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.

 

[DickDV]

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.

 

[rockman7892]

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?

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[itsmoked]

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

 

[rockman7892]

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?

 

[itsmoked]

Yes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

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

 

[Biomatt]

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!