500Hp VFD hitting FLA at ~60% speed

[Biomatt]

I've got a 500 Hp Altivar 61 VFD from Schneider electric that is in direct drive with a positive displacement compressor. Schneider Tech support indicates that an Altivar71 is what I should have, but in reality the 61 is indeed what I want since my loading curve is smooth.

Anyway, my problem is that I am hitting max amps with my speed only at about 60% of its rated (60Hz machine). Generally I understand that the V/F = constant rule forces the voltage to be lower than line (480) if you are at less than 100% speed. But I am not able to get full flow rate out of my compressor since I can only hit 60% of its speed. The compressor is pretty new and am mostly (I have two of these VFD/compressor combos and they perform the same) confident that it is not pulling more than it should . Line side current makes sense, its only moving about 60 or 70% of the power/current.

I have been told by tech support that changing the type of V/F relationship is only going to give a small change. Is there anything else I should be looking at to better identify my issue?

Thanks for your help.

 

[itsmoked]

What are you looking at to determine that you are hitting the "max amps" at 60% speed? You cannot look with a standard clamp-on.

What is your "max amps"? FLA off the nameplate?

You are saying both comps are showing this same result?

How are you measuring the 'speed'?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Further to what Keith said, horsepower ratings for VFDs should be considered as only an approximation. The VFD needs an output current rating that is equal to the motor FLA nameplate rating - regardless of horsepower.



David Castor

http://www.cvoes.com

 

[ozmosis]

Firstly, I'm surprised you have invested in a 500hp VFD that is designed specifically for centrifugal fans/pumps. It seems on paper to be a high investment to then have the company who supplied it say you have bought the wrong one. Didn't they ask you what the application is? especially a drive rated 500hp!

The Altivar 61 and 71 are more or less the same other than the cost of the 61 when looking at 'power' would appear to be lower and this is because of the lower overload rating of the 61 compared to the 71.
Basically, to get the same 'power' then the overload is reduced to try and squeeze more power out of a smaller VFD. It works ok on quadratic loads such as fans/pumps but constant torque loads you will find the limitations.
The fact that it is hitting overload at 60% of rated load is probably due to the i2t measurement in the drive that is looking at the load at different speeds and, I guess, is looking at the current at 60% and thinking "my variable torque brain surmises that the motor should not be hitting this level of current at this speed!"
I suggest look at the setting that changes the factory default from variable torque to constant torque (linear V/F).
I would also get a good commissioning engineer from Schneider to come along and see what magic they could pull out of the settings in the Altivar 61. Unless of course they'd swap the 61 for a 71 at no charge...

 

[VicP]

What is the setting of the Motor Control Type parameter. The default setting in a ATV61 is Energy Saving which is not good for your type of load. I have seen high currents when using this setting on certain types of loads. Set Motor Control Type to SVC V (open loop voltage flux-vector) and run the Auto Tune function. If the current is still high, try the V/F setting.

 

[DickDV]

I'm with ozmosis. It is likely the V/Hz ratio is set to quadratic which starves the motor for voltage in the midrange. That's fine on a quadratic torque curve load like centrifugal fans and pumps but you have a constant torque pump and need a constant V/Hz ratio.

Find that parameter and change it to Linear. I don't see where changing from Open Loop to SVC will help much. You need the V/Hz curve fixed first.

Don't believe that tech service statement about the shape of the curve not making any difference. At 60% speed, it makes a lot of difference and shame on them for not knowing it and saying it.

 

[jraef]

I agree with Dick and Patrick (ozmosis) however I think you may find that when you do fix that curve issue (taking it out of Energy Saver mode), you may find the drive under-sized for your application and you may end up in a similar place: your drive will go into current limit before you can get full speed from it.

You have made a classic error in judgment in your statement "...but in reality the 61 is indeed what I want since my loading curve is smooth". Smoothness has nothing to do with it. The 61 is a re-rated 71 for a larger HP because it is generally accepted that a centrifugal (quadratic) load is not going to demand as much from the VFD. But you do not have a centrifugal load, you have a constant torque load. So in essence, you have the equivalent of a 400HP 71 drive being used on a load that has a 500HP motor on it, and chances are about 99% that the compressor manufacturer picked a 500HP motor because it needed that much power from it at full speed. As you increase loading, in your case speed of the compressor, you will be asking MORE from the VFD, not less, and the VFD may not have it to give.

If you are lucky, maybe the compressor mfr needed only 440HP and went over sized rather than run the motor into its Service Factor, but my experience with compressors is exactly the opposite. It's more likely that they needed 550HP and didn't want to buy a 600HP motor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Biomatt]

Thanks all for your comments. First off, this is a case of a the EPC (engineer/procure/construct) firm selecting a VFD other than what the compressor mfr wanted, likely to save money. I have brought in many Schneider folks to deal with this, as well as compressor folks, but each says their device is working fine, pointing the finger in the other direction. Moral of the story: allow vendors the full solution so that the finger can only point in one direction.

Motor has an FLA of 548 and a SF of 1.15 (with VFD). VFD can push up to 630 amps to the motor before it trips. Motor control is "V/F 2pt" and vector control is off. We have tried the other motor control types, except V/F 5pt since tech support indicated I could do damage if I select wrong values. The only way I know speed is from the LCD, I have not confirmed speed.

My issues have repeated themselves on both of my compressors (each compressor has its own Altivar61).

