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Old 09-02-2009, 08:12 PM   #1
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Relay/contactor confusion


I am looking for a relay, or a contactor, or whatever it's called...

It needs to be a single pole, double throw design, it needs to switch around 30 amps at 120 volts, and it needs a 120 volt coil.

Such a thing seems illusive in my Google exploits, and I think the trouble may be the combination of double throw and high amperage. Is this thing as "weird" as I'm starting to think?

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Old 09-02-2009, 08:26 PM   #2
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Relay/contactor confusion


Relays are relays and contactors are contactcors what is it your wanting to do?? Relays are dpdt or about whatever you want. A contactor will be single pole, double pole, or 3 pole generally with low voltage coils or 120 volts coils or 230 volts coils.

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Old 09-02-2009, 08:50 PM   #3
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Relay/contactor confusion


http://parts.digikey.com/1/parts/751...-5a51-120.html
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Old 09-02-2009, 11:12 PM   #4
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Relay/contactor confusion


Grainger # 5Z542 will do. SPDT, 30 amp contacts, 120AC coil. Lists for $27.40

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Old 09-02-2009, 11:33 PM   #5
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Relay/contactor confusion


Quote:
Originally Posted by gp_wa View Post

It needs to be a single pole, double throw design, it needs to switch around 30 amps at 120 volts, and it needs a 120 volt coil.
?
the type of load will effect the amperage rating. DC compared to AC also cause different ratings for current limits.
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Old 09-02-2009, 11:48 PM   #6
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Relay/contactor confusion


Yeah... (I think ) they rate contactors at resistive ampacities with the motor starting load (inductive) somewhat lower. Say a 40 amp contactor resistive will be 30 inductive and they might state hp
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Old 09-03-2009, 12:50 AM   #7
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Relay/contactor confusion


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what is it your wanting to do??
Darn you guys and your hard questions

This is for my camper. I have an inverter for (in) it, but it's just kinda rigged in there to run the fridge, and it just bugs me. I want something that lets me flip a nice simple switch between shore power and battery/inverter power, no BS. As it sits, if I make a mistake and connect the inverter while shore power is connected, the inverter goes *poof*. I hate *poof*.

I need to do a bit more research, but I think the camper is wired to accept 30 amps; it's more than 20, in any case. I'm new to RV's, but I think it's a standard RV type plug. Based on that, I thought I'd wire my modifications to be safe with whatever it's already wired to accept.

I'm not finding a multi-pole switch that can handle this kind of amperage, so I went looking at relays/contactors.

So after all that, the load on this relay would be mixed, because there is a cute little panel and outlets involved, to which someone could attach a drill, or a toaster, or a hair dryer, or a battery char... oh wait, that's already in there...

I hope I haven't made this worse...
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Old 09-03-2009, 04:51 AM   #8
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Relay/contactor confusion


I would recommend using a double pole relay, as some inverters have a floating neutral. Here is the link from Grainger. I'd wire it up so that the coil is operated by the "shore power" to minimize power drain on the inverter.



Keep in mind that such a relay is an "open" type and should be mounted within an electrical enclosure.
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Old 09-03-2009, 05:57 AM   #9
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Relay/contactor confusion


I have a friend with an RV, and I just installed a simple 60 manual transfer switch on the RV so he could switch from using the generator and when he's at an RV campsite or what not.
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Old 09-03-2009, 08:54 AM   #10
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Relay/contactor confusion


Something like this?

http://www.campingworld.com/shopping...r-switch/42439
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Old 09-03-2009, 11:28 AM   #11
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Relay/contactor confusion


The AC transfer switching is only half of the puzzle. The switching circuit also has to cut the 12 volt power to the inverter, and the AC power to the battery charger, when shore power is plugged in.

Even if the A/C is isolated, powering the inverter with DC power from the converter can let the magic smoke out. My first inverter suffered just such a fate.

The main switch and the DC power relay are easy, it's just the AC contactor that was looking like a problem. It looks to me like the contactors posted will work just fine :D
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Old 09-03-2009, 02:24 PM   #12
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Relay/contactor confusion


The inverter input might appear to be a capacitive load; if so, you need pretty hefty relay contacts if the relay contacts will be turning this thing on (as opposed to just carrying inverter current and the switching done elsewhere).

You have an inverter schematic you can post?

Last edited by Yoyizit; 09-03-2009 at 02:27 PM.
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Old 09-03-2009, 02:32 PM   #13
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Relay/contactor confusion


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Originally Posted by Yoyizit View Post
The inverter input might appear to be a capacitive load; if so, you need pretty hefty relay contacts if the relay contacts will be turning this thing on (as opposed to just carrying inverter current and the switching done elsewhere).
The inverter input would be the 12 volt DC feed, correct? Yes, the 12 volt relay will need to be hefty for sure. A 500 watt inverter could draw 50 amps or so continuous. We have a little baby inverter right now that runs fine on a 20 amp fuse.

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You have an inverter schematic you can post?
Just got done cleaning it up.
Attached Thumbnails
Relay/contactor confusion-camper_inverter.jpg  

Last edited by gp_wa; 09-03-2009 at 02:37 PM.
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Old 09-03-2009, 02:39 PM   #14
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Relay/contactor confusion


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Originally Posted by kbsparky View Post
I would recommend using a double pole relay, as some inverters have a floating neutral.
Interesting. I'm thinking I should use the second pole to bond the neutral to ground when there's no shore power (and hence no true ground). Sound reasonable?
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Old 09-03-2009, 03:47 PM   #15
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Relay/contactor confusion


The box called "convertor" is a DC power supply or a battery charger?

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