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R3947 Pages: 2

constant load torque and running at 1000 rpm drawing 10 A current from the
supply. Calculate the new speed and armature current if an external armature
resistance of value 10 Q is inserted in the armature circuit. Also find the
stalling current. Neglect armature reaction and saturation.
During Swinburne’s test a 250V DC machine was drawing 3A from the 250V
supply. The resistances are 250 Q and 0.2 Q. Find the constant loss of the
machine. Also find the efficiency of the machine when it is delivering a 20A at
250V.
Why transformers are rated in kVA not in KW?
Develop the equivalent circuit of a transformer.
PART 90

Answer any two full questions, each carries 10 marks.
Two standard tests were conducted on a 1009 ‏,ذ۸‎ 1000/200V transformer.
Current in one test was 2A. Voltage in one test was 15V. Power factors were
0.8 and 0.2. Find the efficiency at 90% full load and 0.8 power factor.
What are the necessary and desirable conditions for successful parallel
operation of two single phase transformers?
Can a Yd transformer be operated in parallel with a Dy transformer? What
additional condition is to be satisfied over and above the conditions listed in
question 16 a).
In Scott connection prove that the 3-phase currents will be balanced if the 2-
phase currents are balanced. Assume upf load.

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