DC Machines: 2 Marks Important Questions with Answers

CHAPTER 2: DC MACHINES (DC Generators and Motors)

Part A

Two Marks Questions and Answers

 

 

1. Distinguish between a dc generator and a dc motor.

An electrical machine which converts mechanical energy into an electrical energy is called a dc generator while an electrical machine which converts an electrical energy into the mechanical energy is called a dc motor.

 

2. List the main parts of a dc machine.

The main parts of a dc machine are Stator and Rotor.

Stator consists of:

(i) Yoke or magnetic frame

(ii) Field system ‒ poles, field winding, interpoles

Rotor has the following parts.

(a) Armature - Armature core, Armature winding

(b) Commutator

(c) Brushes, Bearings

 

3. List the main constituents of a stator of dc machine.

Yoke, poles, Field winding and Interpoles.

 

4. What is the purpose of field coils of a dc machine? Where are they located?

The function of field coil is to carry the current and to produce the magnetic flux. Field coils are located in the main poles.

 

5. What are the functions of brushes and bearing in a dc machine.

The function of brushes is to collect the current from commutator.

Ball bearings are frequently employed for quiet operation. But for heavy duty machines, roller bearings are preferable.

 

6. What are the types of induced emf's?

(i) Dynamically induced emf

(ii) Statically induced emf

 

7. Classify dc machines according to its form of excitation?

(i) Separately excited;

(ii) Self excited.

 

8. Name the different types of dc generators?

(i) Separately excited dc generators

(ii) Self excited dc generator

Self excited generator are further classified as (a) DC shunt generator (b) DC series generator (c) DC compound generator

 

9. What is meant by excitation of a dc machine? What are the methods of excitation?

The process of giving dc voltage to the field winding of dc machine for producing magnetic field is called excitation. Excitation are classified into two types.

(i) Self excited (ii) Separately excited.

 

10. Write down the emf equation of dc generator.

 E_g = (ϕZN/60) × (P/A) V

E_g = Generated emf

ϕ = Flux per pole in Wb

Z = Total No. of Armature Conductors

N = Speed of Armature in rpm.

P = No. of poles,

A = No. of parallel paths

 

11. Draw the connection diagram of long shunt compound generator.

 I_a = I_se = I_L + I_sh

Voltage Equation is E_g = V + I_a(R_a+R_se)

 

12. State the functions of commutator in a dc machine.

To facilitate the collection of current from the armature conductors. To convert the alternating current induced in the armature conductors to the unidirectional current in the case of dc generator. In dc motor, it converts unidirectional current into bidirectional current (Alternating current).

 

13. Why in a dc machine, the armature core should be laminated

To armature is laminated at right angles to its axis to minimise eddy current loss.

 

14. Write the number of parallel paths in a lap and wave connected windings.

For lap connected windings A = no. of poles (P)

For wave connected windings A = 2

 

15. Define critical resistance of a dc generator.

It is the maximum value of resistance in the field circuit which the generator will just build up voltage. Beyond this value of resistance the machine will fail to build up voltage.

 

16. What are the important characteristics of a dc generator?

The three most important characteristics of a dc generator are

(i) Open circuit characteristic

(ii) Internal characteristic

(iii) External characteristic

 

17. What is the condition for maximum efficiency of a dc shunt generator?

Variable loss = Constant loss

 

18. List the different variable and constant losses in an electrical machine.

Variable losses are copper loss in armature and series field winding.

Constant losses are Iron losses, mechanical loss and shunt field losses.

 

19. What is meant by armature reaction?

The interaction of armature mmf with main field mmf is called armature reaction. It has two effects.

(i) Demagnetising effect

(ii) Cross magnetising effect.

 

20. What is meant by back emf in a dc motor?

When the armature of a dc motor rotates in the magnetic field, the armature conductor cuts the magnetic flux. Hence emf will be induced in the conductor according to Faraday's law of electro‒magnetic induction.

