An inverter output reactor for a long motor cable must limit current ripple and voltage-edge effects without saturating, overheating, or creating excessive fundamental voltage drop. Cable length, characteristic impedance, motor surge impedance, inverter rise time, switching frequency, current, and grounding determine the actual requirement.
A long cable behaves as a transmission line during a fast PWM edge. Treating it only as additional resistance misses reflected-wave voltage, common-mode current, motor-bearing stress, and high-frequency cable loss.
Why motor-terminal voltage can exceed inverter voltage
A voltage step travels down the cable and reflects when the cable and motor impedances do not match. If the round-trip delay is significant relative to the inverter edge, the reflected wave can raise motor-terminal voltage substantially. Cable type, length, rise time, motor size, and installation geometry influence the result.
Measure at both the inverter and motor terminals with probes and grounding suitable for fast high-voltage waveforms. A waveform captured only at the drive does not show the motor insulation stress.
Define the reactor operating window
Provide continuous RMS current, peak current, overload duration, fundamental frequency range, switching frequency, modulation method, cable length and type, motor rating, ambient temperature, cooling, and maximum permitted voltage drop. Include the desired reduction in current ripple or voltage slew rate.
The output current contains a fundamental component plus PWM ripple. Fundamental current drives copper heating and flux swing, while switching ripple adds core and AC winding loss. Low-speed high-torque operation can be thermally severe because current remains high while motor and cabinet cooling may be reduced.
Control inductance under peak current
State minimum inductance at a defined current, frequency, and temperature. A zero-bias LCR value cannot prove behavior during overload. The core and air gap should produce a controlled inductance roll-off rather than a sudden collapse that increases ripple and semiconductor stress.
Insulation sees repetitive PWM stress
Turn-to-turn and winding-to-core insulation experience repetitive fast edges, not only the fundamental motor voltage. Creepage, clearance, enamel grade, tape, sleeving, terminals, and impregnation should be reviewed for the actual DC bus and dv/dt. Cable and motor common-mode capacitance also create current through bearings, shields, and protective earth.
Reactor, dv/dt filter, and sine-wave filter are different
An output reactor primarily limits current ripple and softens the edge through series impedance. A dv/dt filter adds capacitance and damping to control voltage rise time. A sine-wave filter reconstructs a near-sinusoidal motor voltage. More filtering is not automatically better: added capacitance, resonance, loss, voltage drop, and control interaction must be considered.
Prototype verification checklist
- Inductance at rated and overload current
- Inverter-side and motor-side voltage waveforms
- Current ripple across speed and load
- Winding, core, gap, and terminal temperature
- Common-mode current and motor-bearing current where relevant
- Audible noise, vibration, and terminal integrity
- Dielectric withstand and insulation distances
BaoHui Tech can evaluate custom reactor designs from inverter waveforms, cable data, motor current, insulation requirements, thermal limits, and mechanical constraints.
Frequently asked questions
Is an output reactor the same as a sine-wave filter?
No. A reactor mainly limits current and edge effects; a sine-wave filter uses an LC network to reconstruct the output waveform.
Does more inductance always protect the motor better?
No. It also increases voltage drop, size, loss, and possible control interaction. The target must match the cable and motor problem.
Where should the reactor be installed?
It is commonly located near the inverter, but the correct arrangement depends on filter topology, cable routing, enclosure, and grounding.