Air Compressor Solution?

[TomMull]

This is a followup to my previous post on air compressor size.
https://www.britishcarforum.com/bcf/showthread.php?105123-Air-Compressors-how-much-is-too-much
I have no idea how this will work out but it was not costly, I like old machines, and it runs so quietly that the seller and I carried out a conversation without difficulty standing no more than 3 feet away from it while it was running.

It has a very large Century Electric motor rated at 21 amps at 220 volts (single phase) although it is rated as 3HP, which is kind of a mystery. As you can see, the motor is huge.

The Compressor appears to be a two stage but the tag is missing. It looks like the compressors made or used by by Ingersoll Rand of the same vintage. The Wood Industrial Co. tank is dated 1971 but is a different color than the motor. It could have been replaced or more likely the pump rebuilt and re-painted at some point. Most likely, Wood Industrial made the unit.

I plan, initially at least, to plug it in and see how it goes. Sorry, phone pictures are not very good.

 

[TR3driver]

It has a very large Century Electric motor rated at 21 amps at 220 volts (single phase) although it is rated as 3HP, which is kind of a mystery. As you can see, the motor is huge.

Is the "3HP" on the motor, or on the compressor/frame/tank? Might be that the motor is capable of more than the compressor needs.

Or it might just be rather inefficient, I've seen some old motors like that. I had the one on my old Halco vertical mill head off the other day and it's nameplate claims it draws 5 amps @ 110v but only makes 1/3 hp.

Good luck with your acquisition. Don't forget to check that the safety pop off valve works. Years ago I had one that got stuck somehow and didn't do it's job when the pressure cutout failed. Fortunately it was a rather worn single stage compressor, so it wasn't able to go high enough to blow the tank, but it was kind of scary when I realized what happened and how close I was standing.

 

[TomMull]

Thanks for the heads up on the safety valve.
Both 3HP and 21 amps at 220 volts are stamped on the motor.
I guess I don't know how the HP numbers are calculated. My other compressor, a 10 or so year old Campbell Hausfeld 3.2 HP 60 gal., is rated at 15 amps at 240 volts. The one I just bought is 3 HP and 21 amps at 220v. Just doesn't seem to make sense. (both single phase).
Tom

 

[TR3driver]

If it's stamped on the motor, it should be the amount of power the motor is capable of putting out on a continuous basis (or a specified duty cycle). Generally calculated by measuring torque and rpm, just like with a car engine. A general rule of thumb is that it takes 1 kva (volts times amps divided by 1000) to produce 1 horsepower; but of course not all motors are the same.

In theory, 1hp is equivalent to 745.7 watts, but all electric motors are less than 100% efficient, meaning they take in more power than they deliver (the difference becomes mostly heat). Another big issue with AC motors is a thing called "power factor". Basically, the motor acts like a big inductor, which means when the voltage comes up for each sine wave peak (AC cycle), the current lags behind the voltage. This lag causes the apparent power (volts times amps) to be higher than the real power. Modern motors are (usually) designed to maximize efficiency and power factor, but many older motors were less optimum. No one really "cared" about power factor until the power companies started charging extra for it. And even then, large installations would frequently use special devices (like big capacitor banks) to reduce their overall power factor and avoid the extra charge. AFAIK most residential meters even today only charge for real power, so running a motor with a low power factor just means you need bigger wires and so on.

Another thing that can greatly complicate the issue is that there is no real standard for how compressor "HP" is measured. Kind of like the horsepower wars with cars, the compressor manufacturers used to try to find ever more creative ways to stick higher "HP" numbers on their compressors. Generally the fine print would explain the number was "peak" horsepower, meaning they could include the flywheel as a power source. Yes, that's crazy, but that's what they did. That approach finally blew up some 15 or 20 years ago, when someone finally sued a compressor manufacturer for false advertising and won.
So, for example, I used to own a compressor that was advertised as "2.5 HP", but the motor nameplate claimed it only drew 12 amps at 120 volts. Clearly either a violation of the laws of physics, or a blatant lie.

My current compressor is much less blatant, but still has an inflated HP figure on the side. It says "7HP", while the same exact unit was sold after the lawsuit as "5HP". The motor nameplate gives 23.5 amps @ 240 volts, which is 5.6 kva, so I'm sure the 5HP number is closer to the truth.

Not sure if that ramble makes any sense or not, but maybe it helps.

 

[TomMull]

Thanks for the detailed reply, Randall. What I'm trying to do is to replace the inadequate compressor I now have, which is marked as follows: 3.2 HP, 15 amps, 220 volts and 11cfm claimed.

I'm replacing it with a compressor claiming 3hp, 21 amps, 240 volts, cfm unknown. On face, the evidence is strongly in favor of the latter, including large but unknown two stage pump, huge flywheel, huge motor. The whole unit weighs several times what it is replacing. What I'd like is some data to support the prima facia evidence. All I can come up with is the amperage and, generally speaking, more amps = more power, but as you pointed out, lots of variables.
I ran some figures on an on line power calculator, all things being equal, which they are not, and assumed 65% efficiency.
https://www.onlineconversion.com/motor_horsepower.htm

4.025hp for the big one and 2.875hp for the other. Encouraging, but little else.
Of course, I have a fascination and respect for old machines and there was no way I could walk away from this one. Even if it doesn't produce as much air as I hope, the gentle thump of the thing running is worth the price. I'm optimistic though, and will report when I get it cleaned and hooked up.
Tom

 

[JPSmit]

can you run them in tandem?

 

[TomMull]

can you run them in tandem?

I did consider that. My other compressor is in by shop in my basement. I wanted another for the new garage, a bit more than 600 feet from the house. I'd like a compressor at both locations and I don't think 600 or 700 feet of hose is practical. I was able to get by with the compressor that's in the basement for years, although I did run ran short of air with the usual things that use a lot. I had initially decided to buy a new 7.5 hp unit for the garage but then this old one came up. 1/10 the cost so worth a try. I don't expect it to measure up to the 7.5 but I'm hoping that it will be significantly better then the one I have in the basement.
Tom

 

[Popeye]

Good advice on the safety pop valve.

Also might want to "tap" on the bottom of the tank with a screwdriver and look for thin metal - i.e. internal rust. There is a lot of potential energy in compressed air. Raising the pressure of the air means water will condense, and it collects in the bottom of the tank, and eventually rusts. You can get a timed drain valve for something like $50.

 

[TomMull]

Good advice on the safety pop valve.

Also might want to "tap" on the bottom of the tank with a screwdriver and look for thin metal - i.e. internal rust. There is a lot of potential energy in compressed air. Raising the pressure of the air means water will condense, and it collects in the bottom of the tank, and eventually rusts. You can get a timed drain valve for something like $50.

Yes, big issue with old compressors. This one easily passes the tap test but other than that, hard t tell. The older compressors are less prone to rust, IMO and made from good heavy steel. I plan to clean and paint the tank and will consider finding a scope to look at the inside.
Tom