Alternating current and electrical machines

Read frequency, RMS voltage and real power

Distinguish RMS voltage, frequency, apparent power and real power, including the effect of power factor at unchanged current.

Mechanism and relationships

Alternating current reverses direction periodically. Frequency is cycles per second; a sinusoidal voltage’s RMS value gives the equivalent heating effect in a resistor. Peak voltage is √2 times RMS for a sine wave. In an inductive or capacitive load, current and voltage can be out of phase. Apparent power is RMS voltage times RMS current in volt-amperes; real power is apparent power multiplied by power factor for the stated steady sinusoidal case.

Reason through the evidence

A transformer uses a changing magnetic field to exchange voltage and current levels between windings; ideal power is conserved, so stepping voltage up steps available current down for a given power. A transformer does not rectify AC into DC. A rectifier performs that conversion, while an inverter produces AC from DC. A transformer-rectifier unit combines functions. Semiconductor control and logic can regulate, switch and monitor power, but each component’s cooling and voltage limits remain important.

Apply the explanation to a different case

Consider two loads that each draw 10 A from a 115 V RMS single-phase source. At power factors of 1.0 and 0.8 their real powers are 1,150 and 920 W, yet both require the stated current capability. It would be wrong to size the second feeder as though it carried only 8 A because the real watts are lower. Reactive behaviour changes phase and power exchange, not the need for the conductors and generator to carry current. Three-phase relationships require the correctly defined line and phase quantities and must not be substituted silently into this single-phase example.

Keep this distinction clear

A transformer’s winding ratio sets an ideal voltage ratio while current scales inversely for conserved power. Frequency is not changed by that winding ratio. Real winding resistance, core losses and heating reduce efficiency, so output capability must stay within the unit’s specified thermal and electrical ratings.

Relate generator frequency to shaft speed

Relate synchronous-generator frequency to shaft speed and pole count, then distinguish frequency control from voltage control.

Mechanism and relationships

A rotating generator induces voltage through relative motion between conductors and a magnetic field. For a synchronous generator, frequency in hertz equals rotor speed in rpm multiplied by the number of pole pairs and divided by 60. A constant-speed drive or integrated drive generator allows a variable engine speed to produce the required generator speed. Other aircraft use variable-frequency architectures designed for that variation. Voltage regulation controls excitation according to the machine and architecture.

Reason through the evidence

An electric motor converts electrical power into mechanical torque through electromagnetic interaction. Back electromotive force rises with speed in a simplified DC motor and reduces current for a fixed applied voltage; starting can therefore demand much more current than steady running. Generator paralleling requires compatible voltage, frequency and phase according to the system’s design. Two AC sources are not made safely interchangeable just by reading the same voltage number.

Apply the explanation to a different case

A generator driven directly by a changing engine shaft would have changing frequency unless the architecture compensates or accepts variable frequency. A constant-speed drive addresses the mechanical-speed relationship, while excitation control addresses voltage; they are not the same regulator. A transformer changes voltage ratio but passes the input frequency, so it cannot repair an incorrect frequency merely by producing the desired voltage. Similarly, a motor’s high starting current must be allowed for in distribution design even when its steady running power is modest. Each conversion preserves the need for a valid input and finite output capability.

Keep this distinction clear

A constant-speed drive can be disconnected according to its installation to isolate a drive problem; restoration may require maintenance on the ground. A generator reset and a mechanical drive reconnection are therefore different actions. A generic electrical model cannot establish which recovery is available in flight.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.