New Concept for Power Electronics in the Range > 100 kW
For motor-driven and generator-driven high-speed applications with high output powers the available standard converter solutions are not sufficient. Since the demand for such converters increases, especially in the field of renewable energy, the Lueneburg based company SIEB & MEYER currently develops a solution on the basis of the three-level technology. The so-called SD2M ensures low rotor losses, which reduces the bearing load and prevents excessive motor heating.
SIEB & MEYER stands for competence in drive and control technology for over 50 years. The company’s recipe of success includes a continuous research of new technologies – at present the Lueneburg based experts concentrate on the development of an innovative high-speed converter for the power range > 100 kW. The development project is funded by the German Federal Ministry for Economic Affairs and Energy as part of the program ZIM (Zentrales Innovationsprogramm Mittelstand ‒ sponsorship for small and medium-sized companies to increase their innovative strength).
“This is a gap in the market we recognized”, Rolf Gerhardt, director sales drive electronics at SIEB & MEYER, explains. Up to now converters for output powers > 100 kW and rotating field frequencies > 2,000 Hz were not available on the market – let alone solutions also capable of controlling synchronous motors without sensor. Due to the German turnaround in energy policy, today such systems are in demand: They make a significant increase in the efficiency of regenerative systems possible. These are rotating energy storage units (flywheel) and turbomachinery like turbo compressors and other compressors, e.g. for wastewater treatment systems or ORC systems to convert waste energy into electric energy. Using such a converter also offers advantages in other high-speed applications – for example in machine tools with grinding, routing and drilling spindles or in high-speed test stations.
Challenges in Heat Dissipation and Motor Design
The technical conditions are complex: High-speed (HS) motors generate power via the speed and not via the torque. Generally, the following rule applies: The rotor volume changes at the same rate as the reciprocal of the speed increase, i.e. at 10 times of the speed the rotor volume has decreased to one-tenth. This causes a problem – the small rotor volume and the resulting rotor surface can dissipate only a limited amount of heat. The negative effects become especially troublesome when the motor is operated in vacuo or in a gas with low thermal conductivity.
In addition, the power/speed ratio required by the application demands special consideration regarding the motor design. “The permissible circumferential speed is to be considered as well as the bending-critical frequency of the corresponding shaft”, Rolf Gerhardt explains. In practice, a synchronous motor with 100 kW at 60,000 rpm, for example, can reach the required power density only by means of a 4-pole motor design; the required rotating field frequency is 2,000 Hz instead of 1,000 Hz.
Increase of Switching Frequency
In order to generate the required rotating field frequency two-level frequency converters were used up to now. They generate the required output frequency via pulse width modulation (PWM). Depending on the used switching frequency and the inductance of the motor this method causes a current ripple of the motor current, though. Increasing the switching frequency might solve the problem – but technical and economic reasons make this approach with two-level frequency converters nonproductive. “With a three-levelfrequency converter, however, increasing the switching frequency is possible”, says Rolf Gerhardt. “And that exactly is our approach.” With this technology the semiconductor switches must switch only half the intermediate circuit voltage of 300 V. This makes using semiconductors with a cutoff voltage of 600 V possible. These semiconductors come with significantly better switching characteristics, which makes the resulting power losses controllable in spite of switching frequencies of up to 32 kHz. Ultimately, the share of harmonic current is reduced and the rotor losses caused by the frequency converter are low.
Beside the PWM switching frequency, another important variable is the voltage deviation added to the motor winding with the PWM pattern. The three-level technology cuts the voltage deviation in half which in turn reduces the current ripple by half in the first approximation. Subsequently, the rotor heat is once again lowered to a considerable degree.
The firmware of the new SD2M is based on the well-proven series SD2S by SIEB & MEYER. To accommodate multi-level technology, though, the existing control algorithms and models for sensorless operation of synchronous motors required some adapting. Therefore the developers changed the structures and codes of these software parts noticeably. The new hardware interfaces were implemented by the firmware. “Last year we presented the first development stage of the new device technology with a rated power of 160 kW”, says Rolf Gerhardt. “We already work on a variant with 250 kW and the development proceeds quickly.”
Frequency Converter SD2M with Multi-level Technology