Lesson Notes By Weeks and Term v4 - SHS 3
CIRCUITS AND MACHINES
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CIRCUITS AND MACHINES
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Subject: Engineering
Class: SHS 3
Term: 2nd Term
Week: 1
Grade code: 3.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.1
Indicator code: 3.2.1.LI.2
Theme: ENE RGY SYSTEMS
Subtheme: CIRCUITS AND MACHINES
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This lesson explores the practical applications of the fundamental electrical machines that power our modern world. From the moment we wake up to the sound of an alarm, use a blender in the kitchen, charge our phones, or travel in a car, we are interacting with transformers, motors, and generators. Understanding their specific uses is crucial for any aspiring engineer. In Ghana, these machines form the backbone of our national power grid managed by VRA and ECG, drive the small-scale industries in our communities (like corn mills and fufu machines), and are essential components in the everyday gadgets we rely on.
This section breaks down each type of electrical machine, explaining its core purpose and common applications with a focus on our local context. A. Transformers What is it? A transformer is a static (it has no moving parts) electrical device that transfers electrical energy from one circuit to another through electromagnetic induction. Its primary purpose is to change the voltage level (either increase it or decrease it) while keeping the frequency the same. Key Principle: Mutual Induction. A changing magnetic field in one coil (the primary coil) induces a voltage in a second coil (the secondary coil). The ratio of turns between the two coils determines if the voltage is stepped up or stepped down. Types and Uses: Step-Up Transformer: Function: Increases voltage (and decreases current). Why? To transmit electrical power over long distances with minimal power loss. Power loss is proportional to the square of the current (P = I²R). By increasing the voltage, the current is reduced, thus drastically cutting down energy wasted as heat in the transmission lines. Ghanaian Example: At the Akosombo Dam (a VRA generating station), huge step-up transformers take the voltage generated (e.g., 16,000 Volts) and increase it to very high levels (e.g., 161,000 Volts) before sending it through the GRIDCo transmission lines across the country to cities like Kumasi and Accra. Step-Down Transformer: Function: Decreases voltage (and increases current). Why? To reduce the high transmission voltage to a safer, usable level for homes and industries. Ghanaian Examples: ECG Poles: The large, cylindrical or box-shaped devices you see on ECG poles in your neighbourhood are step-down transformers. They take the high voltage from the distribution lines (e.g., 11,000 Volts) and step it down to the 230/240 Volts that your home sockets provide. Device Chargers: Your mobile phone charger, laptop adapter, and even your DSTV decoder box contain a small step-down transformer. It takes the 240V from the wall socket and reduces it to a much lower voltage (e.g., 5V or 19V) that the electronic device can safely use. B. Electric Motors What is it? An electric motor is a machine that converts electrical energy into mechanical energy (motion or rotation). This is the principle behind almost anything that spins or moves when you power it on. Key Principle: The Motor Effect. When a current-carrying conductor is placed in a magnetic field, it experiences a force, causing it to move. Types and Uses: DC (Direct Current) Motors: Power Source: Runs on DC power, like batteries or a DC power supply. Characteristics: Good starting torque, easy to control speed. Ghanaian Examples: Automobiles: The starter motor that cranks the engine, the motors for windscreen wipers, power windows, and the fan for the radiator. Portable Devices: Battery-powered toys, cordless drills, portable fans people carry, and the vibration motor in your phone. Small-scale projects: Often used by students and hobbyists for simple robotic projects powered by batteries. AC (Alternating Current) Motors: Power Source: Runs on AC power, the type supplied by ECG to our homes. Characteristics: Generally more robust, require less maintenance, and are very common for fixed applications. Ghanaian Examples: Household Appliances: Standing fans, ceiling fans, blenders, refrigerators (compressor motor), air conditioners, and water pumps used to fill overhead "polytanks". Local Industry: The motor for a corn mill or a fufu pounding machine is a powerful AC motor. Industrial machines in factories like those in the Tema Free Zones. C. Electric Generators What is it? A generator is the opposite of a motor. It converts mechanical energy (rotation) into electrical energy. Key Principle: Electromagnetic Induction. When a conductor moves through a magnetic field (or a magnetic field moves past a conductor), a voltage is induced across the conductor, causing current to flow. Types and Uses: DC (Direct Current) Generators (also called Dynamos): Function: Produces a direct current output. Uses: Less common today for large-scale power generation, but have specific uses. Ghanaian Examples: Car Alternators (with a rectifier): A car's alternator technically produces AC, but it has a built-in rectifier (diodes) to convert it to DC to charge the 12V battery and power the car's electrical system. So, its final output is DC. Bicycle Dynamos: Older bicycles had a small generator that rubbed against the tyre to power a headlight. This was a simple DC generator. AC (Alternating Current) Generators (also called Alternators): Function: Produces an alternating current output. This is the source of almost all electricity on the grid. Uses: The foundation of modern power generation. Ghanaian Examples: VRA Power Plants: The massive generators at the Akosombo and Kpong hydroelectric dams are AC generators. The turbines, spun by the force of the water, rotate the generators to produce electricity for the entire country. The same applies to thermal plants like the one at Kpone. Portable Generators: The "gen sets" that many businesses and homes use during power outages ("dumsor") are powered by a petrol or diesel engine that spins an AC generator to produce electricity for the building. D. Special Electrical Machines
These are machines designed for very precise control of movement and position. Stepper Motors: Function: Rotates in discrete, fixed steps. It doesn't spin freely but moves from one position to the next with high precision. Why? Allows for very accurate positioning without needing a feedback sensor. The computer knows exactly how many steps it has told the motor to turn. Ghanaian Examples: Printers & Scanners: The print head in an inkjet printer or the scanner bar moves across the page using a stepper motor to ensure each line is printed or scanned in the exact right place. Photocopy Machines: The internal mechanics rely on stepper motors for precise paper feeding and toner application. Medical Equipment: Found in laboratory analysis machines where precise sample positioning is critical. Servo Motors: Function: A motor combined with a position sensor (feedback device). It can be commanded to rotate to and hold a specific angle with great precision and force. Why? Used in applications where you need to control the exact position of an object and resist external forces trying to move it. Ghanaian Examples: Robotics: In university engineering departments or SHS robotics clubs (like ASRC), servo motors are used to control the joints of robot arms. Remote-Controlled (RC) Devices: RC cars, airplanes, and drones use servo motors to control steering and flight surfaces (like ailerons and rudders). Industrial Automation: In a factory (e.g., a bottling plant), a servo motor might be used to precisely position a bottle under a filling nozzle.
Guided Practice (With Solutions)
Question 1: You plug your new smartphone into the wall socket to charge it. The charger's label says "Input: 240V AC, Output: 5V DC". Identify at least one key electrical machine inside the charger responsible for the voltage change and explain its specific job. Solution: Machine: A step-down transformer. Explanation: The primary job of the step-down transformer is to take the high 240V AC from the ECG wall socket and reduce it to a much lower AC voltage. This is the first critical step to get the power to a level that is safe for the phone's electronics. (Note: The charger also contains a rectifier to change the low-voltage AC to 5V DC, but the transformer does the voltage reduction).