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L3ctric motor horsepowers

AKnate

Icehole
Joined
Mar 5, 2022
Member Number
4908
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1,264
I have a Champion compressor with an allegedly dead 15hp motor. 1750rpm

Trying to figure if I can power it with less since a replacement is expensive... was told $3500 area.
I paid $105 for the compressor.

Have a 5hp 1750 rpm motor, which I'd imagine wouldn't be enough ass? Even geared down to pump minimum would be about 25% change in ratio, so like 12hp.

Found a 7.5hp single phase 3450rpm motor for about $375. Velvor brand, so Chineese, but some of their stuff seems ok.

R40 pump claim min rpm of 425 rpm and has a 22" pulley.

15hp motor pulley is 9" so that's a pump speed of 715rpm

3450rpm to get 715 with 22" pulley is around 4.5" pulley... half the 9"

It half the power needed then so 7.5hp would be enough??

Can find 10hp motors (3 phase) fairly easy too, just are $1200-1500, plus need a vfd too.

120 gal tank, I think 2003.
20240712_101031.jpg
 
Last edited:
AKnate said:
R40 pump claim min rpm of 425 rpm and has a 22" pulley.

15hp motor pulley is 9" so that's a pump speed of 715rpm
Click to expand...
half the hp, half the pump speed
so set it up to run at the minimum 425 or thereabouts and expect it to maybe still draw over dataplate amps on the motor

you might need to run a jackshaft on pillow blocks in order to get enough pulley ratio to keep from destroying belts daily
 
Changing gearing can't change horsepower.

You can trade speed for torque, or vice versa, but that trade keeps horsepower constant.
 
yozsi said:
Allegedly dead? Id be probably making sure that's the cause and then if so I'd be taking it down to the motor shop for repair.
Click to expand...
I talked to the only motor shop around (in Anchorage) and they said if it's labeled as bad from Rotating Services (place that put like 15 compressors in the auction) there's likely nothing fixable but if I wanted to waste my time and $$ they'd look at it for $120.
If it's a winding issue, they said it would cost multiple times more than a new motor to fix.
 
Have a R15 pump and maybe good 5hp motor I could use, but the R15 pump is way lower on output.
Plus would need to redo all the piping, aftercooler mount would need fab work (designed for the big pulley)

Edit...

Found specs.
R40,
770rpm, 54 cfm at 175psi
890rpm, 59 cfm at 125psi

Going to pump minimum, 425 rpm...math works to roughly 30cfm at 175psi with ~8hp
10 would be around 35cfm.

R15
734rpm, 17cfm at 175psi
805rpm, 19cfm at 125psi

Mostly for general shop stuff, would like to run a sandblaster, that looks to be around 25-30 cfm for a small pot.
 
Last edited:
AKnate said:
Found specs.
R40,
770rpm, 54 cfm at 175psi
890rpm, 59 cfm at 125psi

Going to pump minimum, 425 rpm...math works to roughly 30cfm at 175psi with ~8hp
10 would be around 35cfm.
Click to expand...
now figure it for setting the pressure switch at 125 instead of 175, might be able to sneak by with the 7.5
but you still are gonna have trouble with your pulley ratios since it's a 3600
 
Aknate, your questions makes my head hurt so I maybe off with my answer. So my apologies to the members of this board. The HP needed is in relation to the pump size. The pump size is in CFM/pressure which is determined by the piston sizes So you will not get the same CFM and pressure if you half the HP. I would think what you need to change is the operating pressure as 486 mentioned. I my case I went from a 5 to 7 1/2 with no problems.
 
cj3a said:
Aknate, your questions makes my head hurt so I maybe off with my answer. So my apologies to the members of this board. The HP needed is in relation to the pump size. The pump size is in CFM/pressure which is determined by the piston sizes So you will not get the same CFM and pressure if you half the HP. I would think what you need to change is the operating pressure as 486 mentioned. I my case I went from a 5 to 7 1/2 with no problems.
Click to expand...
I need at least 120 psi for filling tires on the forklift, 110 on my dump truck.
150 off, 120 on isn't quite enough, least with my current setup. I have to use enough air to get the pump on. Easy on a 30 gallon tank, but less with 120 gallons.

Could do a bit less than 175, like 130 on and 160 off, not sure if that would make a ton if difference in hp.
Maybe 10hp with a pulley reduction.

Wonder if 2 5hp motors connected shaft to shaft and pulley in between would work? 🤣
 
Put whatever motor on it and start it up with an amp meter on it. Measure the current at peak and at pull in pressure. Is the pump pressure lube or splash?
 
cj3a said:
Put whatever motor on it and start it up with an amp meter on it. Measure the current at peak and at pull in pressure. Is the pump pressure lube or splash?
Click to expand...
Splash. Everything Champion has in piston pumps seems to be splash.
Holds a gallon of oil though and has a low oil shutdown deal.
 
If you go the 3 phase route with a VFD you can wire in a pressure transducer and have it run at a constant pressure while you sand blast.
 
cj3a said:
If you go the 3 phase route with a VFD you can wire in a pressure transducer and have it run at a constant pressure while you sand blast.
Click to expand...
Kind of leaning that route, as it would allow soft start and max amperage control as well.
 
AKnate said:
Wonder if 2 5hp motors connected shaft to shaft and pulley in between would work? 🤣
Click to expand...

Yes

you can also do some silly shit with capacitors to get the 3ph motor running on single phase with a little derate
 
Alternate idea:
crappy li'l single stage disposacompressor
pressure switch set at 200-250 psi
quick coupler on its inlet, feed it 100psi from the big compressor and it'll squeeze air into your tires
 
[486] said:
Alternate idea:
crappy li'l single stage disposacompressor
pressure switch set at 200-250 psi
quick coupler on its inlet, feed it 100psi from the big compressor and it'll squeeze air into your tires
Click to expand...
Have one I've been using, better than nothing but it's loud as ship, and maxes out at like 130psi, but I turned it up to 150 years ago.

Main compressor I have is a Husky 60 gallon, like bottom shelf one. Barely will handle a minute blast use of die grinder.
Doesn't even have specs, but looked them up and it's around 8cfm at 90psi.
 
What. CFm is realistic for a shop?

The R15 pump is around 17-19. That would cover most things other than sandblasting i think?

The R40 is around 55. I think enough for a decent sandblaster?
 
I would check ohms of resistance on each set of windings and check the start cap. It might be a simple fix.
 
Is nameplate horsepower trustable on most motors?

Found a "10 hp" but it's only 25 amp at 230v.. which isn't mathing, even if not factoring loss and 746 watts to a hp, it's 32 amp.
25 is only about 7.5hp

Screenshot_20240723-201349_Facebook.jpg
 
Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences.
Click to expand...

398x25/746x.82x.917=10
Probably a coincidence
 
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