It provides a brief overview of motors used as high-speed motors and generators, their advantages, and several application areas. In addition, it explains the challenges in the development and manufacturing of high-speed motors and generators.
Introduction
Various requirements have led to a continuously increasing demand for high-speed motors. First of all, there is a constant need to improve power density. Since there is an almost linear relationship between motor speed and shaft power, increasing the rated speed is an effective way to improve power density and efficiency. This approach allows higher shaft power without changing the machine size. On the other hand, the same performance can be achieved in a smaller volume. This is particularly important in machine tool applications, where smaller size, reduced weight, and improved dynamic behavior are critical.
Another key factor in this industry is the cycle time required for machine operations. The faster the tool can move and rotate, the quicker it can complete its task. High-speed cutting operations depend on maximum speed to efficiently process individual parts within the shortest possible time.
Another important application area for high-speed motors is the replacement of traditional systems where standard grid-frequency (50/60 Hz) motors are used together with gearboxes to achieve higher speeds. Replacing this gearbox-motor combination with a variable-speed drive system consisting of a high-speed motor and rectifier can significantly improve overall efficiency and reduce maintenance requirements.
The compressor industry is a typical example, where high efficiency, oil-free operation, and zero emissions make high-speed electric motors the most environmentally friendly compressor drive solution. In zero-emission applications, energy storage systems with high-speed flywheels also rely on high-speed generators. This eliminates fossil-fuel-based generators and significantly reduces maintenance costs. Cryogenic applications have undergone a very similar revolution in drive technology: directly driven motor elements replace traditional gearbox systems and standard 50/60 Hz motors. This improves efficiency, greatly reduces required space, and lowers maintenance costs. As a result, energy recovery systems are becoming increasingly attractive from both financial and environmental perspectives. High-speed motor technology therefore contributes to the continued development of green energy applications.
Challenges
The above advantages of high-speed motors can only be achieved through the use of high-quality motor components. Due to very high rotational speeds, centrifugal forces acting on rotating parts (rotors) can become extremely large, pushing materials close to their mechanical stress limits. Failure of motor components may lead to dangerous accidents affecting the environment or, at minimum, damage to the motor spindle.
To prevent this, a challenging development process is required in which multiple physical aspects must be calculated, including electromagnetic, thermal, mechanical stress, and structural dynamics factors. The applied computational methods must be combined with long-term engineering experience in order to extend practical operating limits while maintaining safety as the top priority.
In addition, the interaction between the inverter (converter) and the high-speed motor must be well understood, as the inverter strongly influences heating, noise, losses, and power consumption. In particular, the interaction between different converter systems and high-speed motor components requires very specific knowledge and experience. Therefore, extensive testing is essential for successful applications. This requires close cooperation between power electronics experts and high-speed motor specialists. Furthermore, the infrastructure required to support performance testing is highly complex and often not readily available on the market. In many cases, the associated costs even exceed the entire development cost of a new motor product line.
Inverters
A typical inverter operates based on pulse-width modulation (PWM), where continuous switching of voltage or current is used to control the output waveform. As the demand for higher-speed motors increases, switching frequencies also rise (modern inverters typically use IGBTs).
Although higher switching frequencies improve noise and efficiency, inverters also introduce disadvantages, especially due to fast switching transients, which can be considered a major source of stray losses. Additional time harmonics caused by switching-mode inverters negatively affect air-gap flux distribution. These harmonics generate additional eddy current losses in motor components (especially the rotor), leading to higher temperatures and potentially reduced mechanical performance.
Switching frequency also affects insulation systems. Insulation is subjected to repeated and steep voltage stress caused by pulse wave fronts. With IGBT devices, a voltage rise from 0 to 650 V within less than 0.1 μs can result in a dV/dt of approximately 10,000 V/μs. This has a detrimental effect on motor insulation.
These steep rising and falling pulses lead to non-uniform voltage distribution inside the motor, especially during switching transitions. Without a deep understanding of both the motor insulation system and inverter technology, insulation degradation and subsequent failures may occur. In such cases, partial discharge effects and rotor overheating are well-known failure mechanisms. The latter may lead to unwanted carbon fiber rupture due to thermomechanical stress in resin-based carbon fiber composites (synchronous machines).
Conclusion
Both asynchronous and synchronous high-speed motors offer multiple advantages, such as reduced installation space for higher power output and elimination of unnecessary gearboxes. These advantages apply across various fields and are widely used in industries such as machine tools, compressors, cryogenic systems, and power generation.
The design and production of these asynchronous and synchronous high-speed motors is a challenging task. During development, the use of modern computational methods is as important as extensive experience and engineering expertise to safely extend operational limits.
Not only knowledge of high-speed motors is required, but also a deep understanding of inverter technology, partial discharge phenomena, and so-called stray or additional losses is essential.
