Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Commutator soldering repair
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Commutator soldering repair
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Subject: Auto Electrical Works
Class: Senior Secondary 3
Term: 3rd Term
Week: 2
Theme: Starting System
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This topic introduces Senior Secondary 3 Auto Electrical Works students to the essential skill of diagnosing and repairing damaged commutators, a critical component in many DC electrical machines found in vehicles and other equipment. Understanding commutator repair is vital for maintaining the efficiency and longevity of starter motors, alternators, and other rotary electrical components common in the Nigerian automotive landscape. It also equips students with practical skills that are directly applicable in the burgeoning informal and formal automotive repair sector across Nigeria, fostering entrepreneurship and self-reliance.
This section provides in-depth explanations of the commutator, its function, common faults, and the detailed process of its repair through soldering and surface conditioning. 2.
1. The Commutator: Function and Structure Definition: A commutator is a mechanical rotary switch in DC (Direct Current) electric motors and generators that periodically reverses the direction of current between the rotor and the external circuit. It consists of multiple segments (bars) of a conductive material (typically copper) insulated from each other by mica or other dielectric material, mounted on the armature shaft.
Function: In a DC Motor: It provides current to the armature windings in such a way that the magnetic field produced by the windings continuously interacts with the stationary magnetic field, creating continuous rotational motion.
In a DC Generator: It collects the alternating current (AC) generated in the armature windings and converts it into direct current (DC) for the external circuit.
Structure: Composed of copper segments, often dovetailed or wedge-shaped, separated by insulating mica. Each segment is connected to the end of an armature winding through a riser or lug. 2.
2. Common Causes and Types of Commutator Damage Commutators are subject to wear and damage due to their continuous contact with carbon brushes and the electrical currents they handle.
Causes of Damage:
1. Brush Friction: Continuous mechanical rubbing by carbon brushes causes wear, leading to grooves, uneven surfaces, and loss of original diameter.
2. Arcing/Sparking: Poor brush contact, incorrect brush spring tension, excessive current, or shorted armature windings can cause arcing between brushes and commutator segments, leading to pitting, burning, and carbon deposits.
3. Overheating: Prolonged heavy current draw or poor ventilation can cause the commutator to overheat, weakening insulation, expanding segments, and potentially melting solder joints.
4. Contamination: Oil, grease, dust, and carbon particles can accumulate on the commutator surface, forming a conductive film that leads to tracking, short circuits, and poor brush contact.
5. Loose Connections (Torn or Worn Risers): Vibrations, thermal cycling, and high current can cause the soldered connections between the armature winding wires and the commutator risers (lugs) to become loose, cracked, or completely detached. This is a primary focus for soldering repair.
6. Segment Lift/Runout: Extreme heat or mechanical stress can cause individual segments to lift or become uneven relative to the armature shaft, leading to excessive sparking and noise. Types of Damage to Detect (Performance Objective 1): Grooves and Uneven Wear: Visible depressions where brushes have worn the copper, leading to poor brush contact.
Pitting and Burning: Localised rough spots or blackened areas caused by arcing.
Loose or Torn Risers/Segments: A critical fault where the armature winding wire detaches from its commutator segment, causing an open circuit in that winding. This can be visually identified or confirmed with a continuity test.
High Mica: When the mica insulation between segments wears slower than the copper, it protrudes, causing brushes to bounce and spark.
Discoloration: Dark areas can indicate overheating or carbon tracking. 2.
3. Principles of Soldering (Performance Objective 2)
Definition: Soldering is a process in which two or more items are joined together by melting and flowing a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjacent metal.
Solder: Typically an alloy, traditionally tin-lead (e.g., 60/40 Sn/Pb), but lead-free solders (e.g., tin-copper, tin-silver) are also common. It must melt and flow well. For automotive applications, a solder with good strength and vibration resistance is preferred.
