Question

Q.1) Ina 50 KVA, star connected 440 V, 3-phase 50 Hz alternator, the effective
armature resistance is 0.25 ohm per phase. The synchronous reactance is 32
ohm per phase and leakage reactance is 0.5 ohm per phase. Torque angle in
radian at 0.75 lagging pf will be (1 marks)
0.3481
Ans.
0.3361
0.3464
19.85​

Answer 1

Answer:

For large alternators, the three indirect methods are used to determine the voltage regulation they are as follows:

Synchronous Impedance Method or EMF method.

Ampere-turn method or MMF method of Voltage Regulation.

Zero Power Factor method or Potier Method.

Answer 2

option (C) is the correct answer

Given:

star connected = 440 V

3-phase 50 Hz alternator

the effective armature resistance is = 0.25 ohm

The synchronous reactance is = 32 ohm per phase

leakage reactance is = 0.5 ohm per phase

Explanation:

V = $400/\sqrt{3}$

F.L output current at UPF is = 50000/root3 = 65.6A

Resistive drop = 65.6 x 0.25 = 16.4A

Leakage resistance drop IXL = 65.6 X 0.5 = 32.8V

Ea = $\sqrt{(V + IRa)^{2} + (IX_{L} )^{2} }$

    =$\sqrt{(254 + 16.4)^{2} + 32.8^{2} }$ = 272 volt

   = $\sqrt{3}$ x 272 = 471 volt

the non-load Emf:

E0 =  $\sqrt{(V + IRa)^{2} + (IX_{S} )^{2} }$ = $\sqrt{(254 + 16.4)^{2} + (65.6 + 3.2)^{2} }$   = 342volt

   =$\sqrt{3}$ x 342 = 592volt

Torque angle in radian at 0.75 lagging pf will be:

= Eo - V/V = (342 - 254)/254 = 0.3465

hence, option (C) is the correct answer