Permanent Magnet Generators Offer Improved Performance for RE Production

Permanent Magnet Generators Offer Improved Performance for RE Production

Permanent magnets have been used to provide the magnetic field in small to medium sized generators for many years. The use of these in large renewable energy (RE)generators may offer performance advantages for future utility-scale generators.

Large (MW sized) permanent magnet generators (PMGs) are mainly found in the wind turbine sector today, but are not confined to this area. PMGs offer a number of advantages to the generator field and are being used in applications from standby plant to hydro generators. The PMG is finding particular application in renewable energy systems, where reduced size and higher efficiency give an advantage. The ability to find application in both low speed and high speed generators is accelerating the use of this type of equipment.

Radial flux PMG

A PMG can be a direct current machine with a rotary collector and brushes or an alternating current synchronous multiphase machine where the rotor and the stator magnetic fields are rotating at a similar speed. This removes the excitation losses in the rotor, which generally amount to between 20 and 30% of the collective generator losses. The reduced losses additionally give a lower temperature rise, which essentially means that a simpler and smaller cooling system can also be used in the generator.

In a PMG the magnetic flux is provided by permanent magnets as opposed to conventional generators where the field is provided by field windings or field coils. This places some restrictions as the magnetic field cannot generally be varied as it is in other machines, but this is overcome by using the generator in combination with external equipment. The main impact of this is that the voltage generated will vary with the speed of the generator, which poses challenges in the case of variable speed applications such as wind turbines and hydropower plant. This has been overcome by the use of hybrid excitation systems, and mechanical variation of the airgap.

Axial flux PMG

Construction

The permanent magnets are generally contained in the rotor. PMG can be of three types: Radial flux, axial flux and transverse flux.

Radial flux machines

In the radial flux machine the magnetic field is at right angles to the axis of rotation , in the radial direction , and the stator coils are aligned with the axis of rotation. Generation occurs when the rotor magnets move the field around the stator coils. this is the conventional configuration used for all types of generator. Fig. 1 shows a typical radial flux PMG. Radial flux PMGs are characterised by a small number of poles and operate at high rotational speeds.

Axial flux machines

In an axial flux machine the magnetic field is aligned with the axis of rotation, and flows in acroos the airgap in an axial direction. The stator coils are arranged in a radial pattern at right angles to the axis of rotation. A typical axial flux machine is shown in Fig. 2.

Low speed generators require a large number of poles and hence a large diameter rotor and stator. For these applications the axial flux configuration has proved to be more suitable. The axial flux configuration also finds application in vertical axis generators, such as vertical axis wind turbines and hydropower plant.

Transverse flux PMGs

The transverse flux principle combines elements of both radial and axial flux machines. In the transverse flux machine, the magnets are located on the rotor , and the flux flows in a transverse direction around the stator yoke. The stator coil is located inside the stator yoke . The TFPM configuration allows multiple layers of rotor magnet and stator coil.

Radial flux and axial flux machines are the most common PMG types.

Permanent magnet materials for PMGs

The development of permanent magnet materials has progressed quickly during the past few decades. First, the materials were based on cobalt-tungsten-chromium iron alloys. Aluminium-nickel-cobalt alloys were discovered in the 1930s, but the development of samarium-cobalt in the 1960s, and finally neodymium-iron-boron based magnets (NdFeB magnets) in the late 1970s, made it possible for generators to benefit from permanent magnet materials. Modern permanent magnets fall into four families: Alnico, ceramics or hard ferrites, Samarium cobolt (SmCo) and neodymium-iron-boron(NdFeB) magnets. The properties of the PM which determine its suitability for usage are listed below.

Br, residual induction or magnetic strength is the value of the flux density remaining when the external field returns from the high value of saturation magnetisation to zero. The remanence is also called the residual magnetization. The higher the value, the higher the useable magnetic flux density. The units are the tesla [T] for SI units and the gauss [G] for CGS units and the remanence is expressed as Br.

