Horse power regarding electric motors.

[Fritz Maurer]

I have a little twin tank compressor like roofers use. The motor is 1 hp belt drive. I would like to make it 12 volt so I can carry it in my pickup for airing up tires. I have procured a heavy permanent magnet motor but it is only 1/2 horsepower at 650 rpm.

Since this is half the rpm of the a.c. motor, I have elected to couple the motor directly to the flywheel of the compressor in lieu of putting a huge pulley on it to maintain the same appoximate compressor speed.

This is where I get a little foggy in speed vs. horsepower... I guess the ultimate question is do I enough power to run the thing? I don't intend to run it up to 175 psi, 90 would be enough.

The current gearing of the compressor now is about 2:1 with the motor at 1750 rpm.

 

[stdsch40]

Just my educated opinion here, but its very doubtful what you are trying to do would work effectively. A horse power is a horse power, its the amount of 'work' that can be done within a period of time. Compressing air takes lots-o 'work'. 'Work' is a force acting through some distance as in work = force X distance. Halving the power will reduce the amount of 'work' done per unit time by half. Yes, it will take less 'work' to compress to 90psig than it would to compress to 175psig, exactly half depends on the compressor mechanics. It somewhat depends on how the compressor is set-up too, does it have unloaders? And how would you couple the DC motor to the compressor? Running rpm makes a difference too regarding compressor operation. It might be easier to use an inverter to make AC power for the original compressor motor, or get yourself a higher horsepower DC motor and match the 1750rpm. Go ahead and try your way and let us know if it worked out for you. BTW, they make nice little 12vdc air compressors already for tires, just add a tank.

 

[johnnmr]

Rather than get buried in all the math required I'll cut to the end and say there's no reason to reinvent the wheel. 12 volt and cordless tool battery powered tire inflators are already available. We use a 18v Milwaukee tire inflator when away from the shop. It's not fast but considering convenience it's an excellent option.

 

[stdsch40]

Rather than get buried in all the math required I'll cut to the end and say there's no reason to reinvent the wheel. 12 volt and cordless tool battery powered tire inflators are already available. We use a 18v Milwaukee tire inflator when away from the shop. It's not fast but considering convenience it's an excellent option.

Absolutely, that's the best option. But if Fritz wants to test out his idea, well go for it.

 

[Tramway Guy]

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.

 

[Janicholson]

Even at 90 psi, the motor will have difficulty restarting at the turn on "low limit". Try it (assuming it is turning the correct rotation) Jim

 

[stdsch40]

Even at 90 psi, the motor will have difficulty restarting at the turn on "low limit". Try it (assuming it is turning the correct rotation) Jim

You are correct. Those little diaphragm style air compressors (as opposed to the recip type) don't usually have unloaders to make starting easier, it might just lock up when starting up if there is still some air pressure there. All depends on the mechanics of the compressor itself.

 

[Fritz Maurer]

The pump is 2 cylinder single stage. It is unloaded through the pressure switch.

 

[stdsch40]

The pump is 2 cylinder single stage. It is unloaded through the pressure switch.

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.

 

[wore out]

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.

Have you ever seen a one HP "two stage" compressor, I never have.

 

[stdsch40]

Have you ever seen a one HP "two stage" compressor, I never have.

Can't say that I have. But then why 2 cylinders on it? Logic to me would say its a 2 stage for compressor efficiency reasons. Then again I've also never seen a 2 cylinder 1hp compressor either. Nowadays I see alot of the rubber diaphragm style air compressors. Dewalt makes them, lots of others too, pretty cheap machines no oil required.

 

[wore out]

Can't say that I have. But then why 2 cylinders on it? Logic to me would say its a 2 stage for compressor efficiency reasons. Then again I've also never seen a 2 cylinder 1hp compressor either. Nowadays I see alot of the rubber diaphragm style air compressors. Dewalt makes them, lots of others too, pretty cheap machines no oil required.

LOTS of small compressors have 2-cylinder single-stage pumps, both in "in-line" and "V" cylinder configurations.

 

[ss55]

I have a little twin tank compressor like roofers use. The motor is 1 hp belt drive. I would like to make it 12 volt so I can carry it in my pickup for airing up tires. I have procured a heavy permanent magnet motor but it is only 1/2 horsepower at 650 rpm.

Since this is half the rpm of the a.c. motor, I have elected to couple the motor directly to the flywheel of the compressor in lieu of putting a huge pulley on it to maintain the same approximate compressor speed.