Flux: A chemical cleaning agent applied to the surfaces to be soldered. It cleans oxidation and impurities from the metal surfaces, allowing the molten solder to flow and adhere properly. Without flux, solder will not wet the surfaces effectively.
Heat Source: A soldering iron or gun, which provides controlled heat to melt the solder and raise the temperature of the components being joined. 2.
4. Tools and Materials for Commutator Soldering Repair Soldering Iron/Gun: A high-wattage iron (e.g., 60-100W) or a soldering gun is preferred for the thick copper and winding wires in an armature, to ensure sufficient heat transfer.
Solder Wire: Appropriate gauge and type (e.g., lead-free or metal surfaces, allowing the molten solder to flow and adhere properly. Without flux, solder will not wet the surfaces effectively.
Heat Source: A soldering iron or gun, which provides controlled heat to melt the solder and raise the temperature of the components being joined. 2.
4. Tools and Materials for Commutator Soldering Repair Soldering Iron/Gun: A high-wattage iron (e.g., 60-100W) or a soldering gun is preferred for the thick copper and winding wires in an armature, to ensure sufficient heat transfer.
Solder Wire: Appropriate gauge and type (e.g., lead-free or tin-lead automotive grade).
Soldering Flux: Rosin-based flux is generally suitable for electrical work, ensuring clean joints. Emery Cloth/Fine Sandpaper (Performance Objective 3): Various grits (e.g., 320-600 grit) for smoothing and polishing.
Multimeter: For continuity testing to identify open circuits in windings or loose connections.
Cleaning Agents: Contact cleaner, isopropyl alcohol (spirit), or a clean, dry cloth for surface preparation.
Small Screwdriver/Scraper: For gently scraping off old solder or carbon deposits.
Armature Puller (if necessary): To remove armature from housing.
Bench Vice: To secure the armature safely during repair.
Safety Glasses: Essential eye protection.
Ventilation: A well-ventilated work area or fume extractor. 2.
5. Step-by-Step Commutator Soldering Repair Process Step 1: Detection and Initial Inspection (Performance Objective 1)
1. Visual Inspection: Carefully examine the commutator surface for: Darkened or Burnt Segments: Indicates arcing or overheating.
Pitting or Grooves: Signs of wear or severe arcing.
Loose or Disconnected Wires: Look closely at the points where the armature winding wires connect to the commutator risers. Wires may be visibly detached, cracked, or show signs of arcing where they should be firmly soldered. Gently push on wires with a small non-conductive tool to check for looseness.
High Mica: Observe if the mica insulation is protruding above the copper segments.
2. Continuity Test with Multimeter: Set the multimeter to resistance (Ohms) or continuity mode. Place one probe on a commutator segment and the other on the corresponding armature winding wire. There should be a low resistance reading or a continuity beep.
Test adjacent segments: There should be no continuity between adjacent segments (they are insulated). Detecting an Open Circuit (Torn Commutator): If a winding wire is torn or a solder joint is loose, the multimeter will show an open circuit (OL or very high resistance) between the segment and its winding, or between successive segments in the winding path. This confirms the need for soldering repair.
Step 2: Preparation for Soldering (Pre-soldering cleaning)
1. Secure Armature: Place the armature securely in a bench vice, ensuring the commutator is accessible. Use protective jaws to avoid damaging the shaft.
2. Clean the Area: Use a small wire brush or a scraper to remove any carbon deposits, old solder residue, or dirt from the loose riser and the winding wire end. Apply a small amount of contact cleaner or isopropyl alcohol to further clean the joint area, then allow it to dry completely. A clean surface is crucial for good solder adhesion.
3. Apply Flux: Apply a small amount of soldering flux directly to the clean area where the wire connects to the commutator riser. Flux helps in removing any remaining oxides when heated.
Step 3: Soldering the Commutator (Performance Objective 2)
1. Heat the Joint: Turn on the soldering iron/gun and allow it to reach its operating temperature. Carefully touch the tip of the heated soldering iron to both the commutator riser and the end of the armature winding wire simultaneously. The goal is to heat both components sufficiently so they can melt the solder.
