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Direct torque control of induction motor using fuzzy logic.

INTRODUCTION

The induction motor finds its place amongst more than 85% of industrial motors as well as in its single phase form in various domestic usages. Markedly a constant-speed motor with shunt characteristic, speed drops only by a few percent from no-load to full load. Hence in the past, induction motors have been used primarily in constant speed applications. Traditional methodologies employing speed control have either been high-priced or very inefficient, unlike the dc motor in which the presence of commutator and brushes require recurrent maintenance make dc motor drives improper for use in hazardous and polluted environments. On the other hand, owing to the simple, rugged, cheaper, smaller and subsequently lighter build of induction motor drives (particularly squirrel-cage type), they are designed for fans, blowers, cranes, traction, conveyers, etc. in spite of finding stiff competition from dc drives for such applications

Principle of rotating magnetic field:

When a three phase voltage is applied to the stator winding, a rotating magnetic field is produced. It is called a rotating field since its poles do not remain in a fixed position on the stator but go on shifting their positions surrounding the stator. The magnitude of this field is constant and equal to 1.5[PHI]m, where [PHI]m is the maximum flux due to any phase. On energizing the three phase stator from a three phase supply, a rotating magnetic field sets up round the stator which rotates at synchronous speed ns. This field passes through the air gap and cuts the stationary rotor conductors. Owing to the relative speed between the rotating flux and the static rotor, electromotive forces are induced in the rotor conductors. For the reason that the rotor circuit is short circuited, currents start flowing in the rotor conductors. Again, these conductors are placed in the magnetic field produced by the stator. As a result, mechanical force acts on the rotor conductors. A torque, produced as a result of this force, tends to move the rotor in the same direction as the rotating field. This is justified by Lenz law, according to which the direction of rotor currents will be such that they have a tendency to oppose the cause producing them. Now, the relative speed between the rotating field and the standstill rotor conductors is the cause generating the rotor currents. Thus to reduce this speed, the rotor starts running in the same direction as that of stator field and tries to catch it. Clearly, the rotor speed N is always less than the stator field speed's".

Speed control of induction motors:

The speed control of induction motors involves more complexity than the control of dc motor, especially if comparable accuracy is desired. The main reason for the same can be attributed tot he complexity of the mathematical model of the induction machine, as well as the complicated power converters supplying this motor. Variable speed induction motor drives employ various control algorithms.

Speed Regulation as a Means of Controlling a Process:

Let us consider the process of driving to work. Driving at the highest possible speed would probably cause an accident. And driving at a single speed that will be safe for every portion of the route will take long to reach to the destination. Hence adjusting the speed which goes well with the route minimizes the time to accomplish the objective of the process within limits of reliable operation. The process control benefits that may be provided by an adjustable speed drive are as follows:

1. Smoother operation.

2. Acceleration control as an added incentive.

3. Varying operating speed for each process.

4. Compensates for fluctuating process parameters.

5. Permits slow operation for set-up purpose.

6. Allows accurate positioning.

7. Provides torque control.

Types of speed control:

Scalar Control:

The induction motor draws the rated current and delivers the rated torque at the base speed. When the load is increased (over-rated load), while running at base speed, the speed drops and the slip increases. The motor can take up to 2.5 times the rated torque with around 20% drop in the speed. Any further increase of load on the shaft can stall the motor. The torque developed by the motor is directly proportional to the magnetic field produced by the stator. So, the voltage applied to the stator is directly proportional to the product of stator flux and angular velocity. This makes the flux produced by the stator proportional to the ratio of applied voltage and frequency of supply. By varying the frequency, the speed of the motor can be varied. Therefore, by varying the voltage and frequency by the same ratio, flux and hence, the torque can be kept constant throughout the speed range. Stator Voltage (V) [varies] [Stator Flux ([PHI])] x [Angular Velocity ([omega])] V [varies] [PHI] * 2 [PI] F [PHI][varies] V/F

This makes constant Volts/hertz the most common speed control of an induction motor. Fig 2.3 shows the relation between the voltage and torque versus frequency. It demonstrates torque voltage and frequency being increased up to the base speed. At base speed, the voltage and frequency reach the rated values as listed in the nameplate. The motor can be driven beyond base speed by increasing the frequency further. However, the voltage applied cannot be increased beyond the rated voltage.