Hci (HcJ) : Intrinsic coercivity resistance to demagnetisation. This means the value of the external magnetic field that brings to zero the magnetization or magnetic flux density of a magnetic body when that external magnetic field is caused to operate in the opposite direction from the orientation of the magnetization of the magnetic body. This means the stability of magnetization in the face of an external magnetic field or the capacity to resist an external magnetic field. Material with high coercive force is hard magnetic material and material with low coercive force is soft magnetic material. This is the most important characteristic for a permanent magnet material.
BH, energy product or energy density: A measure of the energy in the magnet . This is an index expressing the performance of a permanent magnet. The units are the joule per meter cubed [J/m3] for SI units and the maximum energy product is expressed as (BH)max. The product of H (taken as positive) and B at the point (H, B) on the B-H demagnetisation curve is called the energy product and the maximum value of this product is the maximum energy product. This value shows a yardstick for the maximum amount of magnetic flux taken out from the magnet per unit volume. When a magnetic circuit is designed to come to the maximum energy product point, the magnet volume can be minimised..

Curie temperature: The temperature at which a ferromagnetic body or ferrimagnetic body shifts to the paramagnetic state due to temperature rise. The magnetic moments lined up in the magnetic body are constantly vibrating due to heat fluctuation. As the temperature rises, the heat fluctuation becomes stronger and proportional to this, the magnetic moment alignment becomes disrupted. Finally, this order is completely lost. The temperature at which this happens is the Curie temperature. In order to show strong magnetism at normal temperatures, this Curie temperature must be adequately higher than room temperature. Curie temperatures higher than room temperature are attained in the transition elements of iron, cobalt, and nickel and in alloys of these metals and rare earth elements.
The four families of permanent magnet

Ceramic or hard ferrites: Sintered hard ferrite or “ceramic” magnets are made from a combination of either barium or strontium ferrite and iron oxide, and exhibit a high degree of coercive strength, making them more resistant to demagnetisation. Ferrite magnets are the only type of magnets that become substantially more resistant to demagnetisation as temperature increases.

Alnico: Alnico is an alloy made mainly from a combination of aluminium, nickel, cobalt and iron plus varying levels of copper, titanium and niobium. The constituent materials are not rare and therefore the price of alnico products tends to be low.

Samarium cobolt: Among all the rare earth permanent magnets, samarium cobalt has the highest Curie temperature. The main components are samarium, cobolt and iron, with addition of small quantities of other metals such as copper, aluminium and zinc. SMCo magnets have a high resistance to corrosion

Neodymium iron boron: This consists of an alloy of neodymium, iron and boron, with the addition of small quantities of other rare earth metals. It has the highest energy product of all commercially used magnetic materials, but has a lower curie temperature than other materials, and has low resistance to corrosion.

NdFeB is the industry standard in PMGs for wind turbines because it has the highest energy product (up to 477.5kJ/m3), so less volume of NdFeB is required to provide a specified flux density than other permanent magnet materials. The weight of the generator is an important design consideration in wind turbines, so decreasing the weight of permanent magnets is a design goal. NdFeB does have limitations on its operating temperature because of its Curie temperature of 312° C, which effectively means that its highest practical operating temperature is about 170°C. Other permanent materials such as SmCo offer a higher operating temperature and coercivity. NdFeB is preferred for PMG wind turbine application due to its higher maximum energy product, the high cost of Co and the relatively low price of Nd with respect to Sm.

Demagnetisation risk

Permanent magnet materials are stable within certain physical limits. NdFeB magnets must, however, be protected against demagnetization and corrosion. The principal threats to NdFeB magnets are corrosion and high temperature tolerance. At temperatures below the Curie temperature (310 to 400°C for NdFeB depending on the grade), possible demagnetisation is caused by a demagnetising magnetic field strength.

In many PM generator designs the operating point can travel to negative flux density values during short circuits. If the demagnetizing field strength absolute values are significantly lower than the coercive field no permanent polarization loss will occur. The coersivity of NdFeB magnets can be increased by alloying the material with other suitable rare earth metals. The process of permanent magnet demagnetisation is both temperature and time dependent. The time dependent polarization loss has found to be linear on a logarithmic scale. Hence, it is possible to predict the long term polarization loss of permanent magnets in constant demagnetizing field conditions by measuring losses during a short period in elevated temperature and extrapolating the results for longer times.

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