Pancyprian TS Elec. Eng. III ΘΚ flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a linear conductor at constant temperature, voltage equals current times resistance (V = I × R). Power dissipated is P = V × I = I²R = V²/R.

Kirchhoff's current law

At any node in an electrical circuit, the algebraic sum of currents entering the node equals zero (sum of currents in = sum of currents out). This expresses conservation of charge.

Thevenin's theorem

Any linear two-terminal network can be replaced by an equivalent voltage source V_th in series with resistance R_th. V_th is the open-circuit voltage; R_th is the resistance seen with all independent sources killed.

Resonance in series RLC

Series resonance occurs at f₀ = 1/(2π√(LC)), where inductive and capacitive reactances cancel. Impedance is purely resistive and current is maximum. Quality factor Q = (1/R)√(L/C).

Three-phase power

In a balanced three-phase system, total active power P = √3 × V_L × I_L × cos φ, where V_L is line voltage, I_L line current, and φ the phase angle between phase voltage and current.

DC machine EMF equation

Generated EMF in a DC machine is E = (p Φ N Z)/(60 A), where p is poles, Φ flux per pole, N speed in rpm, Z total armature conductors, and A number of parallel paths.

Transformer EMF equation

RMS induced EMF in a transformer winding is E = 4.44 f N Φ_max, where f is frequency, N turns, and Φ_max peak core flux. Voltage ratio equals turns ratio.

Induction motor slip

Slip s = (n_s − n_r)/n_s, where n_s is synchronous speed (120f/p) and n_r rotor speed. At standstill s = 1; at synchronous speed s = 0. Rotor frequency equals s × f.

Synchronous machine power-angle

For a cylindrical-rotor synchronous machine neglecting resistance, active power per phase P = (E_f V / X_s) sin δ, where E_f is excitation EMF, V terminal voltage, X_s synchronous reactance, δ load angle.

Cable current-carrying capacity rule

Protective device rated current I_n must satisfy I_b ≤ I_n ≤ I_z, where I_b is design current and I_z the cable capacity after applying correction factors for ambient temperature, grouping, and thermal insulation.

TN earthing system

A system where the installation's exposed conductive parts are connected to the earthed neutral of the supply. Subtypes: TN-S (separate PE and N), TN-C (combined PEN), TN-C-S (combined then separate).

RCD 30 mA protection

A residual current device with rated residual operating current 30 mA provides additional protection against electric shock for socket outlets up to 32 A and mobile equipment. It does not protect against overload.

Voltage drop limit

For low-voltage installations, voltage drop from the origin to the load should not exceed 3% for lighting and 5% for other uses, per IEC 60364. Higher drops may affect equipment performance.

Buck converter voltage ratio

A step-down DC-DC converter has output voltage V_out = D × V_in, where D is the duty cycle (0 ≤ D ≤ 1). It operates by switching and energy storage in an inductor.

Thyristor phase control

In a fully controlled full-wave rectifier with resistive load, average output voltage V_dc = (2V_m/π) cos α, where α is the firing angle. For α > 90°, output becomes negative (inverter mode).

PWM inverter fundamental voltage

With sinusoidal PWM, the fundamental phase voltage peak is V₁ = (V_dc/2) × m_a, where m_a is the modulation index (ratio of reference to carrier amplitude). Linear range m_a ≤ 1.

V/f control

Variable frequency drive control that keeps the ratio of stator voltage to frequency constant to maintain constant air-gap flux in an induction motor. At low speeds, voltage boost compensates stator resistance drop.

Moving-coil instrument

A permanent-magnet moving-coil (PMMC) meter deflects proportional to average current and measures DC only. The scale is linear. It is used in ammeters, voltmeters, and galvanometers.

Two-wattmeter method

For a balanced three-phase load, total power P = W₁ + W₂. Power factor can be derived from cos[tan⁻¹(√3 (W₁ − W₂)/(W₁ + W₂))]. Readings may be negative for low power factor.

Current transformer safety

A current transformer secondary must never be open-circuited while primary current flows, because dangerous high voltage would be induced. Short the secondary before removing the meter.

Insulation resistance test

Apply a DC test voltage (e.g., 500 V) for 1 minute and measure resistance. Minimum acceptable value for low-voltage installations is typically 1 MΩ. Polarization index (R10min/R1min) above 2 indicates good insulation.

Earth fault loop impedance

The impedance Z_s of the path from the point of fault through the protective conductor and earth back to the source. Disconnection is assured if Z_s ≤ U_0 / I_a, where I_a is the current causing disconnection in the required time.

Protective earthing conductor sizing

For copper, the protective conductor cross-section equals the phase conductor up to 16 mm²; 16 mm² for phase conductors 16–35 mm²; and half the phase conductor above 35 mm², per IEC 60364.

Fuse gG characteristic

A gG fuse provides general-purpose protection with inverse time-current characteristics: it carries overloads for a limited time and clears short circuits. It protects cables and lines against overload and short circuit.

SELV system

Safety extra-low voltage system with voltage not exceeding 50 V AC or 120 V ripple-free DC, electrically separated from earth and other circuits. It is used in wet locations and for portable tools.

Load factor

The ratio of average load to maximum demand over a period. A high load factor indicates efficient use of generation and distribution assets. It is used in power system planning and tariff design.

Per-unit system

A normalization method where quantities are expressed as fractions of base values. Z_base = V_base²/S_base. Per-unit transformer impedance = (Z% / 100) × (S_base / S_rated). It simplifies power system calculations.

Surge impedance

For a lossless transmission line, surge impedance Z_c = √(L/C), where L and C are inductance and capacitance per unit length. Overhead lines are about 400 Ω; cables about 50 Ω. Surge impedance loading SIL = V²/Z_c.

Betz limit

The maximum theoretical power coefficient C_p of a wind turbine is 16/27 ≈ 0.593. Power extracted is P = ½ ρ A v³ C_p, where ρ is air density, A rotor swept area, v wind speed.

Single-line diagram

A simplified drawing of a three-phase power system using one line to represent all phases. It shows generators, transformers, busbars, circuit breakers, and protection devices for analysis and documentation.

IEC 60617 symbols

International standard for graphical symbols used in electrotechnical diagrams. Examples: resistor as rectangle, capacitor as two parallel lines, diode as triangle with bar, earth as three horizontal lines.

Reference designation

Component identification per IEC 61346 using a letter code (e.g., Q for circuit breaker, F for fuse, K for relay, M for motor) followed by a number, used consistently in schematics and documentation.