UNIT III THREE PHASE INDUCTION MOTOR 1. How does the shaft torque differ from the torque developed in 3-phase Induction motor? The mechanical power developed Pd causes the rotor to rotate at a speed Nr due to the torque Td developed in the rotor . Therefore, equation for Pr can be written as Pd __ 1rTd / 60 The remaining power, after the mechanical losses Wm are met with, available in the shaft as mechanical power output Po = Pd – Wm .The mechanical power output Po, which is less than Pd is available in the shaft running at a speed of Nr and with a shaft torque T. Therefore the shaft torque (T) is slightly less than the torque developed Td, Pd_ __ 1rT / 60 Wm = Pd – Po_ _>_ 1r(Td – T)] / 60 2. Name the tests to be conducted for predetermining the performance of 3-phase induction machine. (a) No load test (b) Blocked rotor test Vidyarthiplus Vidyarthiplus OM SAKTHI DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING 3rd YEAR/ 5th SEMESTER 3. What are the informations obtained from no-load test in a 3-phase I M? (i) No –load input current per phase,Io (ii) No load powerfactor and hence no load phase angle (iii) Iron and mechanical losses together (iv) elements of equivalent circuit shunt branch 4. What are the informations obtained from blocked rotor test in a 3-phase I M? (i)Blocked rotor input current per phase at normal voltage (ii) Blocked rotor power factor and hence phase angle (iii) Total resistance and leakage reactance per phase of the motor as refered to the stator 5. What is circle diagram of an I M? When an I M operates on constant voltage and constant frequency source,the loci of stator current phasor is found to fall on a circle.This circle diagram is used to predict the performance of the machine at different loading conditions as well as mode of operation. 6. What are the advantages and disadvantages of circle diagram method of predetermining the performance of 3 –phase I M? The prediction can be carried out when any of the following information is available The input line current., the input power factor, The active power input, The reactive power input, The apparent power input, The output power , The slip of operation, The torque developed, The equivalent rotor current per phase, Maximum output power, Maximum torque developed. The only disadvantage is, being a geometrical solution, errors made during measurements will affect the accuracy of the result. 7. What are the advantages and disadvantages of direct load test for 3 –phase I M? Advantages Direct measurement of input and output parameters yield accurate Results Aside from the usual performance other performances like mechanical Vibration, noise etc can be studied. By operating the motor at full load for a continuous period, the final steady temperature can be measured. Disadvantages Testing involves large amount of power and the input energy and the entire energy delivered is wasted Loading arrangement cannot be provided for motors of large power rating 8. State the characteristic features of synchronous motor. a. the motor is not inherently self starting b. The speed of operation is always in synchronous with the supply frequency irrespective of load conditions c. The motor is capable of operating at any power factor. 9. In what way synchronous motor is different from other motors? All dc and ac motors work on the same principle. Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields. 10. Name any two methods of starting a synchronous motors Vidyarthiplus Vidyarthiplus OM SATHI ADHIPARASAKTHI COLLEGE OF ENGINEERING 27 EE2302 – ELECTRICAL MACHINES II By an extra 3 phase cage induction motor By providing damper winding in pole phases By operating the pilot excitor as a dc motor 11. What is the effect on speed if the load is increased on a 3 phase synchronous motor? The speed of operation remains constant from no load to maximum load in the motor operating at constant frequency bus bars. 12. Why a synchronous motor is a constant speed motor? Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed. Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed. 13. What is the phasor relation between induced emf and terminal voltage of a 3 phase Synchronous motor? The rotating magnetic field is initially established by the prime source of supply V. The main field then causes an emf e to get induced in the 3 phase winding. Hence when the machine operates as a synchronous motor the emf phasor always lags the terminal voltage phasor by the load/torque 14. What are V and inverted V curves of synchronous motor? The variation of magnitude of line current with respect to the field current is called V curve . The variation of power factor with respect to the field current is called inverted V curve. 15. What happens when the field current of a synchronous motor is increased beyond the normal value at constant input? Increase in emf causes the motor to have reactive current in the leading direction. The additional leading reactive current causes the magnitude of line current, accompanied by the decrease in power factor. 16. Distinguish between synchronous phase modifier and synchronous condenser A synchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier. A synchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called a synchronous condenser. 17. How the synchronous motor can be used as s synchronous condenser? Synchronous motor is operated on over excitation so as to draw leading reactive current and power from the supply lines. This compensates the lagging current and power requirement of the load making the system power factor to become unity. The motor does the job of capacitors and hence called as syn chronous condenser. Vidyarthiplus Vidyarthiplus OM SAKTHI DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING 3rd YEAR/ 5th SEMESTER 18. What type of single phase induction motor would you use for the following applications? (i) Ceiling fan (ii) Wet grinder Ceiling fan – capacitor start and run motor Wet grinder – capacitor start motor 19. After servicing a single phase fan it was found to run in reverse direction. What could be the reason? The connection to the starting/ auxiliary winding would have reversed. 20. What will be the direction of rotation of a shaded pole single phase induction motor? The motor rotates in the direction specified by the unshaded to shaded region in the pole phase
Posted on: Thu, 26 Sep 2013 13:59:41 +0000
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