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.
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 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
David Castor
www.cvoes.com
RE: 500Hp VFD hitting FLA at ~60% speed
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
RE: 500Hp VFD hitting FLA at ~60% speed
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
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.
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RE: 500Hp VFD hitting FLA at ~60% speed
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
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 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
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
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.
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RE: 500Hp VFD hitting FLA at ~60% speed
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
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
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
Muthu
www.edison.co.in
RE: 500Hp VFD hitting FLA at ~60% speed
RE: 500Hp VFD hitting FLA at ~60% speed
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: 500Hp VFD hitting FLA at ~60% speed
RE: 500Hp VFD hitting FLA at ~60% speed
I would expect it to be about 276V.
RE: 500Hp VFD hitting FLA at ~60% speed
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
Muthu
www.edison.co.in
RE: 500Hp VFD hitting FLA at ~60% speed
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: 500Hp VFD hitting FLA at ~60% speed
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
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!