0195136047.pdf

552 ELECTROMECHANICS

(a) Obtain expressions for self and mutual in-

ductances, assuming infinite permeability

for the magnetic cores.

(b) Find the expression for the electromagnetic

torque in terms of currentsisandirand angle

θ.

(c) Show that the expression of part (b) is equiv-

alent to

Te=−

πμ 0 rl

g

FsFrsinδ

12.5.1A certain 10-hp, 230-V motor has a rotational

loss of 600 W, a stator copper loss of 350 W, a

rotor copper loss of 350 W, and a stray load loss

of 50 W. It is not known whether the motor is an

induction, synchronous, or dc machine.

(a) Calculate the full-load efficiency.

(b) Compute the efficiency at one-half load, as-

suming that the stray-loss and rotational

losses do not change with load.

12.5.2A synchronous motor operates continuously on

the following duty cycle: 50 hp for 8 min, 100

hp for 8 min, 150 hp for 10 min, 120 hp for 20

min, and no load for 14 min. Specify the required

continuous-rated hp of the motor. (Note:A motor

rating is normally chosen on the basis of the rms

value given by

rms hp=

√ ∑

(hp)^2 (time)

running time+standstill time/k

wherekis a constant accounting for reduced

ventilation at standstill.)