Multilevel technology vs. conventional converters: high rotating field frequencies at low system costs
For motor-driven and generator-driven high-speed applications with high output powers the available standard converter solutions are not sufficient. Especially in the field of renewable energy but also for applications with high-efficient compressed air supply, the demand for converters that enable high rotating field frequencies grows. For this purpose the company SIEB & MEYER based in Lueneburg (Germany) has developed a solution based on three-level technology, which not only reduces the motor losses but also ensures low interfering radiation and insulation stress. Users benefit from low system costs throughout the entire product life cycle.
Nowadays the majority of the frequency converters and servo amplifiers used in drive technology are constructed based on two-level technology. That means, the converters rectify the mains AC voltage into DC voltage (also called DC intermediate circuit voltage) at first, so they can supply an AC voltage with variable frequency and amplitude to the motor. This is done with alternating polarity – plus and minus – on two levels. Many converters use the modulation type PWM (pulse-width modulation) for this purpose. Multilevel converters, on the other hand, use at least one more intermediate voltage level that can be supplied to the motor. Therefore, these converters require a quite different output stage topology. A conventional three-phase two-level converter requires, for example, six electronic power switches (transistors), whereas a three-level converter requires twelve switches.
Application in regenerative systems
Multilevel converters are required, amongst other applications, for innovative systems that were developed with the turnaround in energy policy. The converters enable, for example, a significant increase in the efficiency of 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. With these systems the following rule applies: The higher the speeds, the more efficient the systems work. Up to now converters for output powers > 100 kW and rotating field frequencies > 2,000 Hz were rarely available on the market – let alone solutions also capable of controlling synchronous motors without sensor. That is where the multilevel frequency converter comes into play.
The high-speed (HS) motors installed in these systems make using the innovative converters necessary. HS motors generate power via the speed and not via the torque. As a general rule, the rotor volume changes at the same rate as the reciprocal of the speed increase, that means at 10 times of the speed the rotor volume has decreased to one-tenth. This causes the following 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.
Generating high rotating field frequencies
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 rotor. 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. If a 2-pole design is used, the worsened distribution of the magnetic field and the following unsymmetrical utilization of the magnetic field would increase the rotor volume by 1.5 times. The resulting length of the shaft would not work due to bending-critical frequencies. To operate a motor with 60,000 rpm a rotating field frequency of 2,000 Hz instead of 1,000 Hz is required.
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. In case of HS motors must be considered that the behavior of the effective motor inductance is similar to that of the rotor volume, i.e. it drops when the speed increases. Consequently, the smoothing of the current ripple increases proportionally to the decrease of the motor inductance. The problem is that these high-frequency currents cause additional losses in the motor that are not negligible – around 90 % of the losses caused by the converter are generated in the rotor. These losses lead to increased heat development and bearing load. Since the heat dissipation and the cooling is limited, the losses in the motor/rotor must be reduced sufficiently to ensure safe operation. Limit temperatures of synchronous rotors range between 90 and 150 °C.
Increasing switching frequencies
In the power range > 100 kW the two-level frequency converters available on the market provide maximum admissible switching frequencies of 4 or 6 kHz because an intermediate circuit voltage up to 600 V requires semiconductor switches (IGBTs) with a cutoff voltage of 1,200 V. Increasing the switching frequencies is not productive for technical and economic reasons. The higher switching losses would cause disproportionate heating and a reduction of the ampacity. Based on these facts the maximum possible effective rotating field frequency is between 600 and 800 Hz, as the PWM frequency must be 8..10 times of the rotating field frequency to realize an approximately sinusoidal output current.
A three-level frequency converter makes increasing the switching frequency possible: 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 up to 32 kHz. Ultimately, the share of harmonic current is reduced and the rotor losses caused by the frequency converter are low.
Reducing rotor loads by up to 75 %
Beside the PWM switching frequency, another important variable is the voltage level added to the motor winding with the PWM pattern. The three-level technology cuts the voltage level 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. All in all, three-level converters reduce the losses generated in the rotor by up to 75 % – at the same PWM frequency. Therefore, many applications can work without any motor filters or smoothing chokes between motor and converter. Without these additional components the complete system is not only more light-weight but also the costs and the required space are reduced. Furthermore, users benefit from the optimized overall efficiency (measured between power socket and motor shaft).
Thanks to three-level technology the 'partial discharge problem' feared by many becomes also quite irrelevant. Partial discharge means a gradual destruction of the stator insulation due to voltage peaks at the motor. These are generated by switching edges of the power transistors in modern converters. "Gradual" is a period between seconds and months depending on the load conditions. If the insulation is eventually destroyed completely, the motor is permanently damaged – frequently resulting in a burned stator winding. The effect depends on various factors: One relevant factor is the length of the motor cables (the problem occurs with a cable length of 3 m and longer) but the most important factor is the amplitude of the voltage jumps. Three-level converters use only 50 % of the voltage amplitude at each switching operation reducing that effect even with longer motor cables to a great extent. Therefore, the effect can be neglected for almost all applications.
Details: This is the three-level frequency converter SD2M
The frequency converter SD2M by SIEB & MEYER is based on multilevel technology and allows operating high-speed motors with rotating field frequencies of up to 2000 Hz and motor currents of 424 A at switching frequencies of 16 kHz. The devices reach an efficiency of up to 98 %, which ensures optimal system efficiency and reduces the required cooling measures. Synchronous and asynchronous motors are operated vector-controlled with or without speed sensors, e.g. in turbo compressors or turbo blowers. Applications that require a regenerative system can also benefit from the three-level technology. For this purpose the SD2M variant with a DC voltage supply is combined with power supply units/mains inverters capable of regenerating energy, e.g. for test benches, spindles in machine tools, ORC systems or flywheel energy storage units.