Electric motors. AMP, Volt, HP

old

Well-known Member
So my old big air compressor has been down for a few years now due to a bad electric motor. Old motor was a H-F motor I got 20 plus years ago and was said to be a 6HP 230 volt that had a draw of 15 amps. Was in town today having a couple capacitors tested and they where found to be bad so got 2 new ones for my junk oil-less compress since it has not been starting like it should. Figure it is about time to bring the old big one back to life. While talking to the guy at the motor place he was saying they go more by volt/amps then rated HP. He said there 5HP 230 volt would be likely to be a replacement for the old motor it had on it. Cost is almost $400. Surplus center has 3 different rated HP motors bot a good bit less so now I am not sure if I should go with rated HP or go with amp/volts
 

If it had 15a @ 230 that is what ya put back, maybe go up a few amps but do not go down.

I do not have my chart handy but it takes something like 5 amps per horse @ 230-1 and maybe 9amps @ 115

The HP number means very little in real life.
 
Rich, the dude you talked to may not go by HP but when I was an engineer for Century Electric Motor Company THEY SURE DID. Still, of course, the current draw (at a certain voltage) can give you a good indication of the energy being put into the motor and the more in basically the more out less and subject to inefficiency and heat losses.

Here's the deal, I looked up several different motor current versus HP charts for a 230 Volt Single Phase 60 HZ Induction 5 HP motor and current ranged typically from 23 to 28 amps. Even the charts indicated the values were "typical" for different types and classes and RPM and efficiencies etc etc. SOOOOOOOOOOOO if you see a motor that has a 5 HP nameplate but it only draws say 15 amps I WOULD BE SUSPICIOUS OF ITS ACTUAL TRUE HP it may be a 3 HP with a 5 HP surge or some other hyped up rating grrrrrrrrrrrr I guess that shows perhaps what that dude told you rings true to some extent IF IT DRAWS MORE CURRENT ITS LIKELY MORE HP !!!!!!!!!!!!

I have ran lower then original HP motors on compressors and often they would trip the thermal due to overcurrent. To correct that I changed pulley sizes.

Its different now then when I worked at Century Electric Motor Company. You see obviously SMALL motors on compressors with a tag that may say 5 HP but maybe in fine print or in reality that's 5 HP max short term surge HP capacity grrrrrrrrrrrr

I would buy whatever motor is the best value but if it looks small and only draws maybe 15 amps I wouldn't bet the farm its 5 actual true HP

John T Tooooooooo long retired from that job so noooooooooo freaking warranty lol
 
for single phase motors / 230 vac,

1 hp motor is 8.0 Amps full load
3 hp motor is 17 Amps full load
5 hp motor is 28 Amps full load
7 1/2 motor is 40 Amps full load
 
The Harbor freight motor that I had on this old compressor reads this
HP6SPL
A which I figure is amps 15.00
Volts 230.

It did the job just fine. I have a new surplus center catalog and it has a
%HP def purpose
208-230 VAC
Amp22.5
RPM3450
Cost is $239 which is about $100 less then local has it.
I'm thinking this will probably do even better then the old GE motor I had on it
 
The Harbor freight motor that I had on this old compressor reads this
HP6SPL
A which I figure is amps 15.00
Volts 230.

It did the job just fine. I have a new surplus center catalog and it has a
%HP def purpose
208-230 VAC
Amp22.5
RPM3450
Cost is $239 which is about $100 less then local has it.
I'm thinking this will probably do even better then the old GE motor I had on it. Also the one here local is 15 amp so figure the Surplus supply has to be a bit more load torque
 
I had a very simar situation to yours where I had a compressor duty rated motor from HF that burned up. It was a 5 or 6 hp and I replaced it with a 3 hp farm duty motor. Has run like that for about 10 years now. Nameplate current rating was similar to the burnt up motor. I had to buy a new motor pulley to match the larger shaft and makeup for the slower 1800 rpm motor.
 
