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lash-1295 12-07-2009 12:47 AM

Older air compressor wiring help
Registered Member

Join Date: Dec 2009
Location: texas
Posts: 1 Air compressor wiring (speedaire)(dayton)
Can anyone on here help with the wiring on a speedaire(dayton) air compressor? There are no tags on the motor or compressor so Grainger cannot help with wiring diagrams. There is some sort of switch or treminal block missing in the junction box. The wiring was done by previous owner. I tried wiring the two wires with the missing connection directly together but it blows the breaker. With them disconnected nothing happens at all. The wires appear to come from the low oil cut off switch. Any Ideas. The ground wire is not wired to anything at all in the present configuration.


Thurman 12-07-2009 11:09 AM

Well, it appears that the electricians over at their forum didn't offer much help. I'd like to see a picture of the whole compressor, particularly if it is belt-driven. I'm thinking that you may have a larger unit requiring three-phase electricity. That control box seems to have at least one heater element in it, and possibly two small relay contacts. That is why I'm suspecting three-phase electric supply. A photo taken further back would also show where the yellow wires go/come from. So, maybe a pic of the entire unit, and one of the control box from further back would offer more info. Thanks, David

micromind 12-07-2009 11:27 AM

It's pretty hard to tell from the pics, but it looks like the starter is set up for single phase. A single phase motor needs only one overload, a 3 phase needs at least two, preferably 3.

I think this system can be made to work, but we'll need a few more pics. Get a shot of the whole starter, and make it right-side-up. The sideways ones are hard to read.

It's been so long since I've worked with a Furnas starter, I'll need a good look at it.

Make sure that the coil is 240 volt, it'll likely say 120/240 or 240/480.

Basically, the pressure switch, the overload switch, and the low-oil switch are wired in series with the coil. When the coil is energized, the contacts are pulled in, and the motor is powered up.


lash-1295 12-08-2009 05:20 PM

Ok, Quick update here. First of all I guess the guys over at the other forum were none to happy with me. LOL. Guess it was more clear to them than to me that their forum was only for "professional" electricians. OOPs. I had an "electrician" come and look at the compressor and he couldn't make heads or tails of it. I made a call to a compressor shop and talked to a salty old repair man who seems to have me on the right track. I was able to wire the motor directly to the 220 outlet and it works like a charm. But obviously I still need to wire back in all of the shut off and safety switches. He said it sounded like I was missing a starter coil or a "heater". Did a little more poking around and discovered that underneath the terminal block at the top there is a set of four spring loaded contacts that are just setting loose at the moment. It looks like the part that is missing connects to this block of contacts. On the left side of this contact block the is a white "thing" that looks like it pushes down on the left side of this block to "rock" the contacts on and off. I realize a picture is worth a thousand words so I will get bigger better pictures asap.

Thanks for the help so far.

lash-1295 12-08-2009 05:38 PM

nap 12-08-2009 09:31 PM

you are missing the coil for the contactor. It is what pushes the contacts together to turn this on.

You may not be able to find the parts for that starter anymore. That style is not made anymore that I am aware of and parts would be tough to find.

I would suggest pricing out a new motor starter at an electrical supply house.

they will need the voltage of the motor, the control circuit voltage, the # of phases (appears to be single phase), the FLA of the motor,

and what else micromind? I am sure I am forgetting something.

lash-1295 12-08-2009 11:15 PM

Thanks for the advice. One small problem though. There are no and I mean none at all id plates on the motor or the compressor. So I have no idea how to get any of those voltage requirements. Any suggestions?

Thanks for the help so far.<br><br>

micromind 12-08-2009 11:18 PM

nap is right on the dot. That starter is pretty old, parts will be nearly impossible to find.

A new starter is in order, it would be easiest to get one with a 240 volt coil. No neutral that way.

Wherever you get it from, you'll need the HP of the motor, # of phases (I'm betting single phase too), and the full-load amps (for sizing the heaters). It needs to be 2 pole (more is OK, but a waste of money), and doesn't need the auxiliary contact.

There are pretty much 3 types of starters; NEMA, IEC, and Definite Purpose.

