Frequency Converters: One for all?

One for all – this definitely does not apply to frequency converters. According to Wikipedia, a frequency converter is "an electronic or electromechanical device that converts alternating current (AC) of one frequency to alternating current of another frequency". Which applications actually require such a process, though, and how specialized do the converters have to be for this purpose? "At the bottom of the pyramid are the standard 50 Hz converters, which are suitable for the speed control of standard motors used, for example, in conveyor belts or electric gates," explained Torsten Blankenburg, CTO at SIEB & MEYER AG. "They provide a simple closed-loop control of the speed at low costs." Beside making the processes more flexible, the technology in this area focuses on increasing the efficiency and saving energy of systems that formerly ran without open-loop control. The main issue for manufacturers in this area is the global pricing pressure.

Standard, mobile and stationary

Mobile applications require technically more complex frequency converters – these applications are, for example, drives for vehicles but also blowers for fuel cell drives. Aside from good efficiency, the mechanical properties of such converter designs are in the foreground. The most relevant properties include the dimensions, the weight and the robustness against challenging environmental conditions. In addition, the devices are subject to certified development and production processes and they must comply with specific product standards.

Converters for high-speed applications

"At SIEB & MEYER we focus on high-speed converters for stationary applications," said Torsten Blankenburg. "They are designed to operate motors with speeds up to 480,000 rpm." These special motors must be operated with low motor losses because they are more thermally sensitive compared with standard motors. In addition, it is important to ensure high system efficiency or high process quality. Although all high-speed converters basically have the same objective, the devices differ noticeably in details.

Converters with controlled DC link and pulse amplitude modulation (PAM), like FC2 by SIEB & MEYER, can operate asynchronous spindles with speeds up to 480,000 rpm. They are required, for example, in PCB drilling machines. FC2 makes parallel operation of several spindles with one converter possible, which saves costs compared to converters for only one spindle. The drawback is, however, that all spindles are operated at the same speed. Other features of these converters are the low voltage load on the motor insulation and easy parameterization by means of V/f characteristic curve.

Two-level converters for machine tools and the like

Another device group are two-level PWM (pulse-width modulation) converters as stand-alone designs. SIEB & MEYER covers this group with the SD2S and SD4S converters. The basic construction of these devices is similar to standard 50 Hz converters: they provide passive rectification of the mains supply and come with fix DC link voltage or B6 IGBT output stage. For the operation of high-speed motors, though, the converters require faster switching power semiconductors in the output stage – this makes PWM frequencies up to 32 kHz possible. In addition, more accurate and faster current measurements as well as more powerful processor solutions are necessary. "In this application area, performance is more important than the price," Torsten Blankenburg pointed out. "With their control technology, our devices can drive asynchronous and synchronous motors with or without sensors in a speed range up to 240,000 rpm."

These motors operate, for example, machine tools with one or two processing spindles used for grinding and routing. Keeping the spindle heating as low as possible is especially important because it has a significant impact on the machining quality and the service life of the bearings.

In addition, the converters are used in turbo compressors/blowers with powers up to 30 kW. They usually operate sensorless synchronous motors with speeds up to ca. 180,000 rpm. With these high-speed motors, gears are not necessary, which leads to higher system efficiency. In this application area, high efficiency and low motor losses via low motor heating are also important factors.

Compact converters for multi-axis applications

The SD2 variants from the SIEB & MEYER portfolio are two-level PWM converters for multi-axis applications. These devices differ from the above-mentioned converters because several output stages are powered by a central power supply unit. This way, the construction volume is reduced as well as the wiring work in applications with many processing spindles. Another aspect is the increased energy efficiency, since the processing cycles of multi-axis applications are often asynchronous. Via the shared DC link, the braking energy of one spindle can be used to accelerate another spindle. The converters are employed, for example, in rotary transfer machines used in the clock and watch industry or in multi-axis internal grinding machines.

The third level: for special applications

Three-level converters are the latest state-of-the-art converters. SIEB & MEYER provides the SD2M or SD4M devices with this technology. "Three-level output stage technology differs significantly in terms of costs and construction from two-level converters because twelve power semiconductors instead of six are necessary," explained Torsten Blankenburg. "This additional expense is only worthwhile in special applications." Then, the advantages are obvious: compared to two-level systems, the PWM losses are only 25 % with the same switching frequency. This reduces the motor losses significantly – expensive motor chokes or sine-wave filters are not necessary. Thanks to the 50 % smaller PWM switching amplitudes, the insulation stress of the motor winding is also reduced.

"Our three-level converters SD2M and SD4M can show their full potentials, for example, in the field of turbo compressors and turbo blowers," said Torsten Blankenburg. "On the one hand, these systems benefit from the low motor heating combined with the high system efficiency. With the 24/7 full load operation, the latter causes noticeable energy savings and short return on investment times. On the other hand, the overall system can be produced at lower costs and with a more compact construction volume because motor chokes and sine-wave filters are not necessary."

External current supply: low-volt converters

Last but not least: low-volt converters like SD2B plus by SIEB & MEYER are usually supplied by a controlled, external power supply unit with a DC voltage of 24..80 V. The reason: especially the processing spindles in the performance range up to a few hundred watts have an insulation resistance that does not permit the operation with two-level PWM converters at mains voltage level. Thanks to the low DC supply voltage, the converter components can have a low insulation voltage, which results in a smaller construction volume with the same output currents. SD2B plus is well-proven in the field of low-volt spindles used, for example, in dental CAD/CAM routing machines or fine machining in the watch industry.

The examples show that there is not only one solution in the field of frequency converters. "Over many years of intensive development work, we have created a portfolio that can fulfill the requirements of almost all applications in the high-speed range," concluded Torsten Blankenburg. "Of course, we are happy to advise our customers, which converter is the right one for the individual application." And when the required device must be a special solution, SIEB & MEYER can help as well: the company also specializes in customized solutions that consider individual requirements of the application.