21

If the induced emf in the one coil is Es, ( a ) Find out total induced
emf if two coils are bunched in one slot? ( b ) Find out total induced emf if two
coils are distributed in two slots having phase difference of 300 degree?


Induced
emf
 The total
induced emf when two coils are bunched in one slot = E_{s} + E_{s } = 2E_{s}
 The total
induced emf when two coils are distributed in two slots having phase
difference of 30^{0} = 2E_{s
}Cos ( 30/2 )^{0} = 1.9 E_{s}

22

‘The alternator voltage decreases as the load increases’ comment on the
above statement.


The alternator
voltage decreases as the lagging load increases due to following voltage drop
in the alternator
 Voltage drop
due to the armature resistance
 Voltage drop
due to leakage reactance and
 Voltage drop due
to armature reaction

23

Describe the effect of armature reaction on the alternator at Lagging
power factor, Leading power factor and Unity power factor.


Effect
of armature reaction
Lagging
power factor
 The armature flux is
direct phase opposition to main flux therefore main flux weakens.
 Due to
demagnetizing effect at lagging power factor less emf is generated.
Leading
power factor
 The armature flux is
added to main flux therefore main fluxes strengthen.
 Due to magnetizing
effect at leading power factor more emf is generated.
Unity
power factor
 The armature flux at
the leading pole tips is reduced whereas it increases at trailing pole tips.
 The average field
flux remains constant at unity power factor.
 The armature reaction
is distortional due to cross magnetizing effect.

24

What is synchronous reactance?


Synchronous
reactance
 The synchronous
reactance is combined effect of armature reactance and leakage reactance (
Synchronous reactance X_{S} =
X_{L} + X_{a} )

25

On which factor the voltage regulation of the alternator depends?


The voltage
regulation of the alternator depends upon load current and power factor of
the load.

26

Describe the different methods to find out voltage regulation of the alternator.


Voltage
regulation of the alternator
 Direct loading
method
 Synchronous
impedance method
 MMF method
 Potier
triangle method

27

How to measure armature resistance of the alternator?


The
armature resistance of the alternator is measured by any of the below
mentioned method
 Voltmeter – ammeter
method
 Multi meter or
Wheatstone bridge method
 However it is DC
armature resistance. The AC armature resistance is measured by considering
skin effect.
 Generally R_{ac} = 1.6 R_{dc}

28

Why the synchronous impedance method of voltage regulation is also called
as pessimistic method?


Pessimistic
method
 The
synchronous impedance method is also called as pessimistic method because the
value of synchronous impedance in this method is always more than its value
under the normal voltage and saturation therefore the voltage regulation is
always more than the actual value.

29

Describe the effect of saturation on the synchronous impedance of the
alternator.


Effect
of saturation on the synchronous impedance
 The effect of
armature ampere turns is much more at low saturation as compared to high
saturation therefore the value synchronous impedance is higher at low
saturation.

30

Which method of voltage regulation is known as optimistic method? Why?


Optimistic
method
 The ampere
turn method is also known as optimistic method because the voltage regulation
by this method is somewhat less than the actual value.

31

Why zero power factor method to find out voltage regulation is very
accurate method in the alternator?


Zero
power factor method
 The armature leakage reactance drop
and armature reaction drop in the zero power factor method are considered
separately therefore this method gives accurate result than any other method.

32

What is full load zero power factor curves?
OR
What is watt less load characteristic?


Watt less load characteristic
 The zero power factor curves
represents relation between terminal voltages against field current when
armature is delivering full load current at zero power factor.

33

Describe the effect of armature current on the Potier triangle.


The potier
triangle is constant for given armature current.

34

How to obtain zero power factor curves in the alternator?


The zero power
factor curves is obtained by
 Loading
alternator with pure reactor.
 Field current
is adjusted such that wattmeter shows zero reading and ammeter shows full
load current.
 If machine of
same rating is driven at no load and zero power factor

35

Describe the significance of the potier triangle?


Significance
of Potier triangular
 The vertical
line AC of the Potier triangle indicates voltage drop due to armature leakage
reactance drop.
 The horizontal
line CD indicates field current necessary to overcome demagnetizing effect of
armature reaction at full load.
 The horizontal
line BC indicates field current necessary for balancing armature leakage
reactance drop.

36

What is meant by the d  axis and q  axis in the salient pole machines?


Axis in the
salient pole machines
Direct
axis ( d – axis )
 It is axis
passing through centre of main pole.
Quadrature
axis ( q – axis )
 It is axis
passing through centre of inter pole.

37

Which axis offers higher reluctance in the salient pole machine?


Quadrature
axis ( q – axis )

38

What is meaning of synchronizing of alternators?


Synchronizing
 The operation
of the connecting an alternator in parallel with another alternator or common
bus – bar is called synchronizing of the alternators.

39

What do you mean by infinite bus – bar?


Infinite
Bus bar
 Constant
voltage and constant frequency of the supply system

40

What are the conditions to be satisfied for synchronizing of alternators?


The followings
are the conditions to be satisfied for synchronizing of the alternators :
 The voltage of
the incoming alternator must be same as that of busbar voltage.
 The speed ( frequency
) of the incoming alternator must be same as that of busbar frequency.
 The phase of
the alternator must be identical to that of phase of the busbar voltage.

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