Did you ask Surplus Center how much freight it is going to be?

In my experience they charge too much for shipping most stuff. As if they are making up profit for a good price item.
 
All I have done is look at there catalog. But the local motor is 15 amp and the surplus supply is 22.5 amps so it has more HP/Torque
 
More of a sticker then a name plate but it says HP 6SLP A is 15.00 RPM is 3450 and volts is 230 and that is off the old bad motor.
 

Well, to me, buying an electric motor by the volt/amps it consumes is like buying a tractor by the amount of fuel it uses. The HP output tells you more; than the energy consumed.
 
That SPL in the HP rating indicates a motor overrated by the compressor maker. Most that have that do not also have a HP rating on the motor. It will not develop that 6hp for long without going up in smoke, as it is a peak, not sustained rating. Lots of those around, but they are no longer allowed to do that.

That said, a motor with the same rpm and current draw should perform as well as the original. But if put on another pump that requires a true 6hp, it would fail quickly.

Another way to look at it is pump rating. A real 5hp motor pump combo should move about 15CFM at 100PSI.

What is your rating?

BTW, how is your search for a master cyl for the 1ton going?
 
Don't forget about Service Factor. Many motors, especially compressor-duty motors, are rated a higher hp with lower amperage because they figure the motor will kick in, run for a few minutes, then shut down and have time to cool down. So depending on the mathematical formula any given manufacturer chooses to use, they're likely to chose the formula that will give the most impressive sales pitch.

Most motors are rated at 1.25 SF (service factor), but since compressor-duty motors normally run for short periods, the formulated SF may be 2.0 or even higher! That means essentially that if they list a 6hp compressor, the "actual" continuous-duty hp output of the motor is likely going to be between 2.5hp to 3hp. It's much like with tractors, where some tractors were sold according to drawbar hp and some sold by engine hp, and even others were sold according to a mathematically assumed hp.

Many motors need "some" amount of airflow moving over or through the motor for cooling purposes. Without that airflow, the service factor and horsepower numbers advertised go right down the tubes. However, unless your compressor is used in a VERY dirty area, there's not much need for a TEFC or TENV motor, and ODP often won't have enough self-made cooling capacity without some external fan source blowing additional air, normally accomplished by the larger belt pulley on the compressor.

Then figure in the country of origin, quality control, engineering, whether it's 50/60 HZ or only 60 HZ (have also seen 50 HZ-only for sale here), and finally the quality of the materials used. Better motors will use higher quality windings in their motors, while others will use lower quality wire.

In short, if you intend to work your compressor for many years, would be preferable to look for one with nameplate amps higher than what's listed for your stock motor. That 15A motor that came with your compressor got the job done, but that motor worked much harder than a similar motor rated around 22A to 23A.

Some people have also chosen to speed up their compressor, either by using a motor rated at higher speed or by changing the pulley diameters. While this 'does' work, running the compressor at a faster speed may cause the compressor to heat up and warp, as the cooling fins are designed to work at the rated RPM's.
 
Well, the basic problem is that the "horsepower" ratings of air compressors are usually total fabrications. And as you see, HF has no problem exaggerating their motor ratings, either. It's a mathematical impossibility for a 15 amp/230 volt motor to crank out 6 hp. In reality, it's probably about half that. And that assumes it actually draws 15 amps at full load.

Compressors usually need a capacitor start/capacitor run motor. In other words, one with two capacitors. Find a capacitor start/capacitor run motor that is physically the right size and is rated at 15 full load amps and you'll be good to go.

Personally, I question the wisdom of putting a new motor on a compressor that's over twenty years old. If I understand you correctly, the current motor is a replacement, so the tank is probably 40 to 50 years old. In other words, a bomb waiting to go off.
 
