Review Of Related Studies

In modern industrialized countries, more than half the total electrical Energy used is converted to mechanical energy through AC induction motors. Induction motors are extensively used in industrial and household appliances and consume more than 50% of the total generated electrical energy. Single-phase induction motors are widely used in home appliances and industrial control. During the last few years, the concept of speed and torque control of asynchronous motor drives has gained significant popularity. This way, it has been possible to combine the induction-motor structural robustness with the control simplicity and efficiency of a direct current motor. This evolution resulted to the replacement of the dc machines by induction motors in many applications in the last few years. Earlier only dc motors were employed for drives requiring variable speeds due to facilitate of their speed control methods.

The conventional methods of speed control of an induction motor were either too extravagant or too inefficient thus limiting their application to only constant speed drives. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc. This type of electric motor is so popular due to its simplicity, reliability, less maintenance and low cost. Today, with advancements in power electronics, microcontrollers, and digital signal processors (DSPs), electric drive systems have improved drastically. Initially the principle of speed control was based on steady state consideration of the induction motor. V/f control was the commonly used one for the open-loop speed control of drives with low dynamic requirements which is the focus of this study.

This section presents the conceptual and related literature from various entity where the study anchored upon to provide evidence and significant review of the topic grounded with the current study.

Deraz1, et al.[2004], has been presented a new current- limiting soft-starter for a three-phase induction motor drive system using pulse width modulation (PWM) AC chopper. A novel configuration of three-phase PWM AC chopper using only four insulated gate bipolar transistors (IGBTs) is also proposed. It requires only one current sensor. The duty ratio of the chopper IGBTs is obtained from the closed-loop current control in order to limit the motor starting current at a preset value. Only two complementary gate pulses are obtained from the control circuit to control the four IGBT switches.

Pratibha Naganathanl, et al. [2017],has discussed Two cascaded two-level inverters can synthesize three- level voltage space vector For the cascaded three-level inverter controlled induction motor, this study proposes a five-level torque controller (FLTC)-based direct torque control (DTC) method especially for improving steady-state motor torque performance and retaining the high dynamic performance.

1. Prathap Reddy, et al. [2003], has suggested two cascaded two level inverters can synthesize three-level voltage space vector for. the cascaded three-level inverter controlled induction motor, this study proposes a five-level torque controller(FLTC)-based direct torque control (DTC) method especially for improve steady-state motor torque performance and retaining the high dynamic performance.

Gonzalez-Prieto, et al. [2005], has been presented the most serious and recent competitor to the standard field oriented control (FOC) for induction motors (IM) is the finite control set model predictive control (FCS-MPC), has discussed Direct torque control (DTC) has been widely used as an alternative to traditional field-oriented control (FOC) methods for three-phase drives, has been suggested Direct torque control (DTC) has been recently used for the development of high performance five-phase induction motor (IM) drives, This work analyzes the fault-tolerant capability of six-phase drives with parallel converter supply. Different scenarios considering up to three faults for single and two neutral configurations are examined, optimizing off-line the post- fault currents and modifying accordingly the control strategies. Has discussed A model predictive control scheme for multiphase induction machines, configured as multi three-phase. Complete details about the predictive control scheme and adopted flux observer are included, has been suggested Direct torque control (DTC) has been recently used for the development of high performance five-phase induction motor (IM) drives, where normal operation of the system has been usually considered and the ability of DTC to manage the situation has been analyzed in comparison with different rotor field-oriented control (RFOC) strategies.

Mario Bermudez, et al. [2006], has discussed Three-phase machines are the industry standard for electrical drives, but the inherent fault tolerance of multiphase machines makes them an attractive alternative in applications requiring high reliability. This novel strategy is then combined with minimum losses and maximum torque criteria to obtain a variable current injection method that minimizes the drive derating, reduces the copper losses and improves the braking transients. Experimental results confirm the successful performance in the different zones for the case of a six-phase induction motor drive.

Toliyat et al, (2003), due to the high order and nonlinearity of the dynamics of an induction motor, estimation of the angle speed and rotor position without the measurement of mechanical variables becomes a challenging problem. The advantages of position and speed sensorless induction motor drives are to reduced hardware complexity and lower cost, reduce size of drive machine, eliminate of sensor cable, better noise immunity, increasing reliability and less maintenance requirements.

