Myth: VFD Output Cables Don't Need Special Treatment

“A VFD is just a motor starter. Use the same cable you’d use for a DOL motor — it carries the same current.”

This was arguably true for early six-step inverters. For modern IGBT-based PWM drives with switching frequencies of 4–16 kHz and voltage rise times of 50–200 ns, it is dangerously wrong. The cable between a VFD and its motor carries a waveform that looks nothing like a sinusoid, and it creates failure modes that do not exist in conventional motor circuits.

A modern VFD output is a series of DC voltage pulses at the switching frequency (typically 4–8 kHz for LV drives). Each pulse has:

• Rise time: 50–200 ns (modern IGBTs)
• dV/dt: up to 5–8 kV/μs
• Peak voltage: 1.0–1.414 × DC bus voltage (565–800 V for a 400V drive)
• Common-mode voltage: oscillating between +V_DC/2 and −V_DC/2 at switching frequency

This is not an engineering nuance. It is a fundamentally different electrical stress regime on the cable and motor.

1. Bearing Currents

Common-mode voltage on the motor frame couples capacitively through the motor bearings to the shaft. At each switching edge, a displacement current pulse flows through the bearing oil film. Over millions of pulses per second, the bearing surfaces erode in a pattern called “fluting” — microscopic craters that eventually cause bearing failure, typically within 6–24 months.

IEC 60034-17, Section 7 describes this mechanism and recommends: insulated bearings (DE or NDE), shaft grounding brushes, and — critically — low-impedance, symmetrical cable with a continuous shield grounded at both ends to provide a low-impedance path for common-mode current.

2. Voltage Reflections and Insulation Stress

When a voltage pulse with 100 ns rise time travels down a cable, it reflects at the motor terminals (impedance mismatch). The reflected wave adds to the incident wave, producing peak voltage at the motor terminals up to 2× the DC bus voltage. For a 400V drive with 565V DC bus, the motor terminal voltage can spike to 1130V — well above the 600V insulation rating of standard PVC cables.

The critical cable length depends on rise time. For a 100 ns rise time:

Lcritical = v × trise / 2 ≈ 150 × 106 × 100 × 10−9 / 2 ≈ 7.5m

For cables longer than 7.5m (which is most installations), voltage doubling occurs. Above approximately 30m, the full 2× reflection is established. Cable insulation must withstand these repeated impulse voltages for the 20–30 year life of the installation — standard PVC/XLPE insulation systems are not rated for this duty.

3. Capacitive Leakage Current and EMI

The cable’s capacitance to ground (typically 100–250 pF/m for standard cables) creates a leakage current path at each switching edge:

Icap = C × dV/dt

For a 100m cable at 150 pF/m with dV/dt = 5 kV/μs:

Icap = (100 × 150 × 10−12) × (5 × 109) = 75A peak

This 75A peak capacitive current pulse — at every switching edge, 8,000–16,000 times per second — flows through the cable shield (if present) or radiates as EMI (if not). Without a proper shield, this current radiates into parallel signal cables, instrument loops, and communication networks. With a shield grounded at one end only, the shield carries the full current to one ground point, creating ground potential differences.

A properly specified VFD output cable has these characteristics:

• Symmetrical construction: 3 phase conductors + 3 earth conductors (not 3C+E), arranged symmetrically so that capacitive coupling to the shield is balanced across all phases. This reduces common-mode current.
• Low capacitance: <200 pF/m preferred, achieved through XLPE or EPR insulation with appropriate wall thickness. Standard PVC cables typically have 150–300 pF/m; purpose-built VFD cables achieve 80–150 pF/m.
• Continuous shield: Copper braid or tape with ≥85% coverage, terminated at both ends with 360° compression glands — not pigtail connections. The shield provides a low-impedance return path for common-mode current, keeping it out of the building’s earthing system.
• Insulation rated for impulse voltage: The cable must withstand repeated voltage spikes up to 2× DC bus voltage. Some manufacturers specify “inverter duty” or “VFD rated” cables tested per IEC 60502 with additional impulse withstand requirements.