Problems adapting a VFD to my air compressor

[peter2772000]

Hey guys, I've got a problem getting my VFD to run with my air compressor.

With a smaller drive pulley, it'll start and build up speed with no issues. Soft start of course, but the motor gets to it's 60Hz/3450 rpm without issue. Problem is, I'm only getting 830 rpm at the flywheel. To maximize compressor output, I need 950 rpm.

Originally (and because my original 3.25" adjustable pulley was closed up to the max), I simply bumped up the hertz to 70, which gave me exactly 958 rpm flywheel speed. Utopia, right?

Well, then I started thinking, never a good thing. Is it possible that running the motor to 70 hertz (which gave me a motor rpm of 4k rpm) would/could cause harmonic vibrations? It just felt that the motor was shaking a bit more than I thought it should. So I bumped up the drive pulley to 3.75, and that's when everything took a dump.

Simply put; The compressor never gets up to speed. Starts slowly and builds itself up head-pressure, which it then finds impossible to fight before getting up to speed, and stalls out. If I drain the tank completely of air-pressure, the compressor will start and get to the target compressor rpm, no problem. But if there's even just 10 psi in the tank, I end up with the same problem.

So unless someone else with VFD's can shed light on this problem, or I;

- A, go back to the smaller drive pulley and up the hertz back to 70
- B, install an unloader type solenoid on the discharge line of the compressor which would stay open for the first 10 seconds or so 'till the compressor was up to speed

I realize that simply putting on the smaller pulley would fix this. But it bothers me to have to run my motor at 70Hertz.


Now....... it's personal.....

 

[dutchgray]

I would run it at 70 Hz, or 68 or whatever seems to work, the motor should be fine if its decent.
Alternatively up the max current a little so the VFD is pushing the motor harder (if you have the headroom) and see if it can cope with the higher gearing, fine as long as it runs within limits at full speed, I had to do this on my pillar drill as it would trip out on start if used on top speed range, gave it 15% over.

 

[larry_g]

My best guess is that you don't have the power to spin the compressor at the speed you want. I would suggest that you do an amp draw on the input to the VFD and see what power you are pulling there. Then with the VFD set at 60 hz check the 3 legs going into the motor. Make sure they are balanced and within the limits of the motor. Do these test when the the compressor is near cutoff/ at max load.

Do currents check within the limits of your equipment.

lg
no neat sig line

 

[Handyfarmer]

I would back off the pulley size, and keep the smoke in the converter, and the motor.

Yes you will lose a bit of air capacity volume wise, but I bet you will hardly ever notice the difference.

 

[Handyfarmer]

I am not exactly sure how a VFD works on converting single phase to three phase,

but a static converter, works by using one of the three sets of windings in the three phase motor, as a start winding, and then kicks it out when it is operating, thus only two of the three windings are are used, and one needs to down rate the motor by 1/3 for use, so a three HP only produces two HP,

and you soft start is not really helping either, as the unloader valve only unloads the line be tween the pump to the tank, so it does not start under pressure after a few rotations that line is at pressure and the motor needs full rpm to produce enough power to run the compressor head at pressure, (I do not remember the down rating, but most of the time you slow the rpms down and the HP drops like a rock), one by pass is to put a small tank on the line off the compressor so it has a longer time to get up to pressure when starting, or a lot more line, (call it a cooler if any one asks ,

what I have always heard is if one wants to run a compressor ( Hard starters, things that start under load)_(three phase ran on single phases) is to use a rotary converter, and in the starter have few second delay so the converter gets up to speed, first before the compressor motor kicks in, so it can be generating the third leg that is needed,
that is how I ran my walk in cooler and freezer, compressors before getting three phase,

soft starters like table saws planers etc that do not start under load do not need the extra that a rotary gives, but your Hp still suffers,

even on a rotary you hp is not the same as a true three phase as the rotary does generate power on the third leg but unless you have a number of motors all running on the system at once you have demished third leg, the more motors running the truer it is as all the motors are helping generate the third leg, (it is like running things off a small home generator instead of the grid on that third leg, there will be some losses),