Therefore, only the frequency can be increased, which results in the field weakening and the torque available being reduced. Above base speed, the factors governing torque become complex, since friction and windage losses increase significantly at higher speeds. Hence, the torque curve becomes nonlinear with respect to speed or frequency.

Vector Control:

AC motors, particularly the squirrel-cage induction motor (SCIM), enjoy several inherent advantages like simplicity, reliability, low cost and virtually maintenance-free electrical drives. However, for high dynamic performance, their control remains a challenging problem because they exhibit significant non-linearity and many of the parameters vary with the operating conditions. Field orientation control or IVC of an induction machine achieves decoupled torque and flux dynamics leading to independent control of the torque and flux. Fig.2.4 shows block diagram of speed control system using vector control.

Vector control is a technique which allows the induction motor to act like a Separately excited DC machine with decoupled control of torque and flux, making it possible to operate the induction motor as a high performance four-quadrant servo drive. The principle of vector control was devised by Hassel and Blacked and was developed by Leonard. In separately excited DC machine with a constant field excitation, torque is directly proportional to armature current. The orthogonal relationship between air gap flux and torque is independent of the speed of rotation so that the torque of the DC machine is proportional to the product of the flux and armature current. If the magnetic saturation is ignored, field flux is proportional to field current and is unaffected by armature current because of the orthogonal orientation of the stator and rotor fields.

Therefore, direct control of armature current gives direct control of motor torque and fast response, because motor torque can be altered as rapidly as armature current can be altered. The vector control technique provides a similar control strategy for the induction motor. The idea behind vector control is that the stator current of the induction motor is decomposed into orthogonal components as a magnetization component (flux producing) and a torque component. These components are controlled individually. In order to obtain high dynamic performance of the induction motor, the magnetizing current component is maintained at its rated level while the torque should be controlled through the torque component for the stator current.

Direct Torque Control:

Introduction:

Direct torque control (DTC) is one method used in variable frequency drives to control the torque (and thus finally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor. Stator flux linkage is estimated by integrating the stator voltages. Torque is estimated as a cross product of estimated stator flux linkage vector and measured motor current vector. The estimated flux magnitude and torque are then compared with their reference values. If either the estimated flux or torque deviates from the reference more than allowed tolerance, the transistors of the variable frequency drive are turned off and on in such a way that the flux and torque errors will return in their tolerant bands as fast as possible.

Direct torque control principle:

Direct Torque Control (DTC) is an optimized AC drives control principle where inverter switching directly controls the motor variables: flux and torque. The measured input values to the DTC control are motor current and voltage. The voltage is defined from the DC-bus voltage and inverter switch positions. The voltage and current signals are inputs to an accurate motor model which produces an exact actual value of stator flux and torque every 25 microseconds. Motor torque and flux two-level comparators compare the actual values to the reference values produced by torque and flux reference controllers. The outputs from these two-level controllers are updated every 25 microseconds and they indicate whether the torque or flux has to be varied. Depending on the outputs from the two-level controllers, the switching logic directly determines the optimum inverter switch positions. Therefore every single voltage pulse is determined separately at "atomic level". The inverter switch positions again determine the motor voltage and current, which in turn influence the motor torque and flux and the control loop is closed.

Stator flux control:

The IM equations, in a stator reference frame, are defined by: where Rs and Rr are the stator and rotor resistances. Ls and Lr are the mutual stator and rotor inductances. The stator flux is estimated from the measure of stator current and voltage and their transformation in the subspace. So The stator flux module and the linkage phase are given by So, the variation of the stator flux is directly proportional to the stator voltage, thus the control is carried out by varying the stator flux vector by selecting a suitable voltage vector with the inverter. A two level hysteresis comparator could be used for the control of the stator flux.

Direct torque control with three-level inverter:

The basic functional blocks used to implement the DTC scheme are represented in Figure. The instantaneous values of the stator flux and torque are calculated from stator variable by using a closed loop estimator. Stator flux and torque can be controlled directly and independently by properly selecting the inverter switching configuration.

Stator Flux and Electromagnetic Torque:

The calculated magnitude of stator flux and electric torque are compared with their reference values in their corresponding hysteresis comparators as are shown in Fig 3.4. Finally, the outputs of the comparators with the number of sector at which the stator flux space vector is located are fed to a switching table to select an appropriate inverter voltage vector.

The selected voltage vector will be applied to the induction motor at the end of the sample time.

