(d) None of the above.

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Dr. Mahalingam College of Engineering and Technology, Pollachi-3 (An Autonomous Institution affiliated to Anna niversity) CCET II (2016 Regulation) Name of Programme: B.E. (EEE) Course Code & Course Title: 16EET41 & Synchronous & Induction Machines Sem: IV Date & Session:14.

1 Dr. Mahalingam College of Engineering and Technology, Pollachi-3 (An Autonomous Institution affiliated to Anna niversity) CCET II (2016 Regulation) Name of Programme: B.E. (EEE) Course Code & Course Title: 16EET41 & Synchronous & Induction Machines Sem: IV Date & Session: & AN1 Duration: 1½ hours Max. Marks: 40 Part- A Objective Questions (10X1=10 Marks) Q. No Question 1 Which of the following statements about a three-phase squirrelcage induction motor are false? (1) It has no external electrical connections to its rotor (2) A three-phase supply is connected to its stator (3) A magnetic flux which alternates and stationary is produced (4) It is cheap, robust and requires little or no skilled maintenance (a) 1,2,3 only (b) 3 and 4 only (c) 3 only (d) None of the above 2 The slip speed of an induction motor depends upon: (a) Armature current (b) Supply voltage (c) Mechanical load (d) Eddy currents 3 When a 50 Hz 3-phase induction motor is operating at 2% slip, the frequency of rotor current is (a) 1 Hz (b) 2 Hz (c) 50 Hz (d) 100 Hz 4 When rotor resistance of a three phase induction motor become equal to its rotor reactance, its starting torque will be (a) Zero (b) Maximum (c) nity (d) Minimum 5 Circle diagram of an induction motor can be used determine its (a) Power Factor (b) Efficiency (c) Slip (d) All of the above 6 Which starting method cannot be used for starting 3 phase squirrel cage induction motor? (a) Star-Delta (b) Rotor Resistance. (c) Line resistance (d) Auto transformer CO No 7 It is advisable to avoid line-starting of induction motor and use starter because (a) Motor takes five to seven times its full load current. (b) It will pick-up very high speed and may go out of step. (c) It will run in reverse direction. (d) Starting torque is very high. 8 The disadvantages of Scherbius system used for speed controlling in 3 phase induction motor is a. It cannot be used for slip ring induction motors b. It cannot be used for squirrel cage induction motors c. It can be used for large induction motors d. None of these 9 Rotor rheostat control method of speed control is used for (a) Squirrel-cage induction motors only. (b) Slip ring induction motors only. (c) Both squirrel-cage and slip ring induction motor. (d) None of the above. 10 The best suited method for smooth speed control of three phase induction motor is V/F Control Part- B Short Answer Questions (5X2=10 Marks) Q. No Question CO No 11 State the difference between slip ring rotor and cage rotor of an induction motor R

2 12 Draw the equivalent circuit of three phase induction motor. 13 Induction Generator. When IM Speed runs above its Synchronous speed it will act as induction generator 14 Draw star delta starter. R 16. (a) Stator: As its name indicates stator is a stationary part of induction motor. A stator winding is placed in the stator of induction motor and the three phase supply is given to it. Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft he rotor of the three phase induction motor are further classified as Squirrel cage rotor Slip ring rotor or wound rotor or phase wound rotor. Stator Stator frame, Stator core, Stator winding or field winding. Advantages of Squirrel Cage Induction Rotor Its construction is very simple and rugged. As there are no brushes and slip ring, these motors requires less maintenance. Applications of Squirrel Cage Induction Rotor We use the squirrel cage induction motors in lathes, drilling machine, fan, blower printing machines etc 15 List out the methods of speed control of cage type 3-phase induction motor. V / f control or frequency control. Changing the number of stator poles. Controlling supply voltage. Adding rheostat in the stator circuit. Part- C Descriptive either or questions 2X10=20 Marks) Q. No Question CO No RMF: A rotating magnetic field is a magnetic field that has moving polarities in which its opposite poles rotate about a central point or axis. Ideally the rotation changes direction at a constant angular rate. This is a key principle in the operation of the alternating-

