Download Exam 3 Practice

Q1 Three single phase transformers with a turns ratio of 1:1, are connected to form a
three phase transformer. The transformer input is a three-wire three-phase 120 V (lineto-line) system. The transformer output supplies a ‘Y” connected three-phase load with
a phase impedance of 600 +j300 Ω.
(a) Sketch a diagram of the system described above.
(b) For each of the transformer configurations in the table below, find the phase
voltage, and phase current at the load, and the input power (real power) supplied by the
source.
Part ‘a’ sketch here.
Input Output Load
Side
Side
VΦ
(volts)
Y
Δ
Δ
Load
Input
IΦ
P
(amps) (watts)
Y
Q2 A 120-V dc motor has an armature resistance of 0.7Ω. At no load, it requires 1.1A
armature current and runs at 1000 rpm. Assume constant field flux.
(a)(1) Draw a schematic diagram for a separately excited DC motor. Label all
components, voltages and currents.
(b)(2) Find the rotational losses { Prot } at 1000 rpm.
(c)(2) Find the output power { Pout } at 952 rpm.
(d)(2) Find the motor speed (in rpm) if the armature current is 12A.
Put sketch for part ‘a’ in this box.
b. Prot at 1000 rpm =
c Pout at 952 rpm =
d. N at 12A =
Q3 A three phase, Y-connected, 480-V (line –line), 50 Hp, 60 Hz, four pole
induction motor has the following equivalent circuit constants in ohms per phase.
R1 = 0.1, R2 = 0.2, RC = 1000, X1 =2.0, X2 = 1.0, Xm = 40
(a) (2) Sketch the induction motor equivalent circuit.
(b) (1) Find the shaft speed (n) for a slip of s = 0.05.
(c) (1) What is the frequency (fr) induced in the rotor at this shaft speed?
(d) (2) Let RC= ∞ and compute the total input power (Pin) delivered to the induction
motor.
(e) (2) Let RC= ∞ and find the power developed (Pdev) by the induction motor.
(f) (2) Let RC= ∞ and find the starting torque (Tstart) of the induction motor.
Part ‘a’ sketch here.
b) n = _______
c) fr = ________
d) Pin=_______
e) Pdev= ________
f) Tstart =________
Q4 A 3 phase, 50 Hz, 6 pole, Y-connected synchronous generator has a synchronous
reactance of j1.0 ohms per phase. Assume the armature resistance (Ra) is zero. The
field current is set to produce a terminal voltage of 480V (line–line) at full load. At full
load the machine supplies 60 amps at 1.0 power factor.
a)(1) Sketch the per phase equivalent circuit showing the field and armature circuits.
b)(1) Find the magnitude of the internal generator voltage (Ea) at no load.
c)(3) Sketch a phasor diagram showing (per phase) full load values of the following
quantities phase voltage (VΦ ), internal generator voltage (Ea), and torque angle δ.
(Place values in box.)
Part ‘a’ sketch here.
b) Ea = _________
c) 1) VΦ = _________
2) Ea = _________
Part ‘c’ sketch here.
3) δ = __________