Thanks Bill and Gunnar – those are good thoughts to help focus my thinking.
I was thinking that it was not really the total inductance that was important, but the change in surge impedance, since dramatically higher surge impedance would cause almost complete reflection of the incoming wave.
But now I realize there is one thing I didn't think about... that reflection phenomenon applies at the junction of two transmission lines. To act like a transmission line, the "ferrite bead" would have to be something on the order of a wavelength or more? (does anyone know what multiple or fraction of a wavelength is required?).
The rise times of interest are typically stated on the order of 0.1 microsecond. The frequency content associated with that rise time would depend on the shape of the rise. The smoothest rise (lowest frequency) possible would be half sinusoid period over 0.1 seconds which would be a full period at 0.2 usec, or a frequency of 5 Mhz. Most likely a lot higher. For a frequency of 5Mhz, the wavelength would be Lambda = c/f = 3E8(**) / 5E6 = 60 meter(!). (** Actually c might be a little lower due to effective relative permittivity and/or permeability greater than 1, giving slightly shorter wavelength). Higher frequency will reduce that but I don't know how much higher the actual frequency content is.
That suggests something like a coil as Bill mentioned might work better to extend the length to the required fraction/multiple of a wavelength, but then the turn-to-turn impedance is a lot lower... independent of presence of any high-mu metal. For air core again I think the wave would propogate turn-to-turn rather than linking all turns at once. But it's something to think about.
Also worthwhile to note (and probably obvious) that there are many variables that affect the propagation of the surge to the motor. An interesting quote from IEEE62-21:
Motor supply cables individually shielded with outer jackets that effectively isolate the shields from the raceway, and the shields bonded at only one end (only at the motor end) to the metallic raceway and to the motor frame and to a low impedance ground or earthing system. (This shield bonding configuration can reduce the surge at the motor by as much as 60% compared to bonding the shields at both ends) [B23].
B23 - [B23] Dick, E. P., Gupta, B. K., Pillai, P., Narang, A., Sharma, D. K., “Practical Calculation of Switching Surges at Motor Terminals,” IEEE Transactions on Energy Conversion, Vol. 3, No. 4, Dec. 1988, pp. 864– 872, with W. G. correspondence.
Of course there are reasons we don't want to leave the supply end cable shield unbonded... would reduce magnetic shielding from the power circuits to surrounding circuits... and could result in local voltage differences in event of power system fault. And system grounding plays a role as well... complicated.
Sorry.. that's a lot of rambling. I think you guys have shown good reasons a short ferrite bead by itself probably wouldn't do much. I think I'd like to study a little more to understand how the surges propagate.
Some questions that are still on my mind:
1 – What is reasonable approach to convert the 0.1 usec rise time to frequency? I guess it will depend on shape of the rise, but perhaps there are some thumb-rules related to typical switching surges?
2 – What length of transmission line (in multiples or fractions of wavelength) is required for a cable etc to adopt transmission-line-like characteristics such as reflection?
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(2B)+(2B)' ?