My understanding of the reason Schneider recommends a 71 for a PD compressor is because of the high start up torques and transient loadings, such as you might get with a rock crusher (when a big rock comes along, you need big torques for a short time). This transient high-torque scenario could be required by some PD compressors. However, our compressor has slide valves to smoothly load/unload and most of the time there is very little adjustment necessary. The VFD was selected over a softstarter not for efficiency/power reasons, but to have better process turndown capabilities.

Assuming I indeed have a constant torque application (still trying to get more info out of the compressor folks), then with a linear V/F relationship, then I should also have a constant current application as well. The increase in power is seen by an increase in voltage as speed goes up. I have performed some tests and seen this to some extent (minimal current increase with speed increase), but there still is a small current increase with spd, and so if I'm close to railed at 60% spd, then I'll hit FLA * 1.15 or VFD trip currents before hitting 100% spd.

Does the 71 offer any more *continuous* capability over the 61?

Per knowing the motor current, I read the current off of the LCD readout (which comes from the stock current clamps), and this current was very close to my Fluke true rms hand clamp, which was very close to my Dranetz power analyzer current clamps. The voltage looks pretty nasty (per the Dranetz and separate Fluke power analyzer), so I havn't attempted to measure it.

Thanks again any/all comments/suggestions.

 

[LionelHutz]

What actually happens at 60% speed? The drive hits a current limit?

Is this a piston compressor or I believe what's called a roots type blower?

Your theory about the V/Hz ratio and the loading sounds OK. In theory, the compressor should only load the motor to FLA so any VFD that can supply FLA should be OK.

I expect there is one of 2 problems. Either it's a drive set-up problem or a mechanical problem with the compressor overloading the motor.

Yes, the 71 should have a higher continuous current capability. It basically would be similar to using say a 600hp 61 model instead of the 500hp 61 model.

 

[DickDV]

I believe it is time for the exact drive model number and the motor nameplate data. Please provide so we can have some solid data to go on.

I have an associate that works as an application engineer for a Sq D distributor and I would trust his judgement on sizing issues.

 

[Biomatt]

Around 60% we start getting really close to FLA * 1.15 (i.e. about 620 amps, motor FLA is 548 amps). In a few instances the VFD/compressor has tripped when it went above 630 amps.

Full VFD part number is ATV61HC31N4. Note that this was packaged by Motortronix, but the only thing they did was to put it into another cabinet.

 

[jraef]

Motortronics... oh oh...

First off, cover the simplest issues. Start with programming page 1.4, drC (Motor Control)
1) bFr (Standard Motor Frequency) The factory default on this drive is 50Hz, but for North America they change it to 60Hz before shipping. However if anyone ever got confused during commissioning and did a factory reset to defaults, it reverts to 50Hz and people often forget to go back and change that to 60Hz.

2) Ctt (Motor Control Type) You say it is set to UF2, which should be correct, that is a linear V/Hz profile. But again, if they THINK they set it to that but then someone did a factory reset, the ATV61 defaults back to nLd (Energy Saver). Make sure that hasn't happened.

3) Also check the associated settings below it that will be activated: U0 (minimum voltage for the V/Hz ratio) should be set to 0; anything else risks saturation. While you are at it, check UnS (rated motor voltage) to make sure it's set to 460V and FrS (Rated motor Freq.) to make sure it's set to 60Hz. Both of those settings also affect the V/Hz ratio.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

If the VFD is putting out 50Hz your symtoms would match nicely.


Also, are you sure the compressor is starting unloaded? Almost no piston compressor can be started if it's loaded. It would choke around the point yours is too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

itsmoked

Are you saying that if the drive is set for 50Hz the V/Hz ratio will increase and the motor can be going into saturation? Why would this only happen at around 60% speed and not at the lower speeds as well?

 

[Biomatt]

Yes the VFD is set for 60 Hz operation and 460 V. The compressor sends a start command to the VFD, and before this it moves its unloading valves all the way open. We don't have issues starting the system, just steady state operation.

As I understand it, U0 is the zero frequency voltage used for the V/F linear relationship. With a slightly higher voltage on the low side of frequency, would this not decrease the current a little bit over the speed range? What problems do you get with higher U0?

 

[edison123]

You could try to reduce the V/F to say, like 440 V/ 60 Hz and see what happens.

Muthu

http://www.edison.co.in

 

[Biomatt]

But this would lower the voltage across the speed range, thereby increasing my current (assuming the same load). Seems that this would take me in the wrong direction (more current). Is there something counter intuitive here?

 

[edison123]

I am just seeing whether reducing the no-load current component could help you ride over the increase in load current. I was testing a 160 KW motor on no-load with a 28 KW drive (same altivar demo piece) at 415 V/50 Hz and kept hitting the current limit in V/F mode. So I reduced it to 400 V/50 Hz and the motor ran just below current limit. Of course, mine was on no-load.

And another thing I learnt to my surprise was that you could actually use frequencies other 50 and 60 Hz. The drive allows it.

Muthu

http://www.edison.co.in

 

[itsmoked]

rockman7892; It would be happening at lower speeds but the power being 'mis-delivered' would not yet reach the current limit that causes tripping or alarm.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Biomatt]

I know measuring voltage and current on the outlet of a VFD is difficult and requires special equipment, but can you measure (accurately) power factor? Might this help understand whats going on any better?