FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
A brushless AC alternator has three phase windings connected in a Y (wye) configuration. Each phase winding produces 115 volts AC. What is the approximate line-to-line voltage available at the alternator output?
- A230 volts
- B199 volts
- C115 volts
- D345 volts
Show answer & explanationAnswer & explanation
Correct answer: B. 199 volts
In a Y-connected three-phase system, line voltage equals phase voltage multiplied by the square root of 3 (1.732). Calculation: 115 V × 1.732 = 199.2 V, so the line-to-line voltage is approximately 199 volts.
Why the other options are wrong
- A. 230 V would be double the phase voltage, which applies to a different (delta or single-phase doubling) scenario, not Y.
- C. 115 V is the phase voltage, not the line voltage in a Y connection.
- D. 345 V would be triple the phase voltage, which is not how Y-connected voltages combine.
Y-Connected Three-Phase Voltage
In a wye (Y) connected three-phase AC system, line voltage equals phase voltage times the square root of 3 (approximately 1.732).
- Line voltage = Phase voltage × 1.732
- Y connection provides both line and phase voltage access via neutral
- Common in aircraft brushless AC alternators
Memory trick: Y-connection: multiply phase voltage by 1.732 to get the line voltage.