This induced emf acts in opposite direction to the applied voltage (as per Lenz's law) and it is referred as back emf or counter emf. The back emf is given by

E_b = (ϕZN/60) × (P/A) V

Where,

ϕ = Flux per pole in Wb

Z = Total No. of Armature Conductors

N = Speed of Armature in rpm.

P = No. of poles,

A = No. of parallel paths

E_b = Back emf

 

21. What is the significance of back emf?

The back emf acts like a governor. It makes the dc motor self regulating machine. i.e it makes the motor to draw as much armature current as is just sufficient to develop torque required by the load.

 

22. Write the torque equation of d.c. motor defining each term.

 T_a = 0.159 ϕ ZI_a (P/A) Nm

⇒ T_a ∝ ϕI_a

Where,

 ϕ = flux per pole in Wb

 Z = Total no. of armature conductor.

 I_a = Armature current in Amp

 P = No. of poles

 A = No. of parallel path

 

23. Give the expression for speed of a dc motor.

Speed N = k (E_b/ϕ)

Where, k = 60A / PZ

E_b = Back emf in volts

 ϕ = flux/pole in Wb

P = No. of poles

Z = Total number of armature conductors

A = Number of parallel path in armature

 

24. Why a dc series motor should always be started with load?

When load is heavy, I_a is large and ϕ is high. Hence speed is low. ( N ∝ 1/ϕ ) But when a load current falls and hence I_a falls to a small value. Speed becomes dangerously high. Hence a series motor should never be started without some mechanical load on it.

 

25. Draw the torque current characteristics of various types of dc motors.

 

26. Draw the speed‒torque characteristics of series and shunt motors.

 

27. State two applications of dc shunt motor and series motor,

DC shunt motor is mainly employed for constant speed applications such as lathe machines, blowers, fans and centrifugal pumps etc.

DC series motor is mainly employed for variable speed applications such as traction work, Electric locomotives, cranes, hoists etc.

 

28. Why is a shunt motor called constant speed drive?

During the running condition, the reduction in the speed of a DC shunt motor (due to increase in load) from no‒load to full‒load is very low. Hence, it is called as constant speed drive.

 

29. List out the important losses occurring in a DC Generator.

In general DC machines losses may be classified as,

(i) Iron losses (or) constant losses.

(ii) Copper losses (or) variable losses.

(iii) Mechanical losses.

In DC generator, the output voltage will be reduced due to,

(a) Armature Reaction.

(b) Armature Voltage drop.

 

30. Classify DC motors and mention its two applications.

Type of Motor

(i) DC shunt motor

Characteristics: • Speed is fairly constant • Speed can be controlled • Medium starting torque

Application: • Lathe machines • Blowers and Fans • Centrifugal pumps

(ii) DC series motor

Characteristics: • Variable speed • No load condition is dangerous • High starting torque

Application: • Electric locomotives • Cranes • Elevators.

(iii) Copulative compound motor

Characteristics: • Variable speed • High starting torque

Application: • Punches • Rolling mills.

(iv) Differential compound motor

Characteristics: Speed increases as load increases.

Application: Not suitable for any practical applications.

 

31. State the principle behind the operation of a DC motor.

It works on the faraday's electromagnetic induction principle which states that, "when a current carrying conductor is placed in a magnetic field, a force will be experienced by the conductor", which will rotate the conductor.

 

32. State faraday's laws of electromagnetic induction.

I^st law [Generator principle]

Whenever a conductor cuts a magnetic field, an emf will be induced in the conductor. In other words, whenever the flux linking with a coil changes an emf will be induced.

II^nd law [Motor Principle]

Whenever a current carrying conductor is conductor is placed in a magnetic field, force (torque) will be experienced by the conductor, which tends to rotate it.

 

33. Write the voltage equation of a motor?

Voltage equation is given by, V = E_b + L_aR_a

Where,

V = Supply voltage

E_b = Back emf of the motor.

I_a = Armature Circuit Current

R_a = Resistance of armature.

 

34. Which type of induction motor would you use for the following applications?

(a) Wet ‒ grinder → split‒phase induction motor.

(b) Food processor and mixer → AC Series motor.

