PWM inverters control motor voltage and frequency by applying fast voltage pulses. Long motor cables behave as transmission lines and can create reflected-wave overvoltage at the motor. Fast common-mode edges also drive bearing and ground current. Output filters reduce these stresses, but a sine filter vs dv/dt filter decision depends on the required waveform improvement and the complete drive-cable-motor system.
What a dv/dt filter does
A dv/dt filter reduces voltage rise rate and peak at the motor terminals. It typically uses series inductance with capacitive and damping elements selected to shape the edge while allowing substantial PWM carrier content to remain. It can be smaller and lower loss than a full sine filter.
This filter is often used to protect motor insulation on longer cables, reduce reflected-wave stress, and lower some common-mode current. It does not necessarily produce a sinusoidal motor voltage or eliminate switching-frequency acoustic and bearing effects.
What a sine filter does
A sine-wave filter is a low-pass network designed to attenuate the PWM carrier so the motor receives a near-sinusoidal voltage and current. It can reduce motor heating, audible switching noise, insulation stress, and cable-related emissions. It may enable operation of motors not designed for direct inverter pulses.
The stronger filtering requires larger inductance and capacitance, controlled damping, and tighter coordination with switching frequency and fundamental output range. Cost, size, voltage drop, reactive current, and filter loss are generally higher.
Cable length and reflected waves
Motor terminal voltage depends on inverter edge time, cable surge impedance, propagation delay, termination, motor impedance, and cable length. A cable may be electrically long at the edge frequency even when the fundamental frequency is low. Use inverter and motor supplier guidance plus measured or modeled terminal voltage.
A dv/dt filter is commonly sufficient when the main goal is limiting rise time and peak. A sine filter is appropriate when the carrier itself must be strongly suppressed or when cable length and motor requirements demand a near-sine waveform.
Common-mode versus differential-mode filtering
Many output filters primarily address phase-to-phase differential voltage. Bearing and earth currents arise from common-mode voltage and parasitic capacitance. A common-mode choke, shield termination, motor grounding, shaft grounding, insulated bearings, or common-mode filter may still be needed.
Do not assume a sine-looking differential waveform means common-mode current is solved. Measure phase-to-earth voltage, shield current, bearing voltage, and protective-earth current where relevant.
Magnetic component design
Filter inductors carry motor fundamental current plus residual switching ripple. They must retain inductance at peak current and temperature, withstand overload and faults, and manage core and winding loss. Three-phase integrated magnetics can reduce size through flux cancellation, but zero-sequence and unbalanced conditions require analysis.
Core material, gap, conductor, insulation, cooling, acoustic behavior, and winding capacitance all matter. PWM common-mode stress can create insulation demands beyond the phase RMS voltage.
Resonance and control interaction
Filter inductance and capacitance create resonances that need damping. The filter can interact with inverter current control, dead-time compensation, sensor bandwidth, motor impedance, and regeneration. Operating below the recommended minimum switching frequency can move the carrier too close to the filter corner.
Selection checklist
- Define inverter bus, switching frequency, modulation, motor, and cable length/type.
- Measure or model motor-terminal peak voltage and rise time.
- Set allowable dv/dt, peak, carrier ripple, bearing current, and acoustic limits.
- Compare filter loss, voltage drop, reactive current, size, and cooling.
- Verify resonance damping and inverter-control stability.
- Test full speed, low speed, regeneration, overload, long cable, and motor disconnect cases.
BaoHui Tech needs inverter and motor waveforms, cable data, current spectrum, switching range, output frequency, filter target, overload, common-mode limits, insulation, cooling, and package to design custom inductors or chokes.
Frequently asked questions
Does a sine filter always eliminate bearing current?
No. Differential filtering may leave common-mode voltage paths. Grounding, cable shield, motor capacitance, and common-mode filtering also matter.
Can a dv/dt filter be used at any switching frequency?
No. Its impedance, damping, loss, and performance are designed for a specified switching and output-frequency range.