GE 6252 Two Marks Questions & Answers - Unit 2

GE 6252 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING 

UNIT 2 ELECTRICAL MECHANICS

TWO MARK QUESTIONS & ANSWERS 

Question 1: What is the purpose of yoke in a dc machine?

Answer: The purpose of yoke in a dc machine is to

1. Act as a protective cover for the whole dc machine

2. Provide a support for poles

3. Provide a magnetic path for the flow of flux lines.

Question 2: Give the different types of dc machine.

Answer: The types of dc machine are,

Question 3: Name the different parts of a dc generator.

Answer: The different parts of a dc generator are,

1. Magnetic frame or Yoke,

2. Poles or Field magnets,

3. Armature,

4. Commutator, brushes,

5. Field windings & Armature windings.

Question 4: State the applications of DC generator.

Answer: The applications of DC generator are,

1.Separately excited DC generator - Used in electroplating and electrorefining of metals.

2.Shunt generator - Used in charging batteries and as main generator in ships

3.Series generator - Used to add voltage to transmission line and to compensate for the line drop.

4.Compound generator - Used to supply power to railway circuits and elevator motors.

Question 5: Write the emf equation of a DC generator.

Answer: The emf equation of a DC generator is,

where, E_g – emf induced in generator (V)

P – Number of poles

Φ – Flux per pole (Wb)

N – Speed of machine (rpm)

Z – Number of conductors

A – Number of parallel paths.

Question 6: Define back emf.

Answer: When a dc motor rotates, the armature conductors rotate and cut the magnetic flux lines. As a result, an emf is induced in armature conductors. This induced emf is called back emf and it opposes supply voltage.

Question 7: Write the voltage equation of a dc motor.

Answer: The voltage equation of a dc motor is, 

V = E_b + I_a R_a

where, V – Applied voltage (V)

E_b – Back emf (V)

I_a – Armature current (A)

R_a – Armature resistance (Ω).

Question 8: State the applications of a DC motor.

Answer: The applications of a DC motor are,
1.Shunt motors - Used in lathes, drills, pumps, conveyors, spinning and waving machines.
2.Series motors - Used in cranes, elevators, vaccum cleaners, sewing machines, air compressors and hair driers.
3.Compound motors - Used in printing presses, shears, reciprocating machines.

Question 9: What is a transformer?

Answer: Transformer is a static machine which transfers electrical energy from one circuit to the other without change in the frequency. It works on the principle of mutual induction.

Question 10: What is the transformer ratio?

Answer: The ratio of secondary induced emf to primary induced emf is called transformer ratio. It is indicated by K.

Es/Ep = Ns/Np = Ip/Is = K

Where, Ep, Np, Ip – Primary emf, number of turns and current

Es, Ns, Is – Secondary emf, number of turns and current

K – Transformer Ratio.

Question 11: In a single phase transformer, Np = 350 turns, Ns = 1050 turns, Ep = 400V. Find Es.
Given: Np = 350 turns, Ns = 1050 turns, Ep = 400V
Formula: Es/Ep = Ns/Np
Solution: Es/Ep = Ns/Np
              Es = (1050/350) * 400
              Es = 1200 V.
Answer: Es = 1200 V.

Question 12: Give the conditions for step up and step down transformer.

Answer: Step up transformer: Ns > Np or K > 1.

The number of turns in the secondary (Ns)is greater than the number of turns in primary (Np).

Step down transformer: Np > Ns or K < 1.

The number of turns in the secondary (Ns) is lesser than the number of turns in primary (Np).

Question 13: What are the types of transformer based on construction?

Answer: The types of transformer, based on its construction are,

1.Core type transformer,

2.Shell type transformer,

3.Berry type transformer.

Question 14: Write the emf equation of a transformer.

Answer: The emf equation of a transformer is,

Ep = 4.44 φ_m f Np volts

Es = 4.44 φ_m f Ns volts

Where, Ep, Es – induced emf in primary and secondary in volts

Np, Ns – Number of turns in primary and secondary

φ_m – maximum value of flux in Wb

f – frequency in Hz.

Question 15: Why the transformer rating is in KVA?

Answer: The copper loss depends on current and iron loss depends on voltage. The total loss in a transformer depends on volt-ampere (VA). In short, loss depends on voltage and current and not on power factor of load. This is the reason that the transformer rating is in KVA.

Question 16:What are the characteristics of an ideal transformer?

Answer: The characteristics of an ideal transformer are,

1.No winding resistance,

2.No magnetic flux leakage,

3.No losses (100 % efficiency) and

4.Zero magnetizing current.

Question 17: What are the different types of single phase induction motors?

Answer: The different types of single phase induction motors are,

1.Split phase induction motor,

2.Capacitor start induction motor,

3.Capacitor run induction motor,

4.Shaded pole induction motor,

5.Capacitor start capacitor run induction motor.

Read:
GE 6252 - Two Marks Questions & Answers - Unit 1
GE 6252 - Two Marks Questions & Answers - Unit 3
GE 6252 - Two Marks Questions & Answers - Unit 4

GE 6252 - Two Marks Questions & Answers - Unit 5