Neglecting the stator resistance, implies that the end of the stator flux vector will move in the direction of the applied voltage vector, as shown in Fig 3.6. Is the initial stator flux linkage at the instant of switching. To select the voltage vectors for controlling the amplitude of the stator flux linkage, the voltage vector plane is divided into six regions, as shown in Fig 3.5. In each region, two adjacent voltage vectors, which give the minimum switching frequency, are selected to increase or decrease the amplitude of, respectively. For instance, vectors and are selected to increase and decrease the amplitude of when is in region one and is rotating in a counter-clockwise direction. In this way, can be controlled at the required value by selecting the proper voltage vectors. Fig 3.5 shows how the voltage vectors are selected for keeping within a hysteresis band when is rotating in the counter clockwise direction.

As shown in Fig 3.5, eight switching combinations can be selected in a voltage source inverter, two of which determine zero voltage vectors and the others generate six equally spaced voltage vectors having the same amplitude. According to the principle of operation of DTC, the selection of a voltage vector is made to maintain the torque and stator flux within the limits of two hysteresis bands. The switching selection table for stator flux vector lying in the first sector of the d-q plane is given in Table 3.2. The basic idea of the DTC concept is to choose the best vector of the voltage, which makes the flux rotate and produce the desired torque. During this rotation, the amplitude of the flux remains in a pre-defined band. In order to control the induction motor, the supply voltage and stator current are sampled.

The estimated values of the torque and stator flux are compared to the command values, Te* and [PHI]s* respectively. It can be seen from Fig 3.3 that the error between the estimated torque"Te" and the reference torque [Te.sup.*] is the input of a three level hysteresis comparator, where the error between the estimated stator flux magnitude "[phi]s" and the reference stator flux magnitude [phi][s.sup.*] is the input of a two level hysteresis comparator. Finally, the outputs of the comparators with stator flux sector, where the stator flux space vector is located, select an appropriate inverter voltage vector from the switching table. The selected voltage vector will be applied to the induction motor at the end of the sample time.

Hardware implementation:

Block diagram:

Power supply system:

A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy (e.g., mechanical, chemical, solar) to electrical energy. A regulated power supply is one that controls the output voltage or current to a specific value; the controlled value is held nearly constant despite variations in either load current or the voltage supplied by the power supply's energy source. Every power supply must obtain the energy it supplies to its load, as well as any energy it consumes while performing that task, from an energy source. A power supply may be implemented as a discrete, standalone device or as an integral device that is hardwired to its load. In the latter case, for example, low voltage DC power supplies are commonly integrated with their loads in devices such as computers and household electronics.

Pic 16f887 microcontroller:

Power supply for pic 16f887:

The PIC16F887 having 40pins and 5 ports. In this PIC micro controller the first pin is connected with the master clear circuit, it is used for the clear purpose. For this circuit we will provide +5v supply. For this project we won't use port A and port E. 11th and 12th pin for the purpose of VSS and VDD. 13th and 14th pin connected with the crystal oscillator. For PIC16F887 we can use 4MHz to 20MHz crystal oscillator frequency here we are using 4MHz. It will provide clock pulse for digital circuit. In port C 17th pin is connected with buzzer circuit. In D port 19, 20, 21, 27, 28, 29, 30 pins are connected with the LCD. For LCD we will provide +5v supply. For RF communication TX and RX C port 25th and 26th pins are used. For RF we will provide +5v supply. In power supply circuit first the step down transformer will provide 12v AC supply. Then this alternating current converted into direct current with the help of bridge rectifier. At last with the help of particular rectified IC we can get particular voltage level.

I/O Ports:

There are as many as thirty-five general purpose I/O pins available. Depending on which peripherals are enabled, some or all of the pins may not be available as general purpose I/O. In general, when a peripheral is enabled, the associated pin may not be used as a general purpose I/O pin.

Oscillator Module (with fail-safe clock monitor):

The oscillator module has a wide variety of clock sources and selection features that allow it to be used in a wide range of applications while maximizing performance and minimizing power consumption. Clock sources can be configured from external oscillators, quartz crystal resonators, ceramic resonators and Resistor-Capacitor (RC) circuits. In addition, the system clock source can be configured from one of two internal oscillators, with a choice of speeds selectable via software. Additional clock features include:

1. Selectable system clock source between external or internal via software.

2. Two-Speed Start-up mode, which minimizes latency between external oscillator start-up and code execution.

3. Fail-Safe Clock Monitor (FSCM) designed to detect a failure of the external clock source (LP, XT, HS, EC or RC modes) and switch automatically to the internal oscillator.

