What to look for when replacing a 3 phase compressor motor?

[bmwpower]

Looking at a 3 phase compressor. I don't have 3 phase power so I wanted to replace the motor. If I look at stuff, say, at Grainger, what do I look for? There's a ton of different motors, but I have idea what I need.

Current motor is 5 HP, 3 phase, 3450RPM.

Also, the setup has a mag starter. Do I just disconnect and remove this?

So far I guess I would:

- choose a motor of the same HP
- choose a motor of the same RPM (3450)
- choose a motor of the same mounting type

 

[W-Cummins]

I would run the mag starter. You will need to change two of the overloads to match your motors amp draw. I wouldn't run a 5 hp motor with out a mag starter ( I know that cheap machines run 5hp?? motors just using the pressure switch, but I will not....

Yep that's it, just match the speed and frame size, and you will want to match the load rating also. I would run a tefc motor as I think that they are better than an open drip proof motor for use on a compressor.


Here is a baldor that should work just fine

I would check e-bay for one also as you will save money buying it there. Here is one there

William....

 

[bmwpower]

Thanks for the reply.

BTW, what does a magnetic starter do differently?

Why do you think the TEFC is better?

Thanks again!

 

[CraigFL]

The magnetic starter isolates the motor from the off/on switch and uses less current-- it's like a relay controlled on/off.

I prefer TEFC (Totally Enclosed Fan Cooled) motors for compressors too because the fan keeps the motor at a much more even temperature during long on cycles.

 

[bmwpower]

Thanks Craig.

Makes sense to me now.

 

[W-Cummins]

The Magnetic starter also provides over current protection for the motor. The Contactor in the magnetic starter is also rated to switch under load the high current and, they have large replaceable contacts.

Another good feature of a tefc motor is dust can't get into it. Normally a compressor has a cooling fan on its large pulley and it blows dust and **** around that gets into an open motor and can cause problems.

BTW these two "features" are also some thing to look for when selecting a good quality compressor.

William....

 

[bmwpower]

Cool. Thanks for the info.

The guy selling it claims I can get a static phase converter for $100 at www.use-enco.com.

I checked over there and it sound like I need something more than the $100 model. It also says static converter produces up to 67% of the rated output of the 3 phase motor. Doesn't sound like it will produce the power I need.

 

[W-Cummins]

Cool. Thanks for the info.

The guy selling it claims I can get a static phase converter for $100 at www.use-enco.com.

I checked over there and it sound like I need something more than the $100 model. It also says static converter produces up to 67% of the rated output of the 3 phase motor. Doesn't sound like it will produce the power I need.

Nope you need a rotary phase converter and you can make one for far less than $100 if your any good at finding used stuff.

Pickup a used 7.5 hp 3phase motor and then a few caps, a switch, and your up and running!

William....

 

[PAToyota]

I'm looking into this phase converter stuff. There seems to be a lot of information on Google and places on eBay willing to sell you plans or kits. William, any suggestions on good places to start on this?

 

[W-Cummins]

Well It might help to know what you are planing on running. Small systems are real easy to setup and infact I bet I could build a simple one for about $20 with a used 3 phase motor and NEW parts for the rest of it. Here is a link to the pictures of the last one I made.
It was a more than "basic" model and had very close line balance, PF correction capacitors etc. ( could have run a cnc machine w/o problems)

I'm not sure that I would want to run a compressor off one unless you:
a) need to run the compressor alot
b) need to run other 3 phase loads and the compressor alot

This is due to the fact you normally need to have the converter running all the time to have the 3 phase power for when the compressor switches on. I guess you could have it ( the converter) start with the pressure switch and then have a delay for the compressor starting. With a few timing relays you could have it start then say run 10min and turn off unless the compressor cycled again ( or if so extend another 10 min etc..)

William...

 

[PAToyota]

Sorry, not a compressor. I've been looking at electric chain hoists and they are more often than not three phase as you get larger. I think that would suit the bill for a phase converter - nothing fancy and I wouldn't be "cycling" the hoist like you would with a compressor.

 

[OldCarGuy]

In my garage workshop I have static converters, rotary phase converters, and VFD’s Variable Frequency Inverters. All three have their advantages and disadvantages. But in the case of an air compressor of 10 HP or less I would highly recommend refitting it with a single phase motor. Additionally it is best to convert any equipment to a single phase motors.

If you choose to go with a three-phase motor on an air compressor, go with a rotary converter twice the size of the air compressor motor's HP. A static converter only creates the third leg for a few seconds to start the motor, and consequently not enough power start an air compressor. And only produces 2/3 of the motor’s nameplate power once started, not enough to keep it running even if it would start it.

I have a 4,000 pound electric chain fall with a 1 HP three-phase motor. The sales company that sold me my 2 HP VFW thought that it would operate my chain fall. However it wouldn’t. The manufacturer acknowledged that it wouldn’t work when I contacted them. As a VFW will not energies the hoist’s magnetic brake. It runs okay with either my 5HP or 10HP rotary converter.

 

[W-Cummins]

Sorry, not a compressor. I've been looking at electric chain hoists and they are more often than not three phase as you get larger. I think that would suit the bill for a phase converter - nothing fancy and I wouldn't be "cycling" the hoist like you would with a compressor.

Get a used 1750rpm 3 hp motor (220v/230v/240v any listed) ( assuming <=1.5 hp motor in the hoist.) Make sure that the unit is tight ( not lots of end play and little to no radial )

Pick up a momentary switch and about 175-200 Mfd (total) of 250V electrolytic capacitor/s and a 30 amp 2 pole AC disconect switch.

Wire the momentary switch so that when you press it you connect the above cap/s to the third leg and one of the hot (120v to ground) wires to start the motor. ( you wire the 2 hots {from the disconect} to the other 2 legs of the 3 phase motor. Then throw the switch on the disconect and hold the momentary switch on, when the motor starts remove your finger from the momentary switch! You now have a basic phase convertor running and generating a 3rd leg. If you want to you can balance the convertor better ( makes the generated leg closer to the same voltage as the other 2 legs)

Check out this link it's got some good information about setting up and tweaking the balance caps if you need them, lots of stuff runs just fine with out them.

If you want to you can add a relay in place of the momentary switch to make the unit start with out you holding the push button down. Also at the link above they show you how to make a self starting convertor with oil filled 370V caps (more cost but not too bad)

William....

 

[PAToyota]

William, many thanks for the explanation and the link!

Thomas