3 current motor. 16. (b) PRINCIPLE OF OPERATION An AC current is applied in the stator armature which generates a flux in the stator magnetic circuit. This lux induces an emf in the conducting bars of rotor as they are cut by the flux while the magnet is being moved (E = BVL (Faraday s Law)) A current lows in the rotor circuit due to the induced emf, which in term produces a force, (F = BIL) can be changed to the torque as the output. OR An 18.65kW, 4 pole, 50Hz, 3φinduction motor has friction and windage losses of 2.5% of the output. Full load slip is 4%. Find full load (a) Rotor copper loss (b) Rotor input (c) Shaft torque (d) The gross electromagnetic torque. 17. (a) Explain the need of starters in starting three phase induction motors? What are the various types of starters? Explain the working of auto transform starter with a neat diagram. Need of starters An induction motor is similar to a poly-phase transformer whose secondary is short circuited. Thus, at normal supply voltage, like in transformers, the initial current taken by the primary is very large for a short while. nlike in DC motors, large current at starting is due to the absence of back emf. If an induction motor is directly switched on from the supply, it takes 5 to 7 times its full load current and develops a torque which is only 1.5 to 2.5 times the full load torque. This large starting current produces a large voltage drop in the line, which may affect the operation of other devices connected to the same line Types DOL Starter Stator resistance starter Auto transformer starters Star delta starter Auto transformer Starter An Auto transformer Starter is suitable for both star and delta connected motors. In this method, the starting current is limited by using a three-phase auto transformer to reduce the initial

4 stator applied voltage. The figure below shows the motor with the Auto transformer starter. 17. (b) OR With the aid of diagrams, explain the principle of the following methods of speed control of a three phase induction motor: (i) Variable frequency CO2 Variable Frequency Control is a method which is used to control the speed of an induction motor. The synchronous speed and therefore, the speed of the motor can be controlled by varying the supply frequency.the synchronous speed of an induction motor is given by the relation shown below. It is provided with a number of tappings. The starter is connected to one particular tapping to obtain the most suitable starting voltage. A double throw switch S is used to connect the auto transformer in the circuit for starting. When the handle H of the switch S in the START position. The primary of the auto transformer is connected to the supply line, and the motor is connected to the secondary of the auto transformer. When the motor picks up the speed of about 80 percent of its rated value, the handle H is quickly moved to the RN position. Thus, the auto transformer is disconnected from the circuit, and the motor is directly connected to the line and achieve its full rated voltage. The handle is held in the RN position by the under voltage relay. The EMF induced in the stator of the induction motor is given by the equation shown below. Therefore, if the supply frequency is changed induced EMF will also change to maintain the same air gap flux. The terminal voltage V 1 is equal to the induced EMF E 1 if the stator voltage drop is neglected. In order to minimise the losses and to avoid the saturation, the motor is operated at rated air gap flux. This condition is obtained by varying the terminal voltage with frequency so as to maintain (V/f) ratio constant at the rate value. This type of control is known as Constant Volts Per Hertz. Thus, the speed control of an induction motor using variable frequency supply requires a variable voltage power source. The variable frequency supply is obtained by the following converters.

5 Voltage source inverter Current source inverter Cyclo converter An inverter converts a fixed voltage DC to a fixed or variable voltage AC with variable frequency. Cyclo converter converts a fixed voltage and fixed frequency AC to a variable voltage and variable AC frequency.the variable frequency control allows good running and transient performance to be obtained from a cage induction motor. Cyclo converter controlled induction motor drive is suitable only for large power drives and to get lower speeds. (ii) Pole changing Pole Changing Method Pole Changing Method is one of the main methods of the speed control of an induction motor. This method of controlling the speed by pole changing is used mainly for cage motor only because the cage rotor automatically develops a number of poles, which is equal to the poles of the stator winding. The number of stator poles can be changed by the following three methods. They are known as multiple stator windings, method of consequent polesand pole amplitude modulation (PAM). Note: The above principle can be extended to all the three phases of an induction motor. By choosing a combination of series and parallel connections between the coil groups of each phase. Also, the star or delta connections between the phase speed change can be obtained with constant torque and constant power operation or variable torque operation. Code for s: Sl. No. Code 1 Remember R 2 nderstand 3 Apply Ap 4 Analyze An 5 Evaluate E 6 Create C

Regulation: R16 Course & Branch: B.Tech EEE

SIDDHARTH GROUP OF INSTITUTIONS :: PUTTUR Siddharth Nagar, Narayanavanam Road 517583 QUESTION BANK (Descriptive) Subject with Code : Electrical Machines-II (16EE215) Regulation: R16 Course & Branch: B.Tech