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Chapter: Electrical machines : Synchronous Motors

Important Short Questions and Answers: Synchronous Motors

Electrical machines - Synchronous Motors - Important Short Questions and Answers: Synchronous Motors



1. What does hunting of synchronous motor mean?


When the load applied to the synchronous motor is suddenly increased or decreased, the rotor oscillates about its synchronous position with respect to the stator field. This action is called hunting


2. What could be the reasons if a 3-phase synchronous motor fails to start?


It is usually due to the following reasons


a. Voltage may be too low. ,

b. Too much starting load.

 c. Open circuit in one phase or short circuit.

d. Field excitation may be excessive

3. What is synchronous condenser?


An over-excited synchronous motor under no load ,used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.


4. Write the applications of synchronous motor.


a. Used for power factor improvement in sub-stations and in industries. b. Used in industries for power applications.


c. Used for constant speed drives such as motor-generator set, pumps and compressors.


5. What is an inverted 'V' curve?


For a constant load, if the power factor is plotted against various values of field exciting current, the curve formed is inverted V Shape and called as inverted 'V' curve. Also draw draws the graph.


6. A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?


a. Exciter voltage may be too low.


b. Field spool may be reversed.


c. There may be either open-circuit or short-circuit in the field.


7. What are the two types of 3-phase induction motor?


a. Squirrel cage induction motor. b. Slip ring induction motor.


8. Write the two extra features of slip ring induction motors.

a. Rotor is having 3-phase winding.


b. Extra resistance can be added in the rotor circuit by connecting through the help of three slip rings for improving the power factor, increasing Starting Torque, limiting the starting current.


9. Can we add extra resistance in series with squirrel cage rotor? State the reason?


We cannot add extra resistance in series with the rotor because all the copper bars of the rotor are short circuited in both the sides by copper end rings to have a closed circuit.


10. Why an induction motor is called rotating transformer?



The rotor receives electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why an induction motor can be called as a rotating transformer


i.e., in which primary winding is stationary but the secondary is free to rotate.


11. Why an induction motor will never run at its synchronous speed?


If it runs at synchronous speed then there would be no related speed between the two, hence no rotor emf, no rotor current so no rotor torques to maintain rotation. That is why the rotor runs at its synchronous speed.


12. Define SCR?


Short circuit ratio (SCR) is defined as the ratio of field current required to produce rated voltage on open-circuit to field current required to produce rated armature current with the terminals shorted, while the machine runs at synchronous speed.


13. When does a synchronous motor get over excited?


If the field excitation of the motor is increased, the field flux will become strong and Eb will increase. As a result Eb will exceed V and the motor will be called an over excited motor.


14. Define pullout torque?


The pullout torque is the torque, beyond which the synchronous link between field poles and resultant flux wave is severed and the machine falls out-of-slip.


15.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


The motor is capable of operating at any power factor.

16.In what way synchronous motor is different from other motors?


Alldcandacmotors work on the same principle. Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields.


17.Name any two methods of starting a synchronous motors


                                                       By an extra 3 phase cage induction motor

                                                       By providing damper winding in pole phases

                                                       By operating the pilot excitor as a dc motor



18.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.


19.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.


20.What is the phasor relation between induced emf and terminal voltage of a 3 phase Synchronous motor?


TherotatingmagneticfieldisinitiallyestablishedbytheprimesourceofsupplyV.T he main field then causes an emf eto get induced in the 3phase winding. Hence when the machine operates as asynchronous motor the emf phasor always lags the terminal voltage phasor by the load/torque



21.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 in verted Vcurve.



22.What happens when the field current of asynchronous 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.



23.Distinguish between synchronous phase modifier and synchronous condenser.


Asynchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier. Asynchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called asynchronous condenser.



24.Howthe 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 be come unity. The motor does the job of capacitors and  hence called as synchronous condenser.


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