Power Output of Salient Pole Alternator (Reluctance Power)

Power Output of Salient Pole Alternator (Reluctance Power) AU : April-01, Dec.-09, 11

The expression for salient pole machines can be derived as follows :

From the Fig. 3.12.1 (a) it can be seen that

Thus the total power consists of a fundamental component and a second harmonic component which is present because the armature reaction flux has a tendency to pass through the field structure along its minimum reluctance path i.e. along field pole axis or direct axis. Since V²/2 [1/X_sq – 1/X_sd] sin 2δ exists because of difference in reluctance along d and q axes and is called reluctance power and the term P_Reluctance / w_s is called reluctance torque. The first term is identical with that obtained for cylindrical machine and this component of power is known as electromagnetic power.

Key Point : Thus in case o/ salient pole machines the expression for synchronizing power contains one more additional term 

1. Power-Angle Characteristics of Salient Pole Alternator

From the output power equation (3.12.1), the power against load angle characteristics can be obtained as shown in the Fig. 3.12.2.

Condition of δ for maximum power can be obtained by equating dP/d δ equation equation (3.12.2) to zero and solving for δ.

Use only the positive sign in the above equation (3.12.3), to avoid ambiguous result. This equation shows that 8 is less than 90° for maximum output power. To find maximum power, obtain 8 from equation (3.12.3) and substitute in equation (3.12.1).

Example 3.12.1 A 3 phase salient pole synchronous generator is rated at 3.5 MV A, 6.6 kV. Its direct and quadrature axis reactances are 9.6 Q and 6 Q per phase. The winding is star connected and resistance is negligible. If the generator is supplying 2.5 MW at rated voltage and at 0.8 p.f. lagging to an infinite bus. Find the voltage regulation. What power generator can supply at rated voltage if field becomes open circuited ?

Solution :

Review Questions

1. For a salient pole machine, derive the expression for the power developed as a function of load angle.

2. Draw and explain the phasor diagram of salient pole alternator supplying full load power current. Show that the power output per phase is given by,