## 10/03/2020

### Alternator Interview Question Answer - 2

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 = 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.

• 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

• 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

• 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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