VFD Reflected-Wave Cable Length Limit
The reflected-wave overvoltage on a long VFD-to-motor cable.
Example
You enter
- Drive rise time (us) 0.1
- Cable velocity (% of light) 50
- System voltage V_LL (V) 480
- Cable run length (ft) 100
You get
- Critical length 24.6
- Peak terminal voltage 1358
- Limit invduty v 1488
Details, formula, and sources
L_crit = rise_time x cable_velocity / 2, and past it the peak terminal voltage doubles to 2 x the DC bus. Checked against the NEMA MG-1 Part 31 inverter-duty limit (3.1 x V_LL) and a general-purpose motor's ~1000 V. The limit is set by the drive rise time, not the horsepower.
cable_velocity = 0.01 x velocity_pct x 984; L_crit = rise_time x cable_velocity / 2; V_bus = sqrt(2) x V_LL; V_peak = 2 V_bus (run > L_crit); limits 3.1 x V_LL (inverter-duty) and ~1000 V (general-purpose).
VFD reflected-wave cable-length limit (NEMA MG-1 Part 31; transmission-line reflection), by name.
The transmission-line reflection relation is public physics; NEMA MG-1 Part 31 defines the inverter-duty withstand. The drive and motor data govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: rise_time_us, velocity_pct, system_voltage_v, run_length_ft, l_crit_ft, v_peak_v, limit_invduty_v
- Critical length L_crit = rise_time x cable_velocity / 2; past it the reflection fully develops (V_peak = 2 V_bus)transmission-line reflection
- Inverter-duty withstand NEMA MG-1 Part 31 ~3.1 x V_LL; general-purpose motors far lower (~1000 V)NEMA MG-1 Part 31