Electrical workshop supplies
An electrical machine does not fail only for electrical reasons: it fails because of a badly greased bearing, an oil with water in it, a joint that runs hot or a varnish that could not take the temperature. Supplies are the greases, oils, alloys, insulation and adhesives used to maintain and rewind machines; choosing them well, using the right amount and storing them safely is part of the trade.
01Oils and greases for machines
A lubricant keeps two moving surfaces apart with a film that shears easily, so that friction and wear are minimal. It also cools, protects against corrosion and, in greases, seals out dust. An electrical workshop deals with two families: oil (a liquid, for gearboxes, sleeve bearings and hydraulic circuits) and grease (an oil held in a thickener, for rolling bearings, that does not run off).
Oils: ISO VG viscosity
The property that decides the oil is viscosity, the resistance to flow. It is measured as kinematic viscosity, in mm²/s (equal to cSt), and it drops when the oil heats up. The ISO VG grade (ISO 3448) is the midpoint of the viscosity at 40 °C, with a ±10 % tolerance: an ISO VG 68 has between 61.2 and 74.8 mm²/s at 40 °C. The higher the number, the thicker the oil. An oil that is too thin cannot hold the film under load; one that is too thick wastes power, heats up and takes too long to reach the surfaces on a cold start. The viscosity index tells how much the viscosity changes with temperature: the higher, the more stable.
| ISO VG grade | mm²/s at 40 °C | Typical uses |
|---|---|---|
| 32 – 46 | 28.8 – 50.6 | Hydraulic circuits, compressors, sleeve bearings of fast motors and generators. |
| 68 | 61.2 – 74.8 | Sleeve bearings of medium machines, pumps, light gearboxes. |
| 100 – 150 | 90 – 165 | Gearboxes under moderate load, slow bearings. |
| 220 – 320 | 198 – 352 | Industrial gearboxes under heavy load. |
The table gives an order of magnitude: the equipment manual always rules, giving the grade, the type (mineral or synthetic) and the additive level. The oil of the internal combustion engine of a generating set uses another classification (SAE and API) and is covered in Power generation and generating sets.
Greases: thickener, base oil and NLGI grade
A grease is a base oil (the part that actually lubricates) held in a thickener that releases it slowly. It is described by three data:
- Thickener: lithium or lithium-complex soap (general purpose), polyurea (common in electric motor bearings, long life and good stability), calcium sulfonate (very good protection against water and corrosion).
- Base oil viscosity: for electric motor bearings it is usually around ISO VG 100; thinner for high speeds, thicker for heavy loads and low speeds.
- Consistency, NLGI grade: from 000 (almost liquid) to 6 (very hard). Most motor bearings use NLGI 2 (worked penetration of 265 to 295 tenths of a millimetre), with the consistency of soft butter.
Two greases with different thickeners may be incompatible: when mixed, the structure breaks down, the grease softens or hardens and the bearing fails. The best-known case is mixing polyurea with lithium soap. Always grease with the grease named on the plate or in the manual; if the type has to change, the bearing and housing are cleaned. And the grease gun is labelled with the grease it holds.
Lubricating a motor bearing: how much and how often
Only a little grease sits in contact with the balls, and much more is stored beside them, in the housing, renewing it slowly. Both too little and too much do harm. With too much, the grease is churned with nowhere to go and heats up (sometimes a motor runs hot just because too much was put in). So the rule is a little grease, at regular intervals.
The first source is the motor's lubrication plate: many large motors carry a small plate with the grams and the interval in hours. If there is none, the manufacturer's manual rules. And if there is nothing, there is a practical rule to estimate the relubrication quantity, which gives a good approximation for deep-groove ball bearings:
| Bearing | d × D × B (mm) | Relubrication G |
|---|---|---|
| 6205 | 25 × 52 × 15 | 3.9 g |
| 6206 | 30 × 62 × 16 | 5.0 g |
| 6208 | 40 × 80 × 18 | 7.2 g |
| 6309 | 45 × 100 × 25 | 12.5 g |
| 6312 | 60 × 130 × 31 | 20.2 g |
| 6314 | 70 × 150 × 35 | 26.3 g |
The 6309 bearing measures 45 × 100 × 25 mm. The relubrication is G = 0.005 · 100 · 25 = 12.5 g. The grease gun is not graduated, so it is calibrated: ten strokes are pumped onto a scale and give 12 g, that is 1.2 g per stroke. It takes 12.5 / 1.2 ≈ 10 strokes. With a grease density close to 0.9 g/cm³, those 12.5 g occupy about 14 cm³.