The NEMA type is pretty much bullet-proof. They're the toughest ones, and the most expensive. They'll take an incredible amount of abuse, and just keep on working. These are referred to by size. For example, a size 2 will start a 25 HP motor on 460 volts 3 phase, a 15 at 230 volts, a 10 at 200 volts, a 7-1/2 at 230 volts single phase, and a 3 at 115 volts.

The IEC type is not as tough, but it'll work OK for an air compressor. Some of these come in two parts, the contactor and the overload. Easy to assemble. These are rated in amps, but have HP rating as well. Always use the HP ratings.

The Definite Purpose type is similar to the IEC type, but it's the leas toughest. Also the least expensive. It'll work for an air compressor if it doesn't get started and stopped very much. It'll work OK in a home garage, but not in a commercial/industrial application. They're made for A/C units where the motor starts fast. An air compressor starts a bit slower, and is harder on the starter.

If we know the HP of the motor, we can help you pick the right starter.


micromind 12-08-2009 11:45 PM

The new pic helps. That's a Baldor motor, likely either a 3 or 5 HP. Measure the shaft diameter (likely 1-1/8") and how high the center of the shaft is from the bottom of the base (likely 4-1/2").

Next, remove the spring shaped thing from the overload block. (Right below the white button). It'll have some sort of number stamped on it. If I remember right, Furnas uses a letter then a number. Like F34, or something. This will tell us the full-load amps, and confirm the HP.

Failing this, hook it up, and measure current at around 100 PSI. If it's 3 HP, it'll be around 16 amps, if it's a 5, about 25 amps.

Keep at it, eventually we'll get it running.


lash-1295 12-08-2009 11:58 PM

Was told it was a 5 hp single phase so what I am missing is the full load amps. Any idea how to determine that? The only labels I can find at all are on the low oil cutoff and I think on the pressure switch itself.


lash-1295 12-09-2009 12:22 AM

You guys are awesome thanks alot. The copper plate reads H-40 on the side. So 17.7 to 18.4 full load amps?

lash-1295 12-09-2009 12:29 AM

Micromind you called it 1 1/8" shaft 4 1/2" inches high

micromind 12-09-2009 12:42 AM

That motor is a NEMA type. It's pretty tough. Just as a wild guess, I'd get overload heaters at 25 amps. That motor very likely has a service factor of 1.15. This means that it can tolerate a bit of overload indefinitely.

I'd use a 50 amp breaker. The code allows a maximum of a 70, and if the 50 trips, that's what I'd use. #10 will be the minimum wire size, unless it's NM cable (Romex), then 8/2 would be minimum.

An air compressor doesn't overload the motor unless something is wrong. The load on the motor goes up as pressure increases. I'd bet a lot that the pump is two-stage. The maximum operating pressure of such a pump is 175 PSI. Anything less than that is less load on the motor.

If it is 5 HP, you'll need a NEMA size 1P starter. This is a two pole model designed for single phase motors. It might be a bit hard to find though, a size 2 three pole will work as well.

The IEC and Definite Purpose size will be 30 amp. They're available in two or 3 pole. Usually, 3 pole is easier to find, but a bit more expensive. If you went a size or two larger, it'll last longer. There's absolutely no need to up-size a NEMA starter though.

A used NEMA starter would be OK, I'd be a bit hesitant about a used IEC or Definite Purpose type though.


micromind 12-09-2009 12:45 AM

The H40 is just a number. The actual current will be a bit different fir different size-type of starters. I'll try to find a current table, but it might take a while, if I can find one at all.


lash-1295 12-09-2009 12:54 AM

Does this help?

H40 Furnas Furnas "H Series" Heater Element - Reconditioned

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Quantity in Stock:

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Weight: 0.10LB
Manufacturer: Furnas
OEM Part Number: H40
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Weight: 0.10LB
Manufacturer: Furnas
OEM Part Number: H40
Category: H Series
UPC: 783087002852
Condition: Reconditioned
Manufacturer List Price: $11.60

H40 "Standard Trip" Furnas Heater Element for Metal Alloy Relays, Sizes 1, 1 3/4, 2 and 2 1/2 Starters. Full Load Motor Amps Range 17.7 - 18.4 - Furnas Catalog Number: H40

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