Materials used in electrical equipment

Magnetic materials are divided into soft and hard materials according to the shape of their B-H (hysteresis) loop. Soft materials are easy to magnetise and demagnetise; hard materials keep their magnetism once magnetised.

B-H curves

 Soft material            Hard material
        B                        B
        |   __..--             __|______
        | /                   /  |     /
   -----|/-----> H      -----+---|----+---> H
       /|                   /    |   /
  --''  |                  /_____|__/
        |                        |
 narrow loop, small Hc    wide loop, large Hc, Br

• A soft material has a tall, narrow loop: small coercive force $H_c$, high permeability, small loop area.
• A hard material has a short, fat loop: large coercive force $H_c$, large remanence $B_r$ and large loop area (large $(BH)_{max}$, energy product).

Comparison

Examples

• Soft: pure (Armco) iron, low-carbon steel, silicon steel (hot rolled and CRGO), nickel-iron alloys (permalloy, mumetal), soft ferrites (Mn-Zn, Ni-Zn) and amorphous metal. Used in transformer cores, armature and stator cores, relays, chokes and high-frequency cores.
• Hard: carbon steel, tungsten steel, cobalt steel, Alnico, barium/strontium ferrite, samarium-cobalt and neodymium-iron-boron (NdFeB). Used in permanent-magnet DC motors, PM synchronous machines, measuring instruments, loudspeakers and magnetos.

Machine cores carry alternating or rotating flux, so the loop is traced many times per second. This is why they need soft materials with a small loop area. A permanent magnet must keep its flux against demagnetising fields, so it needs a hard material.

Insulating materials are grouped into thermal classes by the maximum temperature they can withstand continuously and still give a reasonable service life. The classes are standardised in IS 1271 / IEC 60085.

Newer editions of IEC 60085 also give classes of 200, 220 and 250 °C. These replace the open-ended class C.

Points to remember

• The temperature limit is the hot-spot temperature of the insulation, not the ambient. Permitted temperature rise = class limit − ambient (40 °C) − hot-spot allowance.
• Organic materials (cotton, paper, silk) char at high temperature, so they belong to the low classes Y and A. Inorganic materials (mica, glass, ceramics) belong to the high classes.
• In classes B, F and H the inorganic base material is the same. The class is set by the binder (varnish/resin), because the binder fails first.
• Insulation life roughly halves for every 8–10 °C rise above the class limit (Montsinger's rule). This is why the limits must be respected.

Use in machines

• Small motors and fractional-kW machines: class A or E.
• Most modern industrial induction motors: class F insulation, designed for class B temperature rise, which gives a thermal margin.
• Traction motors and machines in hot places: class H.
• Oil-immersed transformers: class A (paper and pressboard in oil).

Ageing of core is the gradual increase in the hysteresis loss (and the fall in permeability) of the iron core over time, when the machine runs for a long period at its normal working temperature.

• Its cause is impurities, mainly carbon (also nitrogen and oxygen), dissolved in the iron. At working temperatures (about 60–100 °C) these slowly come out of solution as fine particles of iron carbide/nitride.
• The particles pin the domain walls. This widens the hysteresis loop, so the core loss can rise by up to 50–100 % over a few years. Efficiency falls and temperature rise increases.

Ageing is avoided by using low-carbon steel, by alloying with silicon, and by annealing the laminations.

Advantages and disadvantages of adding silicon to iron

Because of these opposing effects, silicon content is a compromise:

• Transformers: about 4–4.5 % Si (or about 3 % in CRGO). Laminations are simple strips, so brittleness matters less, and low loss matters most.
• Rotating machines: about 0.5–3.5 % Si (dynamo grade). Teeth and slots must be punched, the teeth must carry high flux density, and the laminations need mechanical strength.

Silicon (about 0.5–4.5 % by weight) is added to iron to make the silicon steel used in transformer and machine cores. It improves the magnetic properties but spoils the mechanical properties.