The interval depends on speed, bearing size, temperature and environment: it goes from a few hundred hours in harsh conditions to several thousand in medium motors turning at 1,500 rpm, and it is shorter the faster, larger and hotter the bearing. A widely used rule of thumb says the interval halves for every 15 °C of operating temperature above about 70 °C. Sealed bearings (suffixes 2RS or ZZ) come greased for life: they are not greased, they are replaced.
The safest way is to stop, disconnect and lock out the motor before greasing it. If the manufacturer says to grease while running (so the old grease is pushed out better), it is done only with guards in place, with the grease nipple reachable without bringing hands near the fan or the coupling, and with no loose clothing. The nipple is wiped clean before attaching the gun: a grain of sand in the grease is sandpaper inside the bearing.
02Dielectric oil and fuels in the workshop
Transformer oil
An oil-immersed transformer (Transformers) uses the oil for two things: to insulate (it fills the gaps between coils and core, where air would break down at a much lower voltage) and to cool (hot oil rises, gives up heat to the radiator and sinks again). The usual oil is mineral, refined from petroleum and specified by IEC 60296. Its properties are monitored because oil degrades: it oxidizes, takes up moisture and particles, and all of that lowers its insulation.
The most important property is dielectric strength: the voltage at which the oil breaks down, measured between two electrodes a fixed distance apart. And the main enemy is water: a few tens of parts per million (ppm) dissolved already reduce it a lot, because droplets and particles form conducting bridges between the electrodes. That is why transformers have a silica gel breather, which dries the air drawn in when the oil cools and changes colour when saturated: that is the signal to replace it.
What is tested and how the sample is taken
| Test | What it says about the oil |
|---|---|
| Breakdown voltage (IEC 60156) | Dielectric strength: how clean and dry the oil is. Done in the workshop with a portable tester. |
| Water content (Karl Fischer) | Moisture in ppm. The laboratory test that explains a low strength. |
| Acidity number | Degree of oxidation: acids attack the paper insulation and metal parts. |
| Dielectric dissipation factor (tan δ) | Contamination and ageing: how much is dissipated in the oil as heat. |
| Dissolved gases (DGA) | Internal faults developing: overheating and discharges that decompose the oil and release gases. |
The breakdown voltage is determined with an oil tester: a cup with two electrodes 2.5 mm apart, a 50 Hz high-voltage source that raises the voltage steadily (about 2 kV/s) and an instrument that records the value when the arc jumps. The standard test makes six breakdowns on the same sample, with a rest between them, and the average is reported:
As a practical workshop criterion (indicative; the binding value is set by the equipment specification or the utility): 40 kV or more, oil in good condition; between 30 and 40 kV, monitor and repeat the test more often; below 30 kV, the oil is dried and filtered (or replaced).
The six breakdowns give 24, 27, 22, 26, 25 and 24 kV. The sum is 148 kV and the average is 148 / 6 = 24.7 kV. It is below 30 kV: the oil is wet or contaminated. It should go through a vacuum filtering and drying machine and be tested again; if it is still low after treatment, it is replaced. Meanwhile, the transformer should not stay in service with that oil, because an internal breakdown destroys it.
For the sample to be representative it is taken from the bottom valve, after purging a little oil, into a clean, dry glass or metal bottle (not plastic), preferably on a dry day and with the oil warmer than the surroundings so that no moisture condenses. The bottle is labelled with date, equipment and temperature. All of this is done with the equipment out of service and locked out, unless the utility's procedure for that transformer says otherwise.
