Module 3: Electrical Fundamentals

Module 3: Electrical Fundamentals – EASA Part-66 B2

1. Module Overview

Module 3, "Electrical Fundamentals," forms the cornerstone of all aircraft electrical and electronic systems knowledge for the EASA Part-66 B2 licence. This module provides the essential theoretical foundation required to understand, troubleshoot, and maintain the complex electrical systems found on modern aircraft. The syllabus covers a broad spectrum of topics, ranging from basic atomic theory and DC circuit analysis to AC theory, semiconductor devices, and electrical protection systems.

The module is structured to build knowledge progressively, starting with fundamental concepts and advancing to more complex applications. For the B2 category, the knowledge levels required are predominantly Level 2 (general knowledge) and Level 3 (detailed theory), reflecting the depth of understanding needed for certifying staff working on avionic and electrical systems.

2. Key Concepts Explained in Detail

2.1 Electron Theory and Electrical Materials

Atomic Structure and Charge Carriers

All matter consists of atoms containing protons (positive charge), neutrons (neutral), and electrons (negative charge). In metallic conductors, the outermost electrons (valence electrons) are loosely bound and can move freely, forming a "sea of electrons." This free electron movement constitutes an electric current when a potential difference is applied.

Conductors, Insulators, and Semiconductors

Semiconductor Doping

Important: In P-type material, holes move in the same direction as conventional current. This is because conventional current is defined as the flow of positive charge, and holes are positive charge carriers.

2.2 DC Circuit Theory

Ohm's Law and Power

Ohm's law states that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R):

V = I × R

Where:

Electrical power (P) is measured in watts (W), defined as one joule per second:

P = V × I = I² × R = V² / R

Resistors and Colour Coding

Resistors are identified by a colour code system. The standard code uses four or five bands:

Colour values: Black = 0, Brown = 1, Red = 2, Orange = 3, Yellow = 4, Green = 5, Blue = 6, Violet = 7, Grey = 8, White = 9.

Examples:

Series and Parallel Resistances

For two resistors in parallel, the simplified formula is: R_total = (R₁ × R₂) / (R₁ + R₂)

Internal Resistance and Terminal Voltage

Real voltage sources (batteries, generators) have internal resistance (r). When current flows, there is a voltage drop across this internal resistance:

V_terminal = EMF - (I × r)

Where EMF is the electromotive force (open-circuit voltage).

Example: A 24 V battery with internal resistance 0.5 Ω connected to a 5.5 Ω load:

Higher internal resistance causes a greater voltage drop under load, reducing terminal voltage.

Ideal Voltage Source: An ideal voltage source maintains constant terminal voltage regardless of load current, which requires zero internal resistance.

2.3 Capacitance and Capacitors

Basic Principles

A capacitor stores electrical energy in an electrostatic field. Capacitance (C) is measured in farads (F), where 1 F = 1 coulomb per volt. Aircraft applications typically use microfarads (µF = 10⁻⁶ F) and picofarads (pF = 10⁻¹² F).

Charge Storage: Q = C × V (charge in coulombs)

Series and Parallel Capacitors

Charge Sharing (Conservation of Charge)

When a charged capacitor is connected to an uncharged capacitor, charge is conserved:

Q_initial = Q_final

Example: A 10 µF capacitor charged to 200 V is connected in parallel with an uncharged 30 µF capacitor:

Capacitive Reactance

In AC circuits, a capacitor offers opposition to current flow called capacitive reactance:

X_C = 1 / (2πfC)

Where:

Example: 20 µF capacitor on 100 V, 50 Hz supply:

X_C = 1 / (2π × 50 × 20 × 10⁻⁶) = 159.15 Ω

Phase Relationship: In a purely capacitive circuit, current leads voltage by 90°. This occurs because the capacitor charges and discharges, allowing current to flow before voltage builds up across the plates.

2.4 Inductance and Inductors

Basic Principles

An inductor stores energy in a magnetic field. Inductance (L) is measured in henries (H). When current through an inductor changes, a back EMF is induced opposing the change (Lenz's law).

Inductive Reactance

X_L = 2πfL

Inductive reactance is directly proportional to frequency. As frequency increases, inductive reactance increases.

Example: 50 mH inductor on 200 V, 400 Hz supply:

X_L = 2π × 400 × 0.05 = 125.66 Ω

Phase Relationship: In a purely inductive circuit, current lags voltage by 90°.

Time Constant (RL Circuit)

The time constant (τ) represents the time for current to reach approximately 63.2% of its final value:

τ = L / R

Where:

Example: R = 50 Ω, L = 0.5 H:

τ = 0.5 / 50 = 0.01 s (10 ms)

Power in Purely Inductive Circuits: The phase angle is 90°, so the power factor (cos φ) is zero. True power P = V × I × cos φ = 0. All energy is alternately stored in the magnetic field and returned to the source—no real power is dissipated.