Advantages

1. Higher resistivity. Resistivity rises from about 0.1 µΩ·m (pure iron) to about 0.6 µΩ·m (4 % Si). Since $P_e \propto \dfrac{B_m^2 f^2 t^2}{\rho}$, eddy current loss falls sharply.
2. Lower hysteresis loss. The coercive force falls and the hysteresis loop becomes narrower.
3. No ageing. Silicon combines with or removes carbon and oxygen, so carbide precipitation, and with it the rise in loss over time, is avoided.
4. Higher permeability at low and medium flux densities, so magnetising current is less.
5. Less magnetostriction, so transformer hum is lower.

Disadvantages

1. Brittleness. Above about 4–4.5 % Si the steel becomes hard and brittle, so it cannot be rolled thin or punched easily.
2. Tool wear. Dies and punches wear quickly, which raises manufacturing cost.
3. Lower saturation flux density. Silicon is non-magnetic, so it reduces the iron content. The core must work at a lower $B_m$, or be larger.
4. Lower thermal conductivity, so heat leaves the core less easily.
5. Higher cost of the material.

 loss,          brittleness
 rho  ^ resistivity     /
      |      .-------  /
      |   .-'         /
      | .'  core loss/
      |\_           /
      |   ~~--.__  /
      +-----------+----> % Si
      0   1   2   3   4   5
     usual: 0.5-3.5 % (motors)
            4-4.5 % (transformers)

Practical choice

Conducting materials carry current in windings, busbars, commutators and leads. A material of high conductivity (low resistivity) is needed so that $I^2R$ loss and voltage drop are small.

Fundamental requirements of a high conducting material

1. High electrical conductivity (low resistivity), for low copper loss and voltage drop.
2. Low temperature coefficient of resistance, so that resistance and loss do not rise much when hot.
3. High tensile strength to withstand winding tension, electromagnetic forces and centrifugal forces.
4. High ductility and malleability, so it can be drawn into thin wires and strips and bent into coils without cracking.
5. Good solderability and weldability, giving low-resistance joints.
6. Resistance to corrosion and oxidation.
7. Good thermal conductivity, so heat is carried away from hot spots.
8. Low cost, light weight and easy availability.

Classification of electrical conducting materials

Conducting materials are classified by resistivity into two groups.

1. High conductivity (low resistivity) materials carry current with minimum loss.

• Copper ($\rho \approx 1.72\times10^{-8}\ \Omega$m at 20 °C). Annealed (soft) copper is used for windings. Hard-drawn copper is used for overhead lines, commutator bars and busbars.
• Aluminium ($\rho \approx 2.8\times10^{-8}\ \Omega$m). It is light and cheap, but needs about 1.6 times the copper area. Used in squirrel-cage rotors (die-cast), transformer foil windings, busbars and cables.
• Silver and gold: best conductivity, used only for relay and switch contacts.
• Copper alloys: brass for terminals; bronze, cadmium-copper and beryllium-copper for springs, contact wires and slip rings.
• Steel: used only where strength matters, for example ACSR cores and rail conductors.

2. High resistivity materials are used where a resistance is wanted.

• Manganin, constantan (eureka): precision resistors, shunts and standards (very low temperature coefficient).
• Nichrome, kanthal: heating elements (resist oxidation at high temperature).
• Tungsten: lamp filaments. Carbon: brushes and resistors.
• Cast iron and alloy steel grids: starter and rheostat resistances.

Cold rolled grain oriented (CRGO) steel is silicon steel (about 3 % Si) rolled cold and then annealed so that most crystals (grains) have their easy direction of magnetisation along the rolling direction. Hot rolled silicon steel has grains oriented at random, so its properties are the same in all directions but poorer.

In a transformer the flux in the limbs and yokes flows along a fixed direction. The core strips are cut so that this direction is the rolling direction, and the full benefit of CRGO is then obtained.

Advantages of CRGO over hot rolled laminations

Resulting benefits

1. A higher flux density means a smaller core section for the same voltage per turn. Less core and less copper are needed, and the transformer is smaller and lighter.
2. Lower iron loss and magnetising current give better efficiency, especially for distribution transformers that are energised all day.
3. Thinner laminations with a good stacking factor (carlite/phosphate coating) further reduce eddy loss.
4. Less noise and lower no-load current.

Precautions

• Along the cross (transverse) direction the properties are poor. The corner joints must therefore be mitred at 45° so that the flux stays mostly along the grain.
• CRGO is sensitive to mechanical stress. Strips are annealed again after cutting, and bolt holes are avoided, so cores are clamped with bands or resin instead.
• CRGO costs more, but the saving in core size and losses more than pays for this.

