# Recommendations on a VFD for a screw compressor, 1ph > 3ph



## BrandonK (Oct 18, 2011)

Our attraction uses a bit of compressed air and we have decided to do an overhaul on our air system. I'm removing (3) 1ph 5hp recip compressors, to be replaced with a 25hp Chicago Pneumatic QRS screw (230v 3ph).

My first questions (and maybe I should be asking CP first), can a VFD properly run a screw compressor? IE, since the compressor seems to have it's own motor control and logic control built into it, will a VFD work for that? My inital thoughts lead me to believe that a VFD must output directly to a motor and not just any given 3ph load. If that isn't the case, does anyone have any recommendations on vendors?

If that is in fact the case, then we're likely stuck. I had considered a rotary converter, but for a compressor load, you're supposed to go 2 to 3 times the HP rating, which is a ~50hp rotary converter. Those cost nearly as much as the compressor.

Adding a 3ph service is out. It too is prohibitively expensive, if even at all possible. Cliff notes regarding that: our location is inside of a county recreational park. Any utility work has to be done through the county as they own the lines (the local POCO does NOT own the lines, they just supply them, thus I can't just call up the POCO). Dealing with the county is just about a nightmare on anything, to many supervisors of this and managers of that. Not to mention there isn't a 3rd phase near our building, requiring ~2 miles of buildout of the 7.6k mv lines. So as it is, we're stuck on a 1ph 400A service (500 Cu straight from the pig to a 400A main breaker, no meter).

Thoughts?


----------



## JRaef (Mar 23, 2009)

It's not going to be much easier with a VFD either. Still might be your best bet though. 

1) You will have to use a 50HP 230V VFD to up convert from 1 phase to 3 phase, not cheap. 

2) Because of that, you will be required by the NEC to size the circuit for 125% of the maximum VFD amp rating, not the motor rating. So a 50HP 230V VFD will be rated for something like 125A, which means a 156A minimum circuit and 175A breaker. 

3) The VFD cannot have any switching devices down stream, i.e. motor starter or contactor. So you will have to remove what exists and adapt the control circuit to make it work with the VFD. Not too difficult if you are good at controls, but worth mention. 

4) The screw compressor may (likely) have a pressure sensor and timer circuit that checks to make sure the compressor is working. A slow ramp from the VFD may cause nuisance tripping in the screw compressor controls. You may need the ability to find and adjust that in the compressor controller. If you try to make the VFD accelerate the motor faster instead, it may mean an even bigger VFD or a more expensive one that gives you Sensorless Vector Control that is capable of providing up to locked rotor current. Again maybe not a big deal now days, but worth mention.


----------



## micromind (Aug 11, 2007)

JRaef said:


> It's not going to be much easier with a VFD either. Still might be your best bet though.
> 
> 1) You will have to use a 50HP 230V VFD to up convert from 1 phase to 3 phase, not cheap.
> 
> ...


Exactly true.

I would add; use a load reactor between the VFD and the motor. Most rotary screw compressors use motors specifically designed for the application, and they are typically run right at their limit. These motors don't do well with distorted waveforms, and a load reactor will help. 

Also, make sure the VFD will accept single phase input, most will some won't.


----------



## BrandonK (Oct 18, 2011)

Thanks for the replies gent's. I'm going to have to investigate them a little further and more so, see what Central Pneumatic recommends. Since the compressor has it's own control set for start up, load, unload, etc, I gather that I can't just use the output from the VFD directly to the compressor like I could with a rotary phase converter.

That said, I did some further looking into rotary converters and it would appear that the first source I looked at (Phase A Matic) is significantly more expensive than everyone else. I also found that since it's a screw compressor and it starts unloaded, I can drop down to a 30hp converter and be fine. I can pick up a 30hp "CNC" rated converter for less than $1800, or significantly less if I build it myself. 

So, 100A THQB in the panelboard (I know, GE garbage, I didn't spec it), #3 or #2 Cu to the converter, hang a 3ph panel off of the converter and then a 60a breaker and #6 Cu to the compressor. The dryer is basic, 1ph @ 6.5a.


----------



## Netree (Sep 3, 2011)

micromind said:


> Also, make sure the VFD will accept single phase input, most will some won't.


You usually must disable the phase loss protection in the parameters.


----------



## frenchelectrican (Mar 15, 2007)

Netree said:


> You usually must disable the phase loss protection in the parameters.


Disable it on the input side but you still need it on *output* side so check with the VSD unit to see if you have dual setting if so then follow the instruction on phase loss protection.

Merci,
Marc


----------



## Netree (Sep 3, 2011)

Yes. By example, on (certain) Baldor drives parameter H19 is for the handling of the phase loss. You can select incoming & outgoing, incoming only, or outgoing only. You would set this parameter to "1", outgoing only.


----------



## frenchelectrican (Mar 15, 2007)

Netree.,

It is pretty common for most VSD to have few diffrent function on phase loss protection so it pretty much my SOP to do that to use the phase loss protection on output side due from time to time I get few VSD did go bad and loose one the phases and it will force the VSD to drop off line mode.

BrandonK.,

Couple guys did give you few very good tips along the way and with screw compressour there is one more item you will have to program in the VSD is anti phase rotation { or go in reverse mode } otherwise lock the reversing function complety due many screw compressours are very picky on rotation if you run them backward they can do a bit of damage { two thing will come in my mind real quick is oil pump and air flowage } 

And the other thing if your inspector required a local disconnect switch for any reason if you have to use the disconnect switch on VSD side make sure you get interlock switch so it will turn the VSD unit off before you pull the handle otherwise the voltage from VSD can self destruct { I have see it once a while } I rather have a disconnect switch on line side of VSD this is safer methold.

Merci,
Marc


----------