Timer0 Module:

The Timer0 module is an 8-bit timer/counter with the following features:

1. 8-bit timer/counter register (TMR0)

2. 8-bit prescaler (shared with Watchdog Timer)

3. Programmable internal or external clock source

4. Programmable external clock edge selection

Snubber circuit:

The square safe operating area of the IGBT for switch mode operation minimizes the need for snubber circuits in most applications such as the voltage source inverter. However, it is necessary to use a snubber circuit in the matrix converter due to the absence of freewheeling paths. In the matrix converter the load current is always commutated from one controlled switch to another.

This is in direct contrast to a conventional voltage source inverter where commutation is always from a controlled device to a complementary freewheeling diode or vice-versa. 1n a conventional inverter a time delay can be easily introduced between drive signals for complementary devices in order to avoid simultaneous conduction. During this delay time the inductive load current is taken over by a freewheeling diode. There is no such freewheeling path in the matrix converter but it is still necessary to introduce a delay between drive signals to avoid a short circuit of the input lines. During this delay time the inductive load current is taken over by a snubber circuit. In the converter a small R-C turn-off snubber connected across to each bi-directional switch is used to limit the device voltage to an appropriate level. Unfortunately, this simple snubber circuit arrangement has the disadvantage of high current stress in the devices at turn-on.

Optocoupler ic:

There are many situations where signals and data need to be transferred from one subsystem to another within apiece of electronics equipment, or from one piece of equipment to another, without making a direct ohmic. electrical connection. Often this is because the source and destination are (or may be at times) at very different voltage levels, like a microprocessor which is operating from 5 V DC but being used to control a triac which is switching 240V AC. In such situations the link between the two must be an isolated one, to protect the microprocessor from over voltage damage. Relays can of course provide this kind of isolation, but even small relays tend to be fairly bulky compared with ICs and many of today's other miniature circuit components. Because they are electro-mechanical, relays are also not as reliable .and only capable of relatively low speed operation. Where small size, higher speed and greater reliability are important, a much better alternative is to use an opto coupler. These use a beam of light to transmit the signals or data across an electrical barrier, and achieve excellent isolation. Opto couplers typically come in a small 6-pin or 8-pin IC package, but are essentially a combination of two distinct devices: an optical transmitter, typically a gallium arsenide LED (light emitting diode) and an optical receiver such as a photo transistor or light-triggered diac.

The two are separated by a transparent barrier which blocks any electrical current flow between the two, but does allow the passage of light. Usually the electrical connections to the LED section are brought out to the pins on one side of the package and those for the phototransistor or diac to the other side, to physically separate them as much as possible. This usually allows opto couplers to withstand voltages of any where between 500V and 7500V between input and output. Opto couplers are essentially digital or switching devices, so they are best for transferring either on-off control signals or digital data. Analog signals can be transferred by means of frequency or pulse-width modulation.

Current transformer:

A current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.

A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. Electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and Protective relay in the Electrical power industry.

Working:

The torque and stator flux of the induction motor is estimated and compared with the reference values independently. The torque and flux errors produced are given as input to the fuzzy logic controller where the linguistic inputs are processed as per the fuzzy rules table and crisp output is produced. The controlled output can be applied as a firing signals to the inverter through IR2110(Driver IC) and Opt coupler. Teapot isolation of signals is necessary as the control signals from the controller is digital one where as the firing signals to be applied to inverter should be analog one. Thus torque and flux of the induction motor is controlled effectively.

Conclusion:

In this paper, we present a kind of fuzzy torque control system for induction motor based on fuzzy control technique. The simulation results suggest that FLDTC of induction machine can achieve precise control of the stator flux and torque. Compared to conventional DTC, presented method is easily implemented, and the steady performances of ripples of both torque and flux are considerably improved.

The main improvements shown are:

* Reduction of torque and current ripples.

* No flux droppings caused by sector changes circular trajectory.

* Fast torque response.

* Zero-steady-state torque and flux.

REFERENCES

[1.] Takahashi, I. and T. Noguchi, 1986. "A New Quick-Response and High- Efficiency Control Strategy of Induction Motor," IEEE Trans. On IA, 22(5): 820-827.

