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 Total Downloads : 1404
 Authors : Satyaprasanthyalla, Sarmila Har Beagam
 Paper ID : IJERTV1IS7489
 Volume & Issue : Volume 01, Issue 07 (September 2012)
 Published (First Online): 25092012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Evaluation of Direct Torque Control for HighPower Induction Motor Drive
SatyaPrasanthYalla Asst.Prof, EEE Dept Pragati Engineering College
Andhra Pradesh
Abstract
Direct Torque Control with Multilevel Inverter (DTCMLI) has emerged recently in high dynamics AC drives fields for induction machines or permanent magnet machines application. Direct Torque Control (DTC) has the characters such as combined simplicity and robustness with excellent performance of torque control for the drive system. In this paper, DTC technique for drive system fed on threelevel inverter and induction motor is presented, which means to select and compose right voltage vectors varying from speed working in constant torque area. The simulation results confirm that the control method is very effective with greater number of levels in the output voltage waveforms, lower dv/dt, less harmonic distortion and lower switching frequencies

Introduction
Induction machines have several advantages over DC machines. They are robust, require less maintenance, cheaper, and operate at higher speed. Basically, induction machines control methods can be classified into scalar and vector control. In scalar control, only magnitude and frequency of voltage, current, and flux linkage space vectors are controlled. Whereas, in vector control, the instantaneous positions as well as the magnitude and frequency of voltage, current, and flux linkage space vectors are controlled. Constant volt per hertz is a wellknown scalar control method while Field Oriented Control (FOC) and Direct Torque Control (DTC) are the two most popular vector control methods.
Direct Torque Control was first introduced by Takahashi in 1986. The principle is based on limit cycle control and it enables both quick torque response and efficiency operation [3]. DTC control the torque and speed of the motor, which is directly based on the electromagnetic state of the motor [4]. It has many advantages compare to FOC, such as
Sarmila Har Beagam
Asst.Prof, EEE Dept
B.S. Abdur Rahman University,
TamilNadu
less machine parameter dependence, simpler implementation and quicker dynamic torque response. It only needs to know the stator resistance and terminal quantities (v and i) in order to perform the stator flux and torque estimations. The configuration of DTC is simpler than the FOC system due to the absence of frame transformer, current controlled inverter and position
encoder, which introduces delays and requires mechanical transducer. In [1], Takahashi had proved the feasibility of DTC compared to FOC.

Principle of Direct Torque Control
The basic configuration of the conventional DTC drive proposed by Takahashi [1], which consists of a pair of hysteresis comparator, torque and flux estimators, voltage vector selector and a Voltage Source Inverter (VSI)
DTC performs separate control of the stator flux and torque, which is also known as decouple control. The core of this control method is to minimize the torque and flux errors to zero by using a pair of hysteresis comparators. The hysteresis comparators lie at the heart of DTC scheme not only to determine the appropriate voltage vector selection but also the period of the voltage vector selected. The performance of the system is directly dependent on the estimation of stator flux and torque. Inaccurate estimations will result in an incorrect voltage vector selection.

ThreeLevel Inverter Drive
A simulation on the conventional DTC drive is performed for better understanding by using MATLAB/SIMULINK. With the understanding and knowledge of the conventional DTC, three level inverter is proposed for high power drives [3]. A conventional threephase, twolevel, and six switch inverter is able to switch each phase between two voltages. These are the positive and negative rails of the DC bus. A three level inverter is able to produce a phase voltage consisting of three voltage levels (positive, negative, and zero).
By producing an output voltage having more levels, the inverter can better approximate the
required sinusoidal output voltage. A better approximation of a sinusoidal voltage can be achieved for the same device switching frequency, which results in reduced current harmonics in the load [8]. For an induction motor these current harmonics cause increased copper losses, iron losses, torque pulsations, noise and mechanical stresses. The main difference between the conventional VSI and MultiLevel inverter varies with switching states which is shown in Table 1. The multilevel inverter has better edge in performance than former.
Figure 1. The topology of NPC threelevel inverter
A threelevel inverter causes lower losses in the induction motor and therefore results in a more efficient system. With three states (positive, zero, and negative) for each of the three phases, a total of 33(27) valid IGBT combinations can be produced. Each IGBT combination is represented as a mnemonic such as +, , 0 that represents the voltage produced by each of the three phases. The phaseto phase voltage applied to the load is either zero (both phases at the same state), half of the DC bus voltage (one phase at zero and the other either positive or negative), or the full DC bus voltage (one phase positive and the other negative).

