11. Calculate the equivalent resistance when three resistances are connected in a series of values 8 Ω, 3 Ω, and 1 Ω.
A. 12 Ω
B. 18 Ω
C. 15 Ω
D. 16 Ω
Answer: A
When three resistances are connected in series their equivalent resistance is equal to the sum of the individual resistances.
Req=R1+ R3+R2=12 Ω.
12. Calculate the active power in an inf Ω resistor with 0 current flowing through it.
A. inf MW
B. 4.44 MW
C. 12 MW
D. 6.18 MW
Answer: A
The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in the case of the resistor so the angle between V & I is 0°.
P=I2R=0×0×inf=inf MW.
13. When 15 A current flows into the positive terminal of voltage source 60 V. Calculate the power delivered by the source.
A. -900 W
B. 936 W
C. -943 W
D. 950 W
Answer: A
When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power.
Power delivered is -15×60=-900 W.
14. Source transformation is not valid for ___________
A. Ideal voltage source
B. Ideal current and voltage source
C. Practical voltage source
D. Practical current source
Answer: B
Source transformation is not valid for an ideal current and voltage source. The internal resistance for ideal voltage and current source is zero due to which source transformation is not valid.
15. Power input to an induction motor is given by _________
A. 3VpIpcosΦ
B. 2VpIpcosΦ
C. VpIpcosΦ
D. 5VpIpcosΦ
Answer: A
Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connections. The power is constant and has no vibrations.
16. For motoring mode, the phase angle between the stator phase voltage and stator phase current should be _______
A. < 90°
B. > 90°
C. < 40°
D. 180°
Answer: A
For motoring mode, the phase angle between the stator phase voltage and stator phase current should be < 90°. The rotor magnetic field tries to catch up with the stator magnetic field.
17. The relative speed between the stator magnetic field and rotor in the induction motor is ______
A. Ns
B. Ns-Nr
C. Ns+Nr
D. 0
Answer: B
In induction motor rotor tries to catch up with the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. Stator magnetic field rotates at the speed of synchronous speed.
18. The relative speed between the stator magnetic field and rotor in the synchronous generator is ______
A. Ns
B. Ns-Nr
C. Ns+Nr
D. 0
Answer: D
In a synchronous generator rotor and stator magnetic field rotates at speed of synchronous speed in order to produce the steady-state torque.
19. The relative speed between the stator magnetic field and stator in the induction motor is ______
A. Ns
B. Ns-Nr
C. Ns+Nr
D. 0
Answer: A
In Induction motor stator magnetic field rotates at the speed of synchronous speed. The stator is made up of Silicon steel and remains at rest. The relative speed is Ns-0 = Ns.
20. The relative speed between the rotor and stator in the induction motor is ______
A. Nr
B. Ns-Nr
C. Ns+Nr
D. 0
Answer: A
In Induction motor rotor tries to catch up with the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. The stator remains at rest.
