Sine filter (Sinusfilter): what it does and when you need one
A sine filter is a low-pass LC filter fitted to the output of a frequency converter (variable-frequency drive) or inverter. It smooths the switched PWM voltage into a near-sinusoidal waveform so the motor or grid sees a clean sine instead of steep voltage pulses.
"Sinusfilter" is German for sine (wave) filter. It sits on the AC output of a frequency converter or inverter and consists of series inductors (chokes) plus capacitors — an LC low-pass stage. The drive switches DC into high-frequency PWM pulses; the filter attenuates the switching frequency and its harmonics so what leaves the terminals is close to a smooth sine wave at the fundamental frequency.
Both go on the motor side of a drive, but they do different jobs. A du/dt filter only limits how fast the voltage rises (the dv/dt of each pulse) to protect motor insulation; the waveform stays pulsed. A sine filter goes further and reconstructs an actual sine wave, removing the pulse shape almost entirely. A sine filter is the stronger, more expensive option; a du/dt filter is a lighter measure.
These act at different points. A line reactor (Netzdrossel) and EMC filter sit on the grid side of the drive: the reactor smooths current draw and cuts line harmonics, the EMC filter suppresses high-frequency interference back into the supply. A sine filter sits on the motor/output side and shapes the output voltage. They solve different problems and are often combined rather than swapped.
Consider one when cable runs between drive and motor are long (steep pulses cause reflections and voltage overshoot at the motor), when you need to protect older or standard motors not rated for inverter duty, when you want to cut motor heating, audible noise and bearing currents, or when a transformer or grid feed downstream needs a clean sine. The right choice depends on cable length, motor type, grid quality and EMC requirements — not on a blanket rule.
In a battery storage system the power conversion system (PCS) switches DC into AC the same way a drive does. An AC filter — a ring-core inductor plus filter capacitors at the PCS output — smooths that switched AC voltage into a clean sine wave so the plant meets grid power-quality requirements. It is the sine-filter principle applied to grid-tied inverters instead of motors.
Match the filter to the drive's switching (carrier) frequency, rated current and output frequency range, and confirm it is rated for continuous duty at your load. Note that a sine filter adds a small voltage drop and losses, and that it is tuned for a fundamental-frequency range — running well outside that range degrades performance. For sizing, use the drive and filter manufacturer's data rather than a generic rule of thumb.