PEC EPE Electrical flashcards

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

Ohm's Law

Current through a resistor equals voltage across it divided by resistance: I = V/R. Applies to linear resistive elements and is the basis for series and parallel circuit analysis.

Kirchhoff's Voltage Law

The algebraic sum of voltages around any closed loop in a circuit equals zero. It follows from conservation of energy and is used to write loop equations in mesh analysis.

Kirchhoff's Current Law

The algebraic sum of currents entering a node equals zero. It follows from conservation of charge and underlies nodal analysis of electrical networks.

Thevenin's Theorem

Any linear two-terminal network can be replaced by a single voltage source V_th in series with a resistance R_th. V_th is the open-circuit voltage; R_th is the resistance seen with sources deactivated.

Maximum Power Transfer

A source delivers maximum power to a load when the load resistance equals the source's internal (Thevenin) resistance. Efficiency at this point is only 50 percent for a resistive source.

Series Resonance

In an RLC series circuit, resonance occurs when inductive and capacitive reactances are equal, giving minimum impedance and maximum current at f = 1/(2π√(LC)).

Power Factor

The ratio of real power (watts) to apparent power (volt-amperes) in an AC circuit. It equals the cosine of the phase angle between voltage and current.

Three-Phase Power Formula

For a balanced three-phase load, total real power is P = √3·V_L·I_L·cosφ, where V_L and I_L are line voltage and line current and cosφ is the power factor.

Transformer EMF Equation

The RMS induced EMF in a transformer winding is E = 4.44·f·Φ_m·T, where f is frequency, Φ_m is peak flux, and T is turns. It links voltage, frequency, and core flux.

Transformer Maximum Efficiency

A transformer reaches maximum efficiency when its variable copper loss equals its constant core loss. Efficiency is high over a range of loads near this point.

Synchronous Speed

The speed of the rotating magnetic field in an AC machine is n_s = 120f/P rpm, where f is supply frequency in hertz and P is the number of poles.

Induction Motor Slip

Slip is s = (n_s − n_r)/n_s, the fractional difference between synchronous speed and rotor speed. Rotor frequency equals slip times supply frequency.

DC Machine EMF

Generated EMF in a DC machine is E_g = (P·Φ·N·Z)/(60·A), where P is poles, Φ flux per pole, N speed in rpm, Z conductors, and A parallel paths.

Transmission Line Regulation

Voltage regulation is the rise in receiving-end voltage from full load to no load, expressed as a percentage of full-load voltage. It indicates line voltage-drop severity.

Power Factor Correction

Capacitor kVAR needed to improve power factor from cosφ₁ to cosφ₂ equals P·(tanφ₁ − tanφ₂). Capacitors supply reactive power locally and reduce line current.

Per-Unit System

A normalization method where quantities are divided by chosen base values. Impedance base is V_base²/S_base; per-unit values simplify transformer and machine calculations across voltage levels.

Overcurrent Relay Coordination

Inverse-time overcurrent relays are coordinated by pickup current and time-multiplier settings so the device nearest a fault trips first, providing selective protection.

Cable Ampacity Derating

Cable current-carrying capacity is reduced by correction factors for ambient temperature, grouping, and installation method. Design current must not exceed the derated ampacity.

Earth Fault Loop Impedance

The total impedance of the fault path from the source through the installation and back. It must be low enough for protective devices to disconnect within the required time.

Star-Delta Starting

A motor starting method where the stator is connected in star during run-up and delta for normal operation. Starting current and torque are reduced to one-third of direct-on-line values.

TN-S Earthing System

An earthing arrangement where neutral and protective earth conductors are separate throughout the installation. The protective conductor is connected to earth at the source.

Insulation Resistance Test

A DC test, typically at 500 V or 1000 V, measuring resistance in megohms between live conductors and earth to verify insulation integrity before energizing an installation.

Lumen Method

A lighting design formula: N = (E·A)/(Φ·UF·MF), giving the number of luminaires needed for a target illuminance E over area A.

De Morgan's Laws

Boolean identities stating (A·B)' = A' + B' and (A + B)' = A'·B'. They allow conversion between AND and OR forms and simplify logic circuits.

JK Flip-Flop

A clocked bistable with J and K inputs. When both are high, the output toggles on each clock edge; with unequal inputs it sets or resets.

ADC Resolution

The smallest voltage step an analog-to-digital converter can resolve, equal to V_ref/2^n for an n-bit converter. Quantization error is at most half a step.

Nyquist Sampling Theorem

A signal must be sampled at more than twice its highest frequency component to be reconstructed without aliasing. The minimum rate is called the Nyquist rate.

PID Controller

A control law combining proportional, integral, and derivative action on the error signal. Integral action eliminates steady-state error; derivative action improves damping.

RC Low-Pass Cutoff

For a first-order RC low-pass filter, the cutoff frequency is f_c = 1/(2πRC), where output power falls to half and gain drops by 3 dB.

Op-Amp Virtual Short

With negative feedback and ideal infinite gain, the inverting and non-inverting input voltages are nearly equal, while input currents are approximately zero.

PLC Scan Cycle

A programmable logic controller repeatedly reads inputs, executes the user program, and updates outputs. Total scan time limits how fast the controller reacts to input changes.

Motor Full-Load Current

The current a motor draws at rated load, voltage, and frequency. It is used for overload protection sizing, cable selection, and starter contactor rating.