Important Two Marks & 16 Marks Question and Answers

TWO MARKS QUESTION AND ANSWERS

1. State the basic parts of a DC machine.

Stationary Parts: Frame, Main pole, field coils, interpoles, interpole winding

Rotating Parts: Armature core, Armature winding, Commutator, Shaft.

2. Name the various parts of a DC machine that control the magnetic circuit.

Pole, Air-Gap, Armature Core, Yoke.

3. What is prime mover?

The basic source of mechanical power which drives the armature of the generator is called prime mover.

4. How is voltage generated in rotating machines?

In rotating machines voltage is generated in windings or group of coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor.

5. Write down the emf equation for d.c generator.

E = (φNZ/60) (P/A) V Where, P = number of poles, Z = Total number of conductors, A = number of parallel paths, φ = flux per pole, N = speed in rpm.

6. Why is Commutator employed in d.c machines? Or what is the function of a commutator in a DC generator?

i. Conduct electricity between armature and fixed brushes

ii. Converts alternating emf into unidirectional emf and vice versa.

7. How will you change the direction of rotation of a d.c motor?

Either the direction of the main field or the direction of current through the armature conductors is to be reserved.

8. What is back emf in d.c motors?

As the motor armature rotates, the system of conductor come across alternate North and South Pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors is in the direction opposite to the current. As this emf always opposes the flow of current in motor operation it is called back emf.

9. Under what condition the mechanical power developed in a de motor will be maximum?

Condition for mechanical power developed to be maximum is:

10. What is the function of a no-voltage release coil provided in a de motor starter?

As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in he ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus, a no-voltage or under voltage protections is given to the motor.

11. Define critical field resistance in de shunt generator.

Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specific field.

12. Why is the emf not zero when the field current is reduced to zero in a de mon generator?

Even after the field current/magnetizing force is reduce to zero the machine is left out with some flux as residue. Emf due to this residual flux is available when field current is zero.

13. Define the term "critical speed" in de shunt generator.

Critical speed is defined as the speed at which the generator is to be driven to cause self-excited generator to Build up its emf for the given field circuit resistance.

14. On what occasions de generators may not have residual flux?

i. The generator may be put for its first operation after its construction.

ii. In previous operation the generator would have been fully demagnetized.

15. What are the conditions to be fulfilled for a de shunt generator to build up emf?

i. The generator should have residual flux.

ii. The field winding should be connected in such a manner that the flux set up by the field winding should be in the same direction as that of residual flux.

iii. The field circuit resistance should be less than critical field resistance.

iv. Load circuit resistance should be above its critical load resistance.

16. What are the types of DC starters?

i. Two point starters

ii. Three point starters

iii. Four point starters.

17. What are the major categories of losses in a DC machine?

Magnetic losses, Electrical losses, Mechanical losses.

18. Name the different types of DC motors.

Shunt Motor, Series Motor, Cumulative Compound Motor, Differential Compound Motor.

19. Name any four applications of DC series motor.

Electric traction, Food mixies, Hoist work, Drilling machine.

20. Why starters are used for DC motors? Or Why a starter is necessary for a DC motor?

Starters are used in DC motors to limit the starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Apart from starting resistances starters are invariably fitted with protective devices such as No-voltage protection.

21. Why are carbon brushes preferred for de machines?

The high contact resistance carbon brushes help the current in the coil undergoing 9zun commutation to attain its full value in the reverse direction at the end of communication. The carbon brushes also lubricate and give less wear and tear on commutator surface.

22. Name any two applications of DC series generator.

Booster, electric welding, Constant current source, Constant illumination.

23. What is the basic principle of a de generator?

Basic principle of a de generator is Faraday's law of electromagnetic induction. i.e., whenever a conductor is moved in a magnetic field, dynamically induced emf is produced in that conductor.

24. What is the purpose of yoke in a de machine? Or The outer frame of a DC machine serves double purpose. What are they?

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

ii. It carries the magnetic flux produced by the poles. The flux per pole divides at the yoke so that; the yoke carries only half the flux produced by each pole.

25. What are the causes of failure of dc shunt generator to exite?

i. The residual magnetism may not be present in the pole.

ii. The field winding may not be properly connected with armature.

iii. Under no load condition, the shunt field resistance should be greater than the critical field resistance.

iv. Under loaded condition, the shunt field resistance should be less than the critical field resistance.

26. Why a de shunt motor is also called a constant flux motor or constant speed motor?

In shunt motor, flux produced by field winding is directly proportional to the field current. i.e., (α φ I_sh). Here, the input voltage is constant and so the flux is also constant. Therefore, DC shunt motor is also called a constant flux motor or constant speed motor.

27. Why series motor cannot be started without any load?

In dc series motor, flux is directly proportional to armature current. i.e., (α φ I_a). Under no load condition, the armature current is very low and flux also be less. By using the formula N α (φ / 4), here φ is less; the motor speed will be very high. Due to this motor will be damaged. Hence de series motor should always be started with some load on the shaft.

28. What is the function of starters in DC motor?

i. To limit the starting current.

ii. To protect against low voltage and over load condition.

29. List the important parts of a DC starter.

Starting resistance, Handle, over load relay, No voltage relay

30. What are the drawbacks of brake test on DC machines?

i. The brake test can be used for small motors only, because in case of large motors, it is difficult to dissipate the large amount of heat generated at the brake.

ii. This method cannot be used for determining internal losses.

iii. The output of the motor cannot be measured direclty.

