5HP Compressor motor question

Aimsmall

Joined Sep 20, 2011
6
So here's my new compressor, I got it free. It has a 3 phase 5 hp 1730 RPM motor on it so I can't wire it up at my house. I was given 2 motors that are single phase as well. One of them is only a 3hp 3450 RPM and doesn't really suit the big pump but it might work with a tiny pulley, I don't know for sure. The other is a Baldor 5 hp 1725 RPM unit that is perfect for my application but it might need some repairs. Please check out the pics and let me know what part this is and if you think it needs to be replaced. Thanks guys

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strantor

Joined Oct 3, 2010
5,288
Nice compressor! The first thing I would do is make sure it has oil, looks like it leaks. I assume the pictures are of the 5hp motor? I don't see what problem you are referring to. I wouldn't use that 3hp motor because it's twice the RPM and almost half the hp, so thats like 1/4 of the torque. if you take the circumference of the pulley that's on the 3ph motor and divide it by 4, thats the size pulley you would need (actually even smaller, see explanation for 5 hp motor), but I'm thinking you'll probably find that it's smaller than the motor shaft. if your "5hp" single phase motor works, you will probably need to still use a smaller pulley as compressor manufacturers make wild claims about their motor hp. think of this 1hp = 746W, so 5hp = 3.7KW. on 120 service that comes out to 31A. do you have 40A breakers in your house?

Aimsmall

Joined Sep 20, 2011
6
Nice compressor! The first thing I would do is make sure it has oil, looks like it leaks. I assume the pictures are of the 5hp motor? I don't see what problem you are referring to. I wouldn't use that 3hp motor because it's twice the RPM and almost half the hp, so thats like 1/4 of the torque. if you take the circumference of the pulley that's on the 3ph motor and divide it by 4, thats the size pulley you would need (actually even smaller, see explanation for 5 hp motor), but I'm thinking you'll probably find that it's smaller than the motor shaft. if your "5hp" single phase motor works, you will probably need to still use a smaller pulley as compressor manufacturers make wild claims about their motor hp. think of this 1hp = 746W, so 5hp = 3.7KW. on 120 service that comes out to 31A. do you have 40A breakers in your house?
It's a 230V single phase 5hp motor that is pictured, the problem is the little strings tied around the windings are melted so I'm wondering if the windings are burnt (also wondering if this is the main windings or the stator?), I haven't tried hooking it up to power yet but was going to do that tonight possibly.

The pump has oil and was hooked up in my friends shop so I know it works, he moved shops and had a new one already at the new location + no place to store this one.

strantor

Joined Oct 3, 2010
5,288
that would be the stator windings and now that I look again, they look black. probably shouldn't be black, maybe burnt, but it it were burnt I would expect to see the insulation of your leads melted. So, I guess my final answer is I don't know. You could just test it and see.

Aimsmall

Joined Sep 20, 2011
6
that would be the stator windings and now that I look again, they look black. probably shouldn't be black, maybe burnt, but it it were burnt I would expect to see the insulation of your leads melted. So, I guess my final answer is I don't know. You could just test it and see.

I think I will then. That does help too that you can ID the part for me, if it's the stator windings that's a replaceable part isn't it? At least I would know what to ask for when I call repair shops. These motors are $500 and for a free compressor I don't want to spend that kind of dough.$100 would be cool though Thank you!

strantor

Joined Oct 3, 2010
5,288
yeah a motor shop could fix that up for you, if its burnt

Aimsmall

Joined Sep 20, 2011
6
yeah a motor shop could fix that up for you, if its burnt
Really not sure if it's burnt, kinda looks like it just got hot and melted those cheesy little strings. Gonna test it tomorrow and will report back. Any idea what a stator repair might cost? Ballpark?

strantor

Joined Oct 3, 2010
5,288
Any idea what a stator repair might cost? Ballpark?
nope. I always just send em off emergency-like, and they come back. someone else forks over the cash.

someone here will know

....someone?....

SgtWookie

Joined Jul 17, 2007
22,210
Definitely looks like the stator got way too hot. I don't think I'd plug it in. You may wind up with a fire on your hands. Take it to a motor shop and have them megger it.

VoodooMojo

Joined Nov 28, 2009
505
The amount of CFM needed is an important factor to consider for horsepower selection.

very rough math:

12 CFM is an average demand for a small shop. See air tool datasheet for cfm requirements.

hp requirement is determined by gpm times psi divided by the constant 1714.
(not accounting for pump efficiency loss)

1 cfm is about 7.48 gpm.

Compressors generally shut off around 120 psi and turn back on about 95 psi.

12cfm X 7.48gpm X 120psi / 1714 = 6.28 hp

If 12 cfm is required, a 7 hp motor would be desirable.

5 hp will get you about 9 cfm.

You got a very sweet thing going on there...dont disappoint yourself by skimping on the important things.

Graingers handles Speedaire compressors, parts and tools....you should be able to get information on the compressor from them.

you lucky dog you....how much do you want for the thing???

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Aimsmall

Joined Sep 20, 2011
6
The amount of CFM needed is an important factor to consider for horsepower selection.

very rough math:

12 CFM is an average demand for a small shop. See air tool datasheet for cfm requirements.

hp requirement is determined by gpm times psi divided by the constant 1714.
(not accounting for pump efficiency loss)

1 cfm is about 7.48 gpm.

Compressors generally shut off around 120 psi and turn back on about 95 psi.

12cfm X 7.48gpm X 120psi / 1714 = 6.28 hp

If 12 cfm is required, a 7 hp motor would be desirable.

5 hp will get you about 9 cfm.

You got a very sweet thing going on there...dont disappoint yourself by skimping on the important things.

Graingers handles Speedaire compressors, parts and tools....you should be able to get information on the compressor from them.

you lucky dog you....how much do you want for the thing???
This one says it cuts in at 145 and out at 175, not that I'll need that high of pressure but hey why not

I really hope to get the freebie single phase 5hp motor working this weekend but if not I'll check with some local shops to see about fixing it for me.

#12

Joined Nov 30, 2010
18,210
I could swear I bought a new 5 hp motor for about $200, so I checked eBay. Several in the$200 range, and one used motor at 1750 rpm for 99 cents, make bid soon!

I could swear I bought a new 5 hp motor for about $200, so I checked eBay. Several in the$200 range, and one used motor at 1750 rpm for 99 cents, make bid soon!