (compare output voltage wavefomi in Fig. 6a and 7a with Figs. 6b and 7b). Additionally, application of the SVM also reduce seiiiiconductor device voltage stress by avoiding k 1 dc voltage transition and instantaneous current reversal in dc link. The dynamic performance for both schemes under consideration are very similar. Torque and flux tracking abilities for hysteresis DTC and DTCSVM schemes are presented in Fig. 8 and Fig. 9, respectively. "~
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^
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I X"
e,,,and reference stator flux
are delivered to the predictive controller.
.
The relation between error of torque and increinent of load angel A 8 is nonlinear. Therefore PI controller, which generates the load angel increment required to minimize the instantaneous error between reference
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