Permanent magnet machines

Best efficiency and power density

For small to medium sized machines, nothing surpasses the efficiency and power density of the permanent magnet machine, especially when designed to use high energy rare-earth materials. Key to the PM’s superior efficiency is its ‘energy-for-free’ self excitation.

However, this comes with a loss penalty, even when the machine is inactive, which demands careful balancing of its attributes during the design process.

PM machines are highly versatile, and used across all areas of industry, but cost is always a consideration, requiring very careful optimisation to realise a commercially viable cost/performance ratio.

EMX has analysed and designed all the main categories of PM machines and some less familiar types. In particular, our practical experience with very high speed machines gives us a deep understanding of the limitations of permanent magnets which are not immediately obvious to those more familiar with off-the-shelf products.

Reluctance machines

Low cost option

The reluctance machine is a simple, low cost configuration compared with other types of electric machine. In fact, its development has resulted mainly from a quest for simplicity, where the magnetic circuit comprises only iron and air.

Having a mover or rotor without windings or permanent magnets gives an inherent ruggedness and flexibility in size and shape.

Consequently, the reluctance machine has been successfully utilised in a wide range of applications from low-power relays to high-power traction motors. These machines are highly non-linear in behaviour and good design requires a combination of finite-element analysis and a deep understanding of their theoretical basis.

EMX has many years’ experience of designing reluctance machines for a variety of applications and, combined with our skills in designing permanent magnet and induction machines, makes us well placed to recommend the reluctance machine when most appropriate.

Induction machines

Workhorse of industry

The majority of motors in service worldwide are of the 3-phase induction type, earning these machines the ‘workhorse of industry’ reputation. Unlike most other machine types there is no upper or lower size limit, although in the smallest sizes the single-phase induction machine is more appropriate.

Although rugged in construction, they are capable of very smooth, quiet operation with a wider speed range than permanent magnet machines. The induction machine is often the preferred choice for applications with high speed, high temperature or high torque specifications.

When ruggedness and simplicity are desired, the ability to run directly off mains supply without a controller can make the induction machine the only sensible choice. For low cost, variable speed applications, the wide availability of off-the-shelf controllers keeps the induction machine attractive even when compared with reluctance types.

EMX has designed induction machines for automotive and aerospace applications requiring maximum material utilisation to keep volume low, while meeting tough performance criteria. Our design method employs advanced optimisation techniques which, when coupled with finite-element analysis, allows our engineers to identify the key parameters for a successful design.

Electric machines for hybrid EVs

Powertrain to vehicle integration and simulation

Space utilisation is very important in HEVs due to the densely populated engine compartment, and this poses considerable challenges for the designer.

Machines must be able to withstand the harsh underbonnet environment where extreme temperatures, corrosive fluids and high shock loads are commonplace. Additionally, to minimise the powertrain extension, an abnormally thin, large diameter, machine may be required which can lead to conflicting demands. For example, it could serve as starter motor, traction motor and generator, and therefore must be capable of an extremely wide speed range.

For EMX, these challenges require highly sophisticated designs using technologies previously unseen in automotive engineering.

Traction motors for EVs

Efficient power delivery for EV vehicles

Unlike HEVs, which combine conventional powertrains with electric traction, EVs tend to be limited to those types with sufficient space for mass energy storage, usually batteries, and possibly liquid hydrogen in the future. Space is not as constrained with EVs but it’s common to use forced cooling of the traction motor to increase the specific power density. This is important both from the vehicle dynamics perspective and economically, particularly with permanent magnet machines.

One of the most challenging aspects of EV traction motor design is to provide exceptionally efficient power delivery over the full operating range of the vehicle, unlike hybrid machines which usually assist the engine, or are used as the sole traction source over a very limited operating range.

Combined Heat and Power

Generators for CHP applications

CHP is a well known green technology, saving energy by using low grade waste heat to heat water. Applications requiring a lot of hot or warm water are particularly suitable, eg some factories or swimming pools, because the natural output from an engine consists of a small amount of useful work and a large amount of waste heat.

Recovering energy from exhaust gases, or converting work into electricity more efficiently, improves the ratio of work to heat. Small CHP units, particularly in domestic systems, are now reaching the market and have the potential to greatly reduce central heating bills and overall carbon emissions, but these need to have a high work:heat ratio at a low cost. Larger CHP systems have significant potential as well, particularly in colder countries where the demand for hot water is high.

Permanent magnet brushless generators are a popular technology for this kind of application, particularly at low powers or high speeds, and asynchronous induction generators are suitable for the larger applications. For instance, a high speed (100 krpm+) PM alternator, coupled to an automotive turbo-charger, could be used to extract 1 or 2 kW from a typical engine exhaust.

EMX has experience in CHP, and holds a patent which could be applied to such systems. We are looking to continue developing this kind of technology.

Wind energy

Electrical machines for wind energy development
With fossil fuels becoming scarce and prices likely to rise steeply, alternative sources of energy must be taken seriously. If the full life cycle cost of fossil fuels is taken into account, it could be argued that wind power is already cost effective. Many believe that in the near future a high level of public interest, coupled with market pressures, will enable wind energy technology to push forward to a new level. At EMX it is our intention to contribute to this new industry. Our independence means we are not tied to a particular technology or manufacturer and can remain flexible and responsive to new ideas. Our experience of designing and prototyping low-speed, 100kW+ electric machines, coupled with established links to experts in related technical fields, provide the ideal platform for wind energy development.

Other applications

Our skills applied to a wider range of engineering applications
  • Automotive active suspension
  • Industrial ventilation systems
  • Textile manufacturing equipment
  • PCB Machines
  • Sub-sea cable laying
  • Power station cooling systems

Our Technologies

Our patented new cooling technology

EMX has developed a special cooling system for induction machines which advances the art of forced-cooled rotors. This is particularly relevant to EV and HEV applications, where the size of electric machine has to be kept to a minimum, but it could be applied to other industries as well.

Our cooling system employs special conducting tubes which form part of the rotor bars where most of the rotor losses are created. The heat is removed directly at source, keeping rotor temperatures down and minimising thermal expansion of the rotor. This allows a smaller running airgap, which improves the electrical performance of the machine.

The tubes have special features to induce spiral motion of the cooling medium which significantly increases the heat transfer compared with plain tubes. These features also increase the cross-sectional area of the rotor bar compared with the plain tube case, helping to reduce the bar resistance and minimising the electrical consequences of embedded cooling ducts.

Unlike most forced-cooled rotors, the cooling medium in our technology flows within the rotor. The airgap remains dry, avoiding stator winding insulation complications, airgap viscous drag losses and the need for cans which increase airgap size and induce additional losses.