LECTURE NO. 8x

8/7/2019 LECTURE NO. 8x

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AC Machines

B.Sc. Electrical Engineering

Lecture No. 8Synchronous Generator

(Alternator / AC Generator)

Prof. Engr. Riaz Ahmad RanaFaculty of Engineering, UCP, Lahore


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Learning Objectives:

Cooling systems of SG.

Ratings of Syn. Generator.

Faults & Trouble Shooting.

NEMA Insulation Classes.

Armature Reaction in SG.


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3. Apparent Power (KVA): Maximum acceptable armature current sets the

apparent power rating for a alternator. S rated = 3 V L, rated IL, rated

4. Power Factor: The maximum size of E A and I A together sets the rated

power factor of the generator.

5. Speed of alternator.

6. Current of armature.

7. Field Current.

8. Temperature.

Alternator Faults, Effects & Protection:Alternator Faults, Effects & Protection:Alternator Faults, Effects & Protection:Alternator Faults, Effects & Protection:

[1] Thermal Overloading:

Reasons:

(i) Continuous overloading.

(ii) Failure of cooling system.


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Effects:

(i) Overheating of stator windings.

(ii) Insulation failure

Protection:

(i) Thermocouples of resistance thermometer imbedded in stator slots.

(ii) Stators overload protection with over current relays.

[2] Stator Faults:

Reason:

(i) Phase to phase fault.

(ii) Phase to earth fault.

(iii) Inter-turn fault.Effects:

(i) Winding burn-out.


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(ii) Welding of core laminations burn-out.

Protection:

(i) Differential protection.

(ii) Earth-fault protection.

(iii) Inter turn fault protection

[3] Loss of Field:

Reason:

(i) Tripping of field circuit breaker.

Effects:

(i) Alternator will act as motor.

(ii) Speed increases slightly.

Protection:

(i) Field failure protection.


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[4] Motoring of Alternator:

Reasons:

(i) When input to prime mover stops, the alternator draws power from bus-

bar and runs as synchronous motor in the same direction.

Effects:

(i) Depends upon the type of prime mover.

Protection:

(i) Reverse power protection.[5] Over voltages:

Reasons:

(i)

Atmospheric surge-voltages.(ii) Switching surges

Effects:

(i)

Insulation failure


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Protection:

(i) Lightning arresters connected near the alternator terminals.

[6] Under frequency:

Reasons:

(i) Failure of blades of steam turbine.Protection:

(i) Frequency relays.

[7] Bearing Insulation protection:

[8] Grounding protection:

[9] Over speed protection:

[10] Rotor earth fault protection:


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NEMANEMANEMANEMA InsulationInsulationInsulationInsulation ClassesClassesClassesClasses: :::

The insulation of the AC machine breaks down (fails) generally due to

overheating of the windings. In order to prevent the breaking down of

the insulation of winding, it is necessary to limit the temperature of the

windings. This can be done by two ways:

(ii) By providing a cooling air circulation over them.

(i) By increasing the winding temperature limits. NEMA (National Electrical Manufacturing Association USA) has

defined a series of insulation system classes. Each insulation system

class specifies the maximum temperature rise permissible for that class

of insulation.

NEMA Class B Insulation: This insulation class can withstand up to

80 0C.


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NEMA Class F Insulation: up to 105 0 C. NEMA Class H Insulation: up to 125 0 C. IEC (International Electrical Commission) has also defined different

insulation classes.

Why terminal voltage of an alternator decreases on loading?Why terminal voltage of an alternator decreases on loading?Why terminal voltage of an alternator decreases on loading?Why terminal voltage of an alternator decreases on loading?

When the load is increased on the alternator, keeping the field excitation and

speed constant, the terminal voltage of the alternator decreases. This is due to

following reasons:

(1) Voltage drop due to armature resistance (Ia Ra).

(2) Voltage drop due to armature leakage reactance (Ia XL).

(3) Voltage drop due to armature reaction.


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How does armatureHow does armatureHow does armatureHow does armature reaction occur in synchronous generator?reaction occur in synchronous generator?reaction occur in synchronous generator?reaction occur in synchronous generator?

The reaction of armature flux on main field flux. When an alternator is loaded, current flows through the armature and

produces the armature flux which affects (weakens) the main flux (fieldflux). This effect of armature flux on the main flux in the air-gap is called

armature reaction. The armature reaction reduces the generated voltage.

If a generator is not supplying power to a load, no current flows through

its armature winding and no flux is produced by the armature. The only

flux developed in the machine is that due to the field poles. This flux is

called main flux. This flux is distributed symmetrically with respect to

the axes of the poles.


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What are adverse effects of AR?What are adverse effects of AR?What are adverse effects of AR?What are adverse effects of AR?

(i) It weakens or demagnetizes the main flux.

(ii) It distorts or cross-magnetizes the main flux.

What isWhat isWhat isWhat is thethethethe eeeeffect of Resistive Load onffect of Resistive Load onffect of Resistive Load onffect of Resistive Load on AR?AR?AR?AR? When a resistive load ( unity power factor ) is connected to an

alternator, the armature current and voltage are in phase.

The armature flux is at 90 degree to the main flux and is behind themain flux. In this case, the flux in the air-gap is distorted but not

weakened.

Therefore, at unity power factor, the effect of armature reaction ismerely to distort the main field; there is no weakening of the main

field and the average flux practically remains the same.


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What is the eWhat is the eWhat is the eWhat is the effect of Lagging Load on Affect of Lagging Load on Affect of Lagging Load on Affect of Lagging Load on AR? R?R?R?

When a pure inductive load ( Lagging power factor ) is connected to the

terminals of alternator, current lags behind the voltage by 90 degree.

All the flux produced by the armature current opposes the field flux

and therefore, weakens it.

Hence at zero p.f. lagging, the armature reaction weakens the main

flux. This causes the reduction in generated emf.

What is the eWhat is the eWhat is the eWhat is the effect of Capacitive Load on Affect of Capacitive Load on Affect of Capacitive Load on Affect of Capacitive Load on AR? R?R?R?

When a pure capacitive load (leading p.f) is connected across thealternator, the current in armature winding leads the induced emf by 90degrees.

The armature flux helps the main flux and total air-gap flux increases andhence the generated emf is increased.

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LECTURE NO. 8x