Um mergulho profundo em materiais magnéticos para máquinas elétricas

A deep dive into magnetic materials for electrical machines

June 24, 2024 Luciano Bertene

Electrical machines are essential in our modern world, powering everything from household appliances to industrial equipment. The key to its efficiency and performance lies in its magnetic materials. Magnetic materials are crucial for the generation, transmission and conversion of electrical energy.

magnetic properties

The magnetic properties of magnetic materials depend on the direction of the material's crystals and determine the dimensions or instrumentation of machines for a given power, the required excitation, operating efficiency, etc.

The specific properties that the best magnetic material must possess are explained below.

Be less reluctant or extremely absorbent or have a high relative permeability value µ _R .
High permeation induction (to reduce the volume and weight of iron parts).
High electrical resistance, therefore lower eddy voltage and therefore lower eddy current loss.
Conical hysteresis loop or lower coercivity, therefore lower hysteresis loss and high operating efficiency.
A high temperature.
It must have a great energy product value. It is expressed in joules/m ³ .

Magnetic materials are generally classified as paramagnetic, diamagnetic, ferromagnetic, ferrimagnetic and antiferromagnetic. Only ferromagnetic materials have magnetic properties suitable for electrical machines. Ferromagnetic properties are limited exclusively to cobalt, iron, nickel and alloys. The only exceptions are some alloys of metallic elements and some group parts.

Permanent magnets

Permanent magnets have revolutionized the efficiency of electrical machines. Their ability to maintain their magnetic properties without an external field makes them the first choice for countless applications, from motors and generators to magnetic bearings. We delve deeper into the science behind permanent magnets, including rare earth magnets, and explore recent advances in magnet manufacturing that have significantly improved their performance.

Magnetic Materials: The Building Blocks of Electrical Machines

At the heart of every electrical machine is a magnetic core responsible for controlling the flow of electrical current and producing mechanical work. We investigate the properties of various magnetic materials such as iron, steel and magnetic alloys and understand their unique properties ideal for specific applications. From ferromagnetic to paramagnetic materials, each type has different magnetic properties that engineers carefully consider when designing electrical machines.

Conductive materials for electrical machines

The relative permeability µr of ferromagnetic material is much greater than 1.0. Once the magnetic force materials (ferromagnetic materials) are exposed to the force field, the dipoles align in the direction of the smeared field and become strongly magnetized.

Furthermore, magnetic force materials can be divided into hard or soft magnetic materials as well as permanent magnetic materials.

Hard or permanent magnetic material has a very large hysteresis loop and a gradually increasing magnetization curve.

B. Tungsten steel, carbon steel, cobalt steel, hard ferrite, Alnico etc.

Factors to Consider When Designing Electrical Machines

When designing electrical machines, several factors must be carefully considered to create an efficient, reliable and cost-effective device that meets specific application requirements. These factors cover several aspects, including performance, materials, environmental impact and safety. The most important factors to consider during the design process include:

Soft magnetic material

It has a small hysteresis loop and a sharp magnetization curve.

Example: i) Wrought iron, cast steel, rolled steel, solid steel etc. (in solid form).

Commonly used for DC machine poles, turbogenerator rotors, etc., anywhere DC flow is involved.

Silicon steel

It is (iron +0.3% to 4.5% silicon) in laminated form. Adding an appropriate proportion of silicon reduces aging and core loss. Low silicon steel or dynamo steel is used in rotating electrical machines and processed with high flux density. High silicon steel (4% to 5% silicon) or transformer steel (or high strength steel) is used in transformers. More steel sheets are also hot-rolled or cold-rolled. Cold rolled grain oriented steel (CRGOS) is expensive and better than hot rolled steel. CRGO steel is mainly used in transformers.

Special league

Nickel-iron alloys have high permeability and the addition of chromium results in a better magnetic material. Nickel with iron in various proportions leads to:

Nickel-based alloy with high nickel content (iron + molybdenum + copper or chromium) used in magnetic amplifiers, power transformers, etc.
Nickel-based alloy with low nickel content (iron + silicon + chromium or manganese) used in induction coils, coils, transformers, etc.
Terminator (iron + nickel + cobalt).
Mumetal (copper + iron)
Pemendur (iron + cobalt + vanadium) is used for oscilloscopes, microphones, etc.

Amorphous alloys

Amorphous alloys are produced by rapidly hardening the alloy at cooling rates of several million degrees Celsius per second. The alloy solidifies with a glass-like atomic structure, which is a non-crystalline frozen liquid. Rapid cooling is achieved by flowing the molten alloy through an opening in a water-cooled rotating drum. This makes it possible to produce panels with a thickness of 10 µm and one meter or more.

Superconductors: a magnetic wonder

Superconductors have the extraordinary ability to conduct electricity without resistance at low temperatures. We explore how these materials have the potential to revolutionize electrical machines, achieving unprecedented efficiency and power density. Although the practical application of superconductors is still in its infancy, we discuss the current state of research and development and provide insights into their prospects in electrical machines.

Material selection for optimal performance

Selecting the right magnetic material for a specific application is a complex process that requires a balance between several factors, including cost, efficiency and environmental impact. We address the considerations engineers must make when selecting magnetic materials, taking into account the intended application, operating conditions and sustainability requirements.

Switching in DC machines

Switching is fundamental to the operation of DC machines, including DC motors and generators. The key mechanism allows the conversion of electrical energy into mechanical energy in motors, driving movement, mechanical work and, conversely, the conversion of mechanical energy into electrical energy in generators. Commutation ensures that current in the armature winding flows unidirectionally, which is essential for maintaining the desired direction of rotation in motors and producing a stable output voltage in generators.

The commutation process involves the interaction between the commutator and the brushes, with the contact pads maintaining electrical contact with the commutator segments while the rotor or armature rotates. This continuous contact allows current to be transferred from one armature coil to another, ensuring constant rotor rotation in motors or stable voltage output in generators. Proper switching is critical to the smooth and efficient operation of DC machines, and challenges such as sparking, arcing and brush wear must be carefully managed to maintain optimal performance and reliability. Advances in switching technology, such as electronic commutation in brushless DC motors, have further improved efficiency and reduced maintenance requirements, making DC machines a versatile choice for a variety of industrial, automotive and consumer applications.