Drive controllers and filters minimise power losses

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Fabian Fischer of KEB Automation takes a brief look at motor filters and the minimising of power losses in typical applications.

Drive controllers and filters minimise power losses New drive controllers and motor filters for high-frequency (HF) applications are helping designers develop highly efficient machine concepts for turbo blowers and CNC machine tool spindles. The objective is to increase performance and protect the motor windings at output frequencies up to 2,000Hz. As an example, for industrial high-speed applications, KEB has developed particularly powerful drive controllers, which its customers are using to operate all common high-frequency spindles and high-speed motor types. The associated motor filter technology is an important part of the drive package. HF motor chokes, input/output filters and sinusoidal filters based on the latest core materials all ensure a long operating life of CNC main spindles and high-speed compressors.

The two main features are based on the winding protection and the stabilisation of the thermal balance of the HF motors. By using a specially designed HF motor choke, both the voltage level in the winding and the rate of voltage rise [dU/dt] can be reduced to below 500V/us. Both are main factors that affect the lifetime of the insulation. In addition, there is a noticeable reduction in the high-frequency spectrum and therefore an improvement in the power loss balance in the motor. For very thermally sensitive systems such as air bearing spindles or the electromagnetic bearing of the rotor in blower motors, both the current and the phase voltage must be reduced to the torque-forming fundamental frequency. This requires the use of a sine filter. By using a motor filter, the physical properties and therefore the equivalent circuit diagram of the motor is significantly changed due to the filter’s own inductance. In order to adjust the components in the system in such a way that control dynamics, torque and shaft power on the motor side are not significantly reduced, one must start at two points: through filter development, production and EMC test environment, and also through additional control routines in the drive controllers.

Resonance-free operation

For correct operation of the sine filter, the output circuit can be checked for the natural frequency and the filter-internal components can be dimensioned for resonance-free operation. For large series, it is possible to tune an application-specific sine filter to the operating point of the customer’s machine. Here, the power loss balance can be optimised in a way that would not be possible with any currently known modulation method and none of the common power stage topologies on the inverter side. In addition, the motor / sinusoidal filter output network can be adjusted in such a way that the power loss balance is improved by increasing the voltage at the motor terminals; respectively an increase in power at the motor shaft is achieved.

In our case, the COMBIVERT F6 and S6 drive controllers from KEB feature software notch filters that generate resonance-free modulation for all speed-controlled operating modes, whether with or without an encoder.

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