Download Electric Machines ELCT708 Assignment # 3

Electric Machines ELCT708
Assignment # 3
1) A three phase, 208 V, 4-pole, 60 Hz, 10 hp, Y-connected induction motor has
the following characteristics:
X1 = 0.35 
R1 = 0.4 
Xm = 16  X2 = 0.35  R2 = 0.14 
Rotational losses are 360 W. At a rotor speed of 1746 rpm calculate the
following:
a. The stator armature current.
b. The input power
c. The rotor copper loss
d. Induced torque at full load
e. Starting induced torque
f. Shaft torque
g. Motor efficiency
2) A three-phase 125 hp, 440 V, 60 Hz, 8 poles, Y connected squirrel cage
induction motor has the following equivalent per phase circuit parameters
referred to primary: R1 = 0.068 , R2 = 0.052 , X1 = X2 = 0.224 , Xm =
7.68 . The rotational losses are 2400 W. Determine for slip of 3% the
following:
a. The line current and power factor.
b. The developed power and torque.
c. The output power and torque.
d. The motor’s efficiency.
e. Find out (by calculations) if this load can be connected to the
motor while starting or not?
Use the following equivalent circuit.
3) A three phase 20 kVA, 208 V, 4 pole Y-connected synchronous generator has
a synchronous reactance of Xs = 1.5 /phase. The resistance of the stator
winding is neglected. The machine is connected to a 3-phase 208 V balanced
load. Neglect rotational losses,
a) If the field current and the mechanical input power are adjusted so that the
synchronous machine delivers 10 kW at 0.8 power factor lagging. What is
the voltage regulation?
b) If the mechanical input power is kept constant but the field current is
adjusted to make the power factor unity, what is the percentage change in
the field current with respect to the value in part (a), assuming linear
magnetic circuit.