(I am old school and really do not know much about the VFD system)

but have used static and rotary converters for may years),

and if your running this off the 15 amp circuit it may not be enough to satisfy the amps needed or the voltage, do to voltage drop,

 

[454ragtop]

Let me start by saying I love VFD's, have them on lathes, milling machines, band saws, drill presses, but I would never put 1 on a compressor. It's just the wrong app. A compressor doesn't need variable speed or reverse, and is constantly running at near full load of the motor. Add to the fact this is a low end compressor with a 3450 RPM motor, which generates less torque than a 1725 RPM motor. Best bet would be to convert to a single phase motor, which I always recommend with compressors. If you don't want to do that, go back to the smaller pulley. Are you actually running out of air at 60 Hz, or are you just blindly determined to get every last CFM out of the compressor? If you actually need those few more RPM, increase the speed and look at the parameter list for things like carrier frequency to smooth the motor out.

 

[alan camby]

change the setting in the vfd to allow motor to accelerate to full speed faster. Most are factory set to take 5 seconds to get up to speed.

"So I bumped up the drive pulley to 3.75, and that's when everything took a dump.

Simply put; The compressor never gets up to speed. Starts slowly and builds itself up head-pressure, which it then finds impossible to fight before getting up to speed, and stalls out. If I drain the tank completely of air-pressure, the compressor will start and get to the target compressor rpm, no problem. But if there's even just 10 psi in the tank, I end up with the same problem. "

 

[matt_i]

A couple of things.

You should first check if you are out of power. A VFD can take a derated output when powered by single phase input, and that could be the issue right there. I didn't see any HP numbers but check your manual its probably somewhere in the 142 pages .

A VFD is designed to run up to 120Hz, not sure why running @ 70 is a problem.

There might be a parameter in the VFD which helps it deal with loads. I worked on a problem on a slow feed conveyor which was far, far from the control panel. It would stall out sometimes when nothing was jammed, just couldn't start. There was a Vmin parameter (this is on a Lenze SMD drive as I recall) which assisted the motor in starting the load. Not that this is your specific problem but there's probably a parameter that deals with reactance to dynamic mechanical loading (or the like) that may help you resolve this.

 

[peter2772000]

Thx to all for your input/suggestions. I will trying checking the input amperage while the VFD is trying to get the compressor to speed, though pretty sure we went thru everything with tech support.

Increased torque-boost form 100% to 300%, tried increasing start speed etc...

The whole idea of using a VFD was to...cheat. I'm running a 20A circuit and if I'd changed the motor to yet another single-phase but 3hp, I was worried that I'd run too close to tripping the breaker. I figured that with the slow-start of a VFD and the capacity to adjust for maximum amperage draw thru the VFD itself, I'd have the best of both worlds, tuning-wise.

For the record, I'm not worried about the VFD not being able to handle the higher freqs, more about the 3450 motor now spinning at higher-than-designed revs.
Maybe I'm complicating my life for nothing, wouldn't be the 1st time *grin*

 

[Packard V8]

The whole idea of using a VFD was to...cheat.

I would never put 1 on a compressor. It's just the wrong app.

Maybe I'm complicating my life for nothing, wouldn't be the 1st time *grin*

Of course, you're overthinking; as is our wont her on GJ.
Answered your own question. There have been a dozen threads here reiterating advice not to use VFD on compressor.

Yes, it's possible.

No, it's not recommended.

Maybe, accept the compromises or put on the correct motor.

jack vines

 

[Maui]

Run a rotary phase converter or swap out the 3 phase motor for a single phase motor.

 

[marinusdees]

Compressors do not like simulated 3 phase, no matter how it is generated.

 

[bsaint]

Compressors or motors? A motor that is designed for an inverter will run decades before the windings toast. One made for 60hz only its only a matter of time, even if you run a drive at 60hz because its simulated 60hz. Its why a motor on 3khz PWM 60hz has so much harmonics.

But either way 70Hz wont harm anything. Dont waste your time with a smaller pulley. Dont forget you have a service factor.

