Let's take a look at our Motor Protection Overload relay and this one is made by Siemens and it's designed to be paired with a contactor which is basically an industrial relay. In this case it'll be three phase. It's got the three connections and the idea of this is that you clip it onto the contactor and the output of the contactor where you could just run wires from it. You can actually put this in instead and it provides this layer of over current protection.

The best way to describe this is to show you a quick doodle. Let me show you a quick doodle. I shall Zoom down this So here is your incoming I'll just focus on this. Here's your incoming three-phase Supply and it goes through the contactor.

The contactor is basically a electromagnetic switch of the coil under the three contacts. Then it goes through this device which is shown in pink here and this is the overload really. And it has a heater in series of each of the connections and then it goes out to the motor. It has a normally closed contact that these two connections here and as a normally open contact, that's these two connections here.

And normally you'd use the normally closed contact as part of the control circuit because this when it trips, does not actually break the circuit. it just breaks a control circuit. So say, for instance, you've got a 24 volt control circuit going through a switch that turns that motor on and brings in the contactor. It will go through the switch, then it will go through the overload relay contact and then it will go to the coil and then the return path of the control circuit.

And what that means is if this is turned on, the motor is running and the motor stalls for some reason then when this heats up, it will trip this out and uh, that will break the circuit to the contactor and the contact will drop out and kill power to the motor. It's not uncommon to find overloaded machines and workshops. You get a call out to appear. All with the US can have to do is push the reset button, but it's worth finding out what went wrong in the process.

now. I've shown a 24 volt control circuit here that could be 24 volt, 120 volt. it could be 240, it could be 415. I just drew 24 volts.

It just was logical. The coil usually goes to the sort of like the and if you're using a Mains voltage circuit it usually goes to the neutral to avoid accidentally energizing the coil. If there's a fault in an external control circuit, right? let's um, that's a fault to ground an external control circuit. Let's take a closer look at this now.

So if I bring in the meter and I said to continuity I shall Focus back up onto this level so we'll turn this on. It's continuity and you can see there is continuity through from the phase-in from the contactor out to the motor and even when the strips and I can force a trip like this, this is now tripped. Even I've done that, there is still continuity through. Because this does not break the circuit to the motor directly, it does it via the contactor.

So if I reset this, it will show the normal course. Contacts are normally closed and they normally open contacts down here are open. If it trips, the normally closed contacts have now gone open and the normally open contacts are now closed and they can do one of several things. The normally close contacts, they could do nothing or they could just uh light uh indicator light on the front of the panel to show that something's tripped.

Or they could send a signal to a PLC that tells it that the motor stripped and the PLC if it's got loads of Motors in line doing other things associated with that, it can actually shut everything down in controlled manner or it might bring an auxiliary circuit if you could. say for instance, a critical water supply system that is two pumps. then when one pump fails and this trips out, it will then, uh, trigger the next part. the system come in does show a warning light that it is now off in the backup running on the backup pump.

Now there is a little indicator here. let's Zoom down a bit. There's an indicator here that says one and zero and that shows that this is currently in its own. State There's also a test switch and if I press that test switch over, this will click to zero like that and it has actually broken the circuit to the normally closed contacts and effectively turned the motor off.

The way to reset that, if you have one of these situations is make sure the power is off and then press this in and that resets it. This is something that say for instance, an example or one of the last ones I Took a look at it was a lathe and the guy had been cutting a particularly awkward material in the laser that had been very stiff and resonance and it had been giving the lathe a significant sort of resistance to The Cutting and he said the machine just died. It just basically a tripped thermally on this because it had been the motion had been pulling a lot more current. You can adjust the current in the case of this one.

it's got a range of 0.8 Hold On Let's uh, read that it's 0.63 up to one, so say for instance. that way we did it with Hussman with compressors was that uh, when we did the commission, we'd have this set at roughly the currently expected for the compressor, but once we had commissioned it and let it run for a while, we'd clamp the compressor and then supposing it was drawing about say 0.8 amps per phase, we'd nudge it. just a little bit higher than that to allow for wear and tear and actually sort of getting getting older and just drawing a bit more current. But we'd also take a pen a Sharpie more accurately and we'd uh in the drawer of the panel, the door of the panel, we'd write the current uh of the compressor at new.

so the service department the future uh, had a reference that they could check the compressor current. If it was really ridiculously high compared to normal, they'd know that something was wrong. So let's open this and see what we can find inside. What I'm expecting a bimetallic strip is what I'm expecting I've never owned one of these before.

This is good. It's an education. I Don't think they're serviceable as such. This comes off it has that's purely the trip mechanism here.

Does that still work then? Or is that needed? Uh, hold on. How does it trip? No. I think it is needed I Think that is needed? Interesting is that the trip mechanism? Yes, it is. There is a little trip mechanism in there, so this is just an optional uh set of contacts and the little test system.

Okay, what else can we do here? This Loops is kind of flopped over. there. Is this going to come off? Am I Going to destroy This? Probably. Does it matter? Not really.

These days you tend to use variable frequency drives for many things and they have built-in over current, programmable over current protection. So I'm not sure this is how this is supposed to come off, but I'm going to do it anyway. Oh little pins there. This looks as a meat Rock up if this, uh, ticks ages.

I shall pause as I often do just to avoid, uh, fatigue of waiting for exciting things to happen. This front cover is kind of like come off Now the button's not coming off though. It's trapped. Uh, is that going to release anything? Uh, not really.

right. What's the next bit we should take apart? This looks as though it unclips here. Oh, that looks maybe just be the bottom there? Not really. sure.

