A central question that comes up in a magnet trade study is how to decide whether to use a Neodymium magnet or a ferrite magnet for a particular application. A trade study that seems very simple at first glance can quickly become more complex once all the factors are considered. Neodymium vs ferrite magnets?
Designers who want to optimize their product for certain factors may produce similar products that use either type of magnet depending on a certain goal, which may involve many factors such as cost, efficiency, system weight, system size, performance, form factor, aesthetics, lead time and other considerations.
If efficiency, especially efficiency-per-unit volume is the deciding factor, designers often choose Neodymium magnets, which deliver up to 20 times the magnetic field per unit volume compared to ferrite magnets.
If cost is the most important factor, designers often choose ferrite magnets, which deliver 2-3 times the magnetic field per dollar spent, compared to neodymium magnets.
Additional factors play an important part as designers consider other factors as we will see below.
If the Neodymium-vs-ferrite question could be answered so easily, there wouldn’t be too much to talk about. But some surprising lesser-known other factors can affect the decision, so a detailed analysis is often worthwhile.
Here is a short list of some of the lesser-known factors we will cover later:
Let’s look at these and other factors as we dig deeper into the world of ferrite and Neodymium magnets.
There seems to have been a quite a debate in the audio industry about the usage of ferrite magnets vs Neodymium magnets in audio speakers. Excellent audio systems have been –and continue to be made- with both magnet types.
Promotional materials from speaker manufacturers are great at explaining why Neodymium magnets produce such good sound, yet many audiophiles enthusiastically talk about –and continue to love- the sound output of speaker systems made with ferrite magnets.
So… which magnet is the better speaker magnet? The reason so many have debated this is that the decision depends on a number of factors. Is the speaker designed for home? Will it be installed in an automobile? Is the voice coil optimized for the magnet? How well are the other components matched? Are size and weight important factors?
Does the speaker use compression drivers? Compression drivers require a lot of power to drive the sound through an orifice -which might favor Neodymium magnets. But why not design a ferrite compression driver? It can be –and has been- done with both magnets.
Then what about small speakers –like the ones in mobile phones and headphones? These speakers have grown to be a big part of the electroacoustic market. What materials do they use? And why?
Automotive speakers are usually designed to fit in tight automobile spaces. These spaces are often not ideal for the same design that would be placed in a home setting.
The designer might have to make a 100 mm (4”) woofer work in a car, but a home system might have a 200-300 mm (8-12”) woofer. Concert systems are larger.
Then the depth of the speaker might also factor into the equation. A smaller Neodymium magnet can help a designer make the speaker fit into a tighter envelope than a ferrite speaker would allow.
Here are the traditional factors that affect a decision on which magnet to choose for a project:
If a design is dependent on high magnetic field strength, it is best optimized by using a magnet with the strongest magnetic field.
A strong magnetic field will lead to smaller dimensions in other parts of the design. For example, a stronger magnetic field leads to smaller motors, because now the motor can generate more torque in a smaller diameter than can be achieved by a magnet with a weaker magnetic field.
Will the magnet encounter a strong demagnetizing field? Neodymium magnets have the strongest Coercivity at room temperature and even at mildly elevated temperatures. This means they exert a very strong resistance to demagnetization in the presence of an opposing magnetic field at temperatures of up to 230 degrees C.
A smaller motor allows for other component sizes to be minimized. Smaller components are often cheaper than larger components, but this depends on the particular design. A detailed cost/benefit analysis will bring clarity to this decision, but it may require the analysis of a lot of different factors.
A designer may analyze many factors. These factors may point to a peak torque or a sustained power level.
Efficiency and lifecycle costs favor Neodymium magnets –especially for an application with a high duty cycle. Their strong magnetic field reaches across the air gap and interacts with other magnetic components at a greater distance. This creates a stronger electric current with less input from other places.
One of the finer points of magnet design is an understanding of the cost of making small, precision parts. As the part gets smaller, the cost is dominated not by the material, but by the machining required to make the part.
NdFeB is a fairly brittle metal when compared to many other metals, but ferrite is much more brittle and somewhat more difficult to machine. So there is a size/complexity point where a ferrite part actually becomes more expensive to make than an equivalent Neodymium part due to the mechanical processing.
