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: 3
Grade code: 3.2.1.LI.3
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.1
Indicator code: 3.2.1.LI.3
Theme: ENE RGY SYSTEMS
Subtheme: CIRCUITS AND MACHINES
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This lesson focuses on the practical skills required to safely operate common electrical machines. In Ghana, electrical machines are the backbone of our daily lives and industries, from the corn mill (`nikanika`) in our local market and the water pump in our homes to the large motors used in factories. Understanding how to correctly connect, control, and disconnect these machines is not just an academic exercise; it is a critical skill for safety, efficiency, and future careers in engineering and technical fields. This lesson will provide a step-by-step, hands-on approach to mastering these operations, with a strong emphasis on safety protocols.
This section covers the essential knowledge needed to operate electrical machines safely and effectively. We will focus on the DC Shunt Motor as our primary example because its principles of operation and control are fundamental to understanding many other types of machines. 2.1 Safety First: The Golden Rules of Machine Operation
Operating electrical machines can be dangerous if proper procedures are not followed. High voltages can cause severe shock or death, and moving parts can cause injury. ALWAYS adhere to these rules: Personal Protective Equipment (PPE): Always wear appropriate PPE, including safety glasses, insulated gloves (if necessary), and closed-toe shoes. Avoid loose clothing, jewellery, or long, untied hair that could get caught in moving parts. De-energise Before Work: Ensure the main power switch is OFF and locked out before making any connections or adjustments. Never assume a circuit is dead; always test it. Inspect Equipment: Before use, check all cables for frays or damage. Ensure the machine is securely bolted down and that all safety guards are in place. Know Your Emergency Stop: Identify the location of the main circuit breaker or emergency stop button for the workspace. Work with a Partner: When possible, work with a partner who can assist and call for help in an emergency. Clean and Dry Area: Ensure the floor and your hands are dry to prevent electrical shock. 2.2 Components for Operating a DC Shunt Motor
To operate our DC motor, we need more than just the motor and a power source. Here are the key components:
| Component | Function / Explanation | Why It's Important | | :--- | :--- | :--- | | DC Shunt Motor | The machine itself. It has two main circuits: the Armature (rotating part) and the Field (stationary part with windings). Terminals are typically labelled A1, A2 (Armature) and F1, F2 (Field). | This is the load we want to operate. Understanding its parts is key to correct connection. | | DC Power Supply | Provides the Direct Current (DC) voltage needed to run the motor. In a lab, this is a variable DC source. In industry, it could be a rectifier or a generator. | Without the correct power, the machine won't run or could be damaged. | | Three-Point Starter | A special device used to start DC motors. It contains a variable resistor that is gradually removed from the armature circuit as the motor speeds up. | A DC motor draws a very high current at startup, which can damage its windings. The starter limits this "inrush current" to a safe level. It is NOT just an ON/OFF switch. | | Field Rheostat | A variable resistor connected in series with the field winding. | This is our primary tool for speed control. By changing the resistance, we change the field current, which in turn changes the motor's speed. | | Measuring Instruments | - Voltmeter: Measures voltage. - Ammeter: Measures current. - Tachometer: Measures rotational speed (in RPM). | These tools allow us to monitor the machine's performance and understand the effect of our control actions. | 2.3 Standard Operating Procedure (SOP) for a DC Shunt Motor