By TEBAOFLEX Jyen. Originally published on December 23, 2025.
A PWM drive sends fast voltage transitions along the motor cable. When a transition reaches an impedance mismatch at the motor, part of the wave can reflect. Cable length, propagation characteristics, drive rise time, motor impedance and pulse timing all influence the resulting terminal voltage. The drive and motor manufacturers’ limits remain the basis for installation decisions.
Separate three problems
- Reflected-wave voltage stress at motor terminals and within insulation.
- Capacitive charging and common-mode current through the cable, screens and grounding paths.
- Bearing currents and electromagnetic interference in the complete drive installation.
A low-capacitance screened cable can help address some of these issues, but it does not automatically eliminate overvoltage or bearing damage. Protective-earth continuity and high-frequency EMC bonding have related but distinct purposes. Follow the drive’s cable, grounding and termination instructions.
Illustrative transmission-line calculation
The following inputs are hypothetical, not measured TEBAOFLEX cable or motor data. Assume a lossless-line approximation, cable surge impedance Z0 = 58 ohms and motor surge impedance Zm = 1,200 ohms. The terminal voltage reflection coefficient is Gamma = (Zm − Z0) / (Zm + Z0) = 1,142 / 1,258 = 0.90779.
For an assumed incident step of 679 V, the initial incident-plus-reflected step is (1 + Gamma) × 679 = 1,295.39 V. This simple result does not include repeated reflections, pulse interactions, losses, filters or the motor’s frequency-dependent impedance. It is not a predicted maximum voltage for every 480 V drive.
A rough critical-length estimate is Lcrit ≈ v × tr / 2, where v is propagation speed and tr is transition rise time in compatible units. With assumed v = 198 m/microsecond and tr = 0.10 microsecond, Lcrit = 9.9 m. With tr = 0.02 microsecond, the same approximation gives 1.98 m.
These lengths indicate when reflected-wave interaction may become significant in the simplified model. They are not universal maximum safe cable lengths or a rule that all longer cables reach exactly twice the DC-bus voltage. ABB’s published guidance describes critical length as an estimate and explains why real waveforms cannot be safely “tuned” using cable length alone.
Capacitance and insulation selection
The relation i = C × dv/dt explains why cable capacitance matters during a voltage transition. For illustration only, a lumped 10 nF capacitance subjected to 5 kV/microsecond would imply 50 A during that idealized transition. A long motor feeder is a distributed network, so this arithmetic is not a measured drive-current prediction.
Request capacitance data with the measurement configuration stated: core-to-core and core-to-screen are not interchangeable. Insulation type, geometry, screen arrangement and frequency affect cable behaviour. Do not infer a fixed percentage improvement from PE, XLPE or PVC labels alone.
Choose the complete installation
- Drive model, supply and DC-bus conditions, switching strategy and manufacturer-approved cable length.
- Motor insulation capability and permitted pulse rise time/peak voltage.
- Cable voltage and thermal ratings, PWM suitability, conductor size, route and environment.
- Shield and protective-conductor arrangement, EMC terminations and equipotential bonding.
- Whether the approved solution requires a dv/dt filter, sine-wave filter or other output treatment.
A higher continuous temperature rating does not automatically permit a smaller conductor: current capacity, voltage drop, fault protection and terminal ratings still apply. Likewise, a 2,000 V marking alone does not qualify every mining cable for every inverter waveform.
Verification and troubleshooting
If a system trips, exhibits bearing damage or shows insulation distress, have qualified personnel review the complete installation. Any high-frequency voltage measurement needs appropriate rated probes, equipment and procedures. Request traceable cable electrical data and confirm filter settings and cable-length limits with the drive supplier rather than relying on an unverified FAT table.
Sources and application documentation
Discuss the application
Send the application duty, installation drawing and required acceptance criteria to TEBAOFLEX. WhatsApp: +86 185 9591 3361. Request the applicable product documentation before ordering.