You might have heard of maximum energy product, or (BH)max, if you’re interested in magnets.
It’s a crucial property.
It tells us a lot about a magnet’s strength and performance.
In this post, we’ll dive into what maximum energy product is.
We’ll explore why it matters.
And we’ll look at how it impacts real-world applications.
Let’s get started!
What is Maximum Energy Product?
Maximum energy product is denoted as (BH)max.
- It represents the maximum amount of magnetic energy that can be stored in a magnet of a given volume.
- In other words, it’s a measure of a permanent magnet’s ability to do work.
Here are some key points to understand about (BH)max:
It’s defined as the maximum value of the product of the magnetic induction B and magnetic field strength H.
- This is along the demagnetization curve.
Graphically, (BH)max corresponds to the area of the largest rectangle that can fit under the B-H curve.
- This is in the second quadrant.
The SI unit for maximum energy product is kJ/m^3.
- The cgs unit is MGOe.
- 1 MGOe equals 7.958 kJ/m^3.
For rare earth magnets like NdFeB and SmCo, the grade numbers typically represent the (BH)max values.
- For example, N45 and SmCo28 are in MGOe.
Why Maximum Energy Product?
Maximum energy product directly indicates the performance of a magnet.
- A higher (BH)max means a magnet can produce a stronger field for its size.
This has major implications for magnet applications and design.
Magnet Size and Strength
- For a given size, a magnet with higher (BH)max will create a stronger external field.
Alternatively, to achieve a target field strength, the required magnet volume decreases as (BH)max increases.
This allows for more compact, lightweight designs.
Temperature Stability
- High (BH)max magnets maintain their properties better at higher temperatures.
This is especially true for NdFeB and SmCo, compared to lower grade magnets.
This is important for applications in demanding environments.
Cost Effectiveness
High (BH)max magnets may cost more per kg.
- However, they can reduce overall system costs by minimizing the required magnet volume.
- Smaller, lighter assemblies also have lower indirect costs.
Energy Efficiency
In motors and generators, high (BH)max enables better efficiency.
- It does this by providing stronger fields.
- And it reduces losses associated with the magnet volume.
Optimizing Magnet Geometry
To fully utilize a magnet’s (BH)max, its geometry must be optimized for the specific application.
- The length to cross-sectional area ratio is important.
Most practical magnets don’t operate exactly at the (BH)max point.
So careful design is essential.
How (BH)Max is Measured?
To determine a magnet’s (BH)max, we need to measure its B-H curve or hysteresis loop.
This is typically done using specialized equipment:
Vibrating Sample Magnetometer (VSM)
A VSM measures magnetic moment versus applied field to generate the hysteresis loop.
The sample is vibrated.
The induced voltage in pickup coils is proportional to the moment.
SQUID Magnetometer
A SQUID is an extremely sensitive magnetic flux sensor.
Like a VSM, it measures moment versus field.
But it uses a superconducting pickup loop for higher sensitivity.
Permeameter
Permeameters are designed specifically for B-H curve measurements.
- A primary coil magnetizes the sample.
- Secondary coils sense the induced voltage. This voltage relates to the changing flux density.
The B-H data is then processed to find the maximum value of |B·H|.
This maximum value is (BH)max.
It can be visualized as the largest rectangle area under the B-H curve’s second quadrant.
Common High (BH)max Magnet Materials
The most common commercial magnets with high maximum energy products are rare earth magnets.
Specifically, they are NdFeB and SmCo.
Neodymium Iron Boron (NdFeB)
NdFeB has the highest (BH)max of any commercial permanent magnet.
It ranges from about 28-50+ MGOe.
It provides the strongest fields in the most compact package.
Grade numbers like N45 indicate (BH)max values in MGOe.
Samarium Cobalt (SmCo)
SmCo offers (BH)max from around 16-33 MGOe.
This is lower than NdFeB.
But SmCo has better high temperature performance up to 350°C.
It also has excellent corrosion resistance.
Grades like SmCo28 represent (BH)max in MGOe.
Alnico & Ferrite
Alnico and ferrite magnets are also used.
But they have much lower maximum energy products.
Alnico is known for high Curie temperatures.
Ferrite is an economical option for lower strength applications.
Applications of High (BH)max Magnets
The unique properties of high maximum energy product permanent magnets enable and enhance many modern technologies.
Here are some key application areas:
Electric Motors and Generators
In motors, high (BH)max allows for compact, lightweight, and efficient designs.
This is especially important for applications like electric vehicles.
In EVs, NdFeB magnets enable high-performance traction motors.
NdFeB magnets also play a crucial role in EV regenerative braking systems.
- They efficiently convert kinetic energy to electrical energy during braking.
In generators, high (BH)max magnets enable effective conversion of mechanical energy to electricity.
This applies from vehicle alternators to huge wind turbines.
- NdFeB magnets allow for compact, efficient, direct-drive wind turbine generators.
MRI Machines
The strong, stable fields from rare earth permanent magnets are used in compact MRI scanners.
They are used up to about 0.5T.
Higher (BH)max means smaller magnet assemblies for a given field strength.
Magnetic Bearings and Maglev Trains
High (BH)max magnets enable compact, oil-free, low-friction magnetic bearing designs for high-speed applications.
In maglev trains, the strong fields from these magnets provide levitation and propulsion.
Consumer Electronics
Tiny NdFeB magnets are critical in many products.
These include hard drives, speakers, cordless tools and mobile devices.
They enable miniaturization and efficiency.
Industrial Equipment
High (BH)max magnets are used extensively in many types of industrial equipment.
Examples are magnetic separators, pumps, couplings and lifting devices.
These depend on strong magnetic forces.
Closing Thoughts
As you can see, maximum energy product is a fundamental figure of merit.
It is used for evaluating a permanent magnet’s capabilities.
By understanding (BH)max, you’ll have a better grasp of a magnet’s potential.
You can see its potential for size reduction, performance improvement and efficiency optimization in your application.
If you are looking for custom magnets with high (BH)Max, just Osenc a line.