2. Apply Solder: Once the components are hot enough (solder should melt immediately when touched to the heated components, not the iron tip itself), feed the solder wire onto the joint. Allow the solder to flow evenly and completely around the connection, forming a shiny, smooth fillet that encapsulates the wire end and the riser. * Ensure the solder penetrates the joint and makes a strong mechanical and electrical connection.
3. Remove Heat and Hold: Remove the solder wire first, then the both components sufficiently so they can melt the solder.
2. Apply Solder: Once the components are hot enough (solder should melt immediately when touched to the heated components, not the iron tip itself), feed the solder wire onto the joint. Allow the solder to flow evenly and completely around the connection, forming a shiny, smooth fillet that encapsulates the wire end and the riser. Ensure the solder penetrates the joint and makes a strong mechanical and electrical connection.
3. Remove Heat and Hold: Remove the solder wire first, then the soldering iron. Hold the wire steady for a few seconds until the solder solidifies. Avoid moving the joint during cooling, as this can create a "cold solder joint" which is brittle and unreliable.
4. Inspect Joint: Visually inspect the newly soldered joint. It should be shiny, smooth, and concave (wetting out) rather than dull, lumpy, or convex (cold joint).
5. Repeat as Necessary: Solder all identified loose or faulty connections.
6. Post-Soldering Cleaning: After all soldering is complete and the armature has cooled, use a clean cloth and some contact cleaner or isopropyl alcohol to remove any residual flux from the commutator surface. Flux residue can become corrosive over time or attract dirt.
Step 4: Emery-Clothing and Finishing (Performance Objective 3) This step aims to restore a smooth, concentric surface to the commutator for optimal brush contact and minimal sparking.
1. Secure Armature: Re-secure the armature in the bench vice, ensuring it can rotate freely.
2. Select Emery Cloth: Use a fine-grade emery cloth or sandpaper (e.g., 320 to 600 grit). Start with a coarser grit (e.g., 320) if the surface is very rough, then finish with a finer grit (e.g., 400 or 600) for a smooth polish.
3. Emery-clothing Technique: Hold a strip of emery cloth firmly against the commutator surface. While applying light, even pressure, slowly rotate the armature by hand (or carefully use a low-speed lathe if available and trained). Move the emery cloth slightly from side to side across the commutator face to ensure even material removal. Continue until the entire commutator surface is smooth, concentric, and free of grooves, pits, and solder bumps. The copper should have a uniform, bright appearance. Crucial
Note: Never use sandpaper with metallic particles (e.g., silicon carbide paper) as these can embed in the copper and cause shorts. Always use non-conductive abrasive materials like aluminum oxide emery cloth.
4. Check for High Mica: After emery-clothing, ensure that the mica insulation is slightly undercut (below the copper surface) or at least flush. If high mica is present, special tools (mica undercutter) are used, but for basic repair, thorough emery-clothing often addresses minor high mica.
5. Final Cleaning: Thoroughly clean the commutator with compressed air or a clean, dry cloth to remove all emery dust and debris. Residual dust can cause premature brush wear and arcing.
Step 5: Final Testing Perform a final continuity test on all commutator segments and between segments to ensure no short circuits or open circuits remain. * Visually inspect the commutator for smoothness and cleanliness. This section outlines the practical, hands-on activities for both teachers and students to effectively deliver and grasp the concepts of commutator soldering repair. 3.
1. Teacher Activities Introduction & Motivation (10 mins): Display various automotive electrical components containing commutators (e.g., starter motor armature, alternator rotor, small DC motor from a fan). Ask students to identify them and their function. Pose questions about what might happen if these components fail, linking to common vehicle breakdowns in Nigeria. Introduce the commutator as a critical failure point.