It’s important to note that though the cost-per-kg increases as parts get smaller, the unit cost generally keeps falling. The point is that as the parts get smaller, the total cost gets dominated by the surface area. This is because as the parts get smaller the ratio of surface-area-per-unit-volume rises.
Is this why so many small NdFeB magnets are found in mobile phones?
Ferrite magnets have a higher Curie temperature than Neodymium magnets, so they maintain their magnetization better at higher temperatures. This gives designers greater operating margins at higher temperatures than Neodymium magnets offer.
Even though there are Neodymium magnets with high Coercivity for resistance to temperatures in excess of 200 degrees C, these temperature grades cost more than lower temperature grades. Ferrites can operate all the way up to 300 degrees C and their temperature coefficient actually increases by .27%/degree° C. This means that ferrites actually have stronger Coercivity as the temperature rises.
Ferrite magnets do lose some magnetic field at higher temperatures –losing 0.20%/degree C as the temperature rises.
Neodymium magnets often get a bum rap for their lack of corrosion resistance, but they always come with a corrosion-resistant coating unless a designer specifically orders an NdFeB magnet without it.
The default coating is a nickel-copper-nickel (NiCuNi) coating which offers very good overall corrosion resistance and a clean appearance for most common applications with no other coatings required.
This standard coating is very economical and constitutes a very small cost component. Other coatings are available with a small added cost for applications that demand higher corrosion resistance, so NdFeB magnets can be used in many corrosive environments with no problems.
But now consider the contrast with ferrite magnets which in most cases require no coatings whatsoever. This factor influences quite a few design decisions in favor of ferrite magnets.
Of course, ferrite magnets are much cheaper than Neodymium magnets, especially if considered as a unit of cost-per-volume of magnet. If an application does not require the higher magnetic flux of a Neodymium magnet, why pay for that? Many applications are perfect for ferrite magnets.
Some applications involve a more complicated decision due to the duty cycle- the percentage of time they are working. Let’s look at a few applications that could go either way.
A motor for occasional use has different considerations than a motor for continuous use. An occasional use motor –like a washing machine motor or a vacuum machine motor might see a duty cycle of less than 1%, but an air conditioner motor in a hot climate might run 40-70% of the time.
Refrigerator compressor motors are another example of a motor that runs many hours per day.
Efficiency in a continuous-use motor is much more important than in a motor for occasional use, and stronger magnets make for more higher-efficiency motors.
This is not only a matter of lifecycle cost, but a matter of conserving resources. National Energy administrators evaluate industrial and household uses and make recommendations based on their analysis. It’s one of the reasons you see the tags on major appliances indicating their energy efficiency rating.
If a motor is going to be under continuous use, it is more likely to be made from Neodymium magnets than ferrite magnets.
Many perfectly satisfactory high-tech applications have been designed using either Neodymium or ferrite magnets. What are some examples of successful applications of each?
Neodymium magnets are often preferred in the following applications that are sensitive to space constraints:
If you are designing a part or a system and you are trying to choose the right magnet, these are the basic design tradeoffs to consider.
Magnet size can ripple through the entire system. Industrial drive PMDC (Permanent Magnet Direct Current) motors designed for continuous use typically see a 40-70% reduction in size plus increased efficiency compared to induction motors. Many PMDC motors successfully use ferrite magnets, but some others require the extra performance and efficiency that only Neodymium motors can offer.
Neodymium magnets are typically part of a more-efficient motor with a high duty cycle. In other words, NdFeB magnets are more commonly found in motors designed for continuous use, because the higher efficiency of the motor reduces energy use and lifecycle cost for the life of the motor.
For example, in Asia the rapid growth rate of energy usage has led to strict demand-side management, so even residential air conditioner motors with a relatively low duty cycle use Neodymium magnets to achieve higher efficiency than is seen in many other developed countries. This helps utilities reduce demand at peak times.
As technology continues to move forward, designers are constantly taking a fresh look at how to use both Neodymium and ferrite magnets. Each type of magnet has its own unique characteristics that make for winning designs.
Designers who want to optimize their designs will use analysis software or work with a magnet maker that can run the analysis for them. BJMT performs in-house analysis for many new applications.