Technical Papers

As usual small things have a great effect – like the main spindle of a machine tool. Concerning increase in performance, the main spindle is hardly taken into account. But it's time to rethink: If the main spindle is combined with the appropriate frequency converter double-digit growth values can be reached for cutting aluminum alloy.

Normally, the frequency converter for the main spindle is deemed to be the standard component in machine tools. For this reason the converter uptil now was not considered of particular importance with respect to the performance of the machine. This, however, has significant impact on the performance of the machine in the above mentioned field of application. Appropriate technical equipment allows optimizing the machine performance with significant increase in cutting volume. Since up to 90 per cent of the raw materials are cut when a structured component is manufactured from aluminum alloy materials on a machine tool, it is obvious that there is a request for higher cutting volumes.

Machine manufacturers are faced with numerous challenges with respect to the required rigidity of the machine, high dynamics of the axes, tools for higher cutting speeds and the manufacturing spindle. To increase the cutting volume, priority is set on the manufacturing spindle: The spindle performance and the cutting performance are increased proportionally, which can be realized by increasing the spindle speed and/or the spindle torque. In the past years, users primarily achieved higher spindle performances/cutting volumes by increasing the speed to 30,000 rpm, which resulted in higher cutting speeds up to 4,000 m/min. As further increase in speed is currently not recommendable for reasons of system stability, the spindle torque must be increased. This, however, presents various technical challenges for spindle manufacturers: The first point concerns bearing forces, which increase at higher manufacturing performances. The second point concerns the limited available space in and around the machine, so that the power density must be increased. These problems have for example been realized in the field of high-speed manufacturing spindles by replacing synchronous motors by well-proven asynchronous motors with higher efficiency and thus a better performance-required space ratio.

Reducing Losses…

When talking about power loss, the rotor is an important factor, since magnets react sensitively to high temperatures. Due to the high circumferential velocity, the rotor armoring is another critical aspect. Considering the proportions of the rotor in relation to its speed, it is obvious that the rotor of a high-speed motor (HS motor) is much smaller than that one of a slow-turning motor with the same performance. For this reason, thermal storage and the possibility of heat dissipation via the surface is much smaller. Assuming that both motors have the same efficiency and thus the same power loss, the thermal load for the HS motor is higher, so that thermal limits may easily be exceeded.

Beside losses of the spindle, the frequency converter causes further losses. The reason is that conventional frequency converters as components of CNC drive packages provide currents and voltages, which have no ideal sinus curve. This in turn results in harmonic frequency components and eddy current losses in the motor, which mainly occur in the rotor (about 80 to 90 per cent). For the HS motor, which per se allows lower thermal limits, this means that limits may easier be exceeded.

To reduce the harmonic frequency components and thus losses caused by the frequency converter an attempt is made to work with the highest possible switching frequency of the power semiconductors. This, however, is also limited by the losses caused by the frequency converter. For this reason, motor chokes or LC filters are connected into the motor supply lines, thus allowing the implementation of a system in some fields of application, which can be operated within the admissible thermal limits. High-speed machining processes of aluminum alloys, however, present extreme restrictions with respect to the maximum spindle performance. The reason is that the motor choke or LC filter required for this purpose reduce the motor voltage (UMotor << UFU) significantly, which results in a proportionally decreasing motor performance and finally to a smaller cutting performance. In practice the user can reduce the theoretical rated motor performance of 40 per cent or more

…Increasing Performance

These restrictions have been considered when developing the frequency converter series FC71. The provided solution ensures that maximum machine performance is also available in application fields like high-speed machining of aluminum alloys. As the LC filter is integrated in the frequency converter, it can be controlled during operation, thus allowing the generation of output currents and voltages with ideal sinus curves. Motor losses caused by the frequency converter are reduced to almost zero and additional heating in the motor or rotor is avoided as far as possible. At the same time, the frequency converter provides a motor voltage, which increases the mains supply voltage, for example a motor voltage of 560 VAC at a mains supply voltage of 400 VAC. This allows reaching further increase in performance aside from the specified rated motor power without the need to increase the rated motor current. Although the performance is increased, enlarging the cross section of the motor cables is not necessary, as the rated spindle current is not modified.

The frequency converter has already proven ist effectiveness in practice. A manufacturer of machine tools succeeded in realizing the performance potential theoretically provided by the spindle on a high-performance machining center for aluminum cutting. The current product generation provides a spindle performance of 120 kW at 30,000 rpm during S1 operation – which corresponds to a cutting performance of 10 l/min. During tests on the machine equipped with frequency converters of series FC71, cutting performances of 13 l/min were reached at continuous power of 140 kW. This example shows that greater attention must be paid to losses caused by frequency converters operated with HS motors – and synchronous HS motors in particular. The special, application-oriented technology of the FC71 also allows realization of maximum performance rates in the upper speed range. Furthermore, compared to other conventional frequency converter solutions this technology also provides potential for increase in performance of 40 per cent and more.