Help me find the right electrical relay

Factual Questions
1

Specifically from Grainger.com since I can then pick it up locally.

The short version is that, from what I can tell, I need a normally closed relay that operates at 120V on the load side and 208 on the coil side. Right off the bat I seem to be having a hard time finding a relay that’s normally closed. I’m wondering if I should get a 5 pin relay. Also, enclosed would be a huge help.

What I have is a commercial freezer. This relay turns off the (120v) evaporator fans when the (208v) electric defroster comes on.

Ultimately, we ended up figuring out the relay was bad not by testing it. See, we didn’t even know it was a relay, even our refrigeration guy didn’t know that’s what it was. It’s just a sealed black box with no numbers on it and 4 wires coming out of it, we figured it was a junction box of some sort. But the fans would come back to life when you’d tap it. At that point we figured it out and I finally got my hands on a schematic that confirms it.

Now, a question about the wiring, just out of curiosity…
As I was troubleshooting the problem that lead me to the bad relay I found that the leads going to the coil side were giving me odd voltages. When the defroster was off, I was getting 208v from either side to ground and 0 volts across the leads. When the defroster was on, I was getting 208 volts from either side to ground and 208 volts across the leads.
Since (other then a bad relay) everything works just fine, this is more or less moot, I was just curious about why it’s wired this way. Or course, since there’s timers 120v and 208v service involved, without you being here looking it at, it might be heard for you to understand what I’m looking at.

2

Just search for a 208v coil, single pole, double throw relay. Don’t use the NO contact.

3

That’s what I wasn’t sure about. I assumed a 208v relay would be fine, but since I’ve never had to scout one out myself, I wanted to be sure about that. Normally I have a part number to work with. What I’ll probably end up doing is disconnecting it, jumping the fan wires together so they run continuously (turning off the defroster) and cutting open the sealed black box to see if there’s a part number on the relay inside that Grainger can cross reference.

4

I would think this would be a really (no pun intended) non-critical application. Just find one that’s close size-wise and has contacts that can handle the current you need (which is probably really low, if it’s just a blower) - a higher voltage rating on the contacts is just added insurance.

5

Are you sure there’s no part number on the schematic?

But hey, if it fits physically, the coil doesn’t draw too much current, and you aren’t exceeding the voltage specs on the switch side, just about any relay that meets those specs will do the job. As beowulff said, use a double throw relay and don’t use the NO contacts.

A common failure mode for relays is for them to get “sticky” and not want to switch. Giving them a little whack (“percussive maintenance” if you want a technical term :stuck_out_tongue: ) often makes them work again, if only temporarily.

6

OK so this is how relays work:

A coil is energized with current and it magnetizes, thereby changing the position of a little metal thingy inside of it. That little metal thing moves between one of two positions - it touches one contact when the coil is not energized and touches the other contact when it is energized.

The electrical current that is used to energize the coil does not in any way interact with the other contacts, or with the little metal thingy that moves between them. So, you can use a 208V relay to transfer 120V across the other contacts no problem. Vice versa, you could use a 120V relay to transfer 208V across the other contacts also. However, you need to be sure that the device which is controlled by the “dry” contacts (the ones which are not feeding the coil) won’t pull more current than those “dry” contacts are rated for.

So in short - just make sure that the relay has a 208V coil. When that relay gets it’s 208v signal, it will change the state of the dry contacts. The voltage you put across the dry contacts does not matter one bit. It could be 12v, 24v, or 120v. I wouldn’t try putting anything like 460v across them, but in any case, double check the current draw of the device and compare it to the rating on the relay. Usually a little paper comes with them, or it may be printed on the relay, stating how much current it will allow across the dry contacts.

Hope that helps.

Also - an electrical circuit may often have multiple voltages flowing through it. It all depends on the current needs of the individual devices. Sometimes it is practical to have all the devices run on the same voltage for the sake of simplicity, other times it would be impractical because of the current draw of those individual devices. For example, a 460v 3 phase motor needs a lot of current. However, the motor starter circuit shouldn’t be 460v, because a human needs to push a button. That button can’t have 460v going through it, or else in the event of a failure of the insulators a person would get more than a mild 120v shock, they would get knocked on their butt, maybe killed. So the control circuit runs at 120v, and it enables a 460v motor to operate. Then maybe there’s a 230v electric heater involved also, that would require it’s own power supply. Bottom line is, never assume that all the voltages are the same. ESPECIALLY if you see a square shaped thingy that hums when its turned on. That’s a transformer usually, and it means there’s some voltage change happening in there. We could get more complicated than this, but I don’t think it will make your situation any clearer.

