Electrical steels for copper rotors

01/10/2014 01
By Nuno Fernando

1st October 2014

What electrical steels are needed for copper rotors? How do you select the appropriate type? What do copper rotor manufacturers need to know when handling electrical steels? Sigrid Jacobs, Product Portfolio Director Electrical Steels at ArcelorMittal, provides some answers.

 

What’s so special about an electrical steel?

An electrical steel is a specialty steel designed to accomplish two main objectives.

Firstly, electrical steel has a flux concentrating effect that improves the torque development of a motor, for a given electricity current. The intensity of this flux concentration effect is defined by the electrical steel’s permeability. So an electrical steel designed to have a high permeability will reduce the electrical current needed to produce the magnetism in the first place.

Secondly, when the core of an induction motor is magnetized and then demagnetized, because the associated internal structure changes absorb energy, a part of the energy is inevitably wasted in an induction motor through hysteresis, eddy currents and excess losses. Electrical steels must minimize these losses.

 

What components of an electrical steel give it these properties?

The key additive element is silicon, which is added to steel in various amounts up to 3.5%. Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents. Depending on specific electrical steel property targets, small quantities of manganese, aluminium and phosphorus can also be added up to 1%. In particular aluminium has a similar resistivity effect to silicon.

 

What types of electrical steel exist?

Electrical steels largely fall into two categories depending on their structure, which can be grain-oriented or non-oriented. Grain-oriented electrical steel has an asymmetrical structure in which the iron crystals are all arranged in the direction in which the steel plate has been rolled. Non-oriented electrical steel has randomly oriented crystals and similar magnetic properties in all directions. Grain-oriented steels are used in transformers and self-inductance coils, whereas non-oriented steels are used in rotating machinery such as motors, generators and alternators.

That’s the general situation, but every machine type will have other specific requirements, depending on the application. These could be thermal considerations for some heat-sensitive applications, or strength levels when a machine is running at very high speeds. At ArcelorMittal we ensure that every machine gets the optimal type of electrical steel.

In the context of making motors with the highest efficiency, the number one choice for a number of years is a synchronous motor with a permanent magnet. In these cases the air gap flux is produced essentially by the permanent magnet, and the key parameter of an electrical steel is its low loss performance.

 

What electrical steels do you specify for copper rotors?

The use of copper rotors is meeting an interesting and growing need. This is because there are increasing reasons why a synchronous motor with a permanent magnet is not necessarily the optimum solution. The strongest permanent magnets are made from alloys of rare earth elements, notably neodymium (Nd), iron (Fe) and boron (B). The rare earth pricing and supply situation is fairly precarious at the moment, with Chinese manufacturers controlling much of the world’s sources of rare earth ores. In addition, processing rare earths produces tons of contaminated waste water, leading to serious environmental concerns.

The asynchronous induction motor with copper rotor technology provides a viable alternative to using permanent magnets – and leads to the possibility to produce equally high efficiency motors, either for automotive traction, or for industrial machines up to and over IE4 efficiency class.  In these cases, it’s the combination of the electrical steel and copper that provides the air gap flux and hence a similar power density to the permanent magnet. This means that the electrical steel needs to have a high permeability, at the machine’s working point. This high permeability still needs to be accompanied by low losses. Thus the specifications for electrical steel for copper rotor technology are quite different than those for permanent magnet technology.

 

What other factors need to be considered?

An important consideration to take into account is the magnetic performance of the steel that is required. This will depend on the power of the induction motor and its efficiency under different operating conditions such as frequency, permissible volume and weight, operating temperature, duty cycle (continuous or stop/start) and whether the cooling is forced/natural or internal/external.

Another factor to consider is the required mechanical performance of the steel, which is important for both the machine production process and operating conditions such as punchability and the ability to withstand large electromechanical forces.

Other specific requirements relate to the machine’s specific use. Factors here include a corrosive operating atmosphere, the ability to withstand the operations involved in repairing the copper windings, and the chosen fixation system (welding, automatic stapling, bars etc.). Answers to these questions will influence whether a coated electrical steel is necessary.

 

What are the benefits of a coated electrical steel?

A coating of an organic material increases electrical resistance between laminations, resists corrosion, and acts as a lubricant during die-cutting. Coatings are generally applied to the higher performance electrical steels – so frequently copper rotors.

 

What else do copper rotor manufacturers need to know about electrical steels?

Two things. Firstly, the higher silicon content increases the wear and tear on the punching tools used to form the steel into laminations prior to their assembly into stacks. Secondly, companies using the higher performance electrical steels need to be aware that these are often coated. At the high melting temperatures of copper, an efficient extraction hood is essential to ensure that these volatile organic materials are safely removed from the working environment. This latter factor is obviously relevant only to copper die-castors and not fabricators.