Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in
Closed Loop Speed Control of Induction Motor Fed by A High Performance Z-Source Inverter Shalu Jain1 & Alka Thakur2 2H.O.D. , 1Student, M.Tech Department of Electrical Engineering, Shri Satya Sai Universiteit Sehor Abstract-This paper proposes a new closed loop speed control of an induction motor fed by a high performance Z-source inverter (ZSI), the speed control is based on V/F control and indirect fieldoriented control (IFOC) strategies. The main aim of this thesis is the close loop control of induction motor by field-oriented control (FOC) method using Zsource inverter. In this thesis indirect field-oriented control (IFOC) method has been used to control the induction motor because of its simplicity. Since in direct field-oriented control (DFOC) method the rotor flux is measured from hall effect sensors. The installation of flux sensors is difficult due to limitations of air gap space, armature reaction, noise, etc. The close loop control of induction motor is carried out with the help of MATLAB SIMULATION.. Index Terms- Z-source Inverter, V/F control, indirect fieldoriented control. I. INTRODUCTION Electrical Energy already constitutes more than 30 % of all energy usage on Earth. And this is set to rise in the upcoming years. Its huge popularity has been triggered by its efficiency of use, easiness of transportation, ease of generation, and environmentfriendliness. Some portion of the total electrical energy production is used to produce heat, light, in electrolysis, arc-furnaces, domestic heating etc. Another large portion of the electrical energy production is utilized into mechanical energy by means of different types of electric motors- DC Motors, Induction Motors and Synchronous Motors. Induction Motors are often called the “Workhorse of the Industry”. Induction machines have been widely used as industrial workhorse, at least 90% of industrial drive systems employ induction motors due to low cost and high robustness compared with other electric machines. Typically, induction motors have an exceptional torque reserve and load dependence of speed. The induction motor consists of stator and rotor winding. The stator generates a rotating magnetic field and induces voltage in the rotor similar to a transformer, which makes the rotor turn at a speed less than the
Imperial Journal of Interdisciplinary Research (IJIR)
synchronous speed. Based on the construction type of the rotor, the induction motors are classified into two types • squirrel cage type induction motor. • Wound type induction motor.
Fig.1 The Cut View of Squirrel Cage Induction Motor It is also called the leader of the industry. This is the reason it is one of the most widely used motors in the world. It is widely used in transportation and industries, and also in household machines, and laboratories. Some typical applications demand the use of an induction motor drive ranging from consumer to automotive applications, with a variety of power and sizes. The important reasons behind the acceptance of the Induction Motors are: Induction Motors are low-priced compared to DC and Synchronous Motors. In this phase of competition, this is a major requirement for any machine. Due to its low-cost of procurement, installation and use, the Induction Motor is the first choice for the desired operation Squirrel-Cage induction motors are very uneven in construction. Their robustness allow them for usage in all kinds of environments and for long durations of time. Induction Motors have great efficiency of energy conversion. Also the induction motors are very reliable.
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in Owing to their easiness of construction, Induction Motors have very little maintenance costs. These motors do not have any brushes to wear out or magnets to increase the cost. The rotor is a simple steel cage assembly Induction Motors have very high starting torque. This quality is beneficial in applications where the load is connected before starting the motor. Added major advantage of the Induction Motor compared to the other motors is that its speed can be controlled easily. Different applications require different optimum speeds for the motor to run to obtain required operation. Speed control is a essential in Induction Motors because of the following reasons: o It ensures smooth operation. o It provides torque control and acceleration control. o Different processes need the motor to run at different speeds. o It compensates for fluctuating process parameters. o At the time of installation, slow running of the motors is requisite. All these factors present a strong case for the implementation of speed control or variable speed control drives in Induction Motors. The following are some of the recent developments in Induction Motor drives • Better magnetic and insulation materials and cooling systems. • Availability of design optimization tools. • IGBT-based PWM inverters with efficient frequency changing with low losses and high power density. • New and better methods of manufacturing and testing. • High speed and high power applications. With an extensive existence in all kinds of industries, transportation job, and in household applications, the Induction Motor is now termed as the “Racehorse of the Industry”.
1.2 Induction motor control methods: The advent of power electronic converters with forced commutation in 1960s and later with turn-off power semiconductors (BJT, GTO, and IGBT) made possible the use of the induction motor as a variable speed drive (VSD). Researchers at Siemens and Darmstadt University of Technology (Hasse, Jötten) developed the theory of field-oriented control in 1968-1969. Since this date, researchers all over the world have implemented many accurate practical control algorithms depending on this theory.