Benchaib et al (1999) presented a sliding mode controller with rotor flux estimation for induction motor drives. Rotor flux was also estimated using a sliding mode observer. Most methods are basically based on the Model Reference Adaptive System schemes (MRAS) (Cirrincione and Pucci, 2005).

Bilal et al, (2004) used a reactive-power-based-reference model derived in both motoring and generation modes but one of the disadvantages of this algorithm is its sensitivity to detuning in the stator and rotor inductances. The basic MRAS algorithm is very simple but its greatest drawback is the sensitivity to uncertainties in the motor parameters.

Ouhrouche, (2002) proposed another method based on the Extended Kalman Filter (EKF) algorithm. The EKF is a stochastic state observer where nonlinear equations are linearized in every sampling period. An interesting feature of the EKF is its ability to estimate simultaneously the states and the parameters of a dynamic process. This is generally useful for both the control and the diagnosis of the process.

Negm (2000) discloses the advances in microprocessor and power electronics which gives permission to implement modern techniques for induction machines such as field oriented control also known as vector control. This provides higher efficiency; lower operating costs and reduces the cost of drive components. In sensorless field oriented control, the speed and/or position are not directly measurable; their values are estimated using other parameters such as phase voltages and current, that are directly measured. Sensorless drives are becoming more and more important as they can eliminate speed sensors maintaining accurate response. Monitoring only the stator current and voltages, it is possible to estimate the necessary control variables.

Kyo and Frede, (2006) used the EKF algorithm to simultaneously estimate variables and parameters of the IM in healthy case and under different Induction Motor faults. An extended Kalman filter was also used by Kim et al (1994) for speed estimation of vector controlled induction motor drive. Unfortunately, Cheng and Hai, (2002) stated that this approach contains some inherent disadvantages such as its heavy computational requirements and difficult design and tuning procedure. Luenberger Observer for state estimation of Induction motor was used in. The Extended Luenberger Observer (ELO) is a deterministic observer which also linearizes the equations in every sampling period. There is other type of methods for state estimation that is based on the intelligent techniques is used in the recent years by many authors (Sbita and Ben, 2007).

Ignacio González-Prieto1, et al. [2014], has suggested, a direct torque and flux control is described for a six-phase asymmetrical speed and voltage sensor less induction machine (IM) drive, based on non-linear back stepping control approach. First, the decoupled torque and flux controllers are developed based on Lyapunov theory, using the machine two axis equations in the stationary reference frame.

Seyed Mohd. J. R. Fatemi, et al. [2014], has been presented distributed generation (DG) into low voltage (LV) systems demands that the generation system remain grid connected during voltage sags to ensure the operational stability. has been proposed A variable- frequency drive (VFD) having a 440-V front-end current source rectifier (CSR) interface to a voltage source inverter (VSI) feeding a Permanent-Magnet Axial-flux Air Core motor combination is a solution for low-horsepower pump and fan control that is both power dense and compatible with a shipboard environment. Power density and efficiency comparisons are made between equivalent CSR/VSI- and voltage source- conversion-based VFDs to demonstrate that the CSR/VSI-based VFD is more power dense, has presented the implementation of the controller is based on the machine air gap flux which is measured by detecting the third harmonic component of the stator phase voltages. This new controller does not require any sensors in the air gap of the machine nor does it require complex computations. Only access to the stator neutral connection is necessary to measure the air gap flux has been suggested Comprehensive analysis of the starting period of inverter-fed large induction motors reveals that these motors are subjected to additional components of pulsating torsional torque. These torque pulsations may coincide with the natural torsional frequency of the large motor system and produce hazardous shaft torque oscillations. To alleviate the torsional toque problem and limit the motor starting current, a constant air-gap flux using slip frequency control scheme is proposed to operate the motor inverter.

