What You Need to Know About Magnets to Specify the Right Motor

Just making a motor turn is not enough. It has to deliver enough torque, lifetime, and reliability to serve the application. How do you do that? Choose the right magnet, for starters. Factors like composition, fabrication method, packaging, and more have a direct effect on whether the motor you design into your machine delivers the performance you expect. Start with making an informed choice, which comes back to paying attention to details. "With magnets, you learn the first bit very easily,” says Stan Trout, owner of Spontaneous Materials (Denver, Colorado). “The problem is that there are all these little things that can turn around and bite you later on.” To find out what those issues are and how to remain bite free, read on.

Magnets 101
First, let’s review the basics. From an industrial motor perspective, the most important material characteristics of a magnet are the remanence, the coercivity, the energy product, and the Curie temperature. To magnetize a material, we apply a strong external magnetic field. The magnetization increases as a function of the applied field until it reaches a point referred to as saturation magnetization (Ms).

• The remanence (B_r, in kG) is the amount of residual magnetism remaining once the external field is removed. It is essentially a measure of the magnetic strength of the material. Remanence varies as a function of temperature, shown in the table as α, the reversible temperature coefficient of B_r.
• The intrinsic coercivity (H_ci, in kOe) is the amount of opposing magnetic field that needs to be applied to remove any residual magnetization. This is basically a measure of how effectively a material resists demagnetization. Intrinsic coercivity is a function of temperature, shown in the table as β, the reversible temperature coefficient of H_ci.
• The maximum energy product ((BH)_max, in MGOe) is a measure of the maximum amount of energy a magnet can deliver to a magnetic circuit. (BH)_max is mathematically equivalent to the area of the largest rectangle that can be inscribed under the normal demagnetizing curve.
• The Curie temperature (T_c, in °C) is the temperature above which a ferromagnetic material loses its ferromagnetism and becomes paramagnetic. The magnetic properties of permanent magnets can be significantly reduced at temperatures much lower than the Curie temperature.

The four key families of industrial magnetic materials are ferrite or ceramic, aluminum nickel cobalt (AlNiCo), samarium cobalt (SmCo) and neodymium iron boron (NIB). Their characteristics, including cost, vary widely among materials (see Table 1). As with most things in engineering, it comes down to trade-offs. The key factors to consider include performance, size and weight, and environmental factors like temperature and humidity, all balanced against cost. The variety of materials options gives OEMs a broad selection from which to choose but success depends upon making sure that choice is informed.

Picking performance
Performance is the first item on the list, but it’s nearly impossible to decouple from considerations of size and weight. An axis has to develop enough torque to move the load to the specified position at the desired time. That is driven by a combination of the strength of the magnet material and the volume of material present.

Ferrite is cheap and stable, for example, but its low energy product means that developing high motor torque requires large, heavy magnets. The size bump doesn’t stop with the magnet. Everything has to be larger – rotor, stator, copper windings, housing. The motor as a whole becomes both bigger and more expensive than might be expected from just the magnet alone. That may still be the best choice for a ground-based application, particularly one that can’t justify rare-earth-magnet prices. At the other end of the spectrum are applications like electric vehicles. In these cases, performance and size/weight trump cost. They require magnets with very high energy products in order to be able to generate the greatest amount of torque from the smallest, lightest motor possible.

Temperature
A glance at the table shows that in addition to a high energy product and remanence, NIB has intrinsic coercivity levels second only to those of samarium cobalt. High intrinsic coercivity is important, because it describes how effectively a magnet remains magnetized. The key is that the magnet maintains performance over the temperature range of interest, however. “Look at what you really need, at the working environment of the motor,” says Jinfang Liu, Chief Operating Officer at Electron Energy Corp. (Landisville, Pennsylvania). “Don’t just look at a spreadsheet of measurable properties at room temperature. That doesn’t give you the full picture.” NIB magnets exhibit larger negative reversible temperature coefficient of magnetic flux density – the coefficient that describes how magnetic flux density decreases with increasing temperature as a result, its magnetic flux density drops as the temperature goes up and it does so to a greater extent than any of the competing materials.

Indeed, NIB alone can’t really operate effectively above around 80°C. Adding dysprosium increases the effective operating range but the trade-off is higher cost. Even with dysprosium, NIB magnets are really only effective to around 180° C. Samarium cobalt provides another option.

When the term rare earth gets bandied about, most people think of NIB magnets, but samarium is also a rare-earth element. Samarium cobalt offers the highest room-temperature coercivity of any of the industrial magnetic materials. It is also more thermally stable than NIB, as its reversible temperature coefficient shows. The performance of the two materials reaches a crossing point around 150°C. Beyond that, samarium cobalt is a better choice for applications requiring high energy product in a small package, up to around 300°C.