DTC for ThreeLevel Inverter
A threelevel inverter can generate at its output 27 different voltage vectors in the stationary reference frame as shown in Figure 3.3. Some of these
Table 1. Switch States of A ThreeLevel NPC Inverter
ON
ON
OFF
OFF
+ 1\2 VS
OFF
ON
ON
OFF
0
OFF
OFF
ON
ON
– 1\2 VS
voltage vectors can be generated by more than one switching state as listed in the second column of Table 1. In this table, the three digits of a switching state [ka, kb, kJ] denote the connection status of output phases of inverter, [a b c] to one of three poles of DC link, while the values 1, 1 and 0 represent their connection to positive, negative and neutral poles of DC link respectively. The third columns fourth columns of this table introduce the vector number and D3 components of inverter output voltage in stationary reference frame when the respective voltage vector is switched.
The control of torque, speed and the stator inactive power is realized by changing the frequency, amplitude, phase and phase order of the rotor voltages. The motor torque equation given in Eq(1) can be expressed as vector forms in the stator field oriented – reference axes
Te = np r s sr (1)
Where, np is the number of poles,
r , s are respectively stator and rotor flux vectors, Ls , Lr are respectively stator and rotor inductance, Lm is the mutual inductance,
sr is the angle between the stator and the rotor flux Space vectors
For a constant magnitude of the stator and rotor flux space vectors, the angle sr may be used to control the torque of the motor. The following expression may be obtained from the stator voltage equation i.e. Eq(2)of the induction motor model.
(2)
Where, Tn denotes the constant electrical time and Vs denote the stator voltage space vector. This expression is valid for a stator fixed reference frame and a stator resistance equal to zero. It can be seen that the stator voltage directly impresses the stator flu. The stator voltage space vector Vs may assume twentyfour different non zero states and three zero states in NPC inverter as shown in Fig.2.
Switch States
S1
S2
S3
S4
Output Voltage
Figure 2. Distribution of threelevel inverter space vectors
Figure 3 illustrates the layout of a typical DTC controller. Using the torque and flux error command, the controller selects a proper voltage vector from a predefined switching table to satisfy both flux and torque requirements simultaneously. It operates in such a way that the flux and torque errors do not exceed their limits known as hysteresis bands.
Figure 3. Typical Block Diagram of DTC Induction Motor Drive
The stator flux and motor torque are estimated from the measured stator voltage and current signals. In this method a suitable switching table is the most important part of the controller. It affects directly the drive performances. Thus, accurate knowledge about the affecting characteristic of voltage vectors is worthy and allows the designers to guarantee the best performance of the drive using voltage vectors with unchangeable affecting characteristic for each sector in the switching table.

Results
Simulations for both conventional and proposed threelevel inverter drive are carried out using MATLAB/SIMULINK simulation package. A brief description of simulation method is given. The simulation results obtained are compared for verification. MATLAB/SIMULINK is a software package for modeling, simulating and analyzing dynamic systems. Figure 4 illustrates the complete model of DTC drive, which consists of an
induction machine, stator flux and torque estimators, torque and flux controllers, flux orientation, voltage vector selector and ThreeLevel inverter
Figure 4. Matlab Modeled DTC Drive
The steadystate performance of the Direct torque control schemes is evaluated based on the torque and speed response. From the Fig.5 , initial speed response for 500 rpm was achieved in 0.6 sec and second setting of speed reference 0f 1000 rpm which was set at 1.5 sec reached steady state at 2.1 secs
Figure 5. Speed Response of the DTC
Torque Response: Initially condition of motor is no load condition. In order to reach the initial speed reference, DTC system has generated reference torque of 40 NM. At 0.6 sec, motor has reached the reference speed of 500 rpm and torque generated is zero from 0.6 sec. At 0.8 sec, load torque of 100nm is added and response time for DTC system to reach is minimal. This proves the dynamic response of DTC system for transient conditions. At 1.5 sec, reference speed is changed to 1000 rpm and change in generated torque changed to 140 nm to obtain 1000 rpm. Once the motor reached reference speed
2.1 sec, torque generated is 100 nm. The description above is illustrated in Fig.6.
Figure 6. Torque Response of the DTC

Observations
Fig.7 illustrates the dv/dt effect through stator voltage waveform, which gives clear picture of voltage level which jumps from E to E, which is unusual with multilevel inverter output. Fig.8 shows the variation in the neutral point voltage.
Figure 7. Illustration of dv/dt stress on Switches
Figure 8. Illustration of Neutral Point Variation

Conclusion
The main contribution of this paper is to propose a threelevel inverter based DTC system that significantly reduces torque and stator flux ripples.
At the same time demerits of the system when applied to threelevel inverter were found. It states the advantages of ThreeLevel inverter drive stated and also points out main issues like dv/dt stress and neutral point voltage variation
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