Horse power regarding electric motors.

That was the plan. Run a heavy dedicated line to another battery in the bed, like maybe a 31. Was all contingent upon whether this motor will turn it. Was looking to build up a ball bearing starter motor, when my starter guy said "try this salt spreader motor ".
However, I can limp by with a gasoline engine on it for a while, since this unit was set up for continuous running compressor unload.
I agree with all of the replies that talk about "there are already commercial solutions to this." But sometimes, people just like to noodle around with stuff that they have around the farm/shop to make something. What you want to do could definitely be done. It's just a matter of looking at what you have around vs what you would have to purchase and, of course, the opportunity cost of your own time spent MacGuyvering.
 
Either obtain a generator to operate the compressor or a DC to AC inverter . The DC conversion sounds good, problem is that it is not practical.
 
LOTS of small compressors have 2-cylinder single-stage pumps, both in "in-line" and "V" cylinder configurations.
I have an Emglo V-4 single stage compresor. What a nice unit. Have it set to shut off at 125 PSI and restart at 90 with a "develops 5hp " motor. Probably really more like 3 hp.
 
I think ultimately I would replace the motor with a gas engine. I my truck has a cap and I was trying to find a way to avoid crawling up in there to start it.
If you will need to lift the air compressor out of the cap/bed anyway, a small generator about the same weight might be easy to lift out too.

If you already have some battery powered tools, there is probably a battery powered air compressor available that can use your existing batteries.

Trying to match the capacity of a two cylinder compressor could be difficult, most are powered by a 3 HP minimum electric motor. A rule of thumb is to double the HP when replacing an electric motor with an I.C. engine.
 
Closet
I think it will work with no problem exactly as you describe, assuming the original electric motor was 1750 RPM or 1800 nominal.
The torque of a 1/2 HP motor at 650 RPM is roughly equivalent to a 1.35 HP motor at 1750 RPM.

You will be pulling more than 30 amps at 12 volts at full load, but could still achieve the max pressure, although internal leakage in the compressor itself might limit that, as the speed is slower.

DC motors tend to have low inrush current when starting, by the way, compared with AC.
Closer to 45 amps . DC inrush is still 3-12 times rated operating current .
 
If you will need to lift the air compressor out of the cap/bed anyway, a small generator about the same weight might be easy to lift out too.

If you already have some battery powered tools, there is probably a battery powered air compressor available that can use your existing batteries.

Trying to match the capacity of a two cylinder compressor could be difficult, most are powered by a 3 HP minimum electric motor. A rule of thumb is to double the HP when replacing an electric motor with an I.C. engine.
The motor currently on it is 1 hp. It was my desire to not have to move anything, leave the compressor permanently against the front wall of the bed and have a coil of hose hanging back by the tailgate.
If the formula works out the way Tramway Guy points out, then this direct drive should work. I thou there might be a trade-off out there like less rpm equals more torque, but I just couldn't put my finger on it.

For the sake of discussion, the pump is currently turning 575 rpm , while direct drive it will be turning 650 rpm. It is single stage. It can be set up to unload while running or through a bleeder switch when stopped. I have 110 Amp alternator on my S10 so I should not have a problem killing the battery.
 
If its 2 cylinder its likely a 2 stage compressor since you say it goes to 175psig. I doubt its single stage. Look closely 1 of the cylinders will be much larger than the other. Large cylinder on the low pressure side small cylinder on the high pressure side. Okay so now we know its a recip machine with and unloader, your chances of it working to your satisfaction are higher.
Just about all of the "Farm duty" compressors are 2 cylinder single stage. My Campbell Hausfield 60 gallon "Farmhand" model compressor is single stage two cylinder, advertised as 5 hp, 230 volt for what that's worth. 125-135 psi max is normally single and 175-180 psi max is generally 2 stage. He said 175 psi and single stage, which I have never seen. Also have never seen a 1hp 110 volt that will achieve 175 psi.
 