Natural and synthetic esters and silicones are used where fire risk weighs more (transformers inside buildings): their flash point is above 300 °C, against about 140 °C for mineral oil. On the other hand, very old transformers may have been filled with oils containing PCBs (polychlorinated biphenyls), toxic and persistent substances whose management is regulated by Law 25.670. An oil of unknown origin is treated as suspect until an analysis proves otherwise; it is not decanted, burned or poured out.
Fuels and solvents: only what concerns the workshop
Combustion engines and their fuels are covered in Power generation and generating sets. What matters here is what is stored and used in the workshop: small quantities of petrol and diesel for testing portable sets, and above all cleaning solvents (isopropyl alcohol, thinner, degreasers). What defines the risk is the flash point, the minimum temperature at which the liquid gives off enough vapour to be ignited by a spark: petrol is far below 0 °C (it always ignites, even in winter), diesel above about 55 °C.
- Minimum quantity in the workshop, in closed, metal, labelled containers, never in drink bottles. The rest goes in the outdoor store.
- Away from sparks: contactors, grinders, soldering irons and switchboards produce arcs. Vapours are heavier than air and creep along the floor.
- Transfer with a funnel and, in large quantities, with the containers bonded to earth so static electricity does not produce a spark.
- To clean contacts and windings, use a dielectric contact cleaner or isopropyl alcohol, not petrol: petrol is far more dangerous and can damage varnishes.
- A fire extinguisher within reach and signposted. Flammable liquids are class B fires; in an electrical workshop an ABC extinguisher is advisable. Fire protection is governed by Law 19.587 and Decree 351/79 (Occupational health and safety).
03Solders and joints
An electrical joint must conduct like the cable, withstand the mechanical load and not work loose with heating cycles. Depending on how it is made, the joint can be soldered (a filler alloy flows between the pieces), exothermic (copper is melted in place) or mechanical (a terminal is deformed onto the conductor). The boundary between soft and hard soldering is the melting temperature of the filler alloy: 450 °C.
Soft solder: tin
| Alloy | Melting approx. | Where it is used |
|---|---|---|
| Tin-lead (Sn63Pb37, Sn60Pb40) | 183 – 190 °C | Electronics and low-power splices. Easy to work; lead is toxic and restricted in many products. |
| Tin-silver-copper (SAC305, 96.5 % Sn) | 217 – 220 °C | Lead-free electronics (RoHS). Requires a hotter iron. |
| Tin-silver (Sn96.5Ag3.5) | 221 °C | Power joints that run warm and silver-plated parts: resists thermal fatigue better. With 2 % silver and lead (Sn62Pb36Ag2, 179 °C), silver-plated contacts are soldered without the silver dissolving. |
| Tin-antimony (Sn95Sb5) | 235 – 240 °C | Joints that run hotter: power copper conductors, windings, refrigeration installations. |
Tin only wets clean metal: the flux (the rosin in the core of the solder, or a paste) dissolves the oxide and lets the molten metal through. In electrical and electronic work rosin flux is used; acid fluxes («soldering water», zinc chloride) are corrosive and are left to plumbing: they leave a residue that keeps eating the copper. A good solder joint is bright and concave, with the metal «wetting» both sides; a cold joint is dull and lumpy. Magnet wire with polyurethane enamel (the «solderable» kind) is tinned directly with the iron at about 380 °C, without scraping.
A flexible cable with tinned ends clamped in a screw terminal works loose with time: the solder «flows» under pressure and leaves the screw slack. In a screw terminal, a flexible cable takes a ferrule crimped on. And a soldered splice that also carries tension breaks right at the edge of the solder: it is reinforced with insulating wrap or with a crimp terminal.
Hard solder (brazing): copper, phosphorus and silver
Brazing joins above 450 °C with an alloy that wets the base metal without melting it, and gives a joint of far greater mechanical strength and conductivity. In the electrical workshop it is used mainly on the copper connections of large machines (terminals, bars, large-section coil heads, rotor bars) and in refrigeration joints.
