Sinus and Common-Mode Filters

The output voltage of a servo drive is generated using pulse-width modulation (PWM), which rapidly switches between the high and low rails of the DC bus. The example shown here compares the output of 2-level and 3-level PWM with a 700 V DC bus at a PWM frequency of 100 kHz. The target waveform corresponds to a typical spindle motor with 230 μH inductance, rotating at 150,000 rpm under a 5 A load. While 3-level PWM introduces a mid-level voltage to reduce the switching amplitude, both methods produce sharp transitions at the PWM frequency.

The phase voltage is measured between the drive output and earth. In practical operation, this voltage is typically analyzed in two components:

  • Differential-mode (DM) voltage – the useful voltage across the motor windings
  • Common-mode (CM) voltage – the average of all phase voltages, which does not contribute to torque generation
Schematische Darstellung eines Elektromotors

The differential-mode voltage determines the motor current and torque. Common-mode voltage, on the other hand, is a byproduct of PWM switching and typically oscillates across the range of the DC bus voltage at the PWM frequency itself.

The motor’s electrical system acts as a low-pass filter for current, so differential-mode PWM switching results in relatively smooth current and torque. The common-mode voltage has no direct influence on the isolated motor.

Differential-Mode Issues

Although the motor filters the differential-mode voltage, a current ripple remains. This current ripple causes additional stress and heating in the motor windings, and introduces thermal issues on the rotor.

Common-Mode Issues

When machining grounded conductive workpieces, a capacitive path forms between the motor and earth. This allows displacement currents to flow, leading to

  • Electrical arcing at motor bearings or tool contact points, causing damage to both the motor and the workpiece
  • Electrochemical erosion, which gradually wears down mechanical components and tools

Additionally, common-mode voltage is a major source of electromagnetic interference, especially in voice-coil and open motors, leading to system instabilities and failure to meet EMC requirements.

How Triamec Filters Work

Triamec motor-side filters address these issues with a two-stage approach:

  • Sinus filter – suppresses high-frequency spectral components of the differential-mode voltage
  • Common-mode filter – attenuates the common-mode voltage

Triamec filters of the T-series combine these two filter stages in one product. The sinus filter stage elliminates the differential-mode issues. However, the filter’s inductance must match the motor's electrical characteristics, otherwise resonances can occur in the current loop, leading to a significant reduction of the current-controller bandwidth. Triamec's T-series filters are designed for low-inductance motors like spindles.

For other motor types, C-series filters provide an effective solution for common-mode filtering. Thanks to a minimized sinus filter stage, these filters ensure common-mode suppression without limiting the current-controller bandwidth.

Integration Tips

  • Use motor-side filters when machining metallic workpieces or in EMC-critical applications (especially when using voice-coil or open motors)
  • Ensure the PWM frequency of your drive meets the filter’s specifications (typically ≥50 kHz)
  • Install the filter as close as possible to the drive using short motor cables
  • Compare system behavior with and without the filter during tuning and commissioning
  • Qualify the differential- and common-mode voltages during operation using an oscilloscope