Modeling and Testing of Induction Motors
- 2. I. INTRODUCTION II. CONSTRUCTIONAL DETAILS III. PRINCIPLE OF OPERATION IV. TORQUE SLIP CHARACTERISTICS V. TESTS ON THE INDUCTION MOTOR VI. SPEED CONTROL OF INDUCTION MOTOR VII. CONCLUSION VIII.REFFERENCE
- 3. INTRODUCTION: Induction motor is an energy conversion device that converts electrical energy into useful rotational kinetic energy, it is an application of the Faraday's law of induction. AC Motors are required in many modern adjustable-speed drives; the requirement is for precise and continuous control of speed and torque with long-term stability and high efficiency. The DC motor satisfies most of these requirements, but its mechanical commutator and the sparking are disadvantages because they may be dangerous in some areas of applications, plus regular maintenance is required and cannot be done when the motor is used at inaccessible locations.
- 4. A three phase induction motor mainly consists of two mainparts: 1. Stator 2. Rotor I. CONSTRUCTIONAL DETAILS
- 5. 1.Stator The stator consists of a steel frame which encloses a hollow cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery. The insulated conductors are placed in the stator slots and are connected to form a balanced three phase star or delta connection circuit.
- 6. 2.Rotor: The rotor is rotating part of an induction motor. It is a hollow laminated core having slots on its outer periphery. The rotor may be one the following types. I. Squirrel cage type II. Wound type or slip ring type
- 7. Squirrel cage type: It consists of laminated cylindrical core having parallel slots on its outer periphery, in these slots one copper or aluminium bar is placed. All these bars are connected at each end by metal rings, called end rings. These bars, therefore, are permanently short circuited and therefore no external resistance can be connected.
- 8. Wound type or slip ring type: These consist of laminated cylindrical core and carries a three phase winding as in the stator. The open ends of the rotor winding is brought out and connected to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. The three brushes are connected to a three phase star connected rheostat. These resistances are included at starting to limit high circulating current in the short circuited rotor winding and to give a large starting torque. These resistances are gradually reduced to zero as the rotor attains its normal speed.
- 9. II. PRINCIPLE OF OPERATION • A Rotating magnetic field (RMF) is set up in the stator when a 3 phase supply is given. • The stationary rotor cuts the revolving field and due to electromagnetic induction an emf is induced in the rotor conductors. • As the rotor conductors are short circuited , current starts flowing through them. • It becomes a current carrying conductor in the magnetic field and starts rotating.
- 10. Torque-Slip Characteristics: The curve drawn between the torque and the slip for a particular value of rotor resistance is known as torque-slip characteristics. The motor torque under running condition is given by, 2 2 22 2 2 2 2 2 K E sR T R s X
- 11. The following points may be noted by the torque-slip characteristics: 1.at s=0, T=0 so that torque-slip curve starts from the origin 2.at normal speed, slip is small so that is negligible as compared to , 2sX 2R 2 s T R Hence torque slip curve is a straight line from zero slip to a slip that corresponds to full load. 3.as the slip increases beyond the full load slip, the torque increases and becomes maximum at . This maximum torque is known as pull out torque or breakdown torque. It's value is almost twice that of full load torque when the motor is operated at its rated voltage and frequency. 2 2 R s X
- 12. 4.when the slip increases beyond the maximum torque, the term Increases very rapidly so that may be neglected as compared to . 2 2R 2 2 2 s T s X 1 s as is constant 2X Thus the torque is now inversely proportional to slip. Hence the torque slip characteristics is a rectangular parabola. 2 2 2s X 2 2 2s X
- 13. TESTS ON THE INDUCTION MOTOR No Load Test: No load test is performed to determine the no load current, no load power factor, windage and friction losses, no load resistance and reactance. Since there is no power output on no load, the power supplied to the stator furnishes its core loss and the friction and windage losses in the rotor. The circuit diagram for the no load test is shown in Fig. 3. Fig. 3.
- 14. Blocked Rotor test: The blocked rotor test is also known as locked rotor test or short circuit test. This test is employed to find the short circuit current of the rotor circuit. This test is performed at the rated current of the induction motor. The circuit diagram for blocked rotor test is shown in Fig. 4.
- 15. VII. SPEED CONTROL OF INDUCTION MOTOR Methods of speed control of induction motor: A. Pole changing method: B. Stator frequency control: C. Line voltage control: An induction motor essentially runs at constant speed as the load is varied. Some industrial drives, however, require several different speeds or even continually variable speeds (1 )Nr Ns s 120 (1 ) f Nr s P The operating speed of a 3-phasee induction motor is given by, or,
- 16. Conclusion: An Induction motor is a constant speed motor An induction motor is robust in construction, need less maintenance and can operate in hazardous locations . Induction motor have two major parts: •Stator •Rotor Test performed on induction motor •No load test •Blocked rotor test Speed of Induction motor can be controlled by a. Pole changing method b. Frequency control c. Line voltage control
- 17. References: •Generalized Theory of Electrical Machines By Dr. P.s. Bhimbhra fifth edition 2013 •Principal of Electrical Machines By V.K. Mehta & Rohit Mehta •Electrical Technology By B.J. Theraja & A.k. Theraja Volume 2