Pushing the Strength of Motor Magnets By Flemming Buus Bendixen and Anette Nørskov Schultz • Sintex a/s, Denmark
Magnetic design is often challenged by improvements in motor design and thus demands on higher density at preferably a smaller size. Utilizing the magnet properties and securing the magnet protection is essential in the magnetic design. This article will identify the strength of motor magnets, what areas might be improved and alternative solutions.
Introduction to Magnet Development
The development of magnets is formed by many factors. A few of the main factors are magnetic strength, strength against demagnetization, corrosion stability, mechanical strength, temperature stability and easy to assemble. Since the first magnet was developed all these factors has been improved considerably. An example is the magnet strength shown in Figure 1.
Today, a range of good magnets with very different abilities exits on the market, all have pros and cons. For example, the very strong sintered NdFeB magnets for medium temperatures and medium mechanical strength, but they have strong restriction to magnetization direction, low corrosion resistance and are hard to handle in a high volume production. The isotropic epoxy bounded NdFeB magnet is another example. It has freedom in magnetizing direction and is more simple to assemble, but has medium strength and low corrosion stability. Finally, the injection moulded NdFeB magnet is easy to assemble, has medium corrosion stability, but also medium to low strength.
The skilled engineer is able to evaluate these factors, select the right magnet for the design, and at the same time deal with the cons to eliminate or minimize the disadvantages.
Motor Applications
One of the highest volume applications for the magnet is the electrical machine. A range of different motor and generator types exist on the market. Motors will be the area of focus in this article, but trends from the motor area can typically also be used in the generator area as well. Examples on motor types are the simple and cheap brush commutated DC-motor, the robust and efficient sinusoidal supplied brushless PM-motor and the fast spinning wet running submersible brushless DC-motor. The demands on motors are for example light weight, high efficiency, economic, powerful, corrosion resistant and high temperature.
The NdFeB magnets have pushed the motor development and design (and vice versa) since they were developed in the early eighties (See Figure 1). This development is continued today with the IP-magnet as magnetic rotor.
Magnetic Rotor, IP-Magnet
The “IP” in IP-magnet stands for Incapsulated Powder magnet and is a new integrated magnet type. It’s unique and innovative and a highly corrosion resistant system in a 100 percent sealed enclosure, so that exposure to chemicals, water and gasses is an option. The system can in other words run as wet runner and is maintenance free.
The uniqueness of the IP-magnet is for one the higher magnetic strength and eco-friendliness, but also the cost savings in production, material, finishing, assembly and tolerance.
Uniqueness of the IP-Magnet
The IP-magnet consists of magnet powder, a sleeve, two caps and possibly a shaft. The magnet powder is compressed directly into the sleeve and two end caps are fixated. An example of an IP-magnet prototype is shown in Figure 2.
Unique for this processing method is that no epoxy is necessary for binding the powder or the complete magnet to the shaft, there is no need for a costly heat treatment process and the manual assembly process is completely avoided. Everything can run fully automatic, which is very valuable for high volume production. Production without epoxy is also more eco-friendly: there is no human contact with epoxy, which is known to provoke allergic reactions.
Important for the design and effectiveness of the magnet system and thus the motor is that with no binder, there is more room for magnet powder, which will increase the strength of the magnet (at the same volume) or decrease the size of the magnet and the motor (when keeping the same strength). Furthermore, the binder is known to corrode the surface of the NdFeB magnet powder, which is why IP-magnets without binder are more stabile. There is simply no chemicals, from the binder or the environment, left to react with.
Mechanically the sleeve strengthens the rotor considerably. This allows higher speed on the rotors, which will also decrease the size of the whole motor.
New design possibilities, because of higher magnetic strength, are further enhanced with the IP-magnets possibility to mix different compound and composite materials such as magnet and iron. In PM-rotors the flux can only pass magnet material and soft magnetic materials. Generally speaking when the flux lines flow in a magnetic material, the flux will increase, and when flowing in soft magnetic material the flux will be maintained. So if the flux has to flow long distances in the rotor (i.e. in two or four poled motors) the use of soft magnetic material as back iron could be important.