31. What is universal motor?

A universal motor is defined as a motor, which may be operated either on direct seob current or single-phase ac supply.

32. State some application of universal motor.

Used for sewing machines, table fans, vacuum cleaners, hair driers, blowers, etc.

33. What are the advantages of brushless dc motors drives?

i. Regenerative braking is possible.

ii. Speed can be easily controllable.

34. What are the disadvantages of brushless de motors drives?

i. It requires a rotor position sensor.

ii. It requires a power semiconductor switching circuits.

35. Define mechanical commutators?

i. Its arrangement is located in the rotor.

ii. No. of commutators segments are very high

36. Define electronic commutators?

i. Its arrangement is located in the stator.

ii. No. of switching devices limited to six.

37. Mention some applications of PMBL DC motor?

i. Power alternators.

ii. Automotive applications.

iii. Computer and Robotics applications.

38. Why is the PMBLDC motor called electronically commulated motor?

The PMBL DC motor is also called electronically commutated motor because the phase windings of PLMBL DC motor is energized by using power semiconductor switching circuits here the power semiconductor switching circuits act as a commutator.

39. What are the classification of BLPM DC motor?

i. BLPM square wave motor.

ii. BLPM sine wave motor.

40. What are the two types of rotor position sensors?

i. Optical position sensor.

ii. Hall effect position sensor

41. What are the classification of BLPM dc motor?

i. One phase winding and one pulse BLPM dc motor

ii. One phase winding and two pulse BLPM dc motor

iii. Two phase winding and two pulse BLPM dc motor

iv. Three phase winding and three pulse BLPM dc motor

v. Three phase windings and six pulse circuits.

42. What are the features of one phase winding and one pulse BLPM de motor?

i. It is inertia should be high, such that rotor rotates continuously.

ii. Utilization of transistor and windings are less.

43. What are the features of one phase winding and two pulse BLPM dc motor?

i. In this case winding utilization is better, however transistor utilization is less.

ii. Torque developed is more uniform.

44. What are the features of two phase winding and two pulse BLPM de motor?

i. Winding utilization is only 50% which is less.

ii. It provide better torque waveforms.

45. What is synchronous in stepper motor?

It is the one to one correspondence between the number the number of pulses applied to the stepper motor and the number of steps through which the motor has actually moved.

46. Define mid frequency resonance in stepper motor.

The phenomenon at which the motor torque drops to a low value at certain input pulse frequencies.

47. What are the advantages of micro stepping?

i. Improvement in resolution.

ii. DC motor like performance.

iii. Elimination of mid frequency resonance.

iv. Rapid motion at micro stepping rate.

48. Define bandwidth in stepper motor.

It is a measure of the frequencies upto which the actuator or servo motor system can respond.

49. What is meant by full step operation?

Full step operation of single phase on mode is the one in which at a time only one phase winding is energized, due to which one stator winding is energized and causes the rotor to rotate some angle.

50. Mention some application of stepper motor.

i. Floppy disc drives

ii. Quartz watch

iii. Camera shutter operation

iv. Dot matrix and line printers

v. Small tool application

vi. Robotics.

51. What are the advantages and disadvantages of stepper motor?

Advantages:

i. It can be driven in open loop without feedback

ii. It is mechanically simple

iii. It requires little or no maintenance.

Disadvantages:

i. Low efficiency

ii. Fixed step angle

iii. Limited power output.

 

PART-B QUESTIONS

1. Explain the methods of excitation in DC generators.

2. Explain characteristics of DC generator.

3. Explain in detail the armature reaction in the DC machines.

4. Explain the process of commutation and the methods to improve the communications.

5. Explain the three point starter in detail with diagram.

6. Explain the methods speed control of DC shunt motor.

7. Explain the methods speed control of DC series motor.

8. Explain any 2 methods of testing of DC machines.

9. (i) Derive an expression for the emf generated in the armature winding of a de

machine.

(ii) A long-shunt compound generator delivers a load current of 50 A at 500 V and has armature, series field and shunt field resistance of 0.05 Ω, 0.03 Ω and 250 Ω respectively. Calculate the generated voltage and the armature current. Allow VI per brush for contact drop.

10. (i) Explain the construction of de machine with neat diagram.

(ii) A 100 kW DC hunt generator driven by a belt from an engine runs at 750 rpm and is connected to 230 V dc mains. When the belt breaks, it continues to run as a motor drawing 9 k Ω from the mains. At what speed would it run? Given: Armature resistance = 0.018 Ω and field resistance = 115 Ω.

11. (i) Explain the various methods of excitation of de machines.

(ii) Draw the OCC characteristics and external characteristics of DC generator.

12. A 6-pole DC generator has 150 slots. Each slots has 8 conductors and each conductor has resistance of 0.01 Ω. The armature terminal current is 15 A. Calculate the current per conductor and the drop in armature for lap and wave winding connection.

13. Sketch the structure of controller for PMBLDC motor and explain the functions of various blocks.

14. Explain the closed loop control scheme of a permanent magnet brushless dc motor drive with a suitable schematic diagram.

15. Drive the expressions for the emf and torque of a PMBLDC motor.

16. Discuss the use of Hall sensors for position sensing in PMBLDC motor.

17. Sketch the torque-speed characteristics of a PMBLDC motor.

18. Explain the construction and various modes of excitation of VR stepper Motor.

19. Explain the construction and various modes of excitation of PM stepper motor.

20. Explain the construction and working principle of Hybrid Stepper motor.

21. State and explain the static and dynamic characteristics of a stepper motor.