Advice on Building a Compressor

[MEngineer]

Hi everyone, it seems like there is a lot of talk around the subject of using VFD units to power a 3 phase motor, but I wasn't able to find a clear answer to my question, so here it is:

I am currently at the crossroads of a decision. I have access to the pump which will come will a 5hp 3 phase motor and the associated controls.

I also have a line on a 5hp 240v motor as well. For the same price, I can buy a 4kw VFD or a 5hp 240v motor.

What should I do?

Also to note, my compressor has dual unloaders so I was thinking that I might be able to go the VFD route and be able to tune my ramp time to reduce amperage surge at start up.

Also, if I go with the VFD, I don't think I would need a magnetic starter switch?

What am I missing?

Thanks for the help in advance!

 

[texasranger]

Assuming you're planning to feed the VFD from single phase, you'd need to de-rate unless the VFD is rated for single phase input (they rarely are above 3hp or so). I've heard a good rule of thumb is to use a 10 HP VFD to run a 5 HP 3 phase motor.

I'll be interested to hear other input since I'll be hooking up my milling machine which is 3 phase to my residential single phase power with VFD.

 

[md21722]

I would buy the 5HP single phase motor and save the VFD idea for smaller equipment.

When used as a phase converter, you do need to derate a VFD minimum 1/3 but many times 1/2 is recommended. The VFD takes the place of the magnetic starter.

Starting a 5 HP motor is not too difficult for single phase power if you have "decent" service. I don't have anything fancy here and can start and run 2 5HP compressors just fine. They are true 5HP compressors.

 

[racingtadpole]

http://www.danfoss.com/NR/rdonlyres/1FDE93BE-95CF-46F9-B2AE-6F76C69594CB/0/vfdlesson4.pdf

^^This may be of some assisance. It explains constant torque, over torque, variable torque and a some other things about using a VFD. Written based around larger industrial stuff but the principles are the same.

Personally, I would put a single phase motor on the compressor head, it will be cheaper to run than using a single to three phase VFD. 5HP on the three phase output of a VFD requires a whole lot more current in than the 5HP single phase motor.
Having said that, if you want to play with one, get yourself a VFD suitable for constant torque applications and get amongst it.

 

[MEngineer]

So it looks like I should just roll with the 240v option. In that case, do I need a magnetic motor starter, and if so, does anyone have any recommendations?

 

[md21722]

http://www.ebay.com/itm/ST122001AJ-...202090?hash=item3f5c489c2a:g:mjoAAOSwmrlUz9MZ

I bought two from them, one for each of my compressors. They are a good vendor. They ship fast and can also call them if you need something else.

 

[MEngineer]

http://www.ebay.com/itm/ST122001AJ-...202090?hash=item3f5c489c2a:g:mjoAAOSwmrlUz9MZ

I bought two from them, one for each of my compressors. They are a good vendor. They ship fast and can also call them if you need something else.

Thanks I will have to look into them

 

[PSYKO_Inc]

For 5hp, I'd just go with a single phase motor and sell the 3 phase to offset the cost. One less thing to go wrong. Throw on a mag starter (or straight to the pressure switch if the switch can handle the current load) and you should be good to go.

 

[md21722]

They do make pressure switches that can supposedly handle a full 5 HP motor but they aren't cheap, and are still not as good/reliable as a dedicated magnetic starter. The compressor shops I talk to balk at the idea of using them and they make service calls to repair/replace them when they fail. I've heard the Square D are more reliable than the Siemens/Eaton types.

 

[MEngineer]

The 240v 5hp motor I am going to buy came off another guy's compressor when he had a problem with the pump...

Anyway he is going to include the motor controls and pressure switch with the motor when I obtain it from him. I will make the determination at that point if I need a motor starter.