Just about all of the "Farm duty" compressors are 2 cylinder single stage. My Campbell Hausfield 60 gallon "Farmhand" model compressor is single stage two cylinder, advertised as 5 hp, 230 volt for what that's worth. 125-135 psi max is normally single and 175-180 psi max is generally 2 stage. He said 175 psi and single stage, which I have never seen. Also have never seen a 1hp 110 volt that will achieve 175 psi.
I was mistaken about the scroll style compressors, seems there are plenty on the retail market now. I admit I hadn't looked in a few years. And in the 1 to 2 hp range with good high pressures, plenty of air flow for around the farm utility use, nailers, tires, spraying. If I were in the market for a new air compressor the scroll style would be my first choice. Oil free, and quite too.
 
Motor speed has little to do with hp requirements, compressor size and speed does
You said your compressor is pulleyed 2:1 which should give a compressor speed of 875 with a 1750 motor, you can run a compressor with a smaller motor IF you slow the compressor down
My Quincy 325 2stage runs 650 rpm with a 5 hp motor running 1750 rpm, I can run it with a 3 hp 1750 motor if I change pulley ratios to slow the compressor to 460 rpm. However this will reduce the compressors pumping capacity from 17.4 cfm down to 10.4 cfm requiring a much longer run time to fill the same size air tank

If that 1hp compressor is running 875 rpm I don’t thing it will run 650 rpm with a 1/2 hp motor, your probably looking a 450-500 compressor rpm
If the pump is actually running 550 rpm now you for sure can’t run it at 650 with half the motor hp, your probably looking may be able to slow it down and get by with 250-300 compressor rpm

The down side is a slower compressor sped either way require nearly double the run time to fill the tank to the same psi, your probably looking may have enough battery power to fill the tank but it won’t last for many repeat cycles, even with a 110 alternator I don’t think it can keep up with the demand, also keep in mind with the truck at idle your not getting 110 amps from the alternator
I don’t know enough about inverters and their power demands from the battery to make a recommendation but with a cap on the truck that may be better than wrestling a small generator around
 
Motor speed has little to do with hp requirements, compressor size and speed does
You said your compressor is pulleyed 2:1 which should give a compressor speed of 875 with a 1750 motor, you can run a compressor with a smaller motor IF you slow the compressor down
My Quincy 325 2stage runs 650 rpm with a 5 hp motor running 1750 rpm, I can run it with a 3 hp 1750 motor if I change pulley ratios to slow the compressor to 460 rpm. However this will reduce the compressors pumping capacity from 17.4 cfm down to 10.4 cfm requiring a much longer run time to fill the same size air tank

If that 1hp compressor is running 875 rpm I don’t thing it will run 650 rpm with a 1/2 hp motor, your probably looking a 450-500 compressor rpm
If the pump is actually running 550 rpm now you for sure can’t run it at 650 with half the motor hp, your probably looking may be able to slow it down and get by with 250-300 compressor rpm

The down side is a slower compressor sped either way require nearly double the run time to fill the tank to the same psi, your probably looking may have enough battery power to fill the tank but it won’t last for many repeat cycles, even with a 110 alternator I don’t think it can keep up with the demand, also keep in mind with the truck at idle your not getting 110 amps from the alternator
I don’t know enough about inverters and their power demands from the battery to make a recommendation but with a cap on the truck that may be better than wrestling a small generator around
If he uses an inverter an even higher demand on the battery/alternator is required than using straight DC motor, because there is an efficiency loss of at least 10 to 20% with an inverter. So 40 amps DC for 1/2hp DC motor direct goes up to approx 100amps DC to supply the invertor for a 1hp AC motor. Not something I would do but hey, whatever.
 
If he uses an inverter an even higher demand on the battery/alternator is required than using straight DC motor, because there is an efficiency loss of at least 10 to 20% with an inverter. So 40 amps DC for 1/2hp DC motor direct goes up to approx 100amps DC to supply the invertor for a 1hp AC motor. Not something I would do but hey, whatever.
Inverter losses and AC motor efficiency have less losses than DCI2R losses and DC motor efficiency. Plus no tinkering with the compressor is required. Plus the inverter can be used to operate essential loads during power outages such as sump pumps and furnaces .

Understanding 12V inverter efficiency​

Inverters do not achieve 100% energy conversion during operation and the efficiency of an inverter is usually measured as the ratio of output power to input power. For example, a 12V inverter with an input power of 1000W and an output power of 870W has an efficiency of 870W/1000W = 87%.