The IP-magnet can be made as a compound between iron powder in a small diameter working as back iron and on the outside of this magnet powder. An example on this is shown in Figure 3.
Magnetic Rotor Production with Injection Moulding or Powder Compression
When producing magnets for rotors, alignment of anisotropic magnets in the production process is made by powder compression or injection moulding in a magnetic field. This is traditionally done by an advanced high current coil system. The disadvantages by this method are that the current has to be supplied by a big expensive power supply, the coils has to be cooled by costly equipment and the coils are worn out due to mechanical stress and high electrical fields.
A solution for production with injection moulding or powder compression without large scale equipment or use of electricity is thus interesting.
Alternative Alignment in Halback Array
The Halbach array of magnets is state of the art and has been known since 1980 [1]. A Halbach magnet array consist of a number of individual arc shaped magnets arranged in a hollow cylinder as shown in Figure 4. They were originally developed for focusing accelerator particle beams, due to the high permanent magnetic field strength. The high field strength can however also be used in magnet production to make anisotropic magnets. But production of big Halbach arrays has until now been difficult, for instance due to the great mechanical forces acting between the magnets.
What’s new within this field is that the Halbach array system is now being commercialized through the use of special build assembly tools and highly skilled personnel.
The magnetic field inside the Halbach array has a predetermined number of poles and field strength. The number of poles depends on the direction of magnetization of each segment.
Two or Four Poled Halback Array
A theoretical equation for the magnetic flux density inside a two-pole Halbach array is shown here:
Where Br is the remanence of the magnet, Ro is the outer radius and Ri is the inner radius of the magnet array [2].
It only has a component in one direction inside the array. Like shown in Figure 5 where the field strength inside the bore is about 1.65 T.
In a four pole design the field is nearly sinusoidal along the inner circumference of the bore. This gives the possibility to produce four or more poled anisotropic injection or compression moulded magnets for rotors.
Sintex Magnetic Systems
There is no doubt that the challenges to magnetic design place higher demands on utilizing the magnet properties and securing the magnet protection. The new solutions for motor magnets presented here, the IP-magnet and the Halbach array, does exactly that. They were initially developed by Sintex a/s in order to meet the challenges of customers.
Flemming Buus Bendixen is a magnet expert for Sintex a/s, Denmark. He can be reached at fbb-sintex@grundfos.com.
Anette Nørskov Schultz is the sales and marketing coordinator, for Sintex a/s, Denmark. She can be reached at ans-sintex@grundfos.com.
Sintex a/s develops and manufactures complete systems within the area of magnets. We supply magnetic coupling systems, magnetic rotors, magnetic systems and subcomponents, including components in soft magnetic composite material (SMC materials).
Components or modules are manufactured according to customer specifications and are used within areas such as motors, pumps, electrical devices, sensors, transformers etc.
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Today, a range of good magnets with very different abilities exits on the market, all have pros and cons. For example, the very strong sintered NdFeB magnets for medium temperatures and medium mechanical strength, but they have strong restriction to magnetization direction, low corrosion resistance and are hard to handle in a high volume production. The isotropic epoxy bounded NdFeB magnet is another example. It has freedom in magnetizing direction and is more simple to assemble, but has medium strength and low corrosion stability. Finally, the injection moulded NdFeB magnet is easy to assemble, has medium corrosion stability, but also medium to low strength. 
Uniqueness of the IP-Magnet 
New design possibilities, because of higher magnetic strength, are further enhanced with the IP-magnets possibility to mix different compound and composite materials such as magnet and iron. In PM-rotors the flux can only pass magnet material and soft magnetic materials. Generally speaking when the flux lines flow in a magnetic material, the flux will increase, and when flowing in soft magnetic material the flux will be maintained. So if the flux has to flow long distances in the rotor (i.e. in two or four poled motors) the use of soft magnetic material as back iron could be important. 
A solution for production with injection moulding or powder compression without large scale equipment or use of electricity is thus interesting.
Two or Four Poled Halback Array 