All an electrician needs is a HP rating, voltage, phase, and distance to make a code compliant electrical installation. That would include wire size, branch circuit overload protection and motor overload protection.
A real giveaway is when there is no HP rating on the motor. A sticker on the tank means nothing
 
Current motors that I buy or are exposed to are rated at "locked rotor amps". This is tested on a machine that essentially monitors the votage across the motor, the amperage draw and the power factor (V x I x Cosine of the phase angle between them). The machine literally grabs the armature shaft instantaneously and squeezes it simulating max load.

The amperage shoots up and the machine measures the point of max current before the input voltage has fallen (very far...don't know number). That is where the HP is measured.

I have a 6.5 HP rated 60 gallon vertical system. Calculating the VA (PF = 1) it comes out to 2 hp. Makes sense. The last air comp I had before this one was just like this one and rated at 2 hp.

The Power Factor limit is 0.85 for my COOP power company......Cosine of the VA phase angle is 0.85. Cos 0 = 1 meaning you have essentially a resistor for measuring power consumption purposes. Having a resistor, if the power company supplies you with the VA to dissipate a watt you pay for a watt. The farther the power factor is from 1, the larger disparity between the V and A phase angles and the fewer the watts registered for payment purposes.

Electric motors used for air compressors like we use (for one example) need compensating "running" capacitors across them (which mine has) to keep the motor from shifting the power factor in the Inductive direction (Voltage leads Current....ELI the ICE man). If Cosine of the angle were 0, like a 90 degree phase difference the power company wouldn't get paid at all.

The other bulge on top of the motor case is the starting capacitor which helps the motor to start under heavy loads, like pumping up a tank of air. There is an unloader valve on the contactor supplying power to the motor but the tank is sitting with high pressure on it and the motor hasn't gotten up to speed before it is pumping to overcome the existing pressure.

That's my non-professional take on how it all works.
 
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This compressor when I got it had a 12HP gas engine on it and I had no need for a gas powered compressor plus gas engine and enclosed building is not a good thing.

So I put the electric motor on it and used it up till a couple years ago when the motor went bad. I sits in an enclosed area of my shop so even if it popped it would be contained. Plus to replace it with something that would do the same thing would cost 2 or 3 grand
 
Ah but electric motors are like many of the small engine a few years ago. The company that made them made claims there engine HP was this or that but they where not and B/S got sued for that and had to pay out big time.

MANY electric motors say the yare X number of HP but are not. Like the motor I took off 230 volts pulling 15 amps there is no way from what most of the guy here have said could be 6 HP

As for comparing them to tractor why do you think the Neb. test where set up?? It was set up because many companies said there tractor had X HP but they in fact did not
 

It is a stretch to even make 3HP on a single phase 240V motor at 15amps. It has to be a high efficiency motor with large power factor correction capacitors to make an honest 3 HP at 15amps.
 
I burned up two motors on my Ingersoll Rand. They gave me a new one under warranty first time and it lasted about 8 years. Lot of sand blasting so it worked hard and motor got very hot. I would give it a good rest but it still crapped out. I was running 235 volts at the motor under full pressure. I put on a new motor a couple years ago and changed motor pulley to smaller one and slowed down compressor at least 20 percent and motor runs cool as a cucumber.

Electrician said he had changed one to the lower rpm motor with compatible pulley changes to maintain compressor speed and it worked out good for him.
 
Upright compressors commonly have "3HP" or "5HP" motors on them depending on how big the compressor is.

It sounds like yours would have originally been a 3HP motor.

In the grand scheme of things it doesn't make much difference. A 5HP motor is probably larger than what it came with originally, but it will work just fine. The motor will only output what is required to run the compressor.
 
This compressor is like one would see in an old gas station say from in the 70s big cast iron and at one time had a 12 HP gas engine on it
 
Do not know if it is single or dual but it is a 2 cylinder compressor and is well over 25 years old. It was probably old when I got it also
 
(quoted from post at 13:07:23 03/20/18) Do not know if it is single or dual but it is a 2 cylinder compressor and is well over 25 years old. It was probably old when I got it also

If it's a 2 stage, one cylinder will much larger than the other.


Dusty
 

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