In general, the efficiency of an inverter can be between 85% and 95%, depending on the quality of the inverter design, the working load, and the amount of output power. An efficient 12V inverter optimizes the use of energy and ensures that energy is wasted as little as possible when supplying power to devices. This not only saves energy but also reduces the environmental impact of energy consumption. By using a high-efficiency inverter, you can use electricity in a more environmentally friendly way and reduce your energy costs.

Factors affecting inverter efficiency:

Quality of design

The quality of an inverter's design and topology has a direct impact on its efficiency. High-quality inverters tend to have higher efficiencies because they use superior electronics and advanced circuit designs. In addition, inverters improve energy efficiency by using pulse width modulation (PWM) techniques and high-frequency switching, effectively minimizing energy waste.

Load

The efficiency of an inverter is usually highest within a certain load range. Both low and high loads result in lower efficiency. When inverters are designed, a certain load range is usually taken into account to achieve optimum efficiency. In practice, it is possible to ensure that each inverter operates within its optimum operating range by distributing loads appropriately, thus increasing efficiency!

Operating temperature

Inverters can lose efficiency in high-temperature environments because high temperatures affect the performance of electronic components. Inverters are usually designed with heat dissipation and temperature control measures to maintain a low operating temperature.

Waveform types

Different types of inverters (e.g. square wave, modified wave, and pure sine wave inverters) have different efficiencies. In general, pure sine wave inverters have higher efficiencies because they require more complex circuitry to produce outputs that are close to a sine wave.

Is 120V more efficient than 12V?​

In general, a given inverter may be more efficient at converting 12V DC to 120V AC. This is because at higher output voltages, less current is required to transfer the same amount of power, and therefore less resistive losses are incurred.

Assuming that 100 watts of power need to be converted from DC to AC if the input voltage is 12 volts and the output voltage is 120 volts, this corresponds to an input current of 100 watts/12 volts ≈ 8.33 amps and an output current of 100 watts/120 volts ≈ 0.83 amps. In this case, the output current is lower, the resistive losses are lower, and the inverter may be more efficient.

However, factors such as inverter design, load profile, and operating temperature must also be taken into account to determine which voltage level is more efficient. Different inverters can perform differently in terms of efficiency at different voltage levels. Therefore, when selecting an inverter, it is also necessary to take into account the actual application needs, design requirements, and other factors that affect efficiency.
 
My neighbor tried running his HF compressor plugged into the 120-volt receptacle in his truck, and it worked fine, and it's not a real small compressor, it has a decent size tank. Most trucks now have built in inverters up to 400 watts. That's another option, buy an inverter big enough to run your compressor.
 
Would a 1/2 hp, 650 rpm motor have enough guts to run a compressor up to 90 psi? Next question. What kind of amps will it draw?

Most 1/2 to 1 hp, 12 volt motor I have experience with would draw an average of 40 to 80 amps.
 
Would a 1/2 hp, 650 rpm motor have enough guts to run a compressor up to 90 psi? Next question. What kind of amps will it draw?

Most 1/2 to 1 hp, 12 volt motor I have experience with would draw an average of 40 to 80 amps.
I don't think a 12v battery will last but a few minutes powering a 1/2 HP DC motor?
My dump trailer takes 90 seconds to dump and 70 seconds to go down. 160 seconds.
I'm lucky to make 4 dumps before my largest 12v truck battery is running out of juice.
The current is 140 amps going up and down.
 
Would a 1/2 hp, 650 rpm motor have enough guts to run a compressor up to 90 psi? Next question. What kind of amps will it draw?

Most 1/2 to 1 hp, 12 volt motor I have experience with would draw an average of 40 to 80 amps.
1/2 HP is approx 375 watts. Using the formula watts= volts x amps, amps is about 32 at 12 Volts. Of course, that is at 100% efficiency, which is improbable in most cases. So 40 amps for a 1/2HP 12VDC motor is reasonable.
 
I was faced with the same problem, I needed remote air but didn’t want to fool with a full fledged gas powered compressor. What I did was buy a cheap 2,000 watt generator (I think it was $350). It will run my small portable air compressor fine. Now I’m not running air tools like a impact, but it’s more than enough to air up a tire or run things like a air nailer.
 

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