VFD or Static Converter for 7 H.P. Air Compressor?

[Richard D]

My friend gave me a big air compressor. It has a 7 H.P. 3 phase motor, so I will need a converter. I used a VFD on my mill and like it, so I figure i'll do the same on this. It doesn't need variable speed, but maybe it would be nice? Make it run slower=quieter? Is there a cheaper way to convert 3 phase to single phase without loosing power? The 7 H.P. VFD is about $400.

Edit:
I just found this:
http://www.grizzly.com/products/Static-Phase-Converter-3-to-7-HP/G5843
Maybe just the ticket?

 

[Charles (in GA)]

Buy a new single phase 7.5 hp motor of the proper speed and be done with it. While it is certainly possible to run a 3 ph compressor on a converter, it really, in my mind, doesn't make sense.

Charles

 

[Norcal]

A static phase converter is about the worst thing to use for a 3 phase motor, & even worse to use on a compressor, a static converter starts a 3Ø motor & then it just single phases, you lose 1/3 of the HP also, any VFD over 3 HP is 3Ø only so you need double the HP rating + have to be able to disable the phase loss protection in order for it to work, so a 7 1/2 HP motor (7 HP a oddball) would need a 15 HP drive, $$$$. your 3rd option is a rotary phase converter, Practical Machinist has a whole sub forum just for transformers, phase converters, if you wish to learn more,if you log in to OWWM, there is a electrical forum there too. Option #4 is buy a 7.5 HP single phase motor.

 

[Richard D]

Yeah, I just found a 7.5 for $380 shipped.

Will I screw up my mill with the VFD? It's a 3/4 H.P.

 

[Norcal]

3 HP & under VFD's work great for phase conversion, & they can be bought cheap, I have 3, & 2 it was the best choice as the motors were 200V 3Ø & was able to adjust the output voltage for those tools. 3rd one was expensive because I wanted a NEMA type 4 enclosure.

1, is a 7" grinder, 2, a wood shaper, the only one that a motor swap was a option, 3rd is a drill press that could not find a single phase motor that I would want to use, Rockwell used some odd motor frame sizes.

 

[C96]

Option #4 is buy a 7.5 HP single phase motor.

This in my opinion is your best solution. Simple to make the conversion and less to malfunction.

 

[Richard D]

Thanks guys.

 

[rsanter]

VFDs work great for phase conversion up to about 5hp
One thing to watch is that you do not want to run the motors at a much lower RPM for an extended period of time ( like a compressor) because the fan in the motor will not be able to move enough air.
For a mill or lathe it is fine because generally we do not run the machines under heavy load for long periods of time.
I have seen where people running a VFD on a motor where they knew they would be running fairly low RPMs have attached an extra fan to the outside to blow air through the motor to be sure it stays cool

Bob

 

[Letsgobowhunting]

I think a homemade Rotary phase converter would be your best and least expensive option. If you get a big enough motor you can run additional equipment on it at the same time.

 

[Norcal]

VFDs work great for phase conversion up to about 5hpOne thing to watch is that you do not want to run the motors at a much lower RPM for an extended period of time ( like a compressor) because the fan in the motor will not be able to move enough air.
For a mill or lathe it is fine because generally we do not run the machines under heavy load for long periods of time.
I have seen where people running a VFD on a motor where they knew they would be running fairly low RPMs have attached an extra fan to the outside to blow air through the motor to be sure it stays cool

Bob

After 3 HP , the prices get really high for them because of having to derate them 50%, the 3 HP & smaller single phase input models require no derating. I bought a 3 HP model for less then $200 shipped.

 

[rodm1]

Yeah, I just found a 7.5 for $380 shipped.

Will I screw up my mill with the VFD? It's a 3/4 H.P.

No if installed right it will be fine but a VFD will not give you the full 3/4HP not a problem on your mill but you might need the full 7.5HP on the compressor. A VFD adds some neat capability you can use on the mill but not needed on the compressor.

 

[Norcal]

No if installed right it will be fine but a VFD will not give you the full 3/4HP not a problem on your mill but you might need the full 7.5HP on the compressor. A VFD adds some neat capability you can use on the mill but not needed on the compressor.

There is no loss of HP w/ the single phase input VFD's, go for it.

 

[larry_g]

My friend gave me a big air compressor. It has a 7 H.P. 3 phase motor, so I will need a converter. I used a VFD on my mill and like it, so I figure i'll do the same on this. It doesn't need variable speed, but maybe it would be nice? Make it run slower=quieter? Is there a cheaper way to convert 3 phase to single phase without loosing power? The 7 H.P. VFD is about $400.

Edit:
I just found this:
http://www.grizzly.com/products/Static-Phase-Converter-3-to-7-HP/G5843
Maybe just the ticket?

The grizzly unit will not do it for you as said above. One option that has not been put forward to you is going to a smaller single phase motor. A lot of compressor company's rated their pump heads for a range of motor horsepower's. When going to a lower HP motor you also have to change the motor pulley to something smaller to slow down the pump speed and allow it to work in a speed range that the motor is capable of providing without overcurrenting the motor.

I would suggest that you search out he specs on the pump head and find the horse power it is rated at as well as the speed operating range. Give up the manufacture of the pump and the model number and we might be able to help in this.

lg
no neat sig line

 

[wanderer]

I did the same thing with a 3 hp single phase motor. The pump can be run on 7 1/2, but it 3 hp in the right speed the thing puts out about 15 CFM's and is very quiet. I love it!

 

[Doradoguy]

Not sure what compressor pump you have, but most pumps can be run on smaller motors with a smaller pulley. There are several small pumps that are not balanced properly so we up size the pump and run a smaller motor. There is some math involved and you need to know your desired target pressure.

As far as vfd drives go on air compressors, they're used on rotory screw compressors for trimming for a very targeted pressure and need to be used properly. If you run the compressor to slow it will pass oil and if run it wide out you are wasting power.

Back to your problem. If you have enough power for the 7.5 " 80 amps at 110 volt or 40 amps 220 volt". I would replace the motor. Vfd and phase converter or anything with a heat sink is going to use more power. Heat is energy loss. Whatever direction you go, be sure to put an amp clamp on the motor leads and set the cutout pressure at 5 to 10 percent over the full load amps on the name plate of the motor.

If it were me, I would research a 5hp motor, change pulley and set top of pressure at 135 and the bottom at 115 or so. Depends on size of tank.

Rule of thumb 3 cfm per hp for a piston machine and 4 cfm for a rotary screw.

Most efficient compressor is running wide open or shut off. Any other drives or controls is less efficient.

I hope it helps.

 

[Richard D]

I did the same thing with a 3 hp single phase motor. The pump can be run on 7 1/2, but it 3 hp in the right speed the thing puts out about 15 CFM's and is very quiet. I love it!

This may be the way to go, it is to be used in my garage, my compressor now is very loud. I can possibly rob the motor/pulley from my existing compressor. I should have the big compressor here in few days and will check back with you guys.