Lost Another Air Compressor Pressure Switch

extropic: 300$ seems awfully high for a contactor conversion. I would think less than half that, even budgeting a little box to put it in
-Mark
 
markba633csi said:
extropic: 300$ seems awfully high for a contactor conversion. I would think less than half that, even budgeting a little box to put it in
-Mark
Click to expand...

I agree, but here is where that number came from.
McMaster-Carr #7603K158, $336.96 (3 HP, 115V)
Same thing in a NEMA 4X enclosure $452.70.
Or, McMaster High Current Relay #70255K272, $220.03, (3 HP, 120V)
Plus a box, DIN rail, miscelanious bits. And don't forget the new pressure switch at about $40.

I guess I should shop around. Any recomendations?
 
Do not really need a NEMA 4/12 enclosure, you can get some inexpensive electrical enclosures at home depot like a junction box or just a plain enclosure and cut your own knockouts for $20-30.

If you just want a 240VAC coil (other voltages and poles are available) contactor and do not need an overload then you can get something for around $10-20.

Add a thermal overload fusing or disconnect then $30-50
Use with CC or J-Class fuse holder

Alternative contactors with thermal overloads, would need to size overlaod for motor, I use quite a bit of the WEG and Fuji contactors with overload relay (about $40-50 for both):

 
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McMaster is very expensive- Mark has some good suggestions above. Also try places like Newark electronics, Grainger/Zoro or even Ebay or Amazon
 
@mjsj

Thanks for the pointers and links.
I looked around on Automation Direct a bit.

Trying to figure out what contactor/relay to buy. The compressor nomenclature says 2.5 HP, 120V, 14A.
This chart says 13.8FLA (115V) is a 3/4HP motor. Also says I have to go to a 34A contactor (3 HP @ 115V) to cover a 2.5 HP motor.
My guess is that the 2.5 HP is BS (instantaneous ???) rating. Should I rely on the 14 Amp rating and ignore the 2.5 HP, or use a contactor/relay rated over 2.5 HP?

Next question. I see "definite-purpose contactors" also referred to as "infrequent-cycle relays", but I haven't found any definition of what they mean by either "definite-purpose" or "infrequent-cycle". Are there "electrical for dummies" definitions for those terms? Or, should I not sweat it?
 
First probably need to decide if you need a contactor for such a small motor, based on the 120VAC at 14A typically works out to 3/4 Hp, possibly 1 Hp depending on efficiency. I personally would not use a contactor, your vacuum cleaner probably draws as much or more power. I would just use the standard pressure switch and replace it if it fails, they are like $15.

Recommended contactor size @120VAC for 1 Hp would be 18A rated, for 2 Hp would be 25A. Also your motor should already have a thermal overload button on the motor, so would not need one on a contactor but if absent on the motor than you would want one that can be set to 14A. These are 3 phase contactors/overloads but can be used on single phase, not sure how one would wire the overload for 120VAC since it is only L1. Like I said, for the size motor I wouldn't bother with all this.

SC-E05-110VAC Fuji Electric IEC contactor, 25A, (3) N.O. power poles, 120 VAC (60Hz)/110 VAC (50Hz) coil voltage.

TK26E-012 Fuji Electric thermal overload relay, 12-18A adjustable, bi-metallic, direct mount power connection(s). For use with Fuji SC-E04(G), SC-E05(G) 43mm contactors.
 
Grizzly, last I looked, had a complete starter assembly for less than $100. I don't know if that includes the overloads but it does come in an enclosure. If 240 is available as a choice I highly recommend it. HP rating is mostly for ranging, look on the motor for "Full Load Amps" to size the overloads. The "FLA" rating. OSHA "wants" two overloads for single phase motors. You can get by with one, especially on a 120 volt load. Just wire across the other one with a short piece of wire. AWG12 is sufficient for 25-30 amps. AWG14 for a small motor. If the starter is for 3 phase, just use the two outboard and keep the center as a spare.

"Definite Purpose" contactors are basicly very large relays with heavy contacts. They can be used for motors but won't have the overload deck. And usually require a large enclosure. They also, while not specified, prefer to be mounted base down. If you have one, or access to one cheap, go for it. But buying one at even 1/4 retail will cost as much or more than a low end compressor. They are more suited to specialty applications, like an automatic transfer switch for a standby generator.

I have some (salvage) relays, called "ice cube" relays, with 25 amp DC rated contacts. So they might stand up to a 30 amp motor load on AC. They are socketed, so would be easy enough to change if it failed. Just keep in mind that while AC tends to be self extinguishing for an arc, a motor load is a strong inductor that can draw an arc anyway. That's where the starter contacts come into play. They're big enough to deal with the arc and are "double break". This gets real technical real fast so I won't go any further. Just keep in mind that although a relay says 15 amps contact capacity, it means a resistive load and will not stand up to motor loads.
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Welp, 2 weeks since the conversion and all is well, I re-hooked up the automatic drain system and got it working
 
extropic said:
I'm disappointed that Furnas brand switches have been burning up so frequently for you.

I bought a Harbor Freight compressor to keep me going while my primary compressor was down for repair/modification.
Pretty soon, the pressure switch on the HF unit was trouble, but I got by with it and the pump, though loud, seemed to operate well enough.
The sticker on the motor cover says 2.5 HP, 120V, 14A.

Later, I decided to replace the wonky HF pressure switch to improve reliability. I bought one from McMaster-Carr because the catalog says 24A Switching Current at 120V. I received a Furnas brand switch, but it's nomenclature says nothing about "Switching Current" and is only rated for 1.5HP at 120V. WTF???

This thread has got me thinking I may need to implement a contactor to achieve reliability. Changing the PS and adding a contactor, at retail prices ($300+), looks like it will more than tripple the cost of the cheap compressor. What am I doing wrong here?
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First of all the damage to the pressure switches is generally on the "break" action and not the (make" action. Most common is people who have the pressure switch set to high. The motor is straining at full capacity to give another 5-10 psi, then the pressure switch "breaks" to open the circuit and the amperage is maximum.
Look at used contractors and motor overloads, I regularly use used contractors and motor overload units in old machines.
There are lots to replace the contactor contacts and they are far cheaper then a contactor. New small sized contractors are available pretty cheap from automation direct.
 
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