Overcoming Design Challenges in High Performance Motors

Smaller Motors, Bigger Design Challenges

Permanent magnet (PM) motors for EV applications have unique design challenges. As electric motors become smaller and faster, designers must consider the potential effects of electrical drive frequencies, magnetic eddy currents, laminated steel core loss, mechanical stresses on the quickly rotating shaft (rotor dynamics), and electrical control of the motor through the inverter.

Electrical and magnetic phenomena that create excess heat within the motor during operation include:

• “Skin effect” in the copper windings, which increases at higher switching frequencies
• Hysteresis and eddy current losses occurring within the laminations
• Rotor eddy current losses, which are partly dependent on the material used for the retention band on the permanent magnet-containing rotor

To compensate for the increase in heat, it is best to design with materials that help reduce the amount of heat generated and utilizing those that perform well under high temperature conditions. Two such materials are samarium cobalt magnets and thin, higher resistivity silicon-iron (Si-Fe).

Performance Materials Enable Energy Efficiency

Samarium Cobalt Magnets

Highly dense, sintered samarium cobalt magnets strongly resist demagnetization, allowing them to perform exceptionally well even at high temperatures. Higher temperature also has a much smaller effect on flux output when compared with NdFeB. Consider the reversible temperature coefficient of induction of 0.035%/C° for SmCo versus 0.10 %/C° for neodymium. This smaller flux loss results in much flatter operating characteristics across a wide temperature range. And machine performance can be “optimized rather than compromised.”

Arnold’s RECOMA® 33E and 35E SmCo materials allow for the greatest power density at elevated temperatures experienced in high power density equipment when compared to other commercial permanent magnets.

Thin Film Lamination Steel

One of the primary methods to reduce the severity of eddy currents induced in the lamination stack assembly is to use thinner lamination steel. The thinner material limits the eddy currents to much smaller conduction paths as opposed to solid stator cores. By constructing the length from many thinner, electrically insulated laminations, the eddy current paths are greatly restricted. Your high speed motor or generator can therefore achieve a higher level of performance through improved efficiency, all in a smaller package.

Another way to reduce eddy currents is by using a high resistivity steel. The motor designer might assume that increasing R would only increase the eddy current as it is defined as I²R losses. However, the amount of current (I) that flows is reduced, creating less eddy current and lower I²R losses. This is accomplished by using a silicon steel with approximately 3.2 wt% silicon.

Arnold’s Arnon® non grain oriented electrical steel (NGOES), for example, has shown to be particularly advantageous for motors and generators, especially those operating above 400 Hz switching frequency. In these designs, the thinner material reduces the efficiency-limiting effects of increased eddy currents and subsequent heating. Arnon laminations have proven to be the industry leader, exhibiting both low hysteresis and reduced eddy current losses as compared with other commercial Si-Fe products.

With precision thin materials reducing eddy currents and high performance magnets improving temperature capability and stability, designers are able to push their high-speed motor designs to the limit.

Download the High Performance Machine Design Considerations White Paper here.