Um. I kind of the bit. Everyone to see is in here. Now there is a little sticker here.

I'm going to guess that's for calibration that there's going to be a little screw under there. There is a little screw under there for calibration. Don't mess with that. Um, oh, this has now all come off excellent.

What about if I go like this? Nothing. Nothing. Um I get the feeling that uh, it's gonna be something down here, isn't it? Oh, that comes off load scrunchy noises. There might be a hidden screw or pin under here.

I'm not really sure. let's try it. Nope, that's a good start as well. All right, and now this looks as though there might be a little ledge here you can skip to the Uh.

You can skip forward if you want to see all the Gubbins without all the hassle of watching this. Oh, there's the heaters. I'm done. Is this going to come off? This is about to go boring, isn't it? There's going to be a spring involved that's come off.

There are the heating elements. so the current flows through these and what does it heat up to? Actually, there is. These are the bimetallic strips that it's flowing through. Can you see those? Uh, so that as the current flow is in Fiat These pins here.

just plug onto those so it flows in and then it comes out these auxiliary contacts after it's gone through those. Well, let's see if we can get one of them out. Hold on. Let me see if we can detach one of those completely by removing its connection.

and there's some screws here that probably hold that in total. Destruction I Wonder what these Caution: Yeah, probably quite a lot. It's a Siemens component so it will be quite expensive probably. I Bought it on eBay I Just saw the first cheap one I could find and thought, well, you know let's take that apart.

So now is this going to come out? Let me grab my long nose pliers here and we'll try and hike this out that is not working out. Hmm. what is holding that in place? Is it latched? Is it this thing here? We'll have to push this back I wonder if it's there's something else in here? Maybe something sat down to actually lock those in place? It does look like it foreign. Do I have to take all these screws out? Well, let's go for a total Destruction here.

I Guess Oh no. I'm not sure that is the chip mechanism I'm pretty sure that's done. That may have been the trip mechanism. it might be out narrower.

Okay, well, this looks promising I should just hinge this up and then get destructive if it doesn't come out in a hurry because uh, we kind of want to know without actually just taking hours taking this apart. Why is that not coming out? Long nose pliers, give it a good shove that is not coming out there. Why not? How frustrating? is there another bit of plastic trapping this down? I don't know if there is. Maybe I have to take everything out.

Hold on. I'll take more screws out that always fixes things, doesn't it? Usually it's uh, everything then just explosively falls apart and it never works again. As I said, if you get bored of the uh, tear down, you can skip Beyond this. Yeah, we've seen more or less everything that's in it.

Anyway, it's literally some heaters around by metallic strips. We're just trying to get a close-up of one of those uh heaters. Is this going to liberate anything? No, this is the point I may actually get frustrated and actually pause. Why are these not coming out? Is there? These may actually have little tangs on them to actually So they latch in because it goes so far and then it stops.

and this is the point I stab myself forcibly with a screwdriver at two I may actually just start breaking the stuff right? that's a bit broken. mm-hmm and then break that Maybe Not. Maybe I'll find this is the bit that just pops out and it was supposed to pop out in the first place. Nope, that's not going out either.

This is a very frustrating actual. use the pliers to try and shove that through. I Do see that this has a little uh insert going down there that is maybe actually stopping that from coming out. The people who work in the factory will be very frustrated to see me take one apart in this style.

Anyway, here is the bit where the example I'm going to pause and then I'll uh and then we'll resume in a moment. One moment, please. it is out. You'll be glad to know.

And it turns out that when it was pretend there were some little uh tangs that basically gripped it in when it was pushed into it so it was quite hard to get out. Now if I pair this up right, the way these move is, they're set at a slight angle and they move that direction. So if I purr this up now, uh, you may see a slight movement in that direction, but it's going to take a wee while because it isn't. just it's designed to allow for the current motor start current to uh, kick in.

but I am over currenting it quite significantly to the point there's a bit of smoke and I can actually see it curving. I don't know if you can see that, but now that's nice and red hot and smells hot as well. I shall turn the current off. that was three amps I was giving it, which is well above its rating.

but the mechanism. the adjustment literally has this plate here and this plate of it's pushed in will let that mechanism trip. But this mechanism. This plate is actually um, pulled across by the bimetallic strips.

but there's a adjustment screw here for the fine tuning of that. Plus, because this isn't a pivot, uh, against that, it varies its position. If you can see that, now it's veering slightly, it's sliding it up and down. uh, depending the position to vary that thermal trip point very slightly.

and uh, that will determine at which point when the bimetallic strips Flex over the the degree they have to flex over before it trips. Quite neat. I'm sure you're dripping again. Tripping: There is simple mechanical bi-metallic thing, but there we have it.

That is what's inside a Siemens well, probably every other one as well because they all tend to copy each other don't they ultimately? But it's uh, the inside of a thermal motor protection or water protection. Very simple. and as I say, if you ever come across a panel that suddenly something after an overload has died, make sure the power is off before it mess around inside panels. but there's usually a little red button here you can press it if it comes the crunch if you're not sure which one it is in the panel.

uh, look for the little indicator that shows it has tripped like that one has if it's got it. or just push all the red buttons just to reset any that have tripped before powering the panel back up again. That is it though. Quite interesting.

well worth taking apart. That is what's inside a motor over current protection device that's normally stacked on the bottom of a contactor and breaks the connection to the corner protector when it's overloaded.