AC circuits and theory

NAVEDTRA 14104 Ch. 12 — Resistance

Electrical resistance (R) is that property of an electric circuit that opposes the flow of current. The unit of resistance is known as the ohm (S2). WATT Power (P) is the rate of doing work. In a dc circuit, power is equal to the product of the current times the voltage, or P = I x E. The practical unit of power is the watt (W) or kilowatt (kW) (1,000 watts). Power in an ac circuit is computed in a slightly different way. If you are interested in how ac power is computed, see chapter 4 of Introduction to Alternating Current and Transformers, NAVEDTRA 172-02-00-85.

NEETS Mod. 1 §3-1

Ohm's law and electrical power Ohm's law is the foundation of all circuit calculation: the current in a circuit is directly proportional to the voltage and inversely proportional to the resistance, expressed E = I × R, where E is electromotive force in volts, I is current in amperes, and R is resistance in ohms. Rearranged: I = E/R and R = E/I. If any two quantities are known the third is found. Resistance opposes current flow and depends on the conductor's material (resistivity), length (longer = more resistance), cross-sectional area (thicker = less resistance), and temperature (for metals, resistance rises with temperature). Electrical power — the rate of doing electrical work — is measured in watts: P = E × I. Combined with Ohm's law this gives the useful forms P = I²R and P = E²/R. T…

NEETS Mod. 2 §1-1

AC vs DC, generation, and the sine wave Alternating current (AC) periodically reverses direction and continuously changes in amplitude, unlike direct current (DC) which flows in one direction at a steady value. AC is the shipboard standard for generation and distribution because its voltage is easily raised or lowered by transformers, allowing efficient transmission and simple motor design. AC is produced by electromagnetic induction: when a conductor loop rotates in a magnetic field, the induced voltage varies as the sine of the angle between the conductor's motion and the flux, tracing a sine wave. One complete positive-and-negative excursion is a cycle; the number of cycles per second is the frequency in hertz (Hz). Common shipboard frequencies are 60 Hz (US) and 50 Hz. The time for on…

NEETS Mod. 2 §4-1

Reactance and impedance In AC circuits inductance and capacitance oppose current in a way that depends on frequency; this opposition is reactance, measured in ohms. Inductive reactance is XL = 2πfL — it increases with frequency and inductance, so an inductor passes low frequencies more easily than high. Capacitive reactance is XC = 1/(2πfC) — it decreases with frequency and capacitance, so a capacitor passes high frequencies more easily than low and blocks DC (infinite XC at zero frequency). Reactance and resistance combine into impedance (Z), the total opposition to AC, which cannot simply be added arithmetically because the reactive voltages are 90° out of phase with the resistive voltage. Instead they add vectorially: Z = √(R² + X²), where X is the net reactance (XL − XC). Ohm's law fo…

NEETS Mod. 2 §4-2

Phase angle, power factor, and true vs apparent power The net reactance in an AC circuit forces the current out of phase with the voltage by a phase angle (θ). In an inductive circuit current lags voltage; in a capacitive circuit current leads. Only the in-phase (resistive) component of current does useful work, so AC power has three measures. Apparent power, the simple product of RMS volts and amps, is expressed in volt-amperes (VA or kVA) and sizes generators and cables. True (real) power, the power actually converted to work or heat, is in watts (or kW) and equals E × I × cos θ. Reactive power, expressed in VARs, is the power that oscillates into and out of the magnetic and electric fields doing no net work. Power factor is the ratio of true to apparent power, equal to cos θ, ranging f…

NEETS Module 16 §2.2

POWER MEASUREMENTS You may be required to check the power consumption and the input-signal power levels of electronic equipment. The determination of dc power is fairly simple; recall that the unit of power, the watt, is the product of the potential in volts and the current in amperes (P = E × I). As discussed in NEETS, Module 2, Introduction to Alternating Current and Transformers, the phase angle of the voltage and current must be considered for accurate ac power measurements. The measurement of ac power is further complicated by the frequency limitations of various power meters. If there is no phase angle difference, you can compute ac power in the same manner as dc power; that is, by determining the effective value of the product of the voltage and current. For equipments that operate…