- Copper-phosphorus (with 0 to 15 % silver): brazes copper to copper without flux, because phosphorus reduces copper oxide. It is not suitable for brass, bronze or steel: with iron it forms brittle phosphides.
- Silver (for example 45 % silver, with copper and zinc, cadmium-free): brazes copper, brass, bronze and steel, with flux. It is the most fluid and needs the least temperature; it is also the most expensive.
The workpiece is heated, not the alloy: the copper is brought to a dull red with the flame and the rod is touched to the joint, which draws it in by capillary action. It is left to cool without quenching in water and the flux is cleaned off. Nearby coil heads are protected from the heat with a damp cloth or heat-sink paste. The fumes from fluxes and silver alloys are extracted, not breathed.
Exothermic welding in earthing systems
Exothermic welding (the system known as Cadweld, although that is a brand) melts copper in a graphite mould: a charge of copper oxide and aluminium powder, once ignited, reacts, releases a great deal of heat (above 1,000 °C) and leaves molten copper that bathes and joins the cable to the rod, the flat bar or another cable. The joint is a continuous piece of copper, with the section of the conductor, it does not loosen or corrode and carries the fault current without heating more than the cable. That makes it the preferred joint in buried earthing systems, where nothing can be retightened. The AEA 90364 regulations require earthing connections to be permanent and of low resistance; the measuring method is in Earthing, insulation and generator testing.
The mould and the conductors must be dry: moisture vaporizes at once and throws molten metal. Wear a face shield or goggles, gloves, long sleeves and closed shoes, in a ventilated place with no flammable materials around. Choose the mould and cartridge that match the conductor diameters (the manufacturer gives the table), ignite with the igniter supplied and do not look at the reaction from above. The mould stays very hot for several minutes.
Crimp terminals
The crimp terminal (tinned copper, for copper) is joined to the cable with a crimping tool that deforms it against the strands until they form a solid block, with no air and no heat needed. A good crimp depends on three things: a terminal of the right cross-section for the cable (an «approximate» one will not do), the die of the tool that matches that terminal and a cable stripped to just the right length, with the insulation outside the barrel and all the strands inside. It is checked with a firm hand pull and by eye: the crimp is centred and even, with no loose strands. For aluminium cable, aluminium or bimetallic terminals and an anti-oxide paste are used, because aluminium covers itself with an insulating oxide in seconds.
What any of these joints aims at is keeping the contact resistance low, because the power dissipated in the joint is:
A well-made crimp has a resistance of the order of 0.1 mΩ: it dissipates 100² · 0.0001 = 1 W. A crimp with loose strands or corroded can reach 2 mΩ (illustrative values): it dissipates 100² · 0.002 = 20 W, twenty times more, concentrated in a spot the size of a coin, and the voltage drop across the joint is 0.2 V. That difference is what thermography shows as a hot spot (Maintenance of electrical components) and what a micro-ohmmeter measures.
04Plastics, insulation, varnishes, tapes and adhesives
The basic concepts of insulators (dielectric strength, insulation resistance, thermoplastics and thermosets) are in Electrical materials. Here we look at the actual materials that are bought and their temperature limits.
Cable plastics and insulation
| Material | Conductor temperature | Characteristics and use |
|---|---|---|
| PVC (polyvinyl chloride) | 70 °C continuous · 160 °C in short circuit | The most common insulation of installation cables. Cheap and flexible; it softens with heat and, when burning, releases corrosive gases. |
| PE (polyethylene) | 70 °C continuous | Excellent insulator with low water absorption; used in coaxial and communication cables. |
| XLPE (cross-linked polyethylene) | 90 °C continuous · 250 °C in short circuit | The chains are linked by cross-linking: more temperature and more current than PVC with the same section. Power cables. |
| Halogen-free (LSZH) | depends on the compound | Low smoke emission and no corrosive gases when burning. For places where smoke is critical: tunnels, places of public assembly. |
| Silicone | up to about 180 °C | Flexible from −60 °C to 180 °C. Oven and heating cables, sleeving and tapes. Beware of silicone vapours near contacts. |
| PTFE (Teflon) | up to about 260 °C | Very good insulation and chemical resistance; high-temperature cables. |
Resins are two-component liquids that harden by chemical reaction: epoxy (adhesion and rigidity, encapsulation of dry-type transformers and coils), unsaturated polyester (winding impregnation) and polyurethane (more flexible, encapsulation of circuits and splices). They are weighed in the proportion given by the manufacturer, mixed without whipping (to avoid bubbles) and used within the pot life; after that they start to gel.
