Electrical Machines Short Answers

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1. Why almost all large size Synchronous machines are constructed with rotating field system type?
The following are the principal advantages of the rotating field system type construction of Synchronous machines:

  • The relatively small amount of power, about 2%, required for field system via slip-rings and brushes.
  • For the same air gap dimensions, which is normally decided by the kVA rating, more space is available in the stator partof the machine for providing more insulation to the system of conductors, especially for machines rated for 11kV or above.
  • Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.
  • Stationary system of conductors can easily be braced to prevent deformation.
  • It is easy to provide cooling arrangement for a stationary system of conductors.
    • Firm stationary connection between external circuit and system of conductors enable he machine to handle large amount of volt-ampere as high as 500MVA.

2. Write down the equation for frequency of emf induced in an Alternator.
Frequency of emf induced in an Alternator,f ,expressed in cycles per second or Hz, is given by the following equation
F = (PN)/120 Hz,
Where P- Number of poles
N-Speed in rpm

3. How are alternators classified?
According to type of field system

  • Stationary field system type
  • Rotating field system type

According to shape of field system

  • Salient pole type
  • Smooth cylindrical type

4. Name the types of Alternator based on their rotor construction.
Alternators can be classified into the following two types according to its rotor construction

  • Smooth cylindrical type alternator
  • Salient pole alternator

5. Why do cylindrical Alternators operate with steam turbines?
Steam turbines are found to operate at fairly good efficiency only at high speeds. The high speed operation of rotors tends to increase mechanical losses and so the rotors should have a smooth external surface. Hence, smooth cylindrical type rotors with less diameter and large axial length are used for Synchronous generators driven by steam turbines with either 2 or 4 poles.

6. Which type of Synchronous generators are used in Hydro-electric plants and why?
As the speed of operation is low for hydro turbines use din Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

7. What are the advantages of salient pole type construction used for Synchronous machines?
Advantages of salient-pole type construction are :

  • They allow better ventilation
  • The pole faces are so shaped that the radial air gap length increases from the pole center to the pole tips so that the flux distribution in the air-gap is sinusoidal in shape which will help the machine to generate sinusoidal emf
  • Due to the variable reluctance the machine develops additional reluctance power which is independent of excitation

8. Why is the stator core of Alternator laminated?
The stator core of Alternator is laminated to reduce eddy current loss.

9. How does electrical degree differ from mechanical degree?
Mechanical degree is the unit for accounting the angle between two points based on their mechanical or physical placement. Electrical degree is used to account the angle between two points in rotatingelectrical machines. Since all electrical machines operate with the help of magnetic fields, the electrical degree is accounted with reference to the magnetic field. 180 electrical degree is accounted as the angle between adjacent North and South poles.

10. What is the relation between electrical degree and mechanical degree?
Electrical Degree Ѳe and The Mechanical Degree Ѳm are related to one another the number of poles P, the electrical machine has, as given by the following equation
Ѳe=(P/2)Ѳm

11. What is distributed winding?
When coil-sides belonging to each phase are housed or distributed in more than one slot under each pole region then the winding is called distributed winding A full pitch coil has width of coil otherwise called coil-span as 180ºε where β angle between adjacent slots in electrical degree and x=1,2,3…

12. Why is short pitch winding preferred over full-pitch winding ?
Advantages

  • Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated.
  • Conductor material , copper , is saved in the back and front end connections due to less coil-span.
  • Fractional slot winding with fractional number of slots/phase can be used which in turn reduces the tooth ripples.
  • Mechanical strength of the coil is increased.

13. Write down the formula for distribution factor.

Kd = sin(mβ/2)/m sin(β/2) or Kdn=sin(mn β/2) /m sin (n β/2)
m= number of slots/ pole/ phase
β=Angle between adjacent slots in Electrical Degree
n = order of harmonic

14. Define winding factor.
The winding factor Kd is defined as the ratio of phasor addition of emf induced in all the coils belonging to each phase winding to their arithmetic addition.

15. Why are Alternators rated in kVA and not in kW?
The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.

Apart from the constant loss incurred in Alternators is the copper loss, occurring in the 3 –phase winding which depends on I2 R, the square of the current delivered by the generator. As the current is directly related to apparent – power delivered by the generator , the Alternators have only their apparent power in VA/kVA/MVA as their power rating.

16. What are the causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on load condition are due to the following three causes:

  • Voltage variation due to the resistance of the winding, R
  • Voltage variation due to the leakage reactance of the winding, Xt
  • ·Voltage variation due to the armature reaction effect, Xa

17. What is meant by armature reaction in Alternators?
The interaction between flux set up by the current carrying armature conductors Φa and the main field flux Φm is define as the armature reaction.

18. What do you mean by synchronous reactance?

Synchronous reactance X s= (Xl + Xa)
The value of leakage reactance Xl is constant for a machine based on its construction. Xa depends on saturating condition of the machine. It is the
addition of Xa , which represent the armature reaction effect between two synchronously acting magnetic fields that makes the total reactance Xa to be called syncheornous reactance.

19. What is meant by synchronous impedance of an Alternator?
The complex addition of resistance, R and synchronous reactance , jXs can be represented together by a single complex impedance Zs called synchronous impedance.
In complex form Zs = (R + jXs )
In polar form      Zs =I Zs I< Ѳ
and where the  I Zs I=√(R²+X²s)
and                  Ѳ=Tan-1 (Xs/R)

20. What is meant by load angle of an Alternator?
The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , U during the load condition of an Alternator is called load angle.

21. Upon what factors does the load angle depend?
The magnitude of load angle δ increases with the increase in load.Further the load angle is positive during generator operation and negative during motor operation.

22. An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?
The nature of the load is of leading power factor , load consisting of resistance and capacitive reactance.

23. Define the term voltage regulation of Alternator.
The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to load condition expressed as a fraction or percentage of terminal voltage at load condition ; the speed and excitation conditions remaining same.
Voltage regulation in percentage , URP = [(|E|-|U|)/|U| ]x 100

24. What is the necessity for predetermination of voltage regulation?
Most of the Alternators are manufactured with large power rating , hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

25. Name the various methods for predetermining the voltage regulation of 3-phase Alternator.
The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators

  • Synchronous impedance / EMF method
  • Ampere-turn / MMF method
  • Potier / ZPF method

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