[2.] Depenbrock, M., 1988. "Direct self - Control (DSC) of Inverter- Fed Induction Machine," IEEE Trans. Power Electronics, 3(4): 420-829.

[3.] Casadei, D., F. Profumo, G. Serra and A. Tani, 2002. "FOC and DTC: Tox Viable Schemes for Induction Motors Torque Control," IEEE Trans. Power Electronics. On PE, 17(5),

[4.] Casadei, D. and G. Serra, 2002. "Implementation of Direct Torque Control Algorithme for Induction Motors Based on Discrete Space Vector Modulation," IEEE Trans. Power Electronics, 15(4).

[5.] Pujol, A.A., 2000. Improuvment in Direct Torque Control of Induction Motors, These de doctor at de L'UPC, Novembre 2000.

[6.] Chapuis, Y.A., Controle Direct du Couple d'une Machine Asynchrone par L'orientation de Son Flux Statorique, These Doctorat INP, Grenoble 1996.

[7.] Xia, Y., W.Oghanna, "Study on Fuzzy Control of Induction Machine With Direct Torque Control Approach", IEEE Catalog Number: 97TH8280-ISIE97- GuimarBes, Portugal

(1) C. Sakthivel, (2) Dr. T. Venkatesan, (3) K. Selva kumar, (4) R. Guru prasath

(1) Assistant professor JCT College of Engineering And Technology, Coimbatore

(2) professor K.S.R College of Technology, TamilNadu, india

(3) Assistant professor SRM University, Chennai TamilNadu, India

(4) Assistant professor Sri Krishna College of Technology, Coimbatore TamilNadu ,India

Received 28 January 2017; Accepted 22 March 2017; Available online 28 April 2017

Address For Correspondence:

C. Sakthivel, Assistant professor JCT College of Engineering And Technology, Coimbatore

Caption: Fig. 2.3: Speed Torque Characteristics with V/f Control

Caption: Fig. 2.4: Vector control of induction motor.

Caption: Fig. 3.1: Block diagram of DTC

Caption: Fig. 3.2: Functional block diagram for DTC of induction motor.

Caption: Fig. 3.3: DTC scheme for AC motor with three level inverter.

Caption: Fig. 3.4: stator flux and torque hysteresis comparator.

Caption: Fig. 3.5: Partition of d-q plane into six angular vectors.

Caption: Fig. 3.6 : An example for flux deviation.

Caption: Fig. 5.1: Functional block diagram.

Caption: Fig. 5.2: Block diagram for producing regulated DC.

Caption: Fig. 5.3: Power supply circuit for PIC 16F877.

Caption: Fig. 5.10: RC snubber circuit for MOSFET.

Caption: Fig. 5.11: Optocoupler IC inner view.

Caption: Fig. 5.15: Current transformer circuit.
Table 3.2: switching table for conventional DTC.

     Sector                                 1           2
      Flux               Torque

[DELTA][PHI] = 1   [DELTA][GAMMA] = 1   [V.sub.2]   [V.sub.3]
                   [DELTA][GAMMA] = 0   [V.sub.7]   [V.sub.0]
                   [DELTA][GAMMA] = 1   [V.sub.6]   [V.sub.1]
[DELTA][PHI] = 0   [DELTA][GAMMA] = 1   [V.sub.3]   [V.sub.4]
                   [DELTA][GAMMA] = 0   [V.sub.0]   [V.sub.7]
                   [DELTA][GAMMA] = 1   [V.sub.5]   [V.sub.6]

     Sector            3           4           5           6
      Flux

[DELTA][PHI] = 1   [V.sub.4]   [V.sub.5]   [V.sub.6]   [V.sub.1]
                   [V.sub.7]   [V.sub.0]   [V.sub.7]   [V.sub.0]
                   [V.sub.2]   [V.sub.3]   [V.sub.4]   [V.sub.5]
[DELTA][PHI] = 0   [V.sub.3]   [V.sub.6]   [V.sub.1]   [V.sub.2]
                   [V.sub.0]   [V.sub.7]   [V.sub.0]   [V.sub.7]
                   [V.sub.1]   [V.sub.2]   [V.sub.3]   [V.sub.4]
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Author:Sakthivel, C.; Venkatesan, T.; Kumar, K. Selva; Prasath, R. Guru
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Apr 30, 2017
Words:4385
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