Today we are going to talk about normally open contacts, normally closed contacts, relays, and contactors. And whether we are talking about this push button, this relay, or this contactor, the principles are all the same. We're going to look at all of them to help us understand how these contacts work and just like when you were a kid, sometimes the best ways to learn is by taking things apart.
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But to start with the push buttons have a clear plastic housing so we can look at them without taking them apart. The top contact with the white plastic is a normally open contact and the bottom contact with the black plastic is a normally closed contact. As we look at the side of it there are two stationary conductors on each contact that lead to a set of screws that we would wire to other devices to complete our circuit. Then there is one movable conductor on each contact that is used to switch the circuit. As you can see our normally open movable contact is not touching the stationary contacts when the button is pressed but our normally closed contact is. As we press the push button the normally open movable contact touches the stationary contacts and our normally closed movable contact moves away from the stationary contact breaking the circuit.
Let's break down the words normally open and normally closed. Normally is the state that the contact is in when something else is not affecting it. If it is a relay then it is not energized. If it is a switch, then it is off. If it is a high limit such as a temperature alarm then the current temperature is below the limit.
Normally open - Is a contact that does not flow current in its normal state. Energizing it and switching it on will close the contact, causing it to allow current flow.
Normally closed - Is a contact that flows current in its normal state. Energizing it and switching it on will open the contact, causing it to not allow current flow.
Don't over think these two concepts. That is all there is to it. Also a very important note for those continuing on to our PLC lessons. These two symbols don't mean normally open and normally closed in ladder logic. Right now we are learning about wiring. We will address what these symbols mean later.
Now let's look at our relays and contactors to make sure we understand normally open and normally closed contacts and understand how they work with relays. Let's take the cover off of our cube relay. As you can see the movable contact pivots on this end and is sandwiched between the normally open contact and the normally closed contact. There is a spring on the back that pulls it towards the normally closed contact then there is a coil of wire wrapped around a piece of iron that when voltage is applied to, creates a magnetic field that pulls the movable contact towards the normally open contact.
Let's make a quick crude electric magnet to help you understand how this works using a screw drivers, some wire, a 9 volt battery, and some staples. As you can see the screw driver can't pick up the staples. Take some wire and wrap it around your screw driver. You could also use a nail, drill bit, or any other metallic object. The tie wraps are not necessary but I find they help hold the wire tight. Now connect it to a battery. Let's punch out a few staples here. And as you can see now the screw driver is magnetize and can pick up the staples. The same principle allows the movable contact to switch to the normally open contact when you apply power to the coil.
Now let's take apart our contactor. The principle is the same so this should be review at this point. Once we take the cover off of the contactor we can see the stationary contacts connected to the screw and the movable contacts above it. Can you tell whether these are normally open or normally closed contacts? If you said normally open then you are correct. Now let's remove the contacts completely. First we see a spring. This keeps the contacts disengaged while the contactor is not energized. Then we see the steel that will be attracted to the coil. Looking at the base we can pull the coil out and you can see two fine wires going into the plastic film. If we remove the plastic film you can see hundreds or possible thousands of wire wraps that make up the coil.
Check out the rest of our normally open, normally closed, contactor, and relay lesson at the following link where we continue our discussion of what single pole single throw, double pole double throw, 2 way, three way, and many other terms used in the devices me.
6 авг 2016