Thermal classes
What decides how long an insulation lasts is temperature. Standards group insulation systems into thermal classes (IEC 60085), each with the maximum continuous operating temperature for a reasonable service life:
| Class | Max. temp. | Typical materials |
|---|---|---|
| A | 105 °C | Impregnated paper, cotton and silk; common enamels for copper. |
| E | 120 °C | Special films and resins. Little used today. |
| B | 130 °C | Mica and glass fibre with organic binders; polyester laminates. |
| F | 155 °C | Modified polyester enamels, epoxy and polyester resins, aramid papers: the class of most motors. |
| H | 180 °C | Polyesterimide, silicones, mica with silicone, glass cloth with silicone. |
| N, R, 250 | 200 / 220 / 250 °C | Polyamide-imide and polyimide, mica, glass, ceramics. |
A rule of thumb, which comes from the chemistry of ageing, says the insulation life halves for every 10 °C above its class temperature, and doubles for every 10 °C below it:
A motor with class F insulation (Tn = 155 °C) heats, at normal load, 80 K above ambient (the class B temperature-rise limit), and 10 K more are allowed for the hot spot. At 40 °C ambient: 40 + 80 + 10 = 130 °C, and the relative life is 2(155 − 130)/10 = 22.5 ≈ 5.7 times the reference. At 55 °C (poorly ventilated room): 145 °C and 21 = 2 times. At 70 °C (next to an oven): 160 °C and 2−0.5 ≈ 0.71: less than the reference. That is why a motor outdoors, covered in dust or with a blocked fan burns out sooner, without anything having «failed».
Varnishes, tapes and adhesives
- They penetrate between the wires, bond the turns (with no play they neither vibrate nor rub), improve heat dissipation and protect against moisture and dust.
- Baking (polyester, epoxy): cures with heat; it is the one for serious repairs, with the thermal class matching the winding.
- Air-drying: for touch-ups; less durable.
- Used at the temperature and for the time on the product datasheet; closed and stirred before use.
- Vinyl (PVC): general use; typical values up to 600 V and about 80 °C. Insulates and identifies phases by colour.
- Self-amalgamating: rubber, fuses onto itself; splices that stay watertight.
- Glass cloth: with silicone or acrylic adhesive, for coil heads and high temperature.
- Polyester and polyimide (Kapton): thin, for binding and insulating coils and windings.
| Product | What it is for | Care |
|---|---|---|
| Cyanoacrylate | Fast bonding of small parts, rubber, plastics. | Brittle under impact and heat. Its vapours leave a white haze on nearby contacts. |
| Two-part epoxy | Structural fixing, filling, encapsulation; conductive types exist. | Exact ratio, working time and cure time. Sensitizes the skin: gloves. |
| Anaerobics | Thread lockers and bearing retainers: they cure without air, between metal parts. | Not used to lock electrical terminals: those are tightened to torque with washers. |
| Silicone (sealant) | Sealing enclosures, gaskets and cable entries. | For electrical work, neutral-cure: acid-cure types (vinegar smell) corrode copper. |
| Hot-melt | Holding cables and components; insulating splices (with heat-shrink). | Softens with heat; not for hot areas. |
| Anti-oxide paste | Aluminium joints and outdoor connectors. | Applied on clean metal, before closing the joint. |
Vapours from silicone sprays, greases and sealants settle on the contacts of relays, switches and contactors and, with the arc, form an insulating layer that stops them conducting. Near electrical contacts, silicone products are avoided, except those made for that use.
