Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Identify and describe common faults in electrical machines.
• Explain the basic principles of electrical systems design.
• Demonstrate how to detect faults in electrical machines.
• Apply problem-solving skills to troubleshoot electrical systems.
• Understand the importance of safety when working with electricity.

Lesson summary

Welcome, future engineers and technicians! Today, we are looking at a very practical and important skill: detecting faults in electrical machines. Think about the many machines we rely on in Ghana: the motor for the corn mill (Nikanika) in our community, the water pump that supplies our homes, the fan that cools us down, or even the alternator in a car. When these machines stop working, it causes a lot of problems. A good designer or technician doesn't just know how to build things; they understand how things can go wrong. By learning to identify common faults, you can design better, more reliable systems and quickly diagnose problems when they occur.

Lesson notes

2.1 What is an Electrical Machine? An electrical machine is a device that converts electrical energy into mechanical energy or vice-versa. The most common examples we will focus on are: Motors: Convert electrical energy to mechanical energy (e.g., fan motor, blender motor, water pump motor). Generators/Alternators: Convert mechanical energy to electrical energy (e.g., a "plant" or generator used during a power outage, car alternator). Transformers: Change AC voltage levels (e.g., the large cylindrical ones on ECG poles, the small block in your phone charger). 2.2 What is a Fault? A fault is any abnormal condition in an electrical machine that interferes with its normal operation. It can cause the machine to perform poorly, stop working completely, or even become dangerous. 2.3 Major Categories of Faults Faults in electrical machines can be broadly grouped into two main types: Electrical Faults: These are problems related to the flow of electricity within the machine's circuits, especially the windings (coils of wire). Mechanical Faults: These are problems related to the moving parts of the machine.

Let's explore the common faults in each category. A. Common Electrical Faults and Their Symptoms

| Fault Type | Explanation | Common Symptoms (How to Detect It) | | :--- | :--- | :--- | | Open Circuit Fault | A break in the path of the current. A wire inside the motor's coil (winding) has broken. | - The machine fails to start at all. - It may make a low humming sound but won't turn. - For a three-phase motor, it may run on two phases, causing excessive vibration and noise (a condition called 'single-phasing'). | | Short Circuit Fault | An unintended low-resistance path for the current. The insulation on two wires has worn out, and they are touching. | - Immediate tripping of the circuit breaker or blowing of the fuse. - Sparks or a loud bang from the machine. - A distinct smell of burning insulation (a sharp, acrid smell). - Smoke coming from the machine. | | Earth (Ground) Fault | A type of short circuit where a current-carrying wire touches the metal casing (frame) of the machine. This is very dangerous. | - Can cause a severe electric shock if a person touches the machine. - Will trip a residual current device (RCD) or earth leakage circuit breaker (ELCB) if one is installed. - May also trip the main circuit breaker. | | Overheating | The machine operates at a temperature higher than its design limit. This is a symptom but also a fault that damages insulation over time. | - The machine's casing is very hot to the touch. (Be careful when checking!). - Discolouration or charring of paint on the motor casing. - A continuous smell of burning varnish or insulation. - The machine may run for a while and then stop (due to a thermal overload protector tripping). |

Causes of Overheating: Overloading: Making the machine do more work than it was designed for (e.g., putting too much corn in the mill at once). Poor Ventilation: Blocked air vents, a broken cooling fan, or installing the machine in a tight, hot space. Low Voltage: When the voltage from ECG is low, the motor draws more current to do the same work, causing it to heat up. B. Common Mechanical Faults and Their Symptoms