Moved Machine Shop to House, Air Compressor Problem

[Kparentmachinist]

Hello all.
Just moved my machine shop back home to the garage. My last machine didn't require air, but my new Robodrill does. I have the mill hooked up to my phase convertor I already had, but the kick on from my 3 phase Champion reciprocating compressor cause the power to fault out. So I am looking for a cheap alternative to run it on its own power source. I am looking at VFDs to be able to do so. Does anyone run their 3 phase compressor on a vfd? I've only used rotary phase convertors, not vfds so its a new world for me. Looking at a 10 HP vfd. Any advice would be appreciated!

 

[13engines]

You mentioned 10HP VFD. Is this compressor 10HP? That might put the following idea out of reach.

Maybe consider swapping out to a 240V single phase motor instead. Don't know a lot about VFD's, but a RPC always needs to see the largest motor started first. And few things are harder to start then a compressor motor. Single phasing the compressor would allow you to not have to give your 3ph power concerns another thought.

 

[dalmatiangirl61]

What is the HP of the compressor motor? RPC usually needs to be 2x larger than largest motor, sometimes 3x. If using single phase thru a vfd you need to oversize the vfd, not sure if there is a rule of thumb on how much to oversize.

Do you have adequate amps to the rpc? Adequate wire size? Please add the specs of the rpc system

 

[Seesecurity]

You mentioned 10HP VFD. Is this compressor 10HP? That might put the following idea out of reach.

Maybe consider swapping out to a 240V single phase motor instead. Don't know a lot about VFD's, but a RPC always needs to see the largest motor started first. And few things are harder to start then a compressor motor. Single phasing the compressor would allow you to not have to give your 3ph power concerns another thought.

A single phase 10hp motor?

 

[Vancbiker]

.......If using single phase thru a vfd you need to oversize the vfd, not sure if there is a rule of thumb on how much to oversize.......

This is not true. Many VFDs will run the rated output regardless of single or three phase input. One needs to check the datasheet for the VFD under consideration to verify.

 

[Pete Deal]

Yes they (single phase 10hp motors) exist. Or with some pulley changing to slow the pump down some you could probably go to 7.5hp.

No way I’d consider using my rpc generated 3 phase to run a compressor.

 

[Vancbiker]

Yes they (single phase 10hp motors) exist. ........

Yep, but probably $1500

 

[matthewlee]

I'm running a 7.5hp three phase compressor on a VFD. I don't have any issues. The VFD is sized the same as the motor. The VFD is a cheapo from Amazon. Been running for two years now.

You may want to look into getting a phase perfect instead of using the rotary converter. I had the same issue running a 40 hp rotary. It was eating up a big chunk of my available amperage just running the motor. Phase perfect eliminated that issue and the supplied electric is way more accurate and balanced.

 

[M. Moore]

I ran multiple machines off of a single rotary converter.
One of them was a compressor. It was setup so the compressor demand would turn on the rpc and then 5 seconds later the rpc would turn on the compressor.
The rpc was correctly sized, the wiring was correctly sized and everything ran perfectly for many years.
If the rpc was already on the compressor would start right away.

Sounds like you need a qualified electrician to give you some advice on site, asking here is not going to get you the answers you need.

 

[dalmatiangirl61]

This is not true. Many VFDs will run the rated output regardless of single or three phase input. One needs to check the datasheet for the VFD under consideration to verify.

Many, not all, so its half true

 

[Rob F.]

This is not true. Many VFDs will run the rated output regardless of single or three phase input. One needs to check the datasheet for the VFD under consideration to verify.

I think this is "generally" right up to about 3HP, over that they are "generally" derated IF using single phase input.
OP has not said what HP his compressor is, could be he has 5hp and knew to look at 10 hp VFD.
There are some posts with guys using the eastwood screw compressor, maybe sell the current one and get something easy to run? Or swap the motor.

 

[Garwood]

I ran multiple machines off of a single rotary converter.
One of them was a compressor. It was setup so the compressor demand would turn on the rpc and then 5 seconds later the rpc would turn on the compressor.
The rpc was correctly sized, the wiring was correctly sized and everything ran perfectly for many years.
If the rpc was already on the compressor would start right away.

Sounds like you need a qualified electrician to give you some advice on site, asking here is not going to get you the answers you need.

I've done that same thing and it didn't work for cnc's. The compressor start current pulls the generated leg voltage down below the cnc's low voltage threshold.

 

[Garwood]

I think this is "generally" right up to about 3HP, over that they are "generally" derated IF using single phase input.
OP has not said what HP his compressor is, could be he has 5hp and knew to look at 10 hp VFD.
There are some posts with guys using the eastwood screw compressor, maybe sell the current one and get something easy to run? Or swap the motor.

The dedicated single to 3 phase vfd's are designed for thier nameplate rating.

 

[DavidScott]

Do you need all 10 hp? I put a smaller motor pulley on my 5hp compressor to reduce the starting load and quiet it down for my neighbors, which worked great for both issues.

 

[Cole2534]

This comes up from time to time, I can't help but think there's a way to help the compressor have an easier start. Something like an unloader or sending tank air into compressor suction (recycle), just ways of reducing DP across the compressor until the motor is up to speed.

Unloader would probably be the easiest, just a solenoid valve on a timer hooked up to the pressure-trol. Maybe a small volume pot to reduce the shock on the system when the unloader closes. Don't forget a check valve at the tank.

 

[706jim]

What cfm does the Robodrill require? Somehow I think your compressor is way oversized for its needs.

 

[Rob F.]

Here is his other thread on it....More details there.....

VFD Single Phase to 3 Phase 5hp Compressor

Hello all. Just moved my shop back home to the garage. My last machine didn't require air, but my new Robodrill does. I have the mill hooked up to my phase convertor i already had, but the kick on from my 3 phase Champion reciprocating compressor cause the power to fault out. So I am looking for...

www.practicalmachinist.com

 

[Rob F.]

The dedicated single to 3 phase vfd's are designed for thier nameplate rating.

Thanks for clarifying that, I was thinking of standard 3 ph units.

 

[Seesecurity]

Yes they (single phase 10hp motors) exist.

Yeah, but that's 110A draw with your 110v supply, do you have domestic supplies that big?

 

[Vancbiker]

Thanks for clarifying that, I was thinking of standard 3 ph units.

Even then derating is not always needed. I have a 5hp continuous, 7.5hp intermittent, 8k rpm Mitsu spindle motor and drive on my Mori Seiki mill. It is designed as a 3 phase unit but I run it on single phase (whole machine actually). Standard accel/decel time constants and max current parameters. It runs fine and has been doing so for nearing 11 years now.

A significant factor is that the 3 phase VFDs are designed to run on voltage as low as 200V and still produce their rated power. It really boils down to Watts. When you run one on 240V single phase the higher voltage helps deliver the Watts needed to maintain the DC bus voltage high enough to produce the rated output.