AC Generator MCQ

An alternating current sinusoidal output waveform is formed by an AC generator, which converts mechanical energy into electrical energy. Steam turbines, gas turbines and internal combustion engines typically provide the mechanical energy; but, with the present emphasis in more sustainable electricity, wind, hydropower and tidal resources are becoming more plausible.

Faraday’s law of electromagnetic induction states that an electromotive force (EMF or voltage) is generated in a conductor that cuts a uniform magnetic field. AC generators rely on this law. This can be accomplished by rotating either a conducting coil in a static magnetic field or the magnetic field that contains the stationary conductor. Because obtaining induced alternating current from a fixed armature coil is easier than from one that is revolving, the standard approach is to maintain the coil stationary while rotating the magnetic field.

The EMF created is determined by the strength of the magnetic field, the number of armature coil turns and the rotational field speed.

Alternators, often known as AC generators, are classed based on the rotor construction. For medium-speed alternators, such as those found in hydropower facilities, salient pole or projected pole rotors are employed. Cylindrical rotors are better suited to higher-speed alternators, such as those found in steam or nuclear power plants, which operate at roughly 3000 RPM.

Q1. At half a revolution per second, a 100-turn coil with a surface area of 0.1 m2 turns. It is positioned in a 0.01 T uniform magnetic field perpendicular to the coil’s rotation axis. Calculate the maximum voltage that the coil can generate.
a) 256.33 V
b) 89.12V
c) 0.314 V
d) 3.1455 V

Answer: (c) 0.314 V.
The maximum voltage generated in the coil,
eo = nBAωeo = nBA × (2πv)eo= 100 × 0.01 × 0.1 × 2π × 0.5eo = 0.314 V.

Q2. An a.c. generator is made up of a 1000-turn coil with a 3m2 cross-sectional area that rotates at 60 rad s-1 in a uniform magnetic field of 0.04 T. The coil has a resistance of 500. Calculate the generator’s maximum current draw.
a) 2500 A
b) 1.44 A
c) 6.25 A
d) 0.55 A

Answer: b) 1.44 A.
eo = nBAωeo = 100 × 0.04 × 3 × 60eo  = 720 V.

The maximum current drawn =eo R =720500
I = 1.44 A.

Q3. Adithya rides a stationary bicycle with pedals attached to a 500 turn coil with an 11 m2 area. The coil rotates at half a revolution per second and is immersed in a 20.5 T uniform magnetic field perpendicular to its axis of rotation. What is the greatest voltage that the coil can generate?
a) 354035 V
b) 85000V
c) 111647 V
d) 46464 V

Answer: a) 354035 V.
The maximum voltage generated in the coil,
eo = nBAωeo = nBA × (2πv)eo = 500 × 20.5 × 11 × 2π × 0.5eo = 345035 V.

Q4. Determine the principle that governs the operation of an a.c. generator.
a) Eddy currents
b) Faraday’s law
c) Lenz’s law
d) Electromagnetic induction

Answer: d) Electromagnetic induction.
The principle of electromagnetic induction governs the operation of an a.c. generator. When a coil is turned around an axis perpendicular to the uniform magnetic field’s direction, an induced emf is created across it and so current is generated.

Q5. An armature coil is made up of 30 wire turns, each with an area of A = 0.05 m2 and a total resistance of 10. It rotates at a constant frequency of 140 Hz in a magnetic field of 0.15T. Calculate the highest induced emf generated in the coil.
a) 1 V
b) 500 V
c) 63 V
d) 43 V

Answer: c) 63 V.
eo= nBAω.eo = 30 × 0.15 × 0.05 × 2π × (140π)eo = 63 V.

Q6. Electrical energy is transferred to mechanical energy in an a.c. generator through electromagnetic induction.
a) True
b) False

Answer: b) False.
An induced emf is produced across a coil when it is turned about an axis perpendicular to the direction of the uniform magnetic field. Mechanical energy is transferred to electrical energy in an a.c. generator through electromagnetic induction.

Q7. In an a.c. generator, a ‘X’ is a rectangular coil made up of a large number of turns of copper wire coiled around a soft iron core. Determine the identity of X.
a) Slip ring
b) Armature
c) Copper brushes
d) Field magnet

Answer: b) Armature.
The armature is a rectangular coil made up of a large number of copper-wrapped turns wound around a soft iron core. The magnetic flux is increased by using the soft iron core.

Q8. An a.c. generator is made up of a 50-turn coil with a 2.5m2 area rotating at 60 rad s-1 in a uniform magnetic field of 0.3 T between two fixed pole pieces. When the current is zero, what is the flux across the coil?
a) Maximum
b) Minimum
c) Zero
d) Independent of current

Answer: a) Maximum.
When the coil is vertical, the current is zero. The flux across the coil is at its highest in this position. When the coil is horizontal, though, the current is greatest. As a result, the flux across the coil is greatest when the current is zero.

Q9. Increasing the number of turns in the coil of an A.C generatorDecreases the Electromotive force (EMF) 

1. Electromotive force (EMF) remains the same 
2. Increases the Electromotive force (EMF) 
3. Electromotive force (EMF) becomes zero

 Answer: 3) Increases the Electromotive force (EMF).

Q10. The generator’s electromotive force (EMF) is based on which of the following factors

1. Area of rotating wire 
2. length of rotating wire 
3. Radius of wire 
4. size of magnet 

Answer: 2) length of rotating wire.

Q11. What are the two major components of an AC generator?

1. Stator 
2. Rotor 
3. Both 1 and 2 
4. None of these 

Answer: 3) Both 1 and 2.

Q12. To convert an AC generator to a DC generator, what replacement is required?

1. Concave magnets with horseshoe magnets 
2. Armature with coil 
3. Slip rings with split rings 
4. None of the above 

Answer: 3) Slip rings with split rings.