High resistivity materials are metals and alloys with high resistivity (typically 40–140 µΩ·cm, compared with 1.72 µΩ·cm for copper). They are used wherever a definite resistance, heat or light is needed rather than low loss.

Where they are used

• Standard resistances, shunts and multipliers in measuring instruments
• Starters, rheostats, field regulators and load banks of machines
• Heating elements of furnaces, heaters, irons and ovens
• Filaments of incandescent lamps
• Brushes, arcing contacts and carbon resistors

Classification according to purpose

Notes

• Manganin has a temperature coefficient close to zero, so resistance standards do not drift with temperature.
• Constantan has a nearly constant resistance over a wide temperature range. It is also used in thermocouples (with copper or iron) because of its large thermo-emf.
• Nichrome forms a protective chromium oxide layer, so it lasts long at red heat. This makes it the standard heating element material.
• Tungsten is used for filaments because its high melting point allows high efficiency (lumen per watt).

Ageing of core is the gradual increase in the hysteresis loss (and the fall in permeability) of the iron core over time, when the machine runs for a long period at its normal working temperature.

• Its cause is impurities, mainly carbon (also nitrogen and oxygen), dissolved in the iron. At working temperatures (about 60–100 °C) these slowly come out of solution as fine particles of iron carbide/nitride.
• The particles pin the domain walls. This widens the hysteresis loop, so the core loss can rise by up to 50–100 % over a few years. Efficiency falls and temperature rise increases.

Effects

• Iron loss increases over time, so efficiency falls.
• Temperature rise increases and insulation life is shortened.
• No-load (magnetising) current rises.

How ageing is minimised

1. Use low-carbon steel. Keep carbon below about 0.005 % by decarburising during manufacture, so there is little to precipitate.
2. Add silicon (about 0.5–4.5 %). Silicon reduces the solubility of carbon and helps remove it, so silicon steel is practically non-ageing. This is the main remedy.
3. Anneal the laminations after punching or cutting, in a decarburising atmosphere (wet hydrogen). This removes residual carbon and relieves stresses.
4. Artificial ageing (stabilising): heat the steel to about 100–150 °C before use, so any precipitation takes place before the core goes into service.
5. Keep the working temperature low through good cooling and ventilation, because precipitation speeds up with temperature.
6. Use modern CRGO or other silicon steels, which do not age.

An insulating material has very high resistivity. It separates conductors from each other and from earthed parts, and must remain reliable under the electrical, thermal, mechanical and chemical stresses inside a machine.

Fundamental requirements of a good insulating material

Electrical

1. High insulation resistance (resistivity) and high dielectric strength.
2. Low dielectric loss (low $\tan\delta$) and suitable permittivity.

Thermal 3. Able to work continuously at high temperature (a high thermal class) without losing its properties. 4. Good thermal conductivity, to pass heat to the iron, and low thermal expansion. 5. Non-inflammable.

Mechanical 6. Good mechanical strength, flexibility and resistance to vibration and abrasion.

Chemical and others 7. Non-hygroscopic (does not absorb moisture), and resistant to oil, acids, alkalis and ozone. 8. Does not deteriorate with age, easy to shape and apply, and cheap.

Merits and demerits of adding silicon to iron

 loss,          brittleness
 rho  ^ resistivity     /
      |      .-------  /
      |   .-'         /
      | .'  core loss/
      |\_           /
      |   ~~--.__  /
      +-----------+----> % Si
      0   1   2   3   4   5
     usual: 0.5-3.5 % (motors)
            4-4.5 % (transformers)

The figure shows that resistivity rises and core loss falls as silicon increases. Brittleness, however, rises sharply beyond about 4 %. Practical limits are therefore about 4–4.5 % for transformer steel and 0.5–3.5 % for rotating-machine steel.

An insulating material separates current-carrying parts from each other and from the iron and frame. In a machine it must also stand heat, vibration, moisture and oil for its whole life.