05Winding materials and new materials
The calculation and technique of winding are in Motor winding. Here we describe the materials bought to do it.
Magnet wire
The conductor of the winding is enamelled copper wire (magnet wire): electrolytic copper coated with a thin layer of insulating enamel, which provides the insulation between turns. It is chosen by three data: the copper diameter (in mm), the enamel type, which sets the thermal class (polyester, 155 °C; polyesterimide, 180 °C; polyamide-imide, 200 °C) and the grade, the thickness of the coat (grade 1, 2 or 3: the higher, the more enamel). There are also wires with an adhesive overcoat that bonds the turns when heated. If the motor will run on a variable-frequency drive (Variable-speed drives), a wire with partial-discharge-resistant enamel is advisable, because the drive's pulses punish the insulation more than a sine voltage does.
The section needed by the conductor comes from the current and the admissible current density J, which in ventilated machines is of the order of 4 to 7 A/mm²:
| Diameter (mm) | Section (mm²) | Current at 5 A/mm² |
|---|---|---|
| 0.50 | 0.196 | 0.98 A |
| 0.80 | 0.503 | 2.51 A |
| 1.00 | 0.785 | 3.93 A |
| 1.25 | 1.227 | 6.14 A |
| 1.60 | 2.011 | 10.05 A |
With J = 5 A/mm², the section is S = 4.2 / 5 = 0.84 mm² and the diameter, d = √(4 · 0.84 / π) = 1.03 mm. A larger commercial diameter is taken, for example 1.12 mm (0.985 mm², which gives 4.2 / 0.985 = 4.26 A/mm²). As an alternative, to make it easier to fit in the slot, two 0.80 mm wires in parallel are used: 2 · 0.503 = 1.005 mm², giving 4.18 A/mm².
Slot insulation and accessories
- Pressboard (presspan): pressed cellulose board; cheap, class A (in oil, the kind used in transformers). Slot paper between 0.25 and 0.4 mm in small and medium motors.
- Aramid paper (Nomex): withstands temperature (class H or higher), humidity and mechanical stress. Used alone or laminated.
- Laminates of paper and polyester film: DMD (polyester-film-polyester), class B to F; NMN (aramid-film-aramid), class F to H.
- Sleeving (insulating tubes): glass fibre with silicone (class H), PVC or polyester. They insulate leads and joints.
- Slot wedges: glass cloth laminate with epoxy, or phenolic laminate; they keep the wires inside the slot. Wooden ones, class A, are almost no longer used.
- Polyester or glass lacing cord or tape, to fix the coil heads.
New materials
An alloy of iron, boron and silicon cooled so fast that it does not crystallize is obtained as ribbons about 25 µm thick. With no grains, the core magnetizes with very little loss: the transformer consumes typically 60 to 80 % less at no load than with grain-oriented steel. In exchange, it saturates earlier (1.56 T against about 2 T), is brittle and costs more; it is used in distribution transformers that are energized all year and therefore lose power at no load all the time.
Neodymium-iron-boron (NdFeB) and samarium-cobalt (SmCo) give fields several times those of ferrite magnets, with smaller and lighter rotors: permanent-magnet synchronous motors, servomotors, wind generators. NdFeB demagnetizes with heat (from about 80 °C in the common grade up to 200 °C in special ones); SmCo withstands up to about 300 °C or more.
Other materials of interest: high-temperature insulation (polyimide, mica, ceramics, glass) that allows compact motors of class H or higher; partial-discharge-resistant enamels for motors on drives; and ester fluids for transformers, mentioned above. Every new material comes with its datasheet and is used as it says: the thermal class and the maximum temperature are data, not suggestions.
Two large NdFeB magnets attract with enough force to crush a finger, and the pieces are brittle and fly off on impact. Magnets interfere with pacemakers, mechanical watches and cards. A magnet rotor is not baked or heated without checking the maximum temperature of the grade (baking a varnish at 150 °C may demagnetize it); a magnet motor keeps generating voltage at its terminals when turned, even if disconnected.
