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Super Mistake Caused Super Voltage

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DanSchwartz
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Iron
It's called Surge Impedance
DanSchwartz   10/12/2012 1:42:11 PM
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It's called surge impedance, and it's the exact same thing that burned loads & nearly caused fires in another Sherlock Ohms episode:
Noise Messed With the Automation System.
Here is my explanation, copied & pasted verbatim:

It's called surge impedance Zo, which is defined as √(L/C), where L is the inductance in Henrys, and C is the capacitance, which for a coil is the interwinding stray capacitance.

The back EMF V= Zo (δI/δT), and it will have an oscillation frequency 1/(2π√LC). When you open up contacts in an inductive load, δI/δT goes to ∞: You see this as an arc when you unplug an iron; and also when relay & motor starter contacts are switching off an inductive load.

This is also why contactors have serious current deratings when switching off DC: Once the arc is established and current flows through the ionized channel, there is no zero crossing to extinguish the arc, as occurs with AC,

When dealing with AC, you design using the peak (not RMS) value of the load current when calculating, because you don't know where in the AC cycle the contacts will open.

Note: Those of you who are RF jocks will quickly recognize Z(o) = √(L/C) as the equation for the characteristic impedance of a transmission line: Yes, it's the same thing.

Dan Schwartz,
Editor, The Hearing Blog

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