Imperial Journal of Interdisciplinary Research (IJIR)
As shown in Figure 1.2, two approaches to control the induction motor are: 1) Scalar control where magnitudes of the stator voltages and the stator frequency are the controlled components. 2) Vector control approach uses the space vector model of the induction motor to precisely control the torque both in steady state and transient operation.
Fig.2 Classifications of the induction motors control strategies
1.3 Field Oriented Control (FOC) OF Induction Motor: An introduction Of the above stated methods, V/f Control is the most widespread and has found extensive use in industrial and domestic applications because of its ease-of-implementation. (V/f) control is most commonly used scalar control method for speed control in which both voltage and flux are varied to keep the ratio constant. The scalar controls gives the slower response, more overshoot and suffers instability for higher order harmonics However, it has poorer dynamic performance compared to field oriented control or vector control. Thus in areas where exactness is required, V/f Control are not used. The speed control of induction motor must be done through Adjustable Speed Drives (ASD). The growth Page 48
Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in and availability of power electronic devices made speed control affordable. Compared with traditional scalar control (V/f) control approach, the Field Oriented Control (FOC) needs more calculation effort, but has the following advantages: 1) Improved torque response. 2) Torque control at low frequencies and low speed. 3) decoupled torque and flux control 4) Dynamic speed precision. 5) operating point in a wide range of speed 6) Decrease in size of motor, cost and power consumption. 7) Four quadrant operation. 8) Short-term overload ability. The FOC (field oriented control) concept, which was first introduced by Hasse in 1969 and Blaschke in 1972, constitutes the most important paradigm in controlling induction motors. Basically, the objective of field orientation is to make induction machine work similar to separately excited DC machines. The reason for implementing this technique on induction motor is to get decoupled control of torque and flux as in separately excited DC motors. The field-oriented control consists of direct and indirect vector control methods. For direct method, the rotor flux angle can be obtained from direct measurement of rotor flux. In this case, the rotor flux angle can be calculated. Alternatively, indirect method obtains the rotor flux angle by exploiting the currents and voltages.
1.4 Z-source inverter – An Introduction: The conventional adjustable speed drive system uses voltage source inverter (VSI) for power supply which is made up of diode rectifier, dc link capacitor, Inverter Bridge. To improve power factor either ac inductor or dc inductor is normally used. The motor drive system which uses voltage source inverter suffers some common limitations. I. Output voltage obtained is quite below the input voltage. II. Undesired voltage sag can interrupt the required operation of a motor drive system. III. Inrush current and harmonic current produced by the diode rectifier can spoil the line performance. IV. Low power factor is additional issue of the motor drive system. V. Performance and reliability are affected by the Voltage source inverter construction, because o Mis-gate pulse given due to the EMI can cause complete destruction of the inverter bridge. o the dead time that is required to evade shootthrough generates distortion and unstable operation at low speeds. o Due to the common-mode voltage shaft current and early failures of the motor occurs
Imperial Journal of Interdisciplinary Research (IJIR)
However the motor drive system which uses current source inverter (CSI) also suffers some common barriers which are as follows: o CSI works as a boost converter for ac to dc conversion and acts as a buck converter for dc to ac conversion. FOR APPLICATION WHICH needs wide range if voltages an additional dc-dc buck (or boost) converter is required, that increases the additional cost and power loss. o Gating pulses are given to the at least one of the upper switch and lower switch for satisfactory operation. Otherwise open circuit occurred due to the mis-gating signal can destroy the entire circuit. o The main devices of the current source converter is needed to block reverse voltage that requires a series diode is to be used in combination with high-speed and high-performance transistors such as insulated gate bipolar transistor (IGBTs). This avoids the direct usage of low-cost and highperformance IGBT modules. Both the voltage source converter and the current source converter suffers following common problems • Both the converters are either boost converter or buck converter and cannot be a buck boost converter. • The main circuitry of the both converter is not interchangeable, it means voltage source. • The main circuitry of the both converter is not interchangeable, it means voltage source converter cannot be used in place of current source converter and vice versa. • Both the converters suffer the EMI problem. Hence a newly developed inverter, the Z-source inverter overcomes the above mentioned problems of both the converters. • Z-source inverter can produce any desired output ac voltage, even greater than the input line voltage irrespective of the input voltage. • It provides ride-through during voltage sags without any use of additional circuits • It improves power factor and lessen harmonic current and common-mode voltage Z-source inverter is applicable to all dc-to-ac, acto-dc, ac-to-ac, and dc-to-dc power Conversion
2. Theory Induction motors have many advantages compared to DC motors and synchronous motors in many aspects, such as size, efficiency, cost, life span and maintainability. Low cost and ease of manufacturing have made the induction motors a good choice for electric and hybrid vehicles [2]. However, one must be able to achieve energy regenerative braking and be able to control the torque and the speed of an induction motor in traction drives such as hybrid electric vehicles [3].
