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Cable reflection

A fast PWM edge on a long motor cable. Past the critical length the motor sees nearly twice the DC link.

Lesson: Cables, bearings and protection around the drive
Try it: one PWM edge on a motor cable

About 400 ns for an older device, 50 to 100 ns for a modern IGBT.

Beyond the critical length: the reflection nearly doubles the pulse at the motor. Fit a filter or an inverter-duty motor.

Critical length L_c
7.5 m
Peak at the motor
1,096 V
Overvoltage
1.94 × V_dc

L_c = t_r v_p / 2 , v_p ≈ 1.5 × 10⁸ m/s ; V_dc ≈ 565 V (400 V supply)