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VFD Cable Length & Reflected Wave Calculator electrical
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VFD Cable Length & Reflected Wave Calculator

Compute transmission line critical cable length, peak motor terminal voltage doubling (Vpeak), and dV/dt filter requirements per NEMA MG-1 Part 31.

VFD Inverter & Cable Length

Motor Insulation Standard

Reflected Wave Peak Results

Peak Motor Terminal Voltage (Vpeak)
-- Volts
from -- V DC bus voltage
Critical Cable Length (Lcrit)
-- ft
Onset of voltage doubling
Insulation Stress Status
--
-- V margin
Cable Length to Critical Ratio: -- : 1
Wave One-Way Transit Time: -- ns
Recommended Filter: --
Impedance mismatch between cable and motor winding reflects PWM pulses, doubling voltage and destroying standard motor insulation.

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Frequently Asked Questions

What causes reflected wave voltage doubling in VFD motor cables?

VFD inverters switch DC bus voltage using IGBTs with sub-microsecond rise times (0.1 µs). As this high-frequency square wave travels down the cable, it hits the motor terminals, which have vastly higher surge impedance than the cable (transmission line open-circuit boundary). The wave reflects back, stacking on top of incoming pulses and doubling terminal voltage to over 1,400V.

What is critical cable lead length (Lcrit)?

Critical cable length is the distance Lcrit = (v·tr)/2 at which pulse propagation transit time equals half the IGBT rise time. For typical cables (v ≈ 500 ft/µs, tr ≈ 0.1 µs), Lcrit is only 25 feet! Any cable longer than 25 feet experiences full voltage doubling.

What does NEMA MG-1 Part 31 Inverter Duty specify?

NEMA MG-1 Part 31 certifies that a motor has reinforced phase-to-phase and turn-to-turn insulation capable of withstanding repetitive voltage spikes up to 1,600 Vpeak with rise times down to 0.1 µs without experiencing corona partial discharge degradation.