Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Accurately apply circuit analysis techniques to solve complex problems.
• Analyze and design basic single-phase transformer circuits.
• Explain the operating principles and characteristics of various electrical machines.
• Design and troubleshoot basic electronic circuits.
• Confidently answer exam-style questions.

Lesson summary

This week is dedicated to intensive revision and exam preparation. Electrical Technology is a crucial subject, providing the foundation for careers in power generation, distribution, maintenance, and the burgeoning renewable energy sector – all vital for South Africa's economic development and addressing its energy challenges. A solid understanding of the concepts covered in this subject is not only essential for passing your final exams but also for your future success in tertiary education or vocational training. We will focus on reviewing core concepts and practicing exam-style questions to improve your problem-solving skills and confidence.

Lesson notes

This week's revision will focus on the following key areas: 2.1 Circuit Analysis (DC and AC)

Ohm's Law: V = IR (Voltage = Current x Resistance). This fundamental law underpins all circuit calculations. Remember to use appropriate units (Volts, Amperes, Ohms).

Kirchhoff's Current Law (KCL): The algebraic sum of currents entering a node (junction) is equal to zero. I.e., what goes in must come out. This is based on the conservation of charge.

Kirchhoff's Voltage Law (KVL): The algebraic sum of voltages around any closed loop in a circuit is equal to zero. This is based on the conservation of energy.

Series and Parallel Resistors: Resistors in series add directly: R total = R 1 + R 2 + ... + R n .

Resistors in parallel: 1/R total = 1/R 1 + 1/R 2 + ... + 1/R n . Remember to invert the final result to get R total .

Thevenin's Theorem: Any linear circuit can be replaced by an equivalent circuit consisting of a voltage source (V TH ) in series with a resistor (R TH ). V TH is the open-circuit voltage at the terminals of interest. R TH is the resistance seen looking back into the circuit from the terminals of interest, with all voltage sources short-circuited and current sources open-circuited.

Norton's Theorem: Any linear circuit can be replaced by an equivalent circuit consisting of a current source (I N ) in parallel with a resistor (R N ). I N is the short-circuit current at the terminals of interest. R N is the same as R TH in Thevenin's Theorem.

AC Circuit Analysis: Includes concepts of impedance (Z), reactance (X L and X C ), phase angles, RMS values, and power factor (PF). Z = √(R 2 + (X L - X C ) 2 ). X L = 2πfL (Inductive Reactance). X C = 1/(2πfC) (Capacitive Reactance). Power Factor = cos(φ), where φ is the phase angle between voltage and current.

Example 1: Applying Thevenin's Theorem Consider a circuit with a 12V source, a 10Ω resistor (R1) in series with the source, and a parallel combination of a 20Ω resistor (R2) and a load resistor (R L ). We want to find the Thevenin equivalent circuit with respect to the terminals of R L .

Find V TH (Open-circuit voltage): Remove R L . The voltage across R2 becomes the Thevenin voltage.

Using the voltage divider rule: V TH = V R2 = 12V (20Ω / (10Ω + 20Ω)) = 12V (20/30) = 8

V. Find R TH (Thevenin resistance): Short-circuit the voltage source (12V). Looking back from the terminals of R L , we see R1 and R2 in parallel. R TH = (10Ω * 20Ω) / (10Ω + 20Ω) = 200/30 = 6.67Ω.

The Thevenin equivalent circuit: consists of an 8V source in series with a 6.67Ω resistor. You can now easily analyze the circuit for different values of R L . 2.2 Transformers Basic Operation: Transformers work on the principle of electromagnetic induction. A changing magnetic field created by the primary winding induces a voltage in the secondary winding.

Turns Ratio: N p /N s = V p /V s = I s /I p , where N is the number of turns, V is the voltage, and I is the current, with subscripts 'p' for primary and 's' for secondary.

Transformer Efficiency: Efficiency (η) = (Output Power / Input Power) 100%. Output Power = V s I s cos(φ). Input Power = V p I p cos(φ).

Core Losses: Hysteresis losses and Eddy current losses. Hysteresis losses are due to the energy required to repeatedly magnetize and demagnetize the core. Eddy current losses are due to circulating currents induced in the core by the changing magnetic field. Laminating the core helps to reduce eddy current losses.

Example 2: Transformer Calculations A transformer has a primary voltage of 220V and a secondary voltage of 12V. The primary winding has 500 turns. Calculate the number of turns in the secondary winding.

Using the turns ratio formula: N p /N s = V p /V s => 500/N s = 220/12 => N s = (500 * 12) / 220 = 27.27 turns. Since you can't have a fraction of a turn, round to the nearest whole number: N s ≈ 27 turns. 2.3 Electrical Machines DC Motors: Understand the principle of operation (force on a current-carrying conductor in a magnetic field), torque equation, speed control methods (armature voltage control, field flux control), and applications.

AC Motors (Induction Motors): Know the concepts of synchronous speed, slip, torque-slip characteristics, starting methods (Direct-on-Line, Star-Delta), and applications.

Generators (DC and AC): Understand the principle of operation (electromagnetic induction), factors affecting generated EMF, and types of generators. 2.4 Electronic Circuits Diodes and Rectifiers: Understand the behavior of a diode, half-wave rectifier, full-wave rectifier (center-tapped and bridge), and filter circuits (capacitor filter). Calculate ripple factor and efficiency.

Transistors (BJT): Understand the transistor as a switch and as an amplifier, biasing techniques, and basic amplifier configurations (Common Emitter).

Logic Gates: Understand the operation of AND, OR, NOT, NAND, NOR, XOR gates. Be able to create truth tables and simplify Boolean expressions using Karnaugh maps.