0195136047.pdf

588 ROTATING MACHINES

(b) Instead of an infinite bus as in part (a), let the power to the motor be supplied by a

1000-kVA, 2300-V, wye-connected, three-phase, 60-Hz synchronous generator whose

synchronous reactance is also 5.00/phase. The generator is driven at rated speed, and the

field excitations of the generator and motor are adjusted so that the motor absorbs 750 kW

at unity power factor and rated terminal voltage. If the field excitations of both machines

are then held constant, and the mechanical load on the synchronous motor is gradually

increased, compute the maximum motor torque under the conditions. Also determine the

armature current, terminal voltage, and power factor at the terminals corresponding to

this maximum load.

(c) Calculate the maximum motor torque if, instead of remaining constant as in part (b), the

field currents of the generator and motor are gradually increased so as to always maintain

rated terminal voltage and unity power factor while the shaft load is increased.

Solution

The solution neglects reluctance torque because cylindrical-rotor theory is applied.

(a) The equivalent circuit and the corresponding phasor diagram for the given conditions are

shown in Figure E13.3.2 (a), with the subscriptmattached to the motor quantities,

Rated voltage per phaseVt=

2300

√

3

=1328 V line-to-neutral

Current per phaseIam=

750 × 103

√

3 × 2300 × 1. 0

= 188 .3A

IamXsm= 188. 3 × 5 = 941 .5V

jXsm

Iam

Iam

jIamXsm

+ −Iam

−

+

−

Vt

Vt

Efm

Efm

(a)

Iag = −Iam

jXsg jXsm

Iag

Iam

jIamXsm

jIagXsg

+

−

+

−

Vt Efm

+

−

Efg

Efg

Efm

Vt

Iam

(b)

Figure E13.3.2