Industrial Engineer · phase 13 of 16
Reflected Wave
Cable-transmission effect that can produce high motor-terminal voltage peaks when PWM pulses reflect on long leads.
The Explain card
- Plain English
- A reflected wave is what happens when a fast PWM pulse travels down a long motor cable and bounces back from the motor. The bounce adds to the next pulse, and the motor terminals can see nearly twice the bus voltage.
- Example
- A 480 volt drive feeds a motor through 120 meters of cable. Peaks of 1300 volts appear at the motor terminals, and the winding insulation fails after a year.
- Why it matters
- Cable length, carrier frequency and IGBT edge speed decide whether it happens, and the motor's insulation rating decides whether it matters. Load reactors, dV/dt filters and inverter duty motors are the standard defences.
- Hook
- The pulse comes back and lands on the one behind it.
Where it sits in the deck
Phase 13: VFD Power Quality, Wiring, and Motor Protection
Harmonics, reflected waves, shielded cable and bearing currents: the side effects a drive inflicts on everything around it.