Fundamental requirements of a good insulating material

1. High resistivity and high dielectric strength, to withstand normal and surge voltages.
2. Low dielectric loss ($\tan\delta$) and stable permittivity.
3. High thermal stability: it must work at the temperature of its class (A, E, B, F, H) without charring or becoming brittle.
4. Good thermal conductivity, so the heat of the copper can pass through it.
5. Mechanical strength and flexibility, to withstand winding forces, vibration and short-circuit forces.
6. Non-hygroscopic, and resistant to oil, chemicals and ozone.
7. Not inflammable, with long life and low cost.

Application of insulating materials in machines

In short, solid insulation holds and separates the conductors, liquid insulation (oil) both insulates and cools, and gases such as air and hydrogen serve as insulation and coolant.

Soft magnetic materials are easily magnetised and demagnetised. They have a narrow hysteresis loop with low coercive force (usually below about 1 kA/m), high permeability and low hysteresis loss, so they are suited to alternating and rotating flux.

Properties

• High permeability and high saturation flux density
• Low coercivity and low remanence
• Small loop area, giving low hysteresis loss
• High resistivity (in alloys and ferrites), giving low eddy loss

          B
          |    ___---
          |  /
   -------|-/-------> H     narrow loop:
         /|                 small Hc, high mu
   ---__/ |
          |

Applications according to commercial use

Machines and transformers cannot work without soft materials. They provide the low-reluctance path for the working flux and keep iron losses and magnetising current small.

Ageing of core is the gradual increase in the hysteresis loss (and the fall in permeability) of the iron core over time, when the machine runs for a long period at its normal working temperature.

• Its cause is impurities, mainly carbon (also nitrogen and oxygen), dissolved in the iron. At working temperatures (about 60–100 °C) these slowly come out of solution as fine particles of iron carbide/nitride.
• The particles pin the domain walls. This widens the hysteresis loop, so the core loss can rise by up to 50–100 % over a few years. Efficiency falls and temperature rise increases.

Minimising ageing

1. Use steel with very low carbon content (decarburised steel).
2. Add silicon, which makes the steel practically non-ageing.
3. Anneal the laminations after punching, in a decarburising atmosphere.
4. Artificially age (heat-treat) the steel before use, and keep working temperatures low.

Merits and demerits of adding silicon

Transformer steel has about 4–4.5 % Si. Rotating-machine steel has 0.5–3.5 % Si.

Ageing of core is the gradual increase in the hysteresis loss (and the fall in permeability) of the iron core over time, when the machine runs for a long period at its normal working temperature.

• Its cause is impurities, mainly carbon (also nitrogen and oxygen), dissolved in the iron. At working temperatures (about 60–100 °C) these slowly come out of solution as fine particles of iron carbide/nitride.
• The particles pin the domain walls. This widens the hysteresis loop, so the core loss can rise by up to 50–100 % over a few years. Efficiency falls and temperature rise increases.

How ageing was removed

The ageing problem was removed by:

1. Adding silicon (about 0.5–4.5 %) to iron. Silicon steel is non-ageing because silicon helps remove carbon and oxygen and stops carbide precipitation.
2. Making very low-carbon steel by decarburising and annealing the laminations in wet hydrogen after cutting.
3. Pre-ageing (artificial ageing) the steel by heat treatment before use.

Advantages after removing ageing (silicon steel)

• Core loss stays constant over the life of the machine, so efficiency does not fall.
• Resistivity is higher, so eddy loss is lower; hysteresis loss is also lower.
• Permeability is higher and magnetising current lower.
• Temperature rise stays steady, so insulation life is longer.
• Magnetostriction and noise are reduced.

Disadvantages

• Silicon steel is brittle and hard, so punching slots and teeth is difficult and tools wear quickly.
• Saturation flux density is lower, so the core must work at a lower $B_m$ (a larger core).
• Thermal conductivity is lower.
• The material and its processing (annealing, decarburising) cost more.

For this reason, transformers use high silicon (4–4.5 %) and rotating machines use lower silicon (0.5–3.5 %).

An insulating material (dielectric) is a material with very high resistivity (about $10^{9}$–$10^{20}\ \Omega$m). It prevents the flow of current between conductors at different potentials, and between conductors and earthed parts.

