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500Hp VFD hitting FLA at ~60% speed

500Hp VFD hitting FLA at ~60% speed

500Hp VFD hitting FLA at ~60% speed

(OP)
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.

RE: 500Hp VFD hitting FLA at ~60% speed

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

RE: 500Hp VFD hitting FLA at ~60% speed

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
www.cvoes.com

RE: 500Hp VFD hitting FLA at ~60% speed

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...
 

RE: 500Hp VFD hitting FLA at ~60% speed

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.

RE: 500Hp VFD hitting FLA at ~60% speed

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.

RE: 500Hp VFD hitting FLA at ~60% speed

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: Eng-Tips.com Forum Policies  

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)
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.

RE: 500Hp VFD hitting FLA at ~60% speed

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.

 

RE: 500Hp VFD hitting FLA at ~60% speed

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.

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)
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.

RE: 500Hp VFD hitting FLA at ~60% speed

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: Eng-Tips.com Forum Policies  

RE: 500Hp VFD hitting FLA at ~60% speed

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

RE: 500Hp VFD hitting FLA at ~60% speed

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?

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)

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?

RE: 500Hp VFD hitting FLA at ~60% speed

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

Muthu
www.edison.co.in

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)
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?

RE: 500Hp VFD hitting FLA at ~60% speed

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
www.edison.co.in

RE: 500Hp VFD hitting FLA at ~60% speed

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

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)
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?

RE: 500Hp VFD hitting FLA at ~60% speed

When the motor reaches FLA at around 60% speed (I assume that speed to be 60 x .6 = 36Hz) try to find the drive display for output voltage and please advise what it is.

I would expect it to be about 276V.

RE: 500Hp VFD hitting FLA at ~60% speed

itsmoked

So you are saying that even know at lower speeds the motor may be going into saturation, the slight saturation itself may not draw enough current, and it wouldn't become evident until more load current was added to this current at a higher speed?

eidson 123

If you moved to 50hz during an unloaded condition wouldn't the magneitzing reactance be smaller and thus lead to more no-load current?  Or does the fact that the voltage is reduced as well offset this?


I have always though that for a constant toruqe load, the tourqe requirement (and therefore current requirement) was constant throughout the entire speed range?  Or are there other conditions that are changing with speed that produce and increased torque requirement?

RE: 500Hp VFD hitting FLA at ~60% speed

rockman - The motor flux is proportional to V/Hz. When that ratio goes down, so too flux but then also the motor torque.

Muthu
www.edison.co.in

RE: 500Hp VFD hitting FLA at ~60% speed

rockman7892; There are levels of saturation.  At the peak of the power cycles is where it starts.  At that point the current starts to increase without bound. However the average may not be particularly high.  As you proceed into more and more saturation the average current rises much faster.  At some point you would be reaching maximum current while still somewhere below the maximum voltage.  This would happen if you were driving a motor with an expected V/Hz curve that was wrong.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 500Hp VFD hitting FLA at ~60% speed


So is the fulx current directly related to the magnetizing reactance current?  In other words, if we lowered the frequency on an unloaded induction motor but kept the voltage the same would the flux increase and possibly go into saturatin simply due to the fact that the magnetizing reactance was less and it would draw more current or are there other physical properties involved?

RE: 500Hp VFD hitting FLA at ~60% speed

For years we were concerned with the voltage to a motor and possible saturation due to over voltage. To address the lower reactance at 50 Hz we used the 5:6, 6:5 ratios.
With the widespread use of drives and the multitude of voltages and frequencies available, we now have to be concerned with the V/Hz ratio, rather than the reactance at a fixed frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 500Hp VFD hitting FLA at ~60% speed

(OP)
I checked my Uo value and it is 10 volts.  Not sure if that is significant, but the original purpose for putting it there was to increase voltage, decrease current.  Can't change while its running, so next opportunity I'll change it to zero.

Generally I am learning that there is not much more I can do with this VFD per this forum's responses and per all of my previous inquiries.

Has anybody ever used a torque measuring coupling?  Its a direct drive.  If I could measure the shaft torque, then I will know for certain which device has the issue (compressor requiring more than thought, motor/VFD not delivering as expected).

Thanks again for your helpful information!

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