In Episode 10 of the FunWithDrives videocast, Andreas Vonderschmidt discusses the topic of long cables. That’s because, once the cable length exceeds 10 meters, certain phenomena occur that are quite significant.
And here, he’s referring less to phenomena like the brakes failing to release because there’s too much voltage drop across the cable—which is also a problem, but one that most people are already aware of.
Andreas Vonderschmidt:
I would like to focus specifically on the issue of line reflections. The fact is that, starting at 10 meters—and especially at 20 or 30 meters—severe voltage spikes can affect the motor windings if no measures are taken in the control cabinet. Voltage spikes of 3 kV are certainly possible, and even higher depending on the configuration.
The reason is that the motor impedance differs from the line impedance, causing the electromagnetic wave to reflect off the line. And whenever something is reflected, it can build up.
…If you don’t know what I’m talking about, try making waves in the bathtub tonight until it overflows. 🛀
And that’s exactly what happens on the power cable, with the result that motors can actually fail—often due to short circuits between windings.
The solution is to practice power supply hygiene, i.e.,
✅install appropriate chokes: sometimes sufficient—meaning just inductance (L)—or
✅voltage filters: often sufficient—L + damping—or
✅sinusoidal filters: an “all-in-one” solution—L + C.
And: It’s not always immediately obvious that the motor is failing outright. Often, the phenomenon manifests later on in the form of motors having a shorter service life than they would have had if measures had been taken.
As a rule of thumb: For cable lengths of 10 meters or more, consider cable reflections.
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