Classification

1. By physical state

• Solid: mica, paper, pressboard, cotton, glass fibre, porcelain, rubber, PVC, polyester films, epoxy resins, varnishes.
• Liquid: mineral transformer oil, silicone oil, synthetic esters (askarels are no longer used).
• Gaseous: air, nitrogen, hydrogen (cooling of large alternators), SF₆ (switchgear).

2. By origin/structure

• Organic: paper, cotton, silk, wood, rubber, resins and plastics. Flexible and cheap, but limited to low temperatures.
• Inorganic: mica, glass, porcelain, asbestos and ceramics. They withstand high temperature but are brittle.

3. By thermal class (IS 1271 / IEC 60085)

Class Y and A are unimpregnated and impregnated organic materials. B, F and H are mica and glass with binders of increasing thermal stability. C covers pure inorganic materials.

An insulating material is a substance of very high resistivity that stops current flowing between parts at different potentials. In machines it insulates turns, coils and phases from each other and from the core and frame.

Classification

(a) By state

• Solid: mica, paper, pressboard, cotton, glass fibre, asbestos, porcelain, rubber, PVC, polyester film, epoxy and varnishes.
• Liquid: mineral transformer oil, silicone fluid, synthetic esters.
• Gas: air, nitrogen, hydrogen, SF₆.

(b) By origin

• Organic (paper, cotton, resins, plastics): flexible, but suited to low temperature.
• Inorganic (mica, glass, porcelain, ceramics): suited to high temperature, but brittle.

(c) By thermal class (IS 1271 / IEC 60085)

Fundamental requirements of a good insulating material

1. High dielectric strength and high resistivity.
2. Low dielectric loss and low power factor.
3. Able to withstand high temperature without deterioration; non-inflammable.
4. Good thermal conductivity and low thermal expansion.
5. Good mechanical strength, flexibility and resistance to vibration.
6. Non-hygroscopic; resistant to oil, chemicals and ozone.
7. Stable with age; easy to work; cheap.

Electrical properties of insulating materials

1. Insulation resistance / resistivity. Volume resistivity (through the body) and surface resistivity (along the surface) should be very high. Both fall with moisture and temperature.
2. Dielectric strength. The maximum voltage gradient (kV/mm) the material withstands before breakdown. It depends on thickness, temperature, moisture and the duration of voltage.
3. Permittivity (dielectric constant $\varepsilon_r$). In composite insulation the stress divides inversely as $\varepsilon_r$, so materials with similar $\varepsilon_r$ are preferred. A low $\varepsilon_r$ is preferred for cables.
4. Dielectric loss and loss angle ($\tan\delta$). The power lost in the dielectric is $P = \omega C V^2 \tan\delta$. A low $\tan\delta$ is needed to avoid heating and thermal breakdown.
5. Surface tracking and arc resistance. The ability to resist forming a conducting carbon track along the surface under discharge and contamination.
6. Partial discharge resistance, which matters in high-voltage machine insulation (mica is excellent here).

Iron (core) loss in a magnetic core carrying alternating flux has two parts: hysteresis loss and eddy current loss.

Hysteresis loss

When the core is taken round a complete magnetisation cycle, the domains are repeatedly re-oriented. The energy spent on this, equal to the area of the B-H loop per cycle per unit volume, is lost as heat.

$$P_h = k_h\, B_m^{x}\, f\, V \quad \text{watts}$$

• $k_h$ = hysteresis constant of the material
• $B_m$ = maximum flux density
• $x$ = Steinmetz exponent (1.6 for older steels, about 1.6–2 for modern ones)
• $f$ = frequency, $V$ = volume of core

Reduction: use a soft material with a narrow loop (silicon steel, CRGO), work at a moderate $B_m$, and anneal the laminations.

