Can't remember if it was this list. The question was raised, does compressor amperage decrease as pressure increases. I set up an informal test, using equipment on hand. Plugged in a Kill A Watt meter, and set it to read watts consumption. Plug in my 3 HP Harbor Freight pancake compressor.
I turned on the compressor, and noted the reading as the pressure went from 0 PSI to 100 PSI. The readings are as follows:
PSI -WATTS
10 - 198
20 - 199
30 - 211
40 - 220
50 - 229
60 - 233
70 - 237
80 - 240
90 - 255
95 - 248 (wasn't sure the compressor would make it to 100)
100 - 250
Hope the information is of some use to someone. If nothing else, it used a few minutes of my afternoon.
I bet if you noted the time that it took to attain the increase in pressure when you saw the apparent dip in watts consumed between 90 and 100 psig you would find that the total load (power consumption)increases.
I suspect if you were also to apply the gas laws to the equation where the temperature of the air in the tank was held constant, you will find that the cost in power would increase for the volume/pressure of air in the tank.
No power is watts. Also known as power dissipation, Power CONSUMPTION is watt hours. Horsepower is also a power rating, and is an INSTANTANEOUS measurement. - A snapshot in time.
A couple weeks ago I put my killawatt on my compressor. I noticed that the amperage went up for a while but toward the end of the cycle it went down. at the time I thought it might just be from things "warming up" or perhaps I imagined it but it looks like there is something to the idea that at sufficiently high pressure the flow resistance drops faster then the pressure resistance increases.
"Common unit of power, the rate at which work is done. In the English system, one horsepower equals 33,000 foot-pounds of work *per minute* - that is, the power necessary to lift a total of 33,000 lbs a distance of one foot in one minute."
So if you fail to consider the amount of time the motor is consuming a lesser amount of Watts, you will not get the whole picture. I took issue with the statement:
"The load on the motor is actually less at 135 PSI than it is at 100."
Think about it this way. Lets say you have a pneumatic nail gun that when it is fed 80 psi it can shoot a 16 penny nail into a board. If you set your compressor to turn on @ 80 psi (regulated) and shut off @100psi you could shoot x number of nails before you consumed enough air to cause the compressor to turn on again.
Now take the number of Watts and multiply it by the amount of time it takes the compressor to shut off.
Now reset the compressor to turn on at 115 psi and off at 135psi and count the number of nails you can shoot (it will be more)
Again take the Watts and multiply by the amount of time.
When you then take the number of nails driven divided into the Watt seconds of power you expended it will give you the cost per nail driven.
I suggest that the cost will be higher per nail at the higher pressure, because you are creating much more waste heat that does not put any air in the tank to drive nails.
I think there are 2 factors that effect power (power is measured in Watts, energy is measured in Watt-hours), one is the pressure that you measured, and the other is the FLOW which you did not measure. Power is probably highest when BOTH pressure and flow are high. When the compressor first comes on and is pumping air into the tank, the flow is high and the pressure is low. As the pressure builds, the power goes up but the flow tends to go down a bit. When you get to max pressure, the flow is almost zero. If you want to do a science experiment, try to arrange for a flow of air out of the compressor and record power vs pressure AND flow.
That gets back to what I said a long tome ago, the compressor is less efficient at higher pressures, as another poster said, it can't exhaust the air into a tank full of pressure but that also means the motor is not working as hard as evidenced by the amps drawn
Just from standing there taking the pictures, I can say that last 30 pounds took a lot longer than the first 30. It is clear, according to mr Boyle, that the flow is less.
That does make some sense. It would only occur as the air delivered approaches zero. The air in the cylinder is compressed, but can't go anywhere, so it just pushes the piston back down, speeding up the crankshaft so it doesn't work so hard going up the next time.
It is also clear that if the flow increases the preasure will drop. To maintain the same pressure with higher flow the pump needs to turn faster - which WILL require more power.
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