Theoretical Explanation (20 mins): Explain the function of the commutator, types of damage (visual aids: pictures/diagrams of worn commutators), and the principles of soldering. Discuss the tools and materials required, emphasizing safety precautions using a poster or projected slides. Detection & Inspection Demonstration (25 mins): Using a damaged armature, demonstrate how to visually inspect for wear, pitting, loose risers, and discolouration. Demonstrate the use of a multimeter to check for continuity between segments and between segments and windings, specifically identifying an open circuit (torn riser). Explain what readings indicate a fault.
Soldering Demonstration (30 mins): Prepare a workbench with all necessary tools (soldering iron, solder, flux, cleaning agents, safety glasses, secure vice). Demonstrate step-by-step the soldering process on a practice piece or a non-critical damaged commutator: Cleaning the joint. Applying flux. Heating the joint correctly. Applying solder to achieve a good, shiny joint. Post-soldering cleaning. Emphasize correct posture, steady hand, and heat control.
Emery-Clothing Demonstration (15 mins): Demonstrate the proper technique for emery-clothing a commutator: securing the armature, selecting the correct grit, applying even pressure, and rotating the armature smoothly. Show how to achieve a uniform, smooth, and bright surface. Demonstrate final cleaning of dust. Guided Practice Facilitation (Remaining Time): Organize students into small groups (2-3 students) and provide each group with: An armature (some damaged, some for practice). Soldering equipment (under strict supervision). Emery cloth, cleaning materials, safety glasses. Circulate among groups, providing individual guidance, correcting techniques, and ensuring safety adherence. Ask probing questions to check understanding and practical application. 3.
2. Student Activities Active Listening and Note-taking (10 mins): Students listen to the introduction and theoretical explanation, taking notes on key definitions and concepts.
Observation & Participation (40 mins): Students observe the teacher's demonstrations of commutator inspection, soldering, and emery-clothing. They ask questions for clarification during demonstrations.
Hands-on Commutator Inspection (20 mins): In groups, students will visually inspect various armatures for signs of wear, pitting, and loose risers. They will practice using a multimeter to perform continuity tests on commutator segments and armature windings, identifying faulty connections.
Practical Soldering Practice (30 mins): Under teacher supervision, students practice cleaning, fluxing, and soldering on designated practice armatures or scrap components. They focus on achieving clean, shiny, and strong solder joints. Practical Emery-Clothing Practice (20 mins): Students practice emery-clothing commutators using various grits of emery cloth, aiming for a smooth, uniform surface. They will clean the commutator surface after polishing. Group Discussion and Problem Solving (10 mins): Students discuss challenges encountered during practical tasks and propose solutions, fostering peer learning. They will review safety procedures.
The knowledge and skills acquired in commutator soldering repair have direct and significant real-life applications in Nigeria: Automotive Repair Industry (Mechanic Workshops): The most direct application is in local mechanic workshops (e.g., in Ladipo market, Mushin, Lagos, or Aba, Abia State). Students can directly apply these skills to repair faulty starter motors and alternators in various vehicles – from commercial 'okada' motorcycles and 'keke napep' tricycles to private cars and heavy-duty trucks. This reduces the cost of vehicle maintenance for owners, as repairing a commutator is often significantly cheaper than replacing an entire armature or component. It also creates employment opportunities for skilled technicians.
Generator Maintenance and Repair: Due to prevalent power challenges in Nigeria, generators (ranging from small "I-better-pass-my-neighbour" to large industrial units) are indispensable. Many generators rely on DC excitation systems or incorporate DC components with commutators. The ability to repair these commutators ensures the longevity and reliability of generators, which are crucial for homes, small businesses, and institutions across the country, saving costs and ensuring continuous power supply.
Entrepreneurship and Self-Employment: Proficiency in commutator repair, along with other auto-electrical skills, provides a strong foundation for students to establish their own small-scale auto-electrical repair businesses. This is particularly relevant in Nigeria where entrepreneurship is a key driver of economic growth. A skilled technician can build a reputation for affordable and reliable repairs, attracting clients and contributing to local economic development.