81. 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

82.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

83   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.

84  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.



85  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

86  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.

87  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.

88  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

89  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.

90  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.
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