For ultra-high temperature applications, AlNiCo magnets deliver the best thermal stability. The trade-off is lower energy product and, hence, size and weight.

It’s important to note that depending on the price of raw materials, samarium cobalt may be a better overall solution than neodymium even for lower-temperature applications. “When I was at Molycorp during the bubble and people would call to say that magnets were outrageously expensive, I’d ask them what they were using,” says Trout. “If they told me sintered neodymium magnets, I’d ask if they had thought about doing an equivalent design using samarium cobalt so that depending on raw material prices, so they’d have two options to offer customers. There was usually a very long pause, and then they’d say, ‘Well, no.’ They’d never really thought about that because they’d started down this path. Was it by actual need or by habit?”

It’s easy to make the same mistake with temperature range. Is it really reflective of operating conditions or is it just an arbitrary safe margin? “That choice dictates a lot of other choices that you make,” says Trout. “Sometimes if you can change either the high number or the low number just a little bit, then things open up a little. I don’t think people always grasp that.”

There is another important nuance regarding temperature. It's essential to take into account not just environmental conditions but how running the equipment beyond spec might affect those values. A key performance chart for magnets is the demagnetization curve, which plots magnetization and magnetic induction versus magnetic field for a range of temperatures (see Figure 1). Note the inflection point, or "knee " on each curve. That is the point at which the magnet is effectively demagnetized, and happens at a lower and lower magnetic field strength for increasing temperature.

If a magnet is overheated so that it goes past the knee point, irreversible damage takes place. Motor designers know this and typically add cushion but if there's one constant in the industrial world, it's that an end user will at some point drive their machine beyond its design spec. It's essential to ensure that the motor and the system as a whole have sufficient cushion to avoid damage from the additional heat. “If the design safety margin is not enough, then the operating point will be over the the knee point and damage the magnet,” says Liu. “If you were to really heat up the motor very quickly, you could almost kill it. If you go past the safety margin for just one minute, even if you lower your demand right away, magnet performance is not going to go back.”

Fabrication methods
Magnets can be formed using one of two techniques: bonding or sintering. Bonding involves mixing finely milled material with polymers or resins, compressing it into a shape, then curing it. Bonded magnets are generally isotropic, which means that they can be magnetized in a variety of orientations. The property makes them particularly well-suited to high-pole-count motors. Anisotropic bonded magnets do exist. They offer higher remanence and energy products but are more difficult to manufacture. As a result, their application has been limited.

Sintering involves pressing the milled material into the desired shape under an applied magnetic field, which aligns the particles while they are being compressed. The compacted material is then sintered to densify it. The high degree of alignment introduced during this fabrication process creates a magnet with high maximum energy product. Sintered NIB magnets have a maximum energy product of 52 MGOe compared to just 10 MGOe for the bonded version. The magnetic orientation of sintered magnets is fixed during pressing and sintering. As a result, they are anisotropic and can only be magnetized parallel or antiparallel to the orientation direction. They are not a good fit for high-pole-count motors.

It’s important to note that there is no cost-benefit to choosing bonded over sintered. The decision of which to use should be driven by the desired performance and end-user application.

Protection
The industrial environment presents some of the most punishing conditions around –high humidity, temperature extremes, shock and vibration, and more. NIB magnets are extremely vulnerable to corrosion and must be coated to protect the material and ensure performance. The most common coating material is nickel or nickel-copper-nickel multilayer plating. In the case of very humid environments, epoxy may be a better fit.

It’s important to note that compromising the coating in any way will cause the material to begin to corrode. Once the process begins, magnet performance will degrade, worsening over time. Even a chip flying through the air to the coating can be a problem. Once damage occurs, the magnet cannot be repaired. The best solution is to replace the motor.

It’s also important to note that all of these materials tend to be very brittle and easily damaged. Depending on the particular grade, a magnet may be more or less fragile. It is important to take shock and vibration into account when specifying a motor. Personnel should also wear appropriate protective gear when handling them.

Motors need to be properly specified in order to ensure that they perform as intended. The best way to achieve this is to accurately describe the operating conditions and requirements of your application, then work closely with your vendor to choose the appropriate device. Adding a small amount of headroom to your designs is a good failsafe. “You need to have a cushion,” says Liu. “Motor designers give it a little bit more room but users should have a cushion too, because you never know when you are going to use it up to its limit or even beyond. In order to increase the longevity of a motor, you really want to make sure you don’t use it to its upper limit, at least not constantly.” This will prevent occasional operation beyond spec from causing catastrophic magnet damage. Do your homework, ask informed questions, and you will build a resilient system capable of performing over the long term.

Acknowledgments
Thanks go to John Calico of Moog for additional background discussions.