Machines Ii 2 Marks

Unit I 1. State the type of synchronous generator used in a hydro – electric power station. Salient pole alternator is generally used in hydro - electric power station. This type of alternator consists of a rotor on which projected poles (salient pole) are mounted on rotor. These alternators are driven at low speeds by prime movers like water turbines or diesel engines. 2. What do you mean by single layer and double layer winding? If slot consists of only one coil side, winding is said to be single layer. While there are two coil sides per slot, one at the bottom and one at the top the winding is called double layer.

3. Define synchronous speed. For fixed number of poles, alternator has to be rotated at particular speed to keep the frequency of the generated voltage (e.m.f.) constant at the required value. Such a speed is called synchronous speed of the alternator denoted as Ns So Ns=120fp where f = required frequency. 4. Name the two types of alternators depending upon the rotor construction. 1. Salient pole alternator and 2. Smooth cylindrical or non – salient pole alternators. 5. What are the various functions of damper winding provided with alternator? 6. Explain the meaning of synchronous reactance. The sum of frictions armature reaction reactance accounted for considering armature reaction effect and the leakage reactance of the armature is called synchronous reactance of the alternator denoted as Xs So Xs = XL + Xar Ω/ph. 7. Define voltage regulation of an alternator. The voltage regulation of an alternator is defined as the change in its terminal voltage when full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage. So if Vph = Rated terminal voltage Eph = No load induced e.m.f. The voltage regulation is defined as, % of regulation= Eph- VphVphx 100 8. What is load characteristics? The relationship between load current and the terminal voltage is called load characteristics of an alternator. Such load characteristics for various load power factor conditions are shown in fig.

9. State the condition for paralleling two synchronous generators. To have effective synchronization with out any interruption there are certain conditions to be fulfilled. These conditions are, 1. The terminal voltage of the incoming machine must be same as that bus bar voltage. 2. The frequency must be same as that of the incoming machine as well as that of the bus bar. This necessitates that speed must be properly adjusted (f = PN/120). 3. With respect to the external load, the phase of alternator voltage must be identical with that of the bus bar voltage. Alternatively we can say that phase sequence for the two voltages must be same. 10. What is meant by synchronizing? The process of switching of an alternator to another alternator or with a common bus bar without any interruption is called synchronizing. Alternatively it can also defined as the process of connecting the two alternator in parallel without any interruption. Unit II 1. What are the main parts of the synchronous motor? a. Stator b. Rotor c. Slip rings. 2. List out the starting methods of synchronous motor. The various methods to start the synchronous motor are, 1. Using pony motors 2. Using damper winding 3. As a slip ring induction motor 4. Using small d.c machine coupled to it. 3. Does the change in excitation affect the power factor of the synchronous motor? Yes, the changes in excitation affect the power factor of the synchronous motor. 4. What is synchronous capacitor? Over excited synchronous motor operating on no load condition is called as synchronous condenser or synchronous capacitor, because like a capacitor it takes a leading current. 5. Explain what happens when the load on a synchronous motor is changed. When the load on the synchronous motor increases, there is no change in its speed. But what gets affected is the load angle ‘δ’ i.e., the angle by which rotor axis retards with respect to stator axis. Hence the load increase, δ increases but speed remains synchronous. 6. A synchronous motor always runs at synchronous speed, why? When the load on the synchronous motor increases or decreases, there is no change in its speed. But what gets affected is the load angle ‘δ’ i.e., the angle by which rotor axis retards with respect to stator axis. Hence the load increase or decreases, δ increases or decreases but speed remains synchronous. 7. What is meant by hunting? Oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous motor. 8. What are the uses of damper winding in synchronous motor? Damper winding is used in the synchronous motor to start the motor and to reduce the hunting effect. 9. What is V curve? If graph of armature current drawn by the motor (Ia) against field current (If) is plotted, then its shape looks like an English alphabet V. if such graphs are obtained at various load conditions we get family of curves, all looking V. Such curves are called V – curves of synchronous motor. These are shown in fig.

