Frequency Converters Improve the Climate Footprint

"In international comparison, Germany has a top-ranking position regarding energy prices," explained Torsten Blankenburg, CTO at SIEB & MEYER AG. “This ensures a great motivation to improve the efficiency of industrial processes and simultaneously save costs and CO₂. On top of this comes the CO₂ price that will continue to increase in the future. That means, the two drivers for CO₂ reduction are economical and, of course, environmental considerations.”

The German energy mix still contains a high share of coal-based electricity. Therefore, in 2022 the CO₂ emissions per kWh in Germany were 385 gCO₂/kWh according to calculations by the independent energy think tank Ember. For comparison: the USA ranks slightly below Germany with 367 g/kWh, China is well above with 531 g/kWh. Companies can use this value to quantify and calculate the future CO₂ costs of their processes. In addition, it allows an economic assessment of investments in improving efficiency and the return of investment.

Unique selling point: high PWM frequencies 

With the SD4x frequency converter family for high-speed applications, SIEB & MEYER directly contributes to a higher efficiency in various applications, which saves cash in the end.

"Our frequency converters reduce the motor heating and simultaneously increase the system efficiency by several percentage points," stated Torsten Blankenburg. "A 200 kW application, for example, achieved an efficiency increase of 2 %. With 24/7 operation, that means 39 MWh a year, which is 10,000 € at an electricity tariff of 0.26 € per kWh. In addition, the CO₂ emission were reduced by 15 tons. With the carbon tax of currently 30 €/t, this saves additional 450 €." According to Blankenburg, the two-percent increase of the efficiency were confirmed by test series of different customers.

But what exactly distinguishes the frequency converter technology by SIEB & MEYER from the competition? "On the one hand, our unique selling point are the high PWM frequencies that our SD4x generation can provide," explained Torsten Blankenburg. “Our standard rated frequency is the maximum value of many competitors. That means, our devices are designed for high PWM frequencies and therefore suited to high-speed applications. Users need not compromise regarding the performance of their devices. Furthermore, the maximum frequencies of our SD4S series, for example, is twice as high as that of comparable devices.”

Owing to the high switching frequency of SIEB & MEYER frequency converters, the ripple current relevant for motor heating can be significantly reduced, especially with low-inductive motors. With a frequency of 8 kHz, for example, the output signal deviates greatly from the desired sine wave but with 32 kHz the sine curve is almost perfect. The more perfect the sine curve, the lower is the heating in the rotor.

High output frequencies enable compact motor design

Beside the high PWM frequencies, the SD4x family features correspondingly high output frequencies. These not only enable high motor speeds, but also the use of 4-pole motor designs instead of 2-pole motors. "For a 4-pole design, the motor manufacturer requires twice the output frequency, but he can construct the complete motor much shorter and this way minimize mechanical oscillations during high-speed operation," Torsten Blankenburg described the relations. “Our high output frequencies give motor manufacturers leeway to build compact motors with high power density. These in turn are a prerequisite for high-speed applications with high efficiency such as turbo blowers, which cannot be realized with a 2-pole motor design. With our frequency converters, we are therefore an enabler for taking the step towards higher efficiency in the first place.”

Another technological knack for increasing the efficiency of an application is the use of PWM modulation by means of a multilevel output stage. The latter reduces the eddy-current losses in the rotor and therefore the motor heating. Moreover, only half the voltage must be switched so that the insulation stress is reduced. SIEB & MEYER provides the multilevel technology with the SD4M series – the M in the type designation standing for multilevel. 

Optimized control of IPM motors

Another talent of the SD4x family is the optimized control of IPM motors. IPM motors make significantly higher demands on the frequency converter. For one, the current angle in dependence of the operating point must be optimally applied to the machine to achieve the maximum possible torque at each operating point.In addition, the frequency converter must consider the dependency of the motor inductance on the current and frequency to reach optimal efficiency. Using special control structures, the SD4x frequency converters can accurately determine the described dependencies and optimally control them at the respective operating point, which results in the optimal torque at the lowest possible motor current. This minimizes the losses in the motor and converter, improves the energy balance and reduces CO₂ emissions.

Application 1: turbo compressors and blowers for wastewater treatment

The effects of using a SIEB & MEYER frequency converter on efficiency, CO₂ emissions and costs shows the practical example of a wastewater treatments plant that is ventilated by turbo compressors or turbo blowers. This application is characterized by round-the-clock operation of the compressors or blowers; 365 days a year. Therefore, the plant efficiency is an important factor and keeping the motor heating as low as possible the big challenge. SIEB & MEYER meets these requirements with the SD4M and its multilevel output stage, which increases the efficiency of the turbo compressors or blowers by several percentage points.

Application 2: flywheel energy storage unit for fast charging stations

Another application example for the SD4M series by SIEB & MEYER is fast charging solutions for areas with a weak grid structure. The fast charging station works in combination with a flywheel energy storage unit that provides the energy required for charging the vehicle and stores energy between the charging processes. 

The big challenge of this application is that the rotor of the flywheel works in a high vacuum. With the resulting high insulation effect, the heat can only dissipate as radiant heat. Moreover, this system uses magnetic bearings, i.e. without contact, so that the rotor heat cannot be transferred outward via the bearings. "That means, this system requires the lowest possible heating to work properly," summarized Torsten Blankenburg. "Our frequency converters generate minimal heat and can therefore optimally support such applications and even make them possible in the first place." 

Application 3: air conditioners with water as refrigerant

Nowadays, electronic data processing centers have extremely high expenses for cooling their systems. Therefore, highly efficient air conditioners are in demand, ideally operated with low-risk refrigerants. Using turbo technology in combination with water as refrigerant offers a sustainable solution in this area. The required high-speed synchronous motor is a special challenge, though, since it must come with high power density, low rotor heating and needs a highly efficient drive. In addition, the foil bearings reduce the possible heat dissipation of the rotor to a minimum.

"In this application, we are practically the enabler as well," commented Torsten Blankenburg, "because a compressor with a high power density is necessary for this application and the motor must be driven with suitably high efficiency but low heat development. This is the prerequisite to make such a system work. The solution is our SD2S, the predecessor of SD4S, that we customized for the specific requirements." 

Application 4: ORC and micro gas turbines

In the area of green power generation via ORC (organic Rankine cycle) or micro gas turbines, the SD4M series by SIEB & MEYER provides the required high level of efficiency. For the conversion of biogas as well as waste heat into electricity via the ORC process, the high-speed generators need to be operated efficiently to achieve best possible performance. Both processes are based on a turbo compressor that feeds the turbine with combustion gases or the vaporized refrigerant from the ORC process. The available SD4M variants with DC supply enable the connection with external regenerative power supply units that are optimized for the various grid conditions around the world.

What does the SD4x device family offer?

At present, the SD4x family comprises the series SD4B, SD4S and SD4M. SD4S comes in the sizes B, C and D with a total of 7 device variants for different voltage and power ranges. SD4M is available in 4 sizes with 7 power classes in the range of 83 kVA to 554 kVA. "Currently, we work on extending the power range of the SD4S series," said Torsten Blankenburg regarding the future. "It is our declared goal to replace the complete SD2S series with the SD4S series in 2024."

Conclusion:

With the SD4x family, SIEB & MEYER offers its customers a model kit comprising various drive functions so that the optimal drive is available for every high-speed application field. Whether used for routing or grinding spindles, turbo compressors or blowers, turbines, press and tightening machines, servo positioning or wafer dicing – SIEB & MEYER not only has a solution but the optimal solution. The software as well as different encoder interfaces support a wide variety of applications.