Eddy current loss

The alternating flux induces emfs in the core itself. These drive circulating (eddy) currents in closed paths in the iron, causing $I^2R$ loss.

$$P_e = k_e\, B_m^2\, f^2\, t^2\, V \quad \text{watts}$$

For thin laminations of thickness $t$ and resistivity $\rho$, the loss per unit volume is

$$p_e = \frac{\pi^2 B_m^2 f^2 t^2}{6\rho} \quad \text{W/m}^3$$

Reduction:

• Laminate the core (0.23–0.5 mm sheets) insulated by varnish or oxide, so the eddy paths are broken up. The loss varies as $t^2$.
• Raise the resistivity by adding silicon.
• Use ferrite or powdered cores at high frequency.

 solid core: large eddy loop    laminated: small loops
 +-------------+               +--+--+--+--+
 |  .-------.  |               |()|()|()|()|
 |  |  ->   |  |               |  |  |  |  |
 |  '-------'  |               |()|()|()|()|
 +-------------+               +--+--+--+--+

An electrical insulator is a material of very high resistivity (about $10^{9}$ to $10^{20}\ \Omega$m) and high dielectric strength. It does not allow appreciable current to flow through it, and is used to separate parts at different potentials.

Insulating materials used in different machines

General groups

• Solid: mica, paper, cotton, glass fibre, porcelain, rubber, plastics, resins and varnishes.
• Liquid: mineral oil, silicone fluid, ester oils.
• Gas: air, hydrogen, nitrogen, SF₆.

The insulation must be chosen for its thermal class (Y 90, A 105, E 120, B 130, F 155, H 180, C >180 °C). Its life depends mainly on the operating temperature.

In a DC machine, conducting materials carry the armature and field currents and the commutator current. Insulating materials keep these currents in their intended paths: between turns, between coils and core, and between commutator segments.

Special notes for DC machines

• Commutator bars use hard-drawn copper (or silver-bearing copper) for strength and wear resistance. Mica is used between bars because it withstands heat and sparking and wears at a similar rate to copper (it is undercut to avoid high mica).
• Brushes are carbon or graphite. Their fairly high contact resistance helps commutation.
• Field coils use enamelled or cotton-covered copper, and are insulated from poles by pressboard or mica.

Electrical sheet steels are graded by silicon content and loss. Transformer grade steel is high-silicon, low-loss steel for static cores. Dynamo grade steel is low or medium silicon steel for rotating machines.

Reason for the difference

• A transformer core is made of simple strips with almost no punching, and it is energised continuously. Low loss therefore matters most, and high silicon and grain orientation are used.
• A rotating machine core needs many slots and teeth punched accurately, needs mechanical strength against centrifugal forces, and carries high tooth flux density in all directions. A less brittle, non-oriented steel with higher saturation (dynamo grade) is therefore chosen.

The core of a transformer provides a low-reluctance path for the mutual flux. Since it is magnetised continuously at supply frequency, the material must give low iron loss, low magnetising current and a compact core.

Requirements of transformer core material

1. High permeability, so the magnetising current (no-load current) is small.
2. High saturation flux density, so a higher working $B_m$ can be used. This reduces the core area, the turns and the size.
3. Low hysteresis loss: a narrow B-H loop (soft magnetic material).
4. High resistivity, to reduce eddy current loss.
5. Availability in thin sheets (0.23–0.35 mm) with a thin, strong insulating coating. This gives low eddy loss and a high stacking factor.
6. No ageing: the loss must not increase over time.
7. Low magnetostriction, for low noise (hum).
8. Mechanical properties: it can be cut and assembled without losing its magnetic quality, and is not too brittle.
9. Low cost and easy availability.

Material meeting these needs

• CRGO silicon steel (about 3 % Si, grain oriented) is the standard choice for power and distribution transformers. It has very low loss and high permeability along the rolling direction, and works at 1.5–1.75 T.
• Hot rolled silicon steel (4–4.5 % Si) was used earlier and is still used for small transformers.
• Amorphous metal is used in energy-efficient distribution transformers (very low no-load loss).
• Ferrites and nickel-iron alloys are used in high-frequency and instrument transformers.

CRGO (cold rolled grain oriented) steel is about 3 % silicon steel that is cold rolled and then annealed so the grains line up with their easy direction of magnetisation (the cube edge) along the rolling direction. This is called the Goss texture. Along this direction its magnetic properties are much better than those of ordinary hot rolled steel, so it is called a superior magnetic material.

