New Frequency Converter with Optimal Performance: Test Bench Drive "Reloaded"

Due to the required high rotating field frequency and the small rotor volume, high-speed motors need a frequency converter that is suitable for their operation. If this is not ensured, problems will inevitably arise. "Unfortunately, this was the case at one of our Asian customers", said Dr. Bernd Möller, Head of Engineering Spindle Technology at GMN. "Since 2011, we have already delivered three prototypes of high-speed motors where the spindles failed to reach their full performance." In the meantime, the customer was content with the reduced performance. However, in 2019, they contacted GMN and SIEB & MEYER – requesting to find a more suitable frequency converter that meets the requirements, all down the line.

A partnership that solves problems

SIEB & MEYER and GMN maintain a long-standing and successful partnership. The common denominator of the two companies is that both specialize in high-speed applications. GMN does not only produce machine spindles but also high-precision ball bearings, freewheels and contact-free seals. SIEB & MEYER develops and manufactures, among other things, frequency converters that represent the perfect supplement to the high-speed spindles by GMN. "Our devices have been deployed in combination with GMN spindles for many years now", explained Rolf Gerhardt, Director Sales Drive Electronics at SIEB & MEYER. "This is particularly beneficial for spindles used for internal grinding, which requires high speeds." Besides many standard applications, the two companies together have implemented a series of special projects: Among them, routing spindles for the high-performance aluminum processing with a continuous power of 120 kW @ 30,000 rpm, which are used in the aerospace sector. A hydrogen pump drive with 120 kW @ 21,000 rpm and a test bench motor with max. 40,000 rpm and with a power of 63 kW S1 also appear on the list of joint projects.

In the present case, GMN supplies its customer with an individually designed test bench drive. The high-speed motor is based on an off-the-shelf spindle for internal grinding but has been adapted to the special requirements. It is a HV-P 120 spindle – an all-round product for grinding, routing and drilling applications with high stiffness and performance requirements. In this special design, an integrated synchronous motor allows the required speeds and a performance of 35 kW S1 @ 60,000 rpm. Due to the 4-pole motor, a maximum rotary field frequency of 2 kHz can be reached. However, there is no standard frequency converter in the market that is able to support  these figures.The customer decided to still try it. The result was that the specified performance data of the grinding spindle could not be exploited.

Too high losses, too low performance

"The deployed frequency converter could not achieve the high frequencies and generated too high losses", explained dr. Möller. "It would be like implementing a diesel engine in a Formula One car – this simply won't work." In a first step, SIEB & MEYER explored which frequency converter from the wide range of the SD2x series might be the most suitable one for this special application. Finally, SD2M was selected. This device is based on the multi-level technology  and in high-speed applications with high performances turns out to be a true all-rounder. Especially in applications such as in machine tools or test benches, the solution shows its advantages. "We think that SD2M is the only device on the market that offers the required performance for such motors", stressed Dr. Möller. "Apart from complicated solutions with additional LC filters that in this power range are twice as expensive and much more unwieldy."

Common tests have proven: SD2M is actually the ideal frequency converter for this test bench application. "We have thoroughly examined the motor performance in combination with our frequency converter", commented Rolf Gerhardt. "We could achieve all required performance data." The innovative three-level technology of the frequency converter SD2M and switching frequencies of up to 16 kHz ensure an excellent current quality, which reduces motor losses and increases overall efficiency. This prevents excessive rotor heating and reduces the load on the bearings as well as insulation stress. Customers can do without expensive LC filters. Last but not least, the device has a very compact design, thus needs less space and significantly reduces the system costs. "Our customer is very satisfied with the solution", concluded Dr. Möller. "Thus, we can record another successful joint project."

High speed for machine tools

Frequency converters for machine tools are a specialty of SIEB & MEYER. "In this area, we are dealing with very special requirements", said Rolf Gerhardt. "Thus, during internal grinding, for example, a sensorless operation of asynchronous and synchronous spindles up to 250,000 rpm must be enabled. The motor heating, however, must be kept very low to avoid damaging the sensitive materials and to ensure high machining quality. " Another important factor is the flexibility of the machine: Simple and quick exchange of the grinding spindles is essential. For this purpose, the frequency converter should allow storing several motor parameter sets. Our customers often demand special functions like high-dynamic active load detection.

Customized frequency converters in this area are developed by SIEB & MEYER based on the frequency converter SD2S that allows for an extraordinary comfortable operation of grinding machines: The system can store up to 64 motor parameter sets. The parameter set selection is either coded via the digital inputs or alternatively determined by different fieldbus systems such as PROFINET or EtherCAT. In addition, the integrated technology functions for gap elimination (tool touches work piece) ensure efficient machine operation. Solutions based on the multi-axis system SD2 for multi-axis applications are also possible. Moreover, customers can use the sensorless safety functions SFM (Safe Frequency Monitor) and SLOF (Safe Limited Output Frequency). Universal motor encoder interfaces and different connection possibilities to a higher-ranking control allow easy adaptation to changing system requirements. Thus, users stay fully flexible – and the required performance is always ensured.

Info box:

Frequency converters for tool spindles

High-speed spindles are the key components of a machine tool. Only when operated in an optimal way, the desired machining accuracy, productivity and service life of the whole machine is guaranteed. The used high-speed motors require a suitable frequency converter due to the necessary high rotating field frequency and the low rotor volume. The frequency converters by SIEB & MEYER provide the necessary rotating field frequency and guarantee a low motor heating thanks to the special control technology. The motor heating has a significant impact on the machining accuracy and the service life of high-speed spindles, especially when it comes to the service life of bearings.

Info box:

Motor spindles by GMN

Motor spindles with high performance, high stiffness and high reliability are in many application fields a key requirement for a cost-effective production. Depending on the manufacturing task it is important to be able to deploy a wide range of different tools, according to their performance. Large tools require high power demands at relatively low speeds. Small tools often do with relatively low power demands. With GMN high-speed spindles, these requirements can be largely covered with only one spindle version. They provide – with regard to their design size – a high torque and thus allow for a high manufacturing performance in the lower speed range. This is reached thanks to the asynchronous and synchronous motors that are especially designed for this application area and an efficient liquid cooling of the spindles. The motors feature a high power density and an excellent efficiency.