Suppose that we take two coils - one rated for 24 volts DC - and the other rated for 24 volts AC. In the DC coil circuit, the only thing that limits the amount of current flow is the actual resistance of the copper wire itself. (Remember that in our simple examples, the applied voltage (24 volts) will not change.) In other words, you can simply take a resistance reading of the coil (in ohms) and work through the Ohms law formulas to calculate the amount of current (in amps) which will flow through the coil. But in the AC coil circuit, TWO things are available to limit the amount of current flow. First: the actual resistance of the copper wire itself. Second: the Counter EMF (think: backwards voltage) which is developed by the constantly changing AC current. It works like this: As the AC current constantly changes, the magnetic field around the coil is constantly building up and collapsing. And as the field builds up and collapses, it CUTS THROUGH the conductors of the coil. This relative motion (where the magnetic lines of flux actually CUT THROUGH a conductor) satisfies one definition of a GENERATOR. In other words, the coil itself is generating a voltage. Now guess: Do you think that this new generated voltage is going to HELP you push more current through the coil - or is it going to WORK AGAINST you and LIMIT the amount of current in the coil? Before you answer, remember theres no such thing as a free lunch. (Answer: It will OPPOSE the initial voltage - and tend to limit the amount of current flow.) Now here we could start talking about inductance and impedance - but things would quickly get too complicated for this simple discussion. Now since we have only one thing to limit the amount of current through the DC coil, then we need a LOT of wire wound into the coil. On the other hand, the AC coil - since it has two things to limit the amount of current - will be wound with much less wire. Secret handshake: theres nothing magical about the wire in the DC coil - theres just a lot more of it. Now suppose that we mistakenly connect an AC-rated coil to a DC voltage source. The DC current doesnt constantly change - so theres no GENERATOR effect - and so there is no Counter EMF - and so the only thing available to limit the amount of current flow is the resistance of the copper wire itself. The problem is: theres not enough wire in the AC coil - and so it will get too hot and burn out. Now suppose that we mistakenly connect a DC-rated coil to an AC voltage source. The AC current is constantly changing - so here we DO get a GENERATOR effect - and so we DO have a Counter EMF. When we add in the amount of built-in resistance of the copper wire, we wont have enough current flow. So the magnetic field wont be strong enough to pull in the relay armature. The DC coil wont burn out - but the relay wont work. A troubleshooting tip worth remembering: Suppose that were using a properly rated AC coil, but something mechanical (trash, rust, etc.) is physically keeping the relays armature from properly pulling in and seating in place. In this case, the magnetic field can suffer - and not be strong enough to generate that good-old Counter EMF that weve been counting on to limit the current flow through the coil. The coil can get too hot and burn out. Unknowledgeable technicians will often simply replace the coil without correcting the underlying (mechanical) cause of the problem. Result: Another burned out coil.
Posted on: Mon, 03 Feb 2014 16:14:40 +0000
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