How rotor bandages made of CFRP increase the efficiency of electric motors
Efficient high-performance motors for the smallest installation space
Lesezeit: 5 Minute(n)
Datum: 29. September 2021

No e-mobility without rotor bandages

We know from experience: Developers of high-performance motors for electric cars face an enormous challenge. Tomorrow's e-mobility requires efficient motors for higher drive dynamics and longer ranges. But how can the performance of electric motors be increased in what is sometimes the smallest installation space? Higher speeds are a key to success, but mean extreme stress for the components. Without a stable rotor bandage, machine failure would be inevitable. Read here why armor sleeves made of carbon fiber-reinforced plastic (CFRP) are the way to efficient high-performance motors.

No e-mobility without rotor bandages

At very high speeds, extreme centrifugal forces act on the installed parts in permanent-magnet or externally excited synchronous motors. In order to reliably fix the drive magnets to the rotor of the electric motor under these conditions, rotor bandages are used, which are often made of very high-quality steel alloys.

Maximum speeds push metal bandages to their limits

However, if maximum speeds are required, metal bandages reach their limits. Here, extraordinary stability is required, which in our view can only be provided by a fiber composite material. Compared with conventional stainless steel bandages for magnetic fixation, armor sleeves made of CFRP are many times stronger and lighter, so that correspondingly lower centrifugal forces act on the system.

→ In our White Paper "Armor sleeves made of CFRP" you can learn even more about the material properties and the customized production of our rotor bandages.

The issue with the air gap ...

With higher speeds and temperatures, the expansion of the rotor also becomes more of a problem. Increasing the distance between the rotor and stator would be an option, but would quickly reduce the torque of the motor. Bandages made of CFRP, on the other hand, minimize the expansion of the outer rotor diameter due to their high strength and a thermal expansion coefficient close to zero. As a result, the air gap between the rotor and stator can be made much narrower than with other armoring concepts and the performance of the motor can be increased. A very thin wall thickness (even down to 1 mm, depending on the application) of the CFRP bandages also enables a more compact design.

Rotor bandages in comparison: This is how armor sleeves made of CFRP and metal perform at high speeds and temperatures.


One component for two requirements

In short: CFRP reinforcing sleeves

  • make high speeds possible, as they reliably fix the drive magnets to the rotor
  • reduce power losses, as they allow a compact design with only a small air gap between rotor and stator.

Currently available materials for CFRP armor sleeves can be used for temperatures of up to 220 degrees Celsius and thus provide sufficient thermal stability for all applications in electric drives.

All the advantages of rotor sleeves made of CFRP at a glance

  • High stability
    • makes new motor speeds possible
    • ensures that all motor components are securely enclosed
    • prevents rotor expansion under high loads
  • Reduced air gap possible
    • due to stable rotor dimensions even at high speeds and temperatures
    • due to low coefficient of thermal expansion close to zer 
  • Low conductivity
    • low eddy current losses
  • Low weight
    • reduces the moment of inertia
    • enables faster motor acceleration
CFRP sleeves made of high-strength carbon fiber reinforced material

Designed for high performance

CFRP armor sleeves are designed for maximum performance. This means that when our customers want to realize high speeds, when smallest installation space or the reduction of the air gap are their challenges, they benefit from our decades of experience in both material development and power transmission.

Basically, CFRP armor sleeves can be used in:

  • High-power permanent magnet motors
  • Externally excited high-performance magnet motors
  • Electrically driven turbochargers or boosters
  • Turbomolecular pumps
  • and more

Materials expertise that comes from batch carriers ...

Components made of composite materials per se are not a novelty for Schunk. We have been working with this material since the 1970s - but initially in a completely different application context. At that time, the task was to develop complex geometries with extremely high temperature resistance for batch carriers in firing processes. Our engineers were the first to use carbon fiber reinforced carbon (CFC) as a composite material for this purpose. With great success. In the 1980s, CFC was used to produce, among other things, special crucibles for the manufacture of silicon crystals for the semiconductor industry, where we are still the market leader today.

... extending to flywheels for Formula 1 racing cars

In the next step, we developed CFRP sleeves for the pump industry. These components, which were already quite thin-walled and weight-reduced, provided a significant increase in efficiency when used in turbomolecular pumps. From the 2000s, our engineers then focused on lightweight solutions for electric drives. The best example is the CFC flywheel for the KERS system for energy recovery in Formula 1 racing cars. At the time, this was a real novelty in the automotive world. The system had to be extremely lightweight and yet remain dimensionally stable even at high rotational speeds.

Customers appreciate added value

The application field of electric drives was derived from this development process. The focus was never on the exclusive benefit of weight reduction. Rather, we have always linked the topic of lightweight construction with a technologically sophisticated extension of functions. Today, this added value engineering is one of the most important features of our development work, which our customers particularly appreciate.

Our service package

We see ourselves as a development partner who works with customers to turn an initial idea into a suitable solution. This is why customers rely on our service package, which accompanies them from an initial FE analysis of the required bandages, through prototype production, assembly trials and validation, to pricing information for mass production. If you are interested in an initial consultation, please feel free to contact us.