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Questions and Answers
What is the characteristic of a three-phase stator connected in a delta configuration?
What is the characteristic of a three-phase stator connected in a delta configuration?
- Line voltages are equal to phase voltages.
- Each line current is equal to 1.73 times the phase current. (correct)
- Line currents are equal to phase currents.
- The phases are not connected end-to-end.
What is the formula to calculate power consumed in a 3-Ø circuit?
What is the formula to calculate power consumed in a 3-Ø circuit?
- P = √2 × V × I × Cosineθ
- P = √3 × V × I × Cosineθ (correct)
- P = √3 × V × I × Sineθ
- P = 2 × V × I × Cosineθ
What is the purpose of the formula P ower (kV A) = ILine × VLine × 1.732 / 1000?
What is the purpose of the formula P ower (kV A) = ILine × VLine × 1.732 / 1000?
- To calculate power in kilo volt-amps (kVA) (correct)
- To calculate power in megawatts (MW)
- To calculate power in kilowatts (kW)
- To calculate power in megavolt-amps (MVA)
AC generators are typically power rated in:
AC generators are typically power rated in:
What is a characteristic of a three-phase stator connected in a wye configuration?
What is a characteristic of a three-phase stator connected in a wye configuration?
What is the formula to calculate power in kilowatts (kW) in a 3-Ø circuit?
What is the formula to calculate power in kilowatts (kW) in a 3-Ø circuit?
What is the power rating of the given AC generator in the WYE connection example?
What is the power rating of the given AC generator in the WYE connection example?
What is the result of transposing phases B and C in a three-phase system?
What is the result of transposing phases B and C in a three-phase system?
In a three-phase star connection, what is the relationship between the line voltage and phase voltage?
In a three-phase star connection, what is the relationship between the line voltage and phase voltage?
What is the advantage of a star connection over a delta connection?
What is the advantage of a star connection over a delta connection?
What determines the output frequency of an alternator voltage?
What determines the output frequency of an alternator voltage?
What is the primary advantage of three-phase connections in power transmission?
What is the primary advantage of three-phase connections in power transmission?
What is the advantage of a star connection in a three-phase alternator?
What is the advantage of a star connection in a three-phase alternator?
What is the relationship between the voltage waveforms generated across each phase in a three-phase alternator?
What is the relationship between the voltage waveforms generated across each phase in a three-phase alternator?
What is the total voltage across any two of the three line leads in a three-phase, Y-connected alternator?
What is the total voltage across any two of the three line leads in a three-phase, Y-connected alternator?
Why is it difficult to see what is happening in a three-phase stator schematic diagram?
Why is it difficult to see what is happening in a three-phase stator schematic diagram?
What is the significance of the 1.73 factor in a three-phase, Y-connected alternator?
What is the significance of the 1.73 factor in a three-phase, Y-connected alternator?
What is the advantage of a three-phase alternator over a single-phase alternator?
What is the advantage of a three-phase alternator over a single-phase alternator?
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Study Notes
Calculating Power in AC Generators
- Power of an AC generator can be calculated using the formula: Power (kVA) = 200 × (115 × 1.732) × 1.732 / 1000, resulting in 69 kVA.
Phase Sequence
- Phase sequence determines the order of voltage peaks during rotor rotation.
- A sequence of A, B, C corresponds to ABC phase rotation; transposing B and C results in ACB, reversing motor rotation.
- Critical for three-phase systems, especially impacting direction of AC motors.
Advantages of Three-Phase Connections
-
Wye (Star) Connection:
- Allows both balanced and unbalanced loads, more suited for balanced loads.
- Line voltage (V_line) is √3 times the phase voltage (V_phase).
- I_line equals I_phase, with output voltage and available earthing options.
- Voltage leads current by 30°.
- Ideal for long-distance power transmission.
-
Delta Connection:
- Operates primarily with locally driven machinery.
- Line voltages equal phase voltages; I_line is 1.73 times the I_phase.
- Facilitates alternate current paths among phases.
Alternator Frequency Control
- Alternator output frequency is influenced by rotor speed and number of poles.
- Single current flow path exists in windings, where line and phase currents are equal.
Power Calculation in Three-Phase Circuits
- Power calculation formula: P = √3 × V × I × Cos(θ), with V and I as line values and θ as the power factor.
- For apparent power (kVA): Power (kVA) = I_Line × V_Line × 1.732 / 1000.
- For true power (kW): Power (kW) = I_Line × V_Line × 1.732 × power factor / 1000.
- AC generators commonly rated in kVA.
Three-Phase Stator and Voltage Waveforms
- A schematic of a three-phase stator reveals complex windings that appear phase-displaced by 120°.
- Three-phase alternators comprise three individual single-phase alternators, output voltages 120° out of phase, ensuring phase independence.
Star and Delta Connections
- Connections reduce numbers of leads in three-phase alternators, forming either wye (Y) or delta connections.
- Wye connections include a neutral terminal for single-phase loads, providing voltages from neutral to each phase.
- In a Y-connected alternator, line voltage is the vector sum of phase voltages, with line voltage being 1.73 times that of phase voltage.
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