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issue: April 2009 APPLIANCE Magazine

Motor Technology
Hysteresis Solved


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A hysteresis solver provides a new way of accounting for losses and predicting motor performance changes due to hysteresis effects in magnetized materials.

The new finite-element hysteresis solver for soft magnetic materials from Vector Fields (Aurora, IL, U.S.; www.vectorfields.com) extended its Opera electromagnetic simulator for electrical machinery applications. The Hysteresis Solver provides designers of motors, generators, and other electrical machinery with an accurate means of accounting for losses and predicting performance changes due to hysteresis effects in the specialty electrical steels that are increasingly being used to enhance efficiency.

“The drive toward greater energy efficiency is the overriding force behind this work to produce a finite-element hysteresis solver for soft magnetic materials,” explains Alex Michaelides, the UK-based business development manager of Vector Fields. “The main technical demand is for more-accurate simulation of an electric motor’s iron losses (made up of eddy current and hysteresis losses). Hysteresis losses are notoriously difficult to compute, and in the past engineers have generally used approximations and assumptions of how hysteresis losses might vary over a machine cycle.

He says that the improved control available from electronic convertor or inverter drives also spurred the development of this capability. Motor designs need to be updated to avoid increased losses from both eddy current and hysteresis effects.

“For some categories of product, such as a hysteresis motor used in cooling fans, hysteresis effects are actually used to affect or determine performance. And accurate modeling is highly desirable here—to replace the analytic and equation-based design approaches typically used to approximate real-world performance,” Michaelides explains. “In a few cases, accurate hysteresis modeling is also highly desirable as its effects on performance might be critical—for example, modeling remanent forces in some kind of actuator with safety implications.”

The solver is believed to be unique and posed a real challenge to develop as a practical solution in terms of compute time and accuracy. “Transient solvers for a complex problem such as hysteresis might end up requiring days of execution time to compute a solution,” Michaelides tells APPLIANCE. “A practical approach has to take somewhere from minutes to a few hours at most. The design that Vector Fields came up with employs actual measured BH (magnetic induction, and applied field) characteristics. The data are used to predict the behavior of arbitrary minor hysteresis loops, providing a good approximation of true physical behavior without requiring extensive computation, and additionally making only realistic demands for materials data.”

The solver is targeted at electric motors in general. “In industrial motors there is a great demand to try and achieve ultimate efficiency (to eat away at the last 3 or 4% of energy conversion losses),” Michaelides says. “Even though motors for consumer appliances are generally less efficient and a number of design improvement approaches are being looked at, being able to squeeze, say, another 2% of efficiency out of a design for the expense of a few more hours of design effort is certainly of interest to top-end appliance manufacturers.”

In-House Motor Design

Michaelides tells APPLIANCE magazine that, from his perspective as a supplier of electromagnetic simulation tools, he sees a gradual progression of motor design in the appliance arena—from outsourced supply of motors to in-house motor design. He notices more innovation of motor types that provide much more suitable performance characteristics in terms of quietness, cost, etc. Today, The focus on efficiency is largely satisfied by progress in materials. “Future improvements will rely a lot on design optimization and this will in turn rely heavily on the flexibility and sophistication of design automation tools,” he points out. “This solver is one element of a much larger software tool that is focused on improving the whole motor design process.

Vector Fields says that motor engineers often rely on inaccurate methods, especially when nonsinusoidal excitation is used, and when there are pronounced nonlinear effects involved—such as detent torque. “When engineers make approximations of hysteresis losses, they use formulae that originated as far back as the 1890s and which were developed for electrical machines with sinusoidal excitation,” explains Michaelides. “Although since that time, formulae modifications have been proposed to account for modern pulsed-type motors, etc., many designers are still not confident of their accuracy, citing the representation of material properties through coefficients as one significant problem.

The proposed technique overcomes these issues, as it is based on the measured static hysteresis loop coupled to well-controlled minor loop algorithm prediction.”
The hysteresis solver is part of the general-purpose Opera simulation toolsuite for static and transient electromagnetic design, or in the supplier’s application-specific tool for motor and generator design, Opera Electrical Machines Environment. In addition to motors and generators, actuators and transformers can also be enhanced using the tool.

“Some devices, such as a hysteresis motor, use the hysteresis phenomenon as the operating principle. The absence of an accurate, modern numerical tool for modeling of such devices, has probably limited their popularity,” Michaelides says. “The new hysteresis solver will enable designers of such devices to accurately evaluate the performance of hysteresis motors, and reliably carry out design changes.

“In addition, in actuator applications, hysteresis effects are responsible for detent forces that can adversely affect performance,” he adds. “Here, design changes can now be effectively and reliably modeled and hence implemented with confidence.”

 

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