Yifan Tang,et al. [2001], has discussed Variable-speed constant- frequency generating systems are used in wind power, hydro power, aerospace, and naval power generations to enhance efficiency and reduce friction. In these applications, an attractive candidate is the slip power recovery system comprising of doubly excited induction machine or doubly excited brushless reluctance machine and PWM converters with a dc link.

1. Rahman, et al.[1995], has discussed steady state and transient operation of thyristor and diode controllers for variable voltage control of three-wire 3-phase induction motors is considered.

Takayoshi Matsuo, et al. [1977], has been presented the usual method of induction motor torque control uses the indirect field orientation principle in which the rotor speed is sensed and slip frequency is added to form the stator impressed frequency. In this paper two new field oriented control schemes are presented which employ rotor end ring current detection and thereby remove the dependence of the controller accuracy on temperature so that the controller is entirely independent of rotor time constant variations. The field orientation schemes do not require an incremental encoder for rotor position sensing. The motor torque can be accurately controlled even down to zero speed operation. has been suggested Adjustable-speed operations of induction motors are required to maintain their maximum efficiency levels. This can be achieved by constant slip operation of induction motors. In applications like submersible motor pumps, variable-speed operation is also needed to obtain maximum efficiency at all loads. To maintain a constant slip operation of induction motors, it is necessary to monitor the motor’s speed from its shaft. Conventional methods use speed sensors attached to the shaft. Speed monitored by these sensors is fed back to maintain constant slip operation in scalar control schemes. Has been proposed a definite relation exists between the flux level, torque and slip speed of a vector controlled induction motor. An untuned vector controller generates an inappropriate slip frequency that changes the operating flux of the machine. The torque characteristic is analysed with three aspects of magnetic state: true saturation curve, hard-limit saturation curve, and constant inductance model. Has been suggested adjustable-speed drive is attractive for pump, compressor, and other centrifugal load applications due to its flexibility and high operating efficiency, compared with the conventional constant-speed drive system. The drive system can successfully run in the full speed range with the proposed control method and control sequences.

Jose Titus, et al. [2005], has studied ability of an electrical drive to continue operation without tripping in spite of short duration voltage sags or power supply interruptions is known as Power Failure Ride through capability. Critical production processes in industries require the drives to smoothly ride-through during momentary power interruptions. The reliability of the drive is dependent on the maximum duration for which a total power supply failure can be tolerated without process interruption, has suggested proposes a simple method for single switch and double switches open-circuit fault diagnosis in pulse width modulated voltage-source inverters (PWM VSIs) for vector controlled induction motor drives, which also applies to secondary open-circuit fault diagnosis. According to the phase angle of one phase current, the repetitive operation process of VSI is evenly divided into six operating stages by certain rules. At each stage, only three of the six power switches exert a vital influence on this operation and the others make a negligible influence, uses a disturbance observer for various types of induction motor control was evaluated. The disturbance observer is designed to have fast response that is ten times faster than the current controller for vector control. The voltage error is efficiently corrected using the proposed method, and the current distortion can be reduced by approximately 1/3.

Limitations

The major problem seen in the above research is that induction motor is sensitive to deviation in estimated motor parameters values used in the control algorithm from their actual values. In addition, the parameters values may need to be updated while the motor is running, if there are large changes in the operating temperature and flux magnitude because resistances values depend on the temperature and inductances values changes due to flux saturation. Direct vector control requires values of the rotor leakage inductance, and the ratio of the rotor inductance to the mutual inductance while the first parameter has a constant value independent of temperature and flux, the second parameter is moderately affected by the saturation of the main flux path in the motor. In addition, there is the need for the special flux sensors, which need frequent maintenance and impose limitation in the motor mounting. If the sensorless form of direct method is implemented, the value of the stator resistance is sensitive to temperature changes. Indirect vector control is more sensitive to parameters values inaccuracy than direct vector control. The value of the rotor time constant (Lr/Rr) is used in the slip speed calculation this parameter is sensitive to temperature, skin effect and flux level. Errors in the slip speed calculation produce coupling between the flux and torque producing currents and result in a torque response with possible overshoot/undershoot and a steady state error. The need of special position incremental encoder is another disadvantage of the method. Beside the above limitations, voltage /frequency method is complex and need fast speed microcontrollers.

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