10. Mention some specific applications of synchronous motor. 1. Constant speed load service 2. Reciprocating compressor drives 3. Power factor correction 4. Voltage regulation of transmission line. Unit III 1. What are the two types of 3 phase induction motor? a. Squirrel cage induction motor b. Slip ring induction motor or wound rotor induction motor. 2. Why induction motor is called asynchronous motor? The induction motor never rotates at synchronous speed. The speed at which it rotates is hence called sub-synchronous speed and motor some times called asynchronous motor. ∴N < Ns . 3. Explain why the no load current of an induction motor is much higher than that of an equivalent trans former. 4. What is the advantages of the cage motor. a. Construction is very simple b. Slip-rings and brushes are absent c. The construction is robust and maintenance free d. Due to simple construction rotors are cheep e. The rotor automatically adjusts itself for the same number of poles as that of stator. f. Rotor copper losses are less hence nave higher efficiency. 5. What is crawling of an induction motor? Some times the motor will run at ns7with X as the operating point. Thus stable operation is obtained near sub-synchronous speed ns7. This is called crawling or asynchronous crawling. Due to crawling there is much higher stator current accompanied by noise and vibration. 6. What is cogging? A special behavior is shown by squirrel cage induction motor during starting for certain combinations of number of rotor and stator slots. If number of stator slots S1 are equal to number of rotor slots S2 or integral multiple of rotor slots S2 then variation of reluctance as a function of space will have pronounced effect producing strong forces than the accelerating torque. Due to this motor fails to start. This phenomenon called cogging. 7. What is induction generator? When the slip of the induction motor is negative i.e. when the induction motor runs faster than synchronous speed, the induction motor runs as a generator then it is called as induction generator. 8. What is synchronous induction motor? When the d.c excitation provided in slip-ring induction motor it is pulled into synchronism and starts running at constant speed. Then it is called as synchronous induction motor. 9. What are the losses are take place in induction motor? a. Copper (I2R) loss in stator winding b. Core loss ( hysteresis and eddy current loss) in the stator core c. Copper (I2R) loss in rotor winding d. Friction loss in the bearings, brush and slip-rings e. Windage loss caused due to the rotation of the rotor in air.

10. Why the rotor slots of a three phase induction motor are skewed? The rotor slots of a three phase induction motor are skewed to the following a. Reduce the crawling effect b. To produce a more uniform rotor field and torque c. To reduce some of the internal magnetic noise when the motor is running. Unit IV 1. State the effect of rotor resistance on starting torque. The stating torque is proportional to the rotor resistance. Hence important advantage of this not only the starting current is limited but starting torque of the motor also gets improved. 2. Explain why the power factor of an induction motor is very low at starting. 3. Define starting current. 4. List out the types of starters used in three phase induction motor. a. Stator resistance starter b. Auto transformer starter c. Star – delta starter d. Rotor resistance starter e. Direct on line starter. 5. List the types of speed control schemes used in 3 phase induction motor. Speed of the induction motor can be controlled by basically two methods : 1. From stator side and 2. From rotor side From stator side, it includes following methods: a. Supply frequency control to control Ns, called V/f control b. Supply voltage control c. Controlling number of poles to control Ns d. Adding rheostats in stator circuit. From rotor side, it includes following methods: a. Adding external resistance in the rotor circuit b. Cascade control c. Injecting slip frequency voltage into the rotor circuit. 6. What is slip power recovery scheme? 7. What are the advantages of star delta starter? a. Simple construction b. Cheapest one c. Maintenance free operation 8. Write the necessity of starter. At the instant of starting 3 phase induction motor its rotor is at standstill and acts like a transformer with short circuited secondary. Therefore it takes a high current for short interval (about 5 to 8 times full load current) if started at full voltage. But starting torque will be about 1.5 to 2.5 times full load torque. This high starting current drawn from supply cause a large voltage drop in the distribution line and the operation of other loads connected to that distribution line is adversely affected. Therefore the starters are needed to reduce the starting current. Unit V 1. What is the rating of single phase induction motor? 2. How will you change the direction of rotation of split phase induction motor? In order to reverse the direction of rotation of the split phase induction motor, either the starting or the main winding terminals should be changed.

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This is due to the fact that the direction of rotation depends upon the instantaneous polarities of the main field flux and the flux produced by the starting winding. Therefore, reversing the polarity of one fields will reverse the torque. What type of motor is used for ceiling fan? Single phase capacitor start and run induction motor is used for ceiling fan. It is also called as single value capacitor run motor. State the applications of shaded pole motor. The shaded pole motors are used in toys, hairdryers, table fans, instruments, ventilators, circulators and electric clocks. Can shaded pole motor be reversed? Since this motor relies on the shading band for the starting torque, there is no easy way to reverse the direction of rotation of this motor. The only method is to install two shading bands around two edges on each pole and to selectively short circuit one or the other. Why single phase motor has low power factor? What is universal motor? It is built like a series d.c motor with the difference that both its stator and armature are laminated. They can be used either on d.c or a.c supply although the speed and power are greater on direct current. They cannot be satisfactorily made to run at less than about 2000 rpm. How universal motor different from dc series motor/ Universal motor is built like a series d.c motor with the difference that both its stator and armature are laminated. State application of universal motor. Universal motors are widely used for food mixers, vacuum cleaners, hairdryers, electric savers, portable drill, sewing machines, motion picture Projects and electric showers State the application of stepper motor. Due to the digital circuit compatibility of the stepper motors, they are widely used in computer peripherals such as serial printers, linear stepper motors to printers, tape drives, floppy disc drives. Memory access mechanisms. The stepper motors are also used in serial printers in type writers or word processor systems, numerical control of machine tools, robotic control systems, number of process control systems, actuators, spacecrafts, watches.