If I’ve been unclear, or if I’ve said anything flat out wrong, call me out on it. I like learning too. All I know I taught myself through years of working on industrial equipment.

7

Thank you, I understand how relays work, in fact I just refreshed myself on it today, I just wanted to be sure I could put 120v though the load side of a 208v relay.

As for multiple voltages, I have freezers and coolers running on combos of 3 phase, 220 and 110 so I’m no stranger to that, I’ve just never run into something wired quite like this so it took me by surprise (individual leads both hot when something is off, but only one hot when it’s on, it works, I just wasn’t expecting it).

The original problem we had was that from time to time our fans would be off. We’d start poking around and they would suddenly turn back on. No amount of poking around would turn them back off so it was hard to troubleshoot since all you could do was wait for them to fail again. We figured there was a loose connection (we checked every connection) or a broken wire (at this point we were hoping it would arc and we could find it that way.
Eventually we found that it wasn’t turning back on after a defrost cycle. That made it easier since all we had to do was turn the defroster on and back off and then start poking around. Literally just moving wires would make them come back to life. Eventually we narrowed it down to this black box.
I believe the reason we could wiggle/tap/poke the wires or box and make them come on, but not do the same to make it turn off was that doing that would allow the contact to move over to the electromagnet, but since it was now on the magnet side, you couldn’t wiggle/tap/poke it and get it to go back…make sense? sort of?

8

Yeah, like the electromagnet wasn’t strong enough to change the state of the relay, but when you “helped” it, the contacts moved, and then it could hold it in position.

Sounds like an old relay.

So you’re describing a relay that doesn’t have terminals where you attach the wires? The wires just stick out of it and you wire-nut your devices to it? I don’t get that type of relay…does it just look like an unlabelled sealed plastic box? Like some oil-burner control boxes that do “magic” things but don’t have their inner workings described on their outside?

9

:smack:

unnecessary second post

10

I see you got it, but yes, it’s just a plain black box with wires coming out of it. The relay is located inside the freezer case so it’s probably like that to keep moisture away from it.
My plan is to replace it with a sealed relay and use spade connectors to make the connections. I’m sure they’ll corrode over time, but I’ll keep an eye on them. Maybe I’ll coat them with hot glue.

I noticed that 6 sides of said box are hard plastic, but the bottom is a soft rubber. The other thing I I was thinking is cutting out the bottom of the box, pulling out the old relay, putting in the new one and filling it with hot glue to seal everything.

We’ll see after I get the part, how I’m going to deal with it. I don’t want anyone that has to work in that area getting a shock or for this to short out if it grounds itself, so I’d prefer not to have any bare metal.

11

So, I just took some measurements. The defroster draws 15 amps while running, however the wire going to the coil side of the transformer doesn’t even ping my meter. The motors are drawing about 1.2 amps and as soon as that guy isn’t standing in front of the freezer I’ll pull one out and see if there’s a HP rating on the bottom since that seems to be one of the ways these are rated…BRB…Okay, well there was no HP rating on the motors, but they were listed as .37amps, so I’ll take that info with me to Grainger and let them help me pick something out.

Also, I’m hoping I can find one with a socket, that way I can mount the socket and not mess around with live terminals that I have to find away to protect.

12

a socketed relay may not be the best for inside the freezer, a harsh environment. those will be affected by temperature changes and moisture. you had a potted unit to seal against moisture (you would need to have replacement sealed as good as that) and hard wired to keep continuity no matter the temperature or its changes and vibrations/movement.

13

Searching Grainger for Potted Relay or Sealed Relay doesn’t really help me, but I’ll stop in later and talk to someone. If it makes any difference, inside the freezer there are plenty of wire nutted connections and plug in outlet/socket style connections, but I understand what you’re saying.
Many of the relays appear to be ‘sealed’ at least with some sort of acrylic/glass/plastic dome around them. Whether or not it’s moisture proof, I don’t know, it’s hard to tell by looking at a picture. Again, I may just have to wait until I get there and talk to someone.

14

Here’s the relay they suggested. I’ll have it on Monday. Once I have it in my hands, I’ll crack open the black box and see what’s going on in there. If there’s any kind of part number on it, I’ll see if they can cross reference it.

The next step will be finding away to protect all those terminals. I’m a bit concerned about that.