Air Compressor Motor Question

[medic502]

I recently acquired a industrial 60 gallon IMC air compressor. However it has a 3 phase motor instead of single phase. I have attached a picture of the plate on the motor.
I want to replace this motor with a single phase... The three phase 5hp shows 1740 rpm and I noticed the single phase

5hp motors show like 3450 rpm. Just trying to find out if the increase in Rpm will be compatible with my compressor? Thanks for any help.

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[Skyking1992]

I don't think you want a 3450 RPM motor. 1725 RPM is available - here's one I found with a quick search on flea bay.

http://www.ebay.com/itm/5-HP-Single...722591?hash=item4163971f5f:g:~rgAAOSw4A5Yp25-

 

[Freejack]

Something on your nameplate is not right. If the motor is 2 pole as indicated on the left, then the RPM would be 3450 not 1725.

If indeed the motor is a 4 pole, 1725 RPM, then I would recommend keeping to that speed, going to a 2 pole, would require some change in the pulley ratios. I don't think you'll be wanting to spin the compressor at double the speed.

There are definitely 5hp single phase 1725 RPM motor is available. A couple options:

http://ecatalog.motorboss.com/Catalog/Motors/COMPRESSORS/Reciprocating-Compressors/F078

http://www.baldor.com/catalog/EL3612T

That said, 5hp single phase motors can get expensive. You may want to look into getting a 5hp VFD with a single phase input. You may find that it's cheaper and adds the benefit of softer starting and not needing to replace the motor.

Jake

 

[medic502]

I looked into the vfd... However from what I understand I would need a three phase rotary converter which is pretty expensive.

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[md21722]

You can get cheap VFD's on eBay for around $280 that do phase conversion. However, do take some work to setup.

In addition to the motor, you'd need to do something about a single phase magnetic starter, or change the heaters in the existing one unless its one of those electronic ones that can be adjusted in range.

It should not be switched through the pressure switch.

 

[matt_i]

Something on your nameplate is not right. If the motor is 2 pole as indicated on the left, then the RPM would be 3450 not 1725.

If indeed the motor is a 4 pole, 1725 RPM, then I would recommend keeping to that speed, going to a 2 pole, would require some change in the pulley ratios. I don't think you'll be wanting to spin the compressor at double the speed.

I agree, a 2pole motor should be 3450 and a 4pole = 1725 rpms. Of course for the same HP, the 2 pole motor will have half of the available shaft torque than the 4pole. Seems like you could just change pulleys, but you can't run a super small motor sheave as there isn't enough belt contact length to grip properly. Likewise the compressor sheave often has fan blades cast into it for air movement and also isn't easily modified.

When you swap motors, the Frame size is the best key to getting a motor which will bolt in place. Always good to confirm shaft sizes and brackets anyway as a trust-but-verify strategy.

If you have a 3 pole starter with an overload heater section, I would just bypass the normally closed contact which inhibits the relay coil. My single phase compressor doesn't have one and runs just fine for 21 years now.

 

[Z2V]

I just bought a 5 hp 1750rpm 1ph for $375 to my door. It is a Leeson #131537 off Amazon

 

[md21722]

There are some wholesalers/surplus Baldor guys on eBay. I'd expect a 5 HP to run you $300 for a name brand like a Baldor.

 

[marinusdees]

I looked into the vfd... However from what I understand I would need a three phase rotary converter which is pretty expensive.

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You would need EITHER a VFD or a rotary converter. That said, in general, air compressor motors do not like anything but three phase power if they are three phase motors. I am not equipped to offer an erudite discussion of the pros and cons in this dogfight.

If it were I, I would buy the Baldor 5 horse on ebay and just swap out the sheave. If the compressor has a magnetic starter, you can use it, just use two legs. Maybe swap out the heaters to match the motor. Google is your friend, Use it.

 

[medic502]

That should be a 4 pole...was hard to see when I looked the picture.

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[medic502]

I agree, a 2pole motor should be 3450 and a 4pole = 1725 rpms. Of course for the same HP, the 2 pole motor will have half of the available shaft torque than the 4pole. Seems like you could just change pulleys, but you can't run a super small motor sheave as there isn't enough belt contact length to grip properly. Likewise the compressor sheave often has fan blades cast into it for air movement and also isn't easily modified.

When you swap motors, the Frame size is the best key to getting a motor which will bolt in place. Always good to confirm shaft sizes and brackets anyway as a trust-but-verify strategy.

If you have a 3 pole starter with an overload heater section, I would just bypass the normally closed contact which inhibits the relay coil. My single phase compressor doesn't have one and runs just fine for 21 years now.

It is a 4 pole... I wrote over it wrong

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[md21722]

A lot of folks buy here buy Baldor L1430T

http://www.ebay.com/itm/NEW-BALDOR-...924731?hash=item2a74d7c73b:g:SBwAAOSw~bFWJ-Fc

for this application. If you are patient, you should be able to get one for around $275-300.

If you need a cheap magnetic starter, then something like this:

http://www.ebay.com/itm/SQUARE-D-MA...313386?hash=item4198e97eaa:g:mjoAAOSwmrlUz9MZ

Some folks will tell you go with a NEMA rather than definite purpose mag starter, but they cost a few hundred bucks. The definite purpose is factory default unless you ask for something better on brands like Champion, etc.

By code, wiring from the panel should be #10 THHN (in conduit) or #8 NM-b (Romex). Disconnect at the unit if panel is more than 50 feet away from panel, or out of sight of panel. Upsize wiring if distance is over 50 feet. Breaker can be up to 70A by code, but 30-40A should work fine. No plugs allowed, must be hardwired. I run my 5 HP air compressors off 30A breakers and have no problem with tripping.