 

[peter2772000]

70Hz didn't result in over-amps. I just wasn't thrilled about running a 3450 rpm motor at over 4k rpm.
As well, I discovered a thread about VFDing your shop equipment on this site(how I missed that, I have no clue...), and a poster on page 19 seemed to concur that the motor bearings on a 3450 motor would take a beating when revved higher. I'll be dicking with other parameters tomorrow, hopefully I'll find the magical parameter setting to end this self-inflicted situation.

 

[Mr. T]

I just wasn't thrilled about running a 3450 rpm motor at over 4k rpm.

I wouldn’t think twice about it. The biggest danger with VFD conversions is running too slow and overheating the motor. If it makes you feel better spend the $25 and put some quality bearings in your motor.

I have motors that aren’t inverted rated at work that have been running 95hz for years and are just fine.

 

[matt_i]

70Hz didn't result in over-amps. I just wasn't thrilled about running a 3450 rpm motor at over 4k rpm.
As well, I discovered a thread about VFDing your shop equipment on this site(how I missed that, I have no clue...), and a poster on page 19 seemed to concur that the motor bearings on a 3450 motor would take a beating when revved higher. I'll be dicking with other parameters tomorrow, hopefully I'll find the magical parameter setting to end this self-inflicted situation.

As I recall when you go over 3450 then the -2RS rubber seal bearings will overheat (seals self-melt from the friction more or less) and you have to use -ZZ double-shielded (which don't touch).

 

[Bert_]

I'd guess you VFD is on the small side for your application. But you might be able to overcome that.

I'll assume your compressor has a simple unloader on the pressure switch not an actual pump speed based one.

- B, install an unloader type solenoid on the discharge line of the compressor which would stay open for the first 10 seconds or so 'till the compressor was up to speed

That could be implemented pretty well, solenoid wired to a VFD output that closes at ~75% of full speed.

You could also try using a longer line between the pump and the check valve. More volume in the line = more cycles of the pump before it builds pressure.

I'd set the VFD to ramp up within a second or so

 

[Mr. T]

As I recall when you go over 3450 then the -2RS rubber seal bearings will overheat (seals self-melt from the friction more or less) and you have to use -ZZ double-shielded (which don't touch).

A 6200-2RS bearing (one I’ve used in pump motors) is rated to 17,000 rpm.

 

[peter2772000]

Guys, thx to you all for trying to help. Truly appreciated.

I threw my hands up in defeat this morning after trying a bunch of different parameter settings. Nothing. So I resigned myself to slapping the smaller drive pulley back on. Now I still have the same problem, so I'm sure I mucked up the default parameters. I'm going to get my supplier to tell me what all the defaults were (if lucky, maybe there's a "return to default" setting), then start over again with the smaller pulley and call it a day.

Again, thanks to you all for your suggestions and guidance. I'll post when the $itch is back up'n running

 

[dutchgray]

There usually is a reset to factory parameter.

 

[manwithtools]

70hz is not a problem for most motors, even the cheap ones. I'd run it at 70 and not lose any sleep. Set the ramp up time to be .25 seconds to eliminate the unloader problem and it should work great. It's a VFD creating 3 phase from single phase input, it'll work just as well as a phase converter in this application if it's properly rated hp wise.

 

[matt_i]

A 6200-2RS bearing (one I’ve used in pump motors) is rated to 17,000 rpm.

Ok...but a 6200 is a tiny thing that's not going to be on a 1-3hp motor. Those are going to be something like 6207s or 6209s....its been too long now, I forget which but the inner race has to be bigger than the output shaft....

 

[Badasssapper67]

What IS IT with air compressors that makes us all crazy??? Hahaha.
I havent seen any specs on the motor, length of line in between the head and the unloader valve, and the distance from the circuit board.
To me and this is just my thinking not saying its the way everyone should think, save up for a compressor that gives you the better performance you need (want) and is not blipping the red line everytime it runs.
How many cfm is your pump rated for? Single stage or two stage? The limiting factor on what motor you need isnt your circuit breaker it's your pump. If you're trying to put a funny car engine in a Corola it isnt a question of what alternator to use. If your pump requires 30 amps, then you need 30 amps and the wire and the protection that comes with it. Also did you say what your vfd hp is rated at? I might have missed it. If you have a 3hp motor and a 7.5 hp vfd you are going to be ok, but if you have a 3hp motor and a 5hp vfd that might be your problem.