2.5 AC Theory

Sinusoidal Waveforms

AC voltage and current vary sinusoidally with time. Key parameters:

RMS-Peak Relationships (for sinusoidal waveforms):

Example: RMS value of 115 V:

V_peak = 115 × 1.414 = 162.6 V

Average Value: The average of a symmetrical sine wave over one complete cycle is zero because the positive and negative half-cycles cancel. Over a half-cycle, the average is 0.637 × peak value.

Phase Relationships in AC Circuits

Impedance in Series RLC Circuits

Impedance (Z) is the total opposition to current in an AC circuit:

Z = √(R² + (X_L - X_C)²)

Example: R = 30 Ω, X_L = 50 Ω, X_C = 20 Ω:

Z = √(30² + (50 - 20)²) = √(900 + 900) = √1800 = 42.43 Ω

Resonance in RLC Circuits

Resonance occurs when X_L = X_C. The resonant frequency is:

f = 1 / (2π√(LC))

At resonance:

AC Power

Example: Apparent power = 5 kVA, PF = 0.8 lagging:

True power = 5 kVA × 0.8 = 4 kW

2.6 Transformers

Operating Principle

A transformer transfers electrical energy between circuits through electromagnetic induction. It consists of a primary winding and a secondary winding on a common magnetic core. Transformers operate only on AC because they rely on a changing magnetic flux to induce voltage. With DC, a steady magnetic field is produced, and no voltage is induced in the secondary (after the initial transient).

Turns Ratio

V_s / V_p = N_s / N_p

Where:

Examples:

Functions in Aircraft: Transformers step up or step down AC voltages for various aircraft systems, provide isolation, and match impedances.

2.7 Three-Phase Systems

Basic Configuration

Three-phase systems consist of three AC voltages separated by 120° electrical. This configuration provides more efficient power transmission and smoother operation of motors.

Star (Wye) Connection

Example: Line voltage = 200 V:

V_ph = 200 / √3 = 200 / 1.732 = 115.47 V

Delta Connection

Power Measurement (Two-Wattmeter Method)

For a balanced three-phase load:

Example: P₁ = 4000 W, P₂ = 1500 W:

P_total = 5500 W

tan φ = √3 × (4000 - 1500) / 5500 = 0.787

φ = 38.2°, cos φ = 0.786

AC Generator Frequency

The frequency of a generator output depends on the number of poles and rotational speed:

f = (P × N) / 120

Where:

Example: 4-pole generator, 400 Hz output:

N = (120 × 400) / 4 = 12,000 RPM

2.8 Semiconductor Devices

PN Junction Diode

A diode is a two-terminal device that conducts current in the forward bias direction and blocks it in the reverse bias direction. This unidirectional behaviour makes diodes essential for rectification.

Zener Diode

A Zener diode is specifically designed to operate in the reverse breakdown region. In this region, the voltage across the diode remains nearly constant over a range of currents. This property makes Zener diodes ideal for:

Bipolar Junction Transistor (BJT)

A BJT has three terminals: emitter, base, and collector. It comes in two types: NPN and PNP.

Operating Regions:

Varistor (Voltage-Dependent Resistor)

A varistor has resistance that decreases significantly when the voltage across it exceeds a threshold. It is used to clamp transient overvoltages and protect sensitive electronic equipment from voltage spikes.

2.9 DC Motors

Operating Principle

A DC motor converts electrical energy into mechanical energy. The interaction between the magnetic field and the current-carrying armature conductors produces torque.

Back EMF

When the armature rotates, it generates a back EMF (E_b) opposing the applied voltage:

V = E_b + (I × R_a)

Where:

Example: V = 28 V, I = 4 A, R_a = 2 Ω:

E_b = 28 - (4 × 2) = 20 V

Commutator Function

The commutator reverses the current direction in the armature windings as the rotor turns. This ensures that the torque remains in the same direction, allowing continuous rotation.

2.10 Electrical Protection and Safety

Circuit Breakers

A circuit breaker is a protective device designed to automatically interrupt the circuit when current exceeds a predetermined value. This prevents damage to wiring and components. Key principles:

Example: A 5 A breaker protecting a 10 A cable—if a fault causes 15 A to flow, the breaker trips, protecting the cable.

Bonding Straps

Bonding straps ensure electrical continuity between metallic parts, providing:

Voltage Regulators

A voltage regulator ensures that the output voltage of a generator or alternator remains stable under varying load and speed conditions. This is critical for protecting sensitive avionics equipment.

Rectification

3. Important Formulas and Relationships

4. Common Relationships Between Concepts

Resistance vs. Reactance vs. Impedance

Frequency Effects

Phase Relationships

Charge Conservation

Transformer Relationships

Three-Phase Relationships

5. Typical Exam Focus Points

Based on the EASA Part-66 Module 3 B2 syllabus and typical examination patterns, candidates should focus on:

Level 2 Topics (General Knowledge)

Level 3 Topics (Detailed Theory)

Common Exam Pitfalls

Practical Applications