(c) Ceiling fan→ Capacitor start ‒ Induction run motor.

(d) Lift → 3 - phase slip ring Induction motor.

 

35. Which type of motor is used in electric traction and why?

DC series motor is used for electric traction.

The reason is, DC series motor is having high starting torque.

 

36. What is a Pole Shoe?

Pole shoe is used to spread out the flux in the air gap, and also, being of larger cross section, to reduce the reluctance.

 

37. What is the purpose of yoke in a dc machine?

(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine.

(ii) It carries the magnetic flux produced by the poles.

 

38. Write short notes on the types of DC machines.

• DC Generator: Separately excited, Self excited

• DC Motor: Shunt, Series, Compound

 

39. How can the alternating current waveform in the armature be converted into a dc waveform?

The basic nature of emf induced in the armature conductor is alternating current wave form. This needs rectification in case of DC generator which is made possible by a device called commutator.

(a) Functions

• To facilitate the collection of current from the armature conductors.

• To convert the alternating current induced in the armature conductors into unidirectional current in the external load circuit.

(b) Choice of material

• It is made up of wedge shaped copper segments.

 

40. Mention the applications of stepper motor.

• Positioning systems

• Floppy disk drives

• Computer printers

• Plotters

• Slot machines etc.,

 

41. Write voltage equation of the motor

Voltage equation is given by, V = E_b + I_aR_a

 

42. A dc shunt generator supplies a load of 10 kW at 220 V through feeders of resistance 0.1Ω. The resistance of armature and shunt field windings is 0.05Ω and 100 Ω respectively. Calculate the terminal voltage.

Solution

 I_L = output power / V

= 10×10³ / 220 = 45.45 A

I_sh = V/Rs_h = 220 / 100 = 2.2 A

I_a=I_L+I_sh = 45.45+2.2 = 47.65 A

E_g = V + I_aR_a + I_LR_feeder

= 220+ (47.65×0.05) + (45.45×0.1) = E_g = 226.925 V

 

43. Write the principle of DC motor.

The DC motor works on the Faraday's electromagnetic induction principle which states that, "when a current carrying conductor is placed in a magnetic field, a force (Torque) will be experienced by the conductor".

The direction of this force is given by Fleming's left hand rule and magnitude is given by,

 F = BIl (Newtons)

 

44. Calculate the emf generated by a 4‒pole, wave - wound armature having 45 slots with 18 conductors per slot when driven at 1200 rpm and the flux per pole is 0.016 wb.

Number of conductors,

Z = ( matrix Number of slots) × ( matrix Number of conductors per slot)

= 45 × 18

Z = 810 conductors

Generated EMF, E_g = PϕZN / 60A

= (4×0.016 × 810 × 1200) / (60×2)

E_g = 518.4 Volts

 

45. List the applications D.C. Generator

Application

i. DC Shunt Generator

• Electroplating

• Battery charging

• Exciters for AC generators

ii. DC Series Generator

• Series Arc Lighting.

• Series incandescent lighting.

• Boosters [to compensate the voltage drop]

iii. DC Compound Generator

• To deliver constant voltage at the transmission line ends.

• Differential compound generator may be used for welding purposes.

iv. DC Separately excited

• For supplying DC motors whose speed is to be varied widely.

• Where a wide range of DC voltage is required for testing purpose.

 

46. Give some application of DC motors

Applications:

(i) DC shunt motor

• Lathe machines

• Blowers and fans

• Centrifugal pumps

(ii) DC series motor

• Electric locomotive

• Cranes

• Elevators

(iii) Cumulative compound motor

• Punches

• Rolling mills

(iv) Differential compound motor

• Not suitable for only pratical applications.

 

47. Sketch the OCC of DC shunt generator.

 

48. Calculate the e.m.f generated by a 4‒pole, wave wound armature having 45 slots with 18 conductors per slot when driven at 1200 r.p.m the flux per pole is 0.016 Wb.

Solution

Eg = PϕZN / 60A

= [4×0.016×(45×18)×1200] / 60×2

Eg = 518.4 V