View Full Version : need air compressor "furnas" mag starter wiring help

07-05-2010, 06:24 PM
For starters its a charge air pro 80 gallon 5hp 18.9cfm at 175psi made in 1988, since then it has had a different mag starter installed on it and a seperate shut off box all of which i am removing the second square d mag starter is only rated at 3hp.

i have all the original parts installed on the compressor it is a furnas box assembly with all furnas parts, here is the numbers as it goes

1phase 230v = 5hp
220v-240v coil
size e

600v 6A.
max buss ktk or equivalent fuse

bimetal o.l. relay non-amb comp.
600vac max

i think i have it almost ready but any help would be greatly appreciated, the red wires coming out of the fuse holder are broken by said fuses one for the top 2 wires the other for the bottom 2 wires this is where it starts to lose me lol.

the yellow wires are coming from the pressure shut off, there is an on/off switch on top of the box that just has a single wire in and out, i think it just breaks the relay circuit.

the furnas mag starter starting from the left screw has continuity between the top left and bottom left and top middle and bottom middle but no continuity between top right and bottom right unless you push the little mag button in.

07-05-2010, 07:10 PM
if i posted this in the wrong area please feel free to move, i was going to put it into electrical and still may

07-05-2010, 07:16 PM
Are you talking about a control contactor? That's what your photo looks like to me. Sometimes big contactors are called "motor starters", but they are basically the same thing. And they are usually rated by amps. Getting one in a 208-240v coil wouldn't be hard - matching the amps on a 5HP motor might.
They used to rate air conditioning compressors at 1HP per ton. So, a 5HP compressor would be a 5 ton. That may not be how they rate them currently, but I can tell you how large a contactor is necessary for a 5 ton unit. Usually 40 amps.
You can always pop up to FURNAS' website and look at the specs of their iems. You might just find the exact OEM replacement.
Hope this helps!

07-05-2010, 07:24 PM
i dont know what it is called just assumed it was a mag starter or contactor or what ever:) furnas doesnt have a website anymore its owned by siemens and there is no information on this model "contactor" but everything in the box was originally sold with this charge air pro compressor so i imagine its spec'ed accordingly.

07-05-2010, 07:54 PM
This picture is assuming that those small two red wires up to are connected to the coil but I can't tell from the picture. Also assume the coil on the contactor is 240v.

If you are reading continuity on the the left and and middle on both the upper and lower terminals I'm willing to make a wager your contactor is shot and the first two poles are welded shut. I would be more willing to bet money on it as you said there was a second starter on it. I would pull the contactor apart and look at the contacts before going any further. I suspect you will either be reinstalling the starter that is to small or looking on ebay for a new starter.

The last pole on the right sounds like it works as it should but was probably the pole that wasn't originally used in the single phase configuration.

It is a single phase magnetic starter but about as low budget as they come.

07-05-2010, 08:01 PM
thank you for that will, so no poles top to bottom should have continuity until that push button is depressed right? i will pull it open again and check to see if they are welded shut. if so then this thread is useless lol.

you are correct in that the two red wires are going to the top of the "coil" a1/a2

in you wiring diagram you show the two bottom red wires connecting to the white and black wires of the top portion of the contactor this would run power between the black/white wires as the fuses that those two red wires connect to are each on the same level if that makes any since, one fuse for the top 2 the other fuse for the bottom 2, thus it would essentially be bridging the white/black together

07-05-2010, 08:16 PM
I wasn't trying to bridge the black and white together. I was trying to get your fuses in the coil circuit.

Does this look better with your fuse block conections.

Generally you should only have continuity when the contactor is pulled in. There are some exceptions I've seen that would be a two pole contactor with a brass bridge in place of one of the contacts. I've never seen a three pole contactor like that and the two pole ones have always had there contacts exposed so you could see what was going on.

07-05-2010, 08:18 PM
you were right they were fused together, i think i am going to rebuild it as there are enough good pads to make two lines

that looks correct for my fuses thanks

i think i will try cleaning these pads up and trying it if not gotta find me a 5hp capable contactor thing

also this thing could be wired for 3phase the contactor that is, thats probably why there are 3 leads

07-05-2010, 08:37 PM
where would a on off switch go, just the simple either on(closed) or off(open)

07-05-2010, 08:55 PM
will thanks to you its fully operational, i rebuilt the contactor(not to hard, just had to figure out how to get it apart, luckily there were enough terminals i was able to make 2 work) ran one of the yellow wires to the on/off switch and ran the other red wire to it and turned the power on it zoomed to life nice and smoothly.

07-05-2010, 09:33 PM
alright now that i got it all up and running, it hits the 175psi peak and the little saftey pop of valves start popping off, the pressure switch doesnt shut it off, so i lossened the screw until it turned it off at 150psi and the little bleed valve just sits and bleeds the air out until it gets low enough for the pressure switch to kick back on.