NCATT Alternating Current (AC) Terms - Level B Flashcards
Define Alternating Current.
Alternating Current—Flow of electrons that continuously changes its value in magnitude and periodically reverses direction (sine-wave). Alternating current is much easier to produce in large quantities as compared to DC. The time required for one cycle of AC to occur is called the period.
Define Frequency.
Frequency—Number of cycles of AC completed in one second. Frequency is expressed in hertz (Hz), 1 Hz = 1 cycle of AC/Second. The frequency of most AC used in aircraft applications is 400 Hz.
Define Impedance (Z)
Impedance (Z)—Electrical characteristic of a conductor. Impedance is the total opposition to the flow of AC in an electrical circuit (resistance, capacitance, and inductance). Values are given in ohms.
Define Capacitive Reactance (XC)
Capacitive Reactance (XC)—Opposition to current flow in a circuit. The affect that capacitance has on an AC circuit is termed capacitive reactance. Capacitance causes current to lead voltage in phase (ICE). Values are given in ohms.
Define Inductive Reactance (XL)
Inductive Reactance (XL)—Opposition to current flow in a circuit. The affect that inductance has on an AC circuit is termed inductive reactance. Inductance causes current to lag voltage in phase (ELI). Values are given in ohms.
Define Sine-Wave.
Sine-Wave—Alternating current wave form produced by a rotary generator. Sine-waves are values of voltage and current that start at zero and smoothly rise to a peak value; the waves smoothly fall from a peak back to zero. Then, the waves rise to a peak value in a negative direction and smoothly return to zero. One AC cycle is produced by 360 degrees of rotation.
Define Root Mean Square (RMS)
Root Mean Square (RMS)—Effective value of sine-wave alternating current. The RMS value is .707 of the peak value.
Define Effective Voltage.
Effective Voltage—Amount of AC that produces the same amount of heat as a corresponding value of DC. The effective voltage of AC is referred to as the RMS value. Effective value is always less than the peak value of AC. Unless otherwise specified, all values of AC are considered effective values.
Define Phase Angle
Phase Angle—Angle between two similarly varying quantities (sine-waves). A phase angle is the difference in angle between two sinusoidally varying quantities that have the same frequency.
Define Power Factor.
Power Factor—Ratio of the actual power dissipated in an electrical system to the input power of volts multiplied by amps. The power factor is the ratio of power dissipated over input.
Define True Power
True Power—Power actually available in an AC circuit. True power is the product of the circuit voltage and the current in phase with this voltage. Values are given in watts.
Define Apparent Power
Apparent Power—Product of effective voltage and effective current which is expressed in Volt Amps (VA) rather than Watts (W) without reference to phase shift, if any, between voltage and current.
Define Wye Wound.
Wye Wound—Windings arranged in the shape of a Y. For example, on a three-phase generator or transformer, one end of each of the three windings is connected to form a common point.
Define Delta Wound
Delta Wound—Windings arranged in the shape of a triangle. For example, on a three-phase generator, motor, or transformer, two of the phase windings are in a series, across the third (phase) winding.
Define Polyphase
Polyphase—Production of two or more phases of AC, or of two or more alternating voltages of the same frequency. The most common type of polyphase uses three phases.
