Table of contents:
- Problem recognized, problem eliminated
- Picture gallery
- Improve the quality of motor currents
- What the use of LC filters brings
- Book tip
- Three-level technologies and switching frequency increase
- Three-level technology cuts losses by 75%
- A new generation of devices on the advance
Video: Prevent Engine Warming From The Start
Approx. 90% of all losses caused by the converter occur in the rotor and can generate harmful heating for the motor,”explains Torsten Blankenburg, Chief Technology Officer at Sieb & Meyer AG. "In addition, the type-specific low rotor volume of a high-speed motor creates additional temperature problems."
Problem recognized, problem eliminated
The control procedures of the SD2x frequency inverters from Sieb & Meyer lead to a low proportion of harmonic frequencies in the motor current. The losses are up to 90% lower than those of competitor products; the heating of the motor is reduced accordingly. Further advantages of the lower motor temperatures are a longer service life of the ball bearings and the positive effects on the machining quality.
Improve the quality of motor currents
But what exactly are the backgrounds and how can the quality of the motor currents be improved? "You have to know that all currents that deviate from the ideal sinusoidal form generate losses in the motor," says Blankenburg. "This motor current component is generated by the converter and presents itself as a so-called ripple current, which superimposes the sinusoidal motor current." Motor inductance.
Small inductors lead to large ripple currents, which is particularly unfavorable for high-speed synchronous motors, as these have very small inductances due to their physical nature. The resulting rotor heating can have extreme effects on the rotor stability, the permanent magnets and the bearings. The problems occur above all at high rated engine speeds.
Avoid harmonics - where's the sinus shape please?
What the use of LC filters brings
To avoid this, standard filters with two-level pulse width modulation (PWM) and low switching frequency often use LC filters. These solutions, which are individually composed of passive electronic components, make it possible either to defuse only the switching edges of the pulse pattern output by the converter (du / dt filter) or even to achieve approximately sinusoidal motor voltages and currents. If you use LC filters, you have to expect additional costs, additional space and weight, as well as losses in efficiency. It is also important to design LC filters in advance for the respective application - this takes time and flexibility.
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Three-level technologies and switching frequency increase
Another solution is to increase the switching frequency for the PWM. If it is doubled, the ripple current is usually reduced by half. Technically and economically, however, this has its limits:
- Firstly, fast switching power transistors in the higher voltage range are more expensive.
- However, the switching losses in the output stage also increase extremely, which has a very unfavorable effect on the efficiency and thus also on the cooling effort.
- In addition, not all motors respond positively to an increase in switching frequency. Due to the construction, it may happen that an increase in the switching frequency brings very little improvement in engine losses. This is mainly the case when it comes to synchronous motors in which there is no segmentation of the permanent magnets.
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Three-level technology cuts losses by 75%
Alternatively, it is possible to use the three-level technology on which the frequency converter SD2M is based, for example. Due to the technology used, the power semiconductors in the output stages are only subjected to half the voltage that occurs with two-level technology. It is therefore possible to work with power semiconductors that are designed for much lower voltages and therefore (due to technology) switch even faster. The result: less switching losses occur in the output stage and the switching frequency can be increased significantly.
At the same time, the motor is only loaded with 50% of the voltage jumps compared to two-level technology. The use of three-level technology alone can reduce the losses in the rotor by approx. 75%. If you now use both three-level technology and switching frequency increase, the losses in the rotor can be reduced by up to 90%. LC filters can then often be completely omitted.
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A new generation of devices on the advance
So far, so good - but it gets even better: Sieb & Meyer recently introduced the new development platform SD4x, on the basis of which the company will now develop a future-oriented device series. The devices support new interfaces and offer a range of additional functions. Users will benefit from higher speeds and significantly improved performance. Thanks to an integrated position controller, SD4x devices can now also carry out independent, highly precise positioning; Speed and current controllers remain constant compared to the SD2x series.
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Optimal design of efficient drive system
"Our goal with these devices is to drive high-speed motors dynamically and with even less power dissipation, " says Blankenburg. "That is why we now also support PWM switching frequencies of 24 and 32 kHz." For an even finer modulation of the sinusoidal signal, a commutation angle control is now also integrated for 32, 48 and 64 kHz. This results in an almost optimal sine, there are almost no harmonic currents. The power loss caused by PWM can be reduced to a fraction.
"The optimized performance, higher speeds and low engine warming without a sine filter - these are the main advantages of the SD4x product family", summarizes Blankenburg. "The improvements will make it possible to improve production quality in existing applications and also open up completely new areas of application."
Supplementary information on SD2M: Inverters with three-level PWM
The SD2M frequency inverter with three-level technology has had a great success story in the few years since its introduction: Sieb & Meyer, for example, has won numerous new customers for series production in the field of high-speed turbomachinery - turboblowers, turbo compressors and turbogenerators. The frequency converter SD2M is still the first choice for applications in machine tools or test benches, where it convinces with maximum performance in a small footprint, low system costs and high efficiency. The innovative three-level technology of the frequency converter SD2M is designed for output powers up to 432 kVA and rotating field frequencies up to 2,000 Hz.
As a basis for customer-specific developments, Sieb & Meyer recently extended the SD2M in terms of performance. It is now possible to implement individual customer solutions with motor currents of up to 650 A - optionally based on air or liquid cooling.