This is where I get a little foggy in speed vs. horsepower... I guess the ultimate question is do I enough power to run the thing? I don't intend to run it up to 175 psi, 90 would be enough.

The current gearing of the compressor now is about 2:1 with the motor at 1750 rpm.

If you are just occasionally airing up vehicle tires, buying a small 12 volt compressor might be your best bet. It will be slow, but it will get the job done.

A 1/2 HP electric motor would be around 500 watts minimum, or 40 plus Amps at 12 volt, double that for 1 HP, even more to start an air compressor against pressure without an unloader. That is a lot to run through a cigarette lighter power source (usually 20 amps max.) . An large inverter or a small generator might be a better choice to power the original 120v compressor motor.

 

[stdsch40]

LOTS of small compressors have 2-cylinder single-stage pumps, both in "in-line" and "V" cylinder configurations.

I'm sure they are out there but I haven't seen them yet. Personally my favorite would be scroll style in that horsepower range, but haven't seen any of those in retail market either.

 

[Geo-TH,In]

I have a little twin tank compressor like roofers use. The motor is 1 hp belt drive. I would like to make it 12 volt so I can carry it in my pickup for airing up tires. I have procured a heavy permanent magnet motor but it is only 1/2 horsepower at 650 rpm.

Since this is half the rpm of the a.c. motor, I have elected to couple the motor directly to the flywheel of the compressor in lieu of putting a huge pulley on it to maintain the same appoximate compressor speed.

This is where I get a little foggy in speed vs. horsepower... I guess the ultimate question is do I enough power to run the thing? I don't intend to run it up to 175 psi, 90 would be enough.

The current gearing of the compressor now is about 2:1 with the motor at 1750 rpm.

I have a little twin tank compressor like roofers use. The motor is 1 hp belt drive. I would like to make it 12 volt so I can carry it in my pickup for airing up tires. I have procured a heavy permanent magnet motor but it is only 1/2 horsepower at 650 rpm.

Since this is half the rpm of the a.c. motor, I have elected to couple the motor directly to the flywheel of the compressor in lieu of putting a huge pulley on it to maintain the same appoximate compressor speed.

This is where I get a little foggy in speed vs. horsepower... I guess the ultimate question is do I enough power to run the thing? I don't intend to run it up to 175 psi, 90 would be enough.

The current gearing of the compressor now is about 2:1 with the motor at 1750 rpm.

I have a little twin tank compressor like roofers use. The motor is 1 hp belt drive. I would like to make it 12 volt so I can carry it in my pickup for airing up tires. I have procured a heavy permanent magnet motor but it is only 1/2 horsepower at 650 rpm.

Since this is half the rpm of the a.c. motor, I have elected to couple the motor directly to the flywheel of the compressor in lieu of putting a huge pulley on it to maintain the same appoximate compressor speed.

This is where I get a little foggy in speed vs. horsepower... I guess the ultimate question is do I enough power to run the thing? I don't intend to run it up to 175 psi, 90 would be enough.

The current gearing of the compressor now is about 2:1 with the motor at 1750 rpm.

Why not put a generator in your truck to power your compressor?

 

[BarnyardEngineering]

Trying to match RPMs with 1/2 the power will not end well. You could get away with running the compressor at around 2/3 speed I'd imagine as power requirements are not linear. That's just a stand-back-10-feet-and-sight-down-my-thumb guess. A, "twice as fast takes four times the power" type of thing.

Your biggest challenge will be keeping up with the electrical demands of the motor. It'll chew through a standard automotive battery quicker than you think, so you'll spend all that time you saved having onboard air waiting for someone to come and jump start you if you're not careful.

 

[Fritz Maurer]

Why not put a generator in your truck to power your compressor?

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.

 

[Fritz Maurer]

Also , I wanted to have a little air storage, a dozen times I needed air to blow out an obstruction or clean cooling fins. Which is why a small tire filler won't work.

 

[will-max dairy]

Also , I wanted to have a little air storage, a dozen times I needed air to blow out an obstruction or clean cooling fins. Which is why a small tire filler won't work.

Does your truck have a trailer electrical outlet? With a little ingenuity, you could trickle charge a 12V battery system in your truck bed when you aren't using the compressor and then be able to get a decent amount of air out of it when you needed it.

 

[Fritz Maurer]

Does your truck have a trailer electrical outlet? With a little ingenuity, you could trickle charge a 12V battery system in your truck bed when you aren't using the compressor and then be able to get a decent amount of air out of it when you needed it.

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.