Compressor on VFD - has anybody actually done it?

[scsmith42]

For several years I ran a 7.5hp IR 2 stage compressor off of a 30hp Phase-a-matic rotary phase converter. A few years back I upgraded to a 10hp 2 stage compressor, and could not get it to start under load off of the RPC. IT would start just fine if the compressor tank was empty, but if I tried to start it at the pressure kick-in (145psi), the motor would spin a few revolutions and then stall out when the compressor head starting developing back pressure. In order to make it work I had to install a pressure relief valve on the compressor head that is timed to close 8 seconds after the compressor kicks on (in order to allow the motor to come up to speed).

Basically the motor will spin up with no back pressure on the compressor, but if the tank is full it won't.

The compressor will start just fine w/o the relief valve if I'm running my 400hp generator and bypassing the RPC. So it appears that the RPC won't provide enough amperage to cover the high current inrush during the 5 seconds or so that it takes to spin up to speed off of the RPC when the tank is at pressure. When the generator is running, spin up time is less.


The external relief valved solve my problem, but I'm puzzled about why some folks recommend a VFD for a compressor. Will a VFD do a better job of providing full power through the start up current demand under load on a compressor than an RPC will? It seems to me that it wouldn't work

 

[rons]

Program the vfd with a soft start ramp time. I use 5-7 seconds.

 

[Firstram]

For several years I ran a 7.5hp IR 2 stage compressor off of a 30hp Phase-a-matic rotary phase converter. A few years back I upgraded to a 10hp 2 stage compressor, and could not get it to start under load off of the RPC. IT would start just fine if the compressor tank was empty, but if I tried to start it at the pressure kick-in (145psi), the motor would spin a few revolutions and then stall out when the compressor head starting developing back pressure. In order to make it work I had to install a pressure relief valve on the compressor head that is timed to close 8 seconds after the compressor kicks on (in order to allow the motor to come up to speed).

Basically the motor will spin up with no back pressure on the compressor, but if the tank is full it won't.

The compressor will start just fine w/o the relief valve if I'm running my 400hp generator and bypassing the RPC. So it appears that the RPC won't provide enough amperage to cover the high current inrush during the 5 seconds or so that it takes to spin up to speed off of the RPC when the tank is at pressure. When the generator is running, spin up time is less.


The external relief valved solve my problem, but I'm puzzled about why some folks recommend a VFD for a compressor. Will a VFD do a better job of providing full power through the start up current demand under load on a compressor than an RPC will? It seems to me that it wouldn't work

Im curious about the difference in inrush current values with and without your bypass valve.

 

[13engines]

Do you have a switchable starting boost circuit built into your RPC? The one I use to use did, and the manufacturer recommended leaving the starting circuit active when trying to start demanding loads or motors close to rating. I always needed that when trying to start my Shaper.

From what I understand, a VFD cannot give you more current capacity then you have to begin with. I mean how could it? But it can play a current limiting function in a soft start scenario as was mentioned. I sort of see a catch 22 in that prolonging the start sequence isn't going to change the effects of a continuously building cylinder pressure as the motor runs up.

I tried a soft start on my 10hp compressor (3ph). I didn't like it. I couldn't stand the is-it-going-to-make-it-to-speed sound of it all. I went back to plain ole hard starting. I'm thinking it will take some trial and error to get the start sequence timed just right.

 

[rons]

You can buy a powerful enough vfd and program the current limit. Then know what the limits are.
I'm a the end of a VFD buying decision. Was thinking yesterday that if I changed my 5 Hp motor to a 10 Hp
the vfd expense would be great and the choices would be fewer. Look at what I researched.

15 hp vfd for 5 hp motor on compressor

The volume is 120 gal so the motor is on longer than for a smaller tank. Should I use 15 Hp and not 10? For $100 more the components are rate 50% higher. 10 Hp. https://wolfautomation.com/vfd-10-hp-3-phase-200-240v-32-44-46-a-1 15Hp...

www.practicalmachinist.com

Yaskawa GA50U2056ABA (20 Hp) $1130
Hitachi SJP1-P1-0800-LFUF (20 Hp) $1549
Hitachi WJ-C1-115LFU2 (20 Hp) $1324
Invertek ODE-3-520610-3F42 (20 Hp) $1800

Those are my top picks for your motor size. Each of those models come in 25 Hp or higher.
With four seven segment displays the error translation and programming is not that fun. If you just do it once then who cares.
But a TFT LCD display is so easy to use but costs roughly $200-$300 more.

I'm looking at the WJ-C1-110LFU2 (15 Hp) for my smaller 5 Hp motor.
A distant thought is to buy one of these and get a 10 Hp motor.

 

[SAF]

For several years I ran a 7.5hp IR 2 stage compressor off of a 30hp Phase-a-matic rotary phase converter. A few years back I upgraded to a 10hp 2 stage compressor, and could not get it to start under load off of the RPC. IT would start just fine if the compressor tank was empty, but if I tried to start it at the pressure kick-in (145psi), the motor would spin a few revolutions and then stall out when the compressor head starting developing back pressure. In order to make it work I had to install a pressure relief valve on the compressor head that is timed to close 8 seconds after the compressor kicks on (in order to allow the motor to come up to speed).


The external relief valved solve my problem, but I'm puzzled about why some folks recommend a VFD for a compressor. Will a VFD do a better job of providing full power through the start up current demand under load on a compressor than an RPC will? It seems to me that it wouldn't work

I Believe you are correct, in your thinking. A slow start is also a poor choice for a piston pump, they need to get up to speed quickly to properly lube themselves, especially if they are splash lubricated.

For a larger pump on a limited power supply the best solution, whether that is for a weak utility supply, phase converter or VFD is to use a mechanical pneumatic solution.

A suction valve unloader system system is the best choice. Most larger and higher quality pumps are capable of this type system. It allows the pump and it's drive system to start unloaded and to get up to speed before the pump is asked to perform any loaded operation. It can also be set up for a constant run operation when the air demand is very high, where the motor stays running and the pump is unloaded and the intake valve is held open.

A second common problem is with the tank check valve, either missing or leaking combined with the regular end of cycle unloader not functioning correctly, whether that is a centrifugal operated valve on the crankshaft or one mounted on the pressure switch, When there is a missing, or nonfunctional unloading system you can expect operational problems, hard starting and burned up drive components.
There is a big difference between intake valve unloaders and end of cycle discharge line unloaders. smaller and less costly pumps use the end of cycle methods on the discharge line. Bigger pumps designed for heavy use, normally use the intake unloaders. Some pumps can be retrofitted with the intake unloader piston actuator if they were not fitted by the manufacturer.

More than 20 years ago I built one of these systems for a busy bump shop that had a weak open delta utility supply, he had no complaints with his compressor but had some issues with his downdraft paint booth do to the weak utility supply.

I will attach a paper that describes the system from a compressor company and see if I can locate a drawing in my old files on how to put together a system.

https://www.psgdover.com/docs/defau...ressor-bulletins/cb039.pdf?sfvrsn=e28ae8f8_18

 

[scsmith42]

Im curious about the difference in inrush current values with and without your bypass valve.

I'll see if I can record some numbers and post them.