Induced
emf
- The total induced emf when two coils are bunched in one slot = Es + Es = 2Es
- The total induced emf when two coils are distributed in two slots having phase difference of 300 = 2Es Cos (30/2)0 = 1.9 Es
‘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
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.
What is
synchronous reactance?
Synchronous
reactance
- The synchronous reactance is combined effect of armature reactance and leakage reactance (Synchronous reactance XS = XL + Xa)
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.
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
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 Rac = 1.6 Rdc
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.
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.
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.
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.
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.
Describe the
effect of armature current on the potier triangle.
- The potier triangle is constant for given armature current.
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
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
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 center of main pole.
Quadrature
axis (q – axis)
- It is axis passing through center of inter pole.
Which axis
offers higher reluctance in the salient pole machine?
Quadrature
axis (q – axis)
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.
What do you mean
by infinite bus – bar?
Infinite
Bus bar
- Constant voltage and constant frequency of the supply system
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 bus-bar voltage.
- The speed (frequency) of the incoming alternator must be same as that of bus-bar frequency.
- The phase of the alternator must be identical to that of phase of the bus-bar voltage.
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