Mechanical Characteristics of Three-Phase Wound Rotor Induction Motor On Simulink Isna Ahsan 2018169 Faculty of Electrical Engineering Ghulam Ishaq Khan Institute Topi, Swabi, Pakistan. u2018169@giki.edu.pk Muhammad Bilal 2018250 Faculty of Electrical Engineering Ghulam Ishaq Khan Institute Topi, Swabi, Pakistan. u2018250@giki.edu.pk Abstract—Electric Machines are the cornerstone of engineering applications such as elevators, pumps, and even electric vehicles, owing to their ability to leverage the electromagnetic induction effect. Induction motors use alternating current (AC) and electromagnetism to generate rotational motion. A special type of AC induction motor, known as wound rotor motors, are the main focus of this study. This project is about simulation of mechanical characteristics of three-phase wound rotor induction motor on Simulink. This project is about simulation of mechanical characteristics of three phase wound rotor induction motor on Simulink. Keywords—induction motor, mechanical characteristics wound rotor, Simulink, I. INTRODUCTION Electric motors fall into two classes, based on the power supply: alternating current (AC) or direct current (DC). AC motors can be single-phase or polyphase. In terms of quantity, single-phase motors are the most common type, mainly because many small motors are used for residential and commercial applications in which single-phase power is readily available. However, several operating constraints on these motors limit their widespread use in industrial applications. In contrast, polyphase motors are used widely in industrial applications. Polyphase motors can be found in almost every industrial process, and they often operate continuously to support production processes. These motors can achieve high efficiencies with favorable torque and current characteristics. The effectiveness and low cost of three-phase motors are major reasons why three-phase power is used so widely in industry. In terms of energy consumption and efficiency improvement opportunities, three-phase motor systems predominate. There are two primary types of AC motors: induction (also referred to as asynchronous) and synchronous. Induction motors are present widely in our daily routine. Even 70% of the load of power station is due to induction motors and 90% of industrial load is caused by induction motor. Induction motors include squirrel-cage and wound-rotor types. The main focus of this report is on the mechanical characteristics of three-phase wound rotor induction motor. A. Construction A three-phase, wound-rotor induction motor consists of a stator core with a three-phase winding, a wound rotor with slip rings, brushes and brush holders, and two end shields to house the bearings that support the rotor shaft. 1) Stator: A typical stator contains a three-phase winding held in place in the slots of a laminated steel core. The winding consists of formed coils arranged and connected so that there are three single-phase windings spaced 120 electrical degrees apart. The separate singlephase windings are connected either in wye or delta. Three line leads are brought out to a terminal box mounted on the frame of the motor. This is the same construction as the squirrel-cage motor stator. Fig. 1 shows a stator. 2) Rotor: The rotor consists of a cylindrical core composed of steel laminations. Slots cut into the cylindrical core hold the formed coils of wire for the rotor winding. The rotor winding consists of three single-phase windings spaced 120 electrical degrees apart. The single-phase windings are connected either in wye or delta. The rotor winding must have the same number of poles as the stator winding. The three leads from the three-phase rotor winding terminate at three slip rings mounted on the rotor shaft. Leads from carbon brushes which ride on these slip rings are connected to an external speed controller to vary the rotor resistance for speed control. The brushes are held securely to the slip rings of the wound rotor by adjustable springs mounted in the brush holders. The brush holders are fixed in one position. For this type of motor, it's not necessary to shift the brush position as is sometimes required in directcurrent generator and motor work. Fig. 2 shows a rotor. II. INDUCTION MOTOR An induction motor or asynchronous motor is an alternating current (AC) electric motor in which the electric current in the rotor, needed to produce torque, is obtained by electromagnetic induction from the magnetic field of the stator winding. With the exception of wound-rotor motors that have slip rings, the rotors of induction motors are not physically connected to any external circuits; instead, the excitation current is induced by a magnetic field. Jawad Asad 2018171 Faculty of Electrical Engineering Ghulam Ishaq Khan Institute Topi, Swabi, Pakistan. u2018171@giki.edu.pk Fig. 1. Stator. Fig. 2. Rotor B. Types. Depending upon the type of rotor used the three-phase induction motor is classified as: Squirrel Cage Induction Motor and Wound Rotor Induction Motor. 1) Squirrel-Cage Induction Motor: A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminium or copper conductors embedded in its surface. In operation, the non-rotating stator winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a transformer except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields of currents in the stator and rotor produce a torque on the rotor. 2) Wound Rotor Induction Motor: A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased. Characteristics of this type of motor include excellent speed control, high-starting torque, low-starting current, ability to handle high-inertia loads, ability to handle frequent starts and stops and to operate at reduced speeds for long periods. Fig. 3. Squirrel-Cage Rotor Fig. 4. Wound Rotor Induction Motor C. Operating Characterisitcs The most important motor operating characteristics are operating speed (measured in RPM) and torque. 1) Speed: The speed of an electric motor is an important element that depends on many factors. The operating speed of an AC motor depends on the rotor type, the number of poles, the frequency of the power supply, and slip characteristics. 