WorksheetsElectrical FE Study Session 2
Total questions: 10
Worksheet time: 30mins
Name
Class
Date
1.
A center-tapped transformer has a turns ratio of 4:1 and 120Vrms is applied across the primary. What is the rms voltage across the upper half of the secondary winding?
a)
15V
b)
30V
c)
120V
d)
60V
2.
The ratio of line to phase voltage in a balanced delta 3-phase connected distribution system is most nearly:
a)
1
b)
3
c)
2
d)
3
3.
Find the armature mechanical torque of a DC motor whose field constant Ka=0.0025 , with armature current Ia=0.75A and (EMF) flux generated by the magnetic field ϕ=250Wb
a)
2.613 N-m
b)
3.844 N-m
c)
4.476 N-m
d)
5.200 N-m
4.
A transformer has a primary input voltage of 120V, a secondary output voltage of 12V, a primary coil current of 0.4A and a secondary coil current of 2A. The efficiency of this transformer is most nearly:
a)
50%
b)
75%
c)
150%
d)
25%
5.
In the given circuit, VAN = VBN =120∠0∘V. If the impedances are Z1=8+j6Ω, Z2=8−j6Ω, and Z3=4−j20Ω, then determine the total power delivered to the system of loads.
a)
1975 W
b)
5701 W
c)
2857 W
d)
3166 W
6.
The primary winding of a 120VA, 10:1 single-phase step-down transformer is fed from a constant 120Vrms supply. Calculate the percentage regulation of the transformer when connected to an impedance of 1.1Ω.
a)
4.4%
b)
8.7%
c)
3.5%
d)
9.2%
7.
Given a 3-phase (3-ϕ), 60Hz transmission system consisting of: a 3-ϕ transmission line; a 3-ϕ generator rated at 15000kVA, 4kV, with a reactance of 25%; a transformer bank of three single-ϕ transformers connected Δ−Y, each transformer rated at 10kVA, 4200−66500/115000Y Volts with low voltage taps.
The operating tap is 3900−66500V, with a leakage reactance of 10% based on rated kVA and tap voltages. Base 3-ϕ kVA and base line-to-line voltage has arbitrarily been chosen as 100kVA and 110kV, respectively, in the transmission line.
What is the base line-to-line voltage in the generator circuit?
a)
1.31 kV
b)
2.40 kV
c)
2.97 kV
d)
3.72 kV
8.
A 3-phase induction motor is operating at a speed of n=2700rpm with an applied line voltage of 240V at 50Hz. The number of poles this motor has is most nearly:
a)
8
b)
6
c)
4
d)
2
9.
A synchronous generator supplies a synchronous motor connected directly to its terminals. The positive-sequence subtransient reactances of the generator and motor on a certain kVA base and the rated voltage of the machines are 40% and 200%, respectively. Resistance is neglected. The current supplied by the generator is 50% of base current. The power factor at the bus is unity and the voltage is 98% of rated voltage.
Find the initial symmetrical rms current in the bus If, the generator Ig, and motor Im, when a three-phase short-circuit fault occurs on the bus.
a)
Ig = 0.75−j1.32A Im = −0.75−j0.40A If=−j1.72A
b)
Ig = 0.50−j2.45A Im = −0.50−j0.49A If=−j2.94A
c)
Ig = 0.50−j1.44A Im = −0.50−j2.13A If=1−j3.57A
d)
Ig =j1.32A Im = j0.75A If=j2.07A
10.
The power (W) dissipated in the 100Ω resistor of the given circuit is most nearly:
a)
4 W
b)
8 W
c)
0.4 W
d)
16 W
100 %