 

[peter2772000]

Posting on the results.

First off, my replacement compressor is rated at 11 CFM @ 950 rpm. I knew that the OEM CH motor rated at 17 amps was approx. 2hp which laughably produced their rated 6hp capacity.

I did not want to pull a new, higher amperage line to the garage although, to be honest, it would have been easier and cheaper in the long run. But again, now it was personal...

We reset the drive to the OEM parameters and entered the motor specs (ie amps/voltage etc). Started it up with the same ****** results. I then simply bumped up the rated motor amps from the tag rating of 8A to 14A. Voila, no more issues.

I haven't built up the courage to start slowly lowering the amperage in .5A increments until finding the rating that is closest to the actual motor rating without stalling. But being the trouble-maker I am, I'm sure I'll end up doing so, and soon.

Thx for tuning in *wink*

 

[laser3kw]

(if lucky, maybe there's a "return to default" setting),

Looks like you're on the right track. Sounds like the vfd was folding back as a precaution. I don't think you'll hurt the vfd as tuned. It may shorten the life, but then again you are not running it continuous.
Also look for a "torque boost" parameter(s). It is made to do exactly what you are trying to do.

 

[454ragtop]

Posting on the results.

First off, my replacement compressor is rated at 11 CFM @ 950 rpm. I knew that the OEM CH motor rated at 17 amps was approx. 2hp which laughably produced their rated 6hp capacity.

I did not want to pull a new, higher amperage line to the garage although, to be honest, it would have been easier and cheaper in the long run. But again, now it was personal...

We reset the drive to the OEM parameters and entered the motor specs (ie amps/voltage etc). Started it up with the same ****** results. I then simply bumped up the rated motor amps from the tag rating of 8A to 14A. Voila, no more issues.

I haven't built up the courage to start slowly lowering the amperage in .5A increments until finding the rating that is closest to the actual motor rating without stalling. But being the trouble-maker I am, I'm sure I'll end up doing so, and soon.

Thx for tuning in *wink*

What are you hoping to gain by lowering the amp setting in the VFD? The motor is going to draw what it needs, not what you set it at. All you'll get when you lower the amp setting is likely an over current fault the first time the motor tries to start on a slightly colder day or something that requires a little more current. Not sure why you thought the 17 amp OEM rating should be set at 8 amps to start with. Starting current for a motor is a lot higher than running current, especially on a compressor.

 

[peter2772000]

Looks like you're on the right track. Sounds like the vfd was folding back as a precaution. I don't think you'll hurt the vfd as tuned. It may shorten the life, but then again you are not running it continuous.
Also look for a "torque boost" parameter(s). It is made to do exactly what you are trying to do.

We initially found the torque boost setting....and changed that parameter with no change. And yes, that's exactly what was happening; the drive was protecting either itself or the motor.

What are you hoping to gain by lowering the amp setting in the VFD? The motor is going to draw what it needs, not what you set it at. All you'll get when you lower the amp setting is likely an over current fault the first time the motor tries to start on a slightly colder day or something that requires a little more current. Not sure why you thought the 17 amp OEM rating should be set at 8 amps to start with. Starting current for a motor is a lot higher than running current, especially on a compressor.

I dunno, it just seems like the equivalent of swapping in pieces of copper tube instead of fuses. As well, the 17A rating was on the OEM CH motor. The replacement motor is a 3-phase rated at 8 amps. So, a setting of the motor FLA at 10 amps was already a stretch. Remember, the drive has parameters for FLA and SFA, which in this case is 9.2A. All I wanted was to get the pump running at the rated 950 rpm without running over 60Hz. I was just so relieved I got the beotch running. I had to install the summer tires on my truck last weekend, it woulda burned my a$$ to have to do it with a break-bar

So in the end, this "genius" finally got the compressor going. Cost $380 for the new pump, and another $900 for the drive and motor. I could have bought a complete air-compressor for less.

But hey, I won, right?