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Chapter: Electrical machines - DC Generators

Important short questions and answers: DC Generators

Electrical machines - DC Generators - Important short questions and answers: DC Generators

DC GENERATOR

 

41. Write the expressions for the synchronous speed.

 

The speed of rotating magnetic field is called synchronous speed.

 

43. Write the mmf equation of dc machine.

 

The fundamental component of mmf wave is given by

 

Where θ = electrical angle measured from the magnetic axis of the coil which coincides with the positive peak of the fundamental wave.

 

44. What is meant by electromagnetic torque?

 

When the stator ad rotor windings of the machine both carry currents, they produce their own magnetic fields along their respective axes which sinusoidally distributed along the air-gaps. Torque results from the tendency of these two fields to align themselves.

 

 

45.         State the torque equation for round rotor machine.

Where P = No. pole

 

D = Average diameter of air gap

l= Axial length if air gap

μo = Permeability of free space = 4π  x 10 -7 H/m

g = air gap length

 

F1 = Peak value of sinusoidal mmf stator wave  

F2 = peak value of sinusoidal mmf rotor wave

Α                      = Angle between F1 and F2 called torque angle

 

46.         Define rotating magnetic field.

 

When a balanced three phase winding with phase distributed in space so that the relative space angle is 120o is fed with balanced 3 phase current, resultant mmf rotates in air gap at speed.

 

47. What is prime mover?

 

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

 

48.         Give the materials used in machine manufacturing

Three materials are used in machine manufacturing.

(i)steel – to conduct magnetic flux

(ii)copper – to conduct electric current

(iii)Insulation

 

49.         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.

 

50. 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 . The direction of the emf induced 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.

 

 

51.         Under what condition the mechanical power developed in a dc motor will be maximum?

 

Condition for mechanical power developed to be maximum is

Eb = Ua /2

 

or Ia= Ua / 2Ra

 

52.         What is the function of a no-voltage release coil provided in a dc 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 the 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 given to the motor.

 

53.         Name the two types of automatic starters used for dc motors.

Back emf type starter

Time delay type starter

 

54. Enumerate the factors on which the speed of a dc motor depends.

N = 1/CE (Ua-IaRm)/ф

 

The speed of dc motor depends on three factors.

Flux in the air gap

Resistance of the armature circuit

Voltage applied to the armature

 

55. List the different methods of speed control employed for dc series motor

Field diverter method

 

Regrouping of field coild\s

Tapped field control

Armature resistance control

Armature voltage control for single motor

Series parallel control for multiple identical motors

 

57.   Name the different methods of electrical breaking of dc motors.

 

(i)     Dynamic braking

(ii)    Regenerating braking

(iii)   Counter current braking or plugging

 

 

58.   Under what circumstances does a dc shunt generator fail to build up?

Absence of residual flux.

 

Initial flux set up by the field winding may be in opposite direction to residual flux

 

Shunt filed circuit resistance may be higher than its critical field Resistance

 

Load circuit resistance may be less than its critical load resistance.

 

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