Reasons

1. Very high permeability along the rolling direction. Only a small magnetising force is needed, so no-load current is very low.
2. High working flux density: 1.5–1.75 T, against 1.2–1.4 T for hot rolled steel. A smaller core area and fewer turns are needed for the same rating.
3. Very low core loss: about 0.9–1.3 W/kg at 1.5 T, 50 Hz, against roughly 2.5–3.5 W/kg for hot rolled steel. The loop is narrow, and the thin sheets have high resistivity.
4. Thin laminations (0.23–0.35 mm) with a smooth surface and a thin inorganic (carlite/phosphate) coating give a high stacking factor (about 0.95–0.97).
5. No ageing, so the loss stays constant over the transformer's life.
6. Low magnetostriction, so the transformer is quieter.

 B ^         CRGO (rolling dir.)
   |      .------------
   |    /    hot rolled
   |   /  .------------
   |  / /
   | //
   +---------------------> H
 CRGO reaches high B at small H

Conditions for using it well

• The flux must flow along the rolling direction. Across the grain the properties are poor, so corner joints are mitred at 45°. This is why CRGO suits transformers (one-direction flux) better than rotating machines.
• The strips must be annealed again after cutting, and bolt holes avoided, because stress spoils the grain properties.

Because CRGO gives smaller, lighter, cooler and more efficient transformers, it is the standard core material for power and distribution transformers.

Adding silicon lowers core loss, but it also makes steel brittle and reduces its saturation flux density. The best silicon content therefore depends on whether low loss or mechanical and magnetic strength matters more. Transformers use about 4–4.5 % Si; rotating machines use about 0.5–3.5 % Si.

Why transformers use higher silicon content

1. The core is energised all the time (24 hours for distribution transformers), so iron loss decides all-day efficiency. High silicon raises resistivity (about 0.6 µΩ·m), which cuts eddy loss, and also lowers hysteresis loss.
2. Simple shape. Transformer laminations are plain rectangular strips that are sheared, not punched with teeth and slots. The brittleness of high-silicon steel does little harm.
3. The core is static, with no centrifugal or vibration forces, so high mechanical strength is not needed.
4. Moderate flux density (1.1–1.7 T) is used in the core, so the lower saturation of high-silicon steel is acceptable.

Why rotating machines use lower silicon content

1. Punching: stator and rotor laminations have many narrow teeth and slots punched with dies. High-silicon steel is brittle, cracks at the teeth and wears the dies quickly.
2. High flux density in teeth: teeth carry about 1.6–2.1 T. Low-silicon steel has a higher saturation flux density and needs fewer ampere-turns.
3. Mechanical strength: rotor laminations must withstand centrifugal forces, vibration and torque, so tougher steel is needed.
4. Air gap dominates the magnetising current in a rotating machine. The small loss benefit of high silicon is less important than in a transformer.
5. Low frequency in the rotor (slip frequency in induction motors) means rotor iron loss is small anyway.

Temperature is the most important factor deciding the life of insulating materials, and so the life of the machine. A machine's rating is in fact limited by the temperature its insulation can safely withstand.

Why temperature matters

• At high temperature the organic materials in insulation (paper, cotton, varnish, resins) undergo slow chemical changes: oxidation, loss of volatile matter and depolymerisation.
• The insulation becomes dry, brittle and cracked. Its mechanical strength falls first; the vibration and electromagnetic forces of normal service then crack it, and its dielectric strength is lost. Breakdown follows.
• The rate of these chemical reactions follows the Arrhenius law, so it rises rapidly with temperature:

$$L = A\, e^{B/T}$$

where $L$ is life, $T$ is absolute temperature, and $A$, $B$ are constants of the material.

Montsinger's rule (10-degree rule)

A practical form of the Arrhenius law: the life of insulation is roughly halved for every 8–10 °C rise in operating temperature above its rated limit.

$$L = L_0 \times 2^{-(\theta-\theta_0)/10}$$

For example, class A insulation running at 115 °C instead of 105 °C would last only about half as long.

Consequences for design

• Insulation is grouped into thermal classes (Y 90, A 105, E 120, B 130, F 155, H 180, C >180 °C). Each class's limit gives an economic life of about 20–25 years.
• The design must keep the hot-spot temperature (not the average) below the class limit. Permissible temperature rise = class limit − ambient (40 °C) − hot-spot margin.
• Short overloads are allowed only while the temperature stays within limits. Thermal time constants decide how long these overloads can last.