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in The traditional adjustable speed drives (ASD) system is based on the voltage source inverter (VSI), which consists of a diode rectifier frond end, dc link capacitor, and inverter bridge. It suffers some common limitations and problems, such as: the obtainable output voltage is limited quite below the input line voltage, the voltage sags can interrupt an ASD system and shut down critical loads and processes, the performance and reliability are compromised by the VSI structure (mis-gating, dead time, common mode voltage) [4]. A recently developed Z-source inverter (ZSI) has a niche for ASD systems to overcome the above problems. It can: produce any desired output ac voltage, even greater than the line voltage, provide ride-through during voltage sags without any additional circuits, improve power factor and reduce harmonic current and common-mode voltage [5]. There are three different topologies for three phase two level ZSI as voltage source: the first topology is the basic ZSI; the second topology is the bidirectional ZSI; and the third topology is the high performance ZSI. The basic version of the ZSI can be changed into a bidirectional ZSI by the replacement of the input diode by a bidirectional switch. The bidirectional ZSI is able to exchange energy between ac and dc energy storage in both directions. Also, the bidirectional ZSI is able to completely avoid undesirable operation modes when the ZSI operated under small inductance or low load power factor. If the input dc source is unidirectional source such as a fuel cell or a photovoltaic array, the bidirectional ZSI cannot be used. So, the high performance ZSI can be used. The high performance ZSI can operate at wide load range with a small Z-network inductor, eliminate the possibility of the dc link voltage drops, and simplify the Znetwork inductor design and system control, so, the high performance ZSI topology appears to be the most suitable topology for HEV applications [6]. The volts per hertz (V/F) induction motor drives with inverters are widely used in a number of industrial applications leading not only to energy saving, but also to improvement in productivity and quality. The low cost applications usually adopt V/F scalar control when no particular performance is required. Variable speed pumps, fans and appliances are the examples. Furthermore, these applications usually do not require zero speed operation. The main advantage of V/F control is its simplicity and for this reason it has been traditionally implemented using low cost microcontrollers. For those applications which require higher dynamic performance than V/F control, the dc motor like control of IM that is called, the field oriented control (FOC), is preferred. The key issue for a FOC drive is how to obtain the decoupled control of motor flux and torque. The indirect field oriented controlled (IFOC) IM drive is widely used in high performance
Imperial Journal of Interdisciplinary Research (IJIR)
applications due to its simplicity and fast dynamic response [7, 8]. This paper presents V/F control and IFOC based closed loop speed control of a three phase IM fed by a high performance ZSI. The IFOC is implemented based on PWM voltage modulation with voltage decoupling compensation. The high performance ZSI with controlled peak dc link voltage is used to drive the three phase IM. The peak dc link voltage is estimated by measuring the input and the capacitor voltages. A dual loop controller is designed based on a small signal model of the ZSI for controlling the peak dc link voltage. MATLAB simulation results verified the validity of the proposed V/F control and IFOC based closed loop speed control of the three phase IM fed by the high performance ZSI. In addition, the simulation results verify that the performance of the indirect field-oriented control is more efficient than V/F control for speed control of induction motor fed by the high performance ZSI.
3.FUTURE RESEARCH WORK In order to verify the obtained simulation results, an experimental setup of a 33 kW high performance ZSI is designed and constructed in our laboratory to derive a 19 kW induction motor. The F2808 DSP will be used for control strategies implementation and LEM current and voltage sensors are used for measuring feedback signals. The required measuring and interface circuits are designed and built. Fig. 3 shows the experimental setup, which is now in testing mode.
Fig. 3 Experimental setup of 33 kW high performance ZSI
4.
CONCLUSION
This paper presents a new closed loop speed control of an induction motor fed by a high performance Z-source inverter based on V/F control and IFOC strategies. The peak dc link voltage is
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in controlled by a dual loop controller. The simulation results verified the validity of the proposed closed loop speed control methods during start up, load disturbance and input voltage change. Also, it verify that the performance of the indirect field-oriented control is more efficient than the V/F control for speed control of the induction motor fed by the highperformance ZSI.
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