06Safety: data sheets, storage and waste
The safety data sheet
Every chemical product comes with its safety data sheet (SDS; in Spanish HDS), which the manufacturer or supplier provides and which must be at hand where the product is used. In Argentina products are classified and labelled under the Globally Harmonized System (GHS), adopted for the workplace by Resolution SRT 801/2015, with red-diamond pictograms (flame, skull, corrosion, exclamation mark, health hazard, environment). The sheet has 16 sections; the ones that matter in the workshop are:
| Section | What is found out |
|---|---|
| 2 | Hazards of the product: pictograms, hazard statements. |
| 4 and 5 | First aid and what to do in case of fire (with which extinguisher). |
| 7 | Safe handling and storage. |
| 8 | Personal protection needed: gloves (of what material), goggles, mask, ventilation. |
| 9 | Properties: flash point, state, odour. |
| 10 | What it is incompatible with and what not to mix. |
| 13 | How to dispose of the product and the container. |
Law 19.587 and Decree 351/79 require the employer to inform workers about the risks of what they handle and to provide personal protective equipment; the general framework is in Occupational health and safety.
Storage
- Original container, labelled and closed; if decanted, the new container carries the same label.
- Segregate by incompatibility: flammables (solvents, petrol) away from oxidizers and any heat source; acids away from bases; epoxy hardeners (amines), which are corrosive, away from the rest.
- With a containment tray under liquids (with a volume at least that of the largest container), in a ventilated, cool, dry place out of direct sunlight.
- Sturdy shelving, with no switches or sockets next to flammables.
- First in, first out, with an opening date: greases, resins, adhesives and varnishes expire, and an expired product does not cure or lubricate.
Waste
Used oils, filters, rags with oil or solvent, empty chemical containers, uncured resin residues, cells and batteries are hazardous waste under Law 24.051 (and the provincial adherence): they are not poured down the sink, onto the ground or into ordinary rubbish. They are collected separately, in closed, labelled containers, and picked up by an authorized carrier and operator, with the corresponding record. Leaded solder and solder dross also go apart.
A rag soaked in oil or drying varnish, crumpled in a corner, heats up as it oxidizes and can catch fire with nobody lighting it. Used rags are kept in a metal bin with a lid and removed with the hazardous waste.
07In the workshop
Materials: three-phase motor stopped and locked out, grease gun, the grease specified for the motor, 0.1 g scale, clean rag, infrared thermometer, worksheet. Steps: read the plate and the bearing designation (for example, 6309); calculate G = 0.005 · D · B; calibrate the gun (grams per stroke, average of ten strokes); wipe the nipple; apply the amount in slow strokes; start the motor and measure the end-cap temperature every 15 minutes for two hours, with the drain open for the first of them. Acceptance criterion: quantity within ±20 % of the calculated value, no excess grease expelled at the end, and an end-cap temperature that levels off (rising no more than 10 °C from the initial one).
Materials: oil tester with a 2.5 mm cup (with the teacher), sample in a clean bottle, lint-free cloth, worksheet, gloves. Steps: rinse the cup with the same sample; fill it carefully without bubbles and wait a few minutes; with the safety lid closed, make six breakdowns with the rest the equipment calls for; record each value; calculate the average; classify the oil with the criterion in section 02; repeat with a sample to which a few drops of water have been added. Acceptance criterion: six values recorded, average calculated to one decimal, a written decision (in service, monitor or treat) and a check that water lowers the breakdown voltage.
Materials: three 30 cm lengths of 2.5 mm² cable, 60 to 100 W soldering iron, rosin-core solder, crimping tool and terminals, torch with copper-phosphorus rod, a source of 5 A or more, millivoltmeter, worksheet. Steps: make one splice with solder, another with a crimp terminal and another with brazing; pass the current the teacher indicates and measure the voltage drop across each joint and across a same-length piece of sound cable; cut each joint open and look inside. Acceptance criterion: the voltage drop across the joint does not exceed that of the same length of unjoined conductor; the crimp has no loose strands and the brazed joint shows the metal wetted along the whole joint.
