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Wiring Fundamentals: Inductors

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Inductors

03.03

00:00 - Earlier when we talked about resistance and loads, we discovered that the resistance of a device determines the current that flows through it, in response to an applied voltage.
00:09 This is absolutely true if we're talking about a device that is purely resistive.
00:14 There is another factor particularly relevant to automotive electrics, that determines how the current flows in the device in response to applied voltage.
00:22 This is inductance.
00:24 What inductance actually is and how it works is outside the scope of this course.
00:28 But we will talk about how it affects our electrical system and wiring harness design.
00:33 Looking at our plumbing analogy again, if we think about our main supply pipe, with the tap fully open, we'll have a high flow of water rushing out of the tap.
00:42 Suppose that we now instantaneously snap the tap shut.
00:45 The water flowing in the pipe has physical mass so it has momentum.
00:49 It's moving pretty quickly in one direction and it now has to come to a complete stop in a hurry.
00:55 Because of its momentum, the water can't stop instantly.
00:58 It continues to move even though the tap is closed.
01:01 The water backs up behind the closed tap causing a large pressure spike.
01:05 As this pressure spike is higher than the 60 psi of pressure our main system is supplying, the backed up water behind the tap now sees a pressure difference in the opposite direction away from the tap.
01:18 It flows in response to this pressure difference and as it does, the pressure spike dissipates until the pressure behind the tap falls back to our 60 psi of main supply pressure.
01:28 There is now no longer any pressure difference across the pipe, so there's no longer any flow.
01:33 In reality this all happens very very quickly and it's known as water hammer.
01:37 It is the pressure spike caused by the water hammers on the tap and the pipe.
01:41 Back in the electrical world we observe the same effect when we're applying a voltage to, an therefore flowing a current through a device that is inductive.
01:50 The more inductive a device is, the more momentum the electrons flowing through it have.
01:55 If we then remove the voltage quickly, the electron flow will continue momentarily causing a large voltage spike.
02:02 We need to be aware of the spike as it can have a couple of different effects.
02:06 At the least it can radiate out electrical noise which may interfere with other devices or sensor signals.
02:11 At worst if it is large enough, it can cause permanent damage to the device that was applying the voltage to the inductive load in the first place.
02:20 So why aren't all automotive electrical devices just designed so they aren't inductive? Well inductance is the crucial element in allowing an electrical device to interact with the physical world.
02:30 It converts electrical energy into magnetic energy, and this is fundamental to the operation of solenoids, fuel injectors, and ignition coils.
02:38 In fact almost all electrical loads in a vehicle are inductive to one degree or another.
02:43 Automotive ECU manufacturers are of course aware of this and put protection circuits in place on their ECU outputs to deal with these voltage spikes.
02:50 You still need to be aware that they exist however, and later on when we discuss shielding you'll see why.