Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Identify different types of faults in electrical machines.
• Recognize the symptoms for each fault.
• Explain diagnostic methods to detect faults.
• Apply knowledge to troubleshoot common issues.

Lesson summary

This lesson focuses on identifying and understanding common faults in electrical machines like motors, generators, and transformers. In Ghana, these machines are everywhere – from the corn mill in our local market and the water pump at home, to the large ECG transformers in our communities and the generators that power our businesses during power outages. When these machines fail, it can cause inconvenience, loss of income, and even danger. By learning to recognize the signs (symptoms) of a developing fault, you can help prevent major breakdowns, save money on costly repairs, and ensure safety.

Lesson notes

A. What is an Electrical Machine?

An electrical machine is a device that converts energy from one form to another, with one of the forms being electrical energy. The main types we will discuss are: Motors: Convert electrical energy into mechanical energy (e.g., fan motor, grinding mill motor). Generators: Convert mechanical energy into electrical energy (e.g., a diesel generator or 'gen set'). Transformers: Transfer electrical energy from one circuit to another, usually changing the voltage level (e.g., the ECG pole-mounted transformer, phone charger adapter). B. What is a Fault?

A fault in an electrical machine is any abnormal condition that interferes with its normal, safe, and efficient operation. It's a defect or a problem that can lead to a partial or complete failure of the machine. Early detection is key to preventing a small problem from becoming a big, expensive one. C. Types of Faults and Their Symptoms

We will focus on four major categories of faults as outlined in our curriculum. Bearing Faults What it is: Bearings are components (usually small metal balls or rollers in a cage) that allow the rotating part of a machine (the rotor or shaft) to spin smoothly with minimum friction. A bearing fault occurs when these components become worn out, damaged, or lose their lubrication. This is one of the most common mechanical faults in rotating machines like motors and generators. Causes: Lack of lubrication (grease or oil dries up). Contamination with dust, dirt, or moisture. Normal wear and tear over a long period. Incorrect installation or misalignment of the shaft. Symptoms: Abnormal Noise: A grinding, screeching, or rumbling sound. This is caused by the damaged metal parts rubbing against each other instead of rolling smoothly. Overheating: The machine's housing near the shaft becomes unusually hot to the touch. This is due to the increased friction from the faulty bearing. Vibration: The machine may shake or vibrate more than usual. Example: Your ceiling fan starts making a loud, continuous `grrrrring` sound and seems to rotate slower than before. The central part of the fan feels very hot after running for a while. This strongly suggests a bearing fault. Stator or Armature Fault What it is: The stator (in AC motors) and armature (in DC machines/generators) contain coils of insulated copper wire called windings. A fault here means there is a problem with these windings. The two main types are: Open Circuit: A wire in a coil has broken. No current can flow through that path. Short Circuit: The insulation on the wires has failed, causing two or more wires to touch, creating a 'shortcut' for the current. This can happen between turns in the same coil or between different coils. Causes: Overheating, which damages the wire's thin insulation. Ageing and degradation of the insulation material. Vibration causing wires to rub against each other. Moisture or chemical contamination. Symptoms: Motor will not start: If there is a major open or short circuit, the magnetic fields needed for rotation cannot be created correctly. The motor might just hum without turning. Generator will produce no output (or very low voltage): In a generator, the armature windings are where the electricity is induced. If they are faulty, no current can be generated and sent out. Overheating and burning smell: A short circuit causes a very large current to flow, generating intense heat and often burning the insulation, which produces a distinct electrical burning smell. Circuit breaker tripping: The high current from a short circuit will often trip the protective circuit breaker or blow a fuse. Broken Rotor Bar and End Ring Faults What it is: This fault is specific to AC induction motors, which are extremely common (e.g., in water pumps, industrial machines). The rotor has conductive bars (like a hamster cage) shorted at both ends by 'end rings'. A fault occurs when one or more of these bars crack or break, or the connection to the end ring is lost. Causes: Stress from frequent starting and stopping. Manufacturing defects. Thermal stress from overheating. Symptoms: Noise: A loud, rhythmic mechanical noise that changes with the load on the motor. Vibration or Wobbling: The broken bar creates an unbalanced magnetic pull on the rotor, causing it to vibrate or 'wobble' as it spins. This can eventually damage the bearings too. Reduced Power/Torque: The motor will not be able to deliver its full power. Overheating: The imbalance and inefficient operation cause the motor to run hotter than usual. Open or Shorted Transformer Windings What it is: Similar to stator/armature faults, this relates to the primary and secondary copper windings inside a transformer. Open Winding: A break in the primary or secondary coil wire. Shorted Winding: Insulation failure between turns of a coil. Causes: Lightning strikes or major power surges. Long-term overloading, causing overheating and insulation breakdown. Ageing and oil degradation (in large transformers). Symptoms: No Output Voltage (or very low): If the secondary winding is open, no power can be delivered. If the primary is open, no energy can enter the transformer in the first place. Overheating: A shorted winding will draw excessive current, causing the transformer to get very hot very quickly. Loud Humming or Buzzing: While transformers normally hum, a shorted winding will cause a much louder, angry buzzing sound due to intense magnetic forces. Bulging or Leaking Casing: In oil-filled transformers, the intense heat can cause the oil to expand and the casing to bulge or even leak. Tripping Breakers: Like any short circuit, it will draw high current and trip the main breaker.

Evaluation guide

• Focal Area : Fault In Electrical Machines
• Nature/ types of faults
• Symptons of each fault
• Faults