2) Torque: Torque is the rotational force that a motor applies to its driven equipment and a fundamental factor in motor performance. The torque capacity of a motor depends on many design characteristics. Pull-up torque is the minimum torque that the electric motor develops when it runs from zero to full-load speed. Full-load torque is the torque produced by the motor at rated horsepower and speed. III. TORQUE-SPEED CHARACTERISTICS OF AN INDUCTION MOTOR Torque-Speed characteristic is the curve plotted between the torque and the speed of the induction motor. As discussed earlier, torque and speed are very important mechanical parameters of an induction motor. The equation of the torque is given as (1). ……………….𝜏 = (k s R2 E202) / (R22 + (s X20)2)………….. (1) At the maximum torque, the speed of the rotor is expressed by (2). ..................................NM = Ns (1 – sM )…………………… (2) Fig. 5. General Torque-Speed Characteristic Curve Fig. 6. Circuit Design on Simulink of Three-Phase Wound Rotor Induction Motor The maximum torque is independent of the rotor resistance. But the exact location of the maximum torque Ʈ-max is dependent on it. Greater, the value of the R2, the greater is the value of the slip at which maximum torque occurs. As the rotor resistance increases, the pull-out speed of the motor decreases. In this condition, the maximum torque remains constant. IV. WORKING PRINCIPLE OF A THREE-PHASE WOUND ROTOR INDUCTION MOTOR Wound rotor motors are a specialized type of AC motor and work in much the same way that other induction motors function. They consist of two main components, the outside stator, and the inside rotor, which are separated by a small air gap. The stator is generally the same across all induction motors, and consist of metal laminations that hold windings of copper or aluminum wire in place. There are three separate coils in the stator, which are fed by a three-phase AC, which simply means they are each powered by a separate AC. These motors are classified as asynchronous motors, where a discrepancy, known as slip, exists between the stator rotational magnetic field speed (synchronous speed) and output speed (rated speed). Wound rotor motors differ in how their rotor interacts with their stator. The rotor windings are connected to a secondary circuit containing slip rings, brushes, and external resistors, and are powered by a separate three-phase AC. Upon start-up, the external resistance imparted on this secondary circuit causes the rotor current to reduce the strength of the stator RMF. This means that the speed of rotation can be controlled by altering the resistance as the motor reaches 100% speed, allowing operators to choose the starting torque and running characteristics. This results in a smooth start-up, high initial torque, low initial current, and the ability to adjust rotational speed, which cannot be achieved with simpler designs. of block libraries. It offers tight integration with the rest of the MATLAB environment and can either drive MATLAB or be scripted from it. Simulink is widely used in automatic control and digital signal processing for multidomain simulation and model-based design. VI. SIMULATION A. Circuit Design Circuit was designed on Simulink as shown in Fig. 6 to show the torque-speed characteristic curve or in other words, the mechanical characteristics of the wound rotor motor. B. Simulation Results Simulation Results are shown in Fig. 7. VII. DISCUSSION The characteristic curve implies a few important pieces of information about the operation of induction motor. The induced torque of the motor is zero at synchronous speed. The torque-speed curve is nearly linear between no load and full load. V. SIMULINK Simulink is a MATLAB-based graphical programming environment for modelling, simulating and analysing multidomain dynamical systems. Its primary interface is a graphical block diagramming tool and a customizable set Fig. 7. Torque-Speed Characteristic Curve after Simulation There is a maximum possible torque that cannot be exceeded. This torque is called the pullout torque or breakdown torque. The starting torque on the motor is slightly larger than its full-load torque, so this motor will start carrying any load that it can supply at full power. If the rotor of the induction motor is driven faster than synchronous speed, then the direction of the induced torque in the machine reverses and the machine becomes a generator, converting mechanical power to electric power. If the motor is turning backward relative to the direction of the magnetic fields, the induced torque in the machine will stop the machine very rapidly and will try to rotate it in the other direction. Since reversing the direction of magnetic field rotation is simply a matter of switching any two stator phases, this fact can be used as a way to very rapidly stop an induction motor. VIII. CONCLUSION This report presented an understanding of what wound rotor motors are, how they work, and what their primary mechanical characteristics are that establish their applications in the industry. IX. ACKNOWLEDGMENT Firstly, I would like to thank Allah Almighty for everything. Then, it is our privilege to express our sincerest regards to our course instructor for his valuable inputs, encouragement, whole-hearted cooperation, and constructive criticism throughout the duration of our project. We deeply express our sincere thanks to our Lab Instructor for encouraging and allowing us to present the project on the topic “Mechanical Characteristics of Three Phase Wound Rotor Induction Motor” at our department premises for the partial fulfillment of the requirements. We take this opportunity to thank all our lecturers who have directly or indirectly helped in our project. Last but not the least, we express our gratitude to our guiders for their cooperation and support. [1] [2] [3] [4] REFERENCES Torque Speed Characteristic of an Induction Motor, Punith.G.S., accessed 23 December 2020, <https://circuitglobe.com/torquespeed-characteristic-of-an-induction-motor.html> Types of Three Phase Induction Motor, Electrical4U, accessed 24 December 2020, <https://www.electrical4u.com/types-of-threephase-induction-motor/> Squirrel-Cage Rotor, Wikipedia, accessed 23 December 2020, < https://en.wikipedia.org/wiki/Squirrel-cage_rotor > Three-Phase Wound-Rotor Induction Motor, Industrial Electronics, accessed 25 December 2020, < https://www.industrialelectronics.com/elecy4_17.html >