Materials: the products in the store, their safety data sheets (printed or on a phone), worksheet, labels. Steps: list each product with its pictogram, flash point and incompatibilities (sections 2, 9 and 10); place them on the shelves according to those incompatibilities; note opening and expiry dates; identify the waste that is generated and where it is collected. Acceptance criterion: every product with a sheet and a label, no adjacent incompatibilities, containment trays under liquids and waste kept separate.
08Common mistakes
| Mistake | Consequence |
|---|---|
| Greasing «until grease comes out» or with far too much | The grease is churned and the bearing overheats; the grease degrades and the bearing life is shortened. |
| Mixing greases with different thickeners | Incompatibility: the grease softens or hardens and the bearing fails. |
| Using a «similar» oil instead of the one the manual calls for | Insufficient film or power loss from excessive viscosity. |
| Leaving the transformer's silica gel saturated | The oil absorbs moisture and the dielectric strength drops; the risk is an internal breakdown. |
| Taking the oil sample in a plastic or damp bottle | The result does not represent the oil, and decisions are made on false data. |
| Cleaning contacts or coils with petrol | Fire risk, and the solvent damages varnishes and enamels. |
| Tinning the ends of a flexible cable to put it in a screw terminal | The solder flows under pressure and the terminal works loose: hot spot. |
| Using acid flux in an electrical installation | The residue corrodes the copper of the joint over time. |
| Acid-cure silicone in a switchboard | Acetic acid corrodes copper and contacts. |
| Baking a rotor with rare-earth magnets | Loss of magnetization above the maximum temperature of the grade. |
| Using a varnish or paper of a lower thermal class than the winding | The insulation ages sooner and the repair lasts half as long as planned. |
| Storing everything together, with no data sheets or containment | Reactions between incompatible products, spills that reach the drain, and nobody knows what to do in an accident. |
09Self-assessment
What does «ISO VG 68» mean and at what temperature is it measured?
It is the viscosity grade of an oil according to ISO 3448: its midpoint kinematic viscosity is 68 mm²/s at 40 °C, within ±10 % (between 61.2 and 74.8). The higher the number, the thicker the oil.
Calculate the grease relubrication quantity for a 6312 bearing (60 × 130 × 31 mm).
G = 0.005 · D · B = 0.005 · 130 · 31 = 20.2 g. It is applied in slow strokes, with the calibrated gun, and the temperature is checked afterwards. If the motor has a lubrication plate, the plate rules.
Six oil breakdowns give 38, 41, 36, 40, 39 and 38 kV. What is reported and what is decided?
The average is (38 + 41 + 36 + 40 + 39 + 38) / 6 = 232 / 6 = 38.7 kV. It is between 30 and 40 kV: the oil is still usable, but it must be monitored and the test repeated more often, without waiting for it to fall below 30 kV.
What is the difference between soft and hard soldering, and which would you use to join copper to copper without flux?
Soft solder melts below 450 °C (tin alloys) and hard solder above. For copper to copper without flux, the copper-phosphorus alloy is used, because phosphorus reduces copper oxide. For brass, bronze or steel, silver and flux are needed.
Why is exothermic welding preferred in a buried earthing system?
Because it forms a continuous copper joint that neither loosens nor corrodes, with the conductor's section and the ability to carry the fault current. In a buried installation it cannot be retightened.
A class F (155 °C) motor runs with its hottest spot at 145 °C. How much does the insulation life change?
L / Ln = 2(155 − 145)/10 = 21 = 2: it lasts twice as long as at 155 °C. For every extra 10 °C, it is halved.
Where do you find what a product is incompatible with and how to dispose of it, and what is done with an oily rag?
In its safety data sheet: section 10 (incompatibilities) and 13 (disposal). The rag with oil or varnish is kept in a metal bin with a lid because it can ignite spontaneously, and it is handed over as hazardous waste (Law 24.051).