Full load current on nameplate

[compguy22]

Hello,

On a electric motor driven rotary air compressor (rated 200 kW and equipped with a variable speed driver), the full load package current is indicated on the nameplate (this is package nameplate, not motor nameplate). However this current value is different than the package current value stated on (OEM) datasheet. I refer here to the generic datasheet of the package (not as built one). The machine is from standard production / pre-engineered package. The only parameters which I can think that could be project specific and potentially affect performance would be the ambient conditions and air condition at inlet of the machine.

My question: is the Service Factor (1.2) normally included in the full load current? That would explain the difference, all though there is still a discrepancy in the values. Or could the difference be due to the fact that the current value reported on nameplate correspond to the one determined during factory testing?

Thank you for providing any pointers.

 

[LittleInch]

If you post the different values and data sheets maybe we can figure it out, but at the moment no one can see what you're looking at.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[compguy22]

Hi LittleInch,

Package Datasheet (excerpt):
========================
Voltage: 440/460 V
Frequency: 60 Hz
Current @max pressure: 233/276 Amp
Input Electrical Power (Package): 215 HP
Service Factor (Motor): 1.2
Power Factor (Motor): 0.91

Nameplate (excerpt):
====================
Power: 215 HP
Full Load Current: 228/333 Amp
Voltage: 460 V

note: Sorry power is 160 kW (not 200kW as I mentioned before by mistake).
Please note this is generic datasheet.

Thank you

 

[jraef]

When motors are driven by VFDs, you cannot use the Service Factor.

It’s hard to decipher what they are saying here, the numbers don’t relate to anything standard. Most likely they are using an IEC motor designed for 400V 50 Hz, then running it at 60Hz 460V so that they get more HP out of it. My best guess is that it is a standard 132kW 400V 50Hz motor to start with, which would be 177HP @ 50Hz, then by using it at 460V 60Hz it becomes 212HP, but they are running it a LITTLE harder at peak pressure (knowing it will shut down soon) to call it 215HP. Compressor people like to play games with their ratings.

The input current will reflect the input amps to the VFD, which will be different from the motor amps.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

VFD's are most often applied to positive displacement compressors, which are a constant torque application. Getting heat out of the motor at lower speeds will most likely be the controlling factor in the motor selection.
The OEM may have provided a 50 HZ motor, or a 60 HZ motor, with the VFD it really does not matter, as when driven by a VFD, input and output are decoupled by the DC link.
At volume an OEM can order a custom VFD program that does anything desired to the motor, including possibly running above base speed and higher than standard amps to get more capacity.

 

[TugboatEng]

The full load current (FLA) does not include the service factor. The safe full current (SFA) is the FLA x Service Factor.

 

[compguy22]

Thank you for your inputs.

jraef, could you please elaborate on why service factor cannot be used for VFD.
It is stated on electric motor datasheet, SF=1.2. (By the way, the motor datasheet also says Y connected, which I do not understand as the motor is intended for VFD application, but this is another subject. I thought Y connected is only for fixed speed application).
It is likely that the motor is originally per IEC and designed for 50 Hz and has been adapted to 60 Hz. Another information I would like to share is that although package voltage is 440/460V, motor voltage is 380V which means there is an internal transformer between VFD and motor.
Please excuse my ignorance and thanks for your insights.

FacEngrPE
"At volume an OEM can order a custom VFD program that does anything desired to the motor, including possibly running above base speed and higher than standard amps to get more capacity."

Could that explain the difference between what is stated in datasheet and nameplate in terms of package current? In fact, max current that is stated on nameplate is way above datasheet values, and reason is not clear to me. Also as per jraef input, this has nothing to do with S.F.

 

[compguy22]

The full load current (FLA) does not include the service factor. The safe full current (SFA) is the FLA x Service Factor.

So for VFD, SFA = FLA because S.F. cannot be used (that is, S.F.=1). Is this correct?

 

[TugboatEng]

Electric motors are rated to run at 100% but their protection needs to have some fudge room to prevent unnecessary trips. Size the motor so the FLA isn't exceeded during normal operation and select the overload relay so that the protection operates at 120% of nameplate.

In the case of the VFD, because they are designed specifically to control motors, these factors are all accounted for. The VFD nameplate needs only to meet or exceed the motor nameplate. Everything else is programmed during commissioning.

 

[FacEngrPE]

To operate a 380 volt motor on a VFD with 480 V input is not tricky at all, even if a bit unusual. The output of the inverter just simulates a 380 volt sign wave. As long as the DC link voltage is above the Peak voltage of a 380 volt sign wave this is an easy task, with no transformer required. This explains the difference in the motor and input currents.
This is not a setting usually available in an off the shelf drive, so the OEM gains some spare parts lock in.

 

[jraef]

Service Factor on NEMA design motors is based upon the effects of heating when running at a load higher than rated by the SF percentage. So for example a 10HP motor with a 1.15SF can survive the heating effects of having to be loaded to 11.5HP, but also other heating effects such as unbalanced voltage, low or high voltage, etc.

But when the same motor is operated from an inverter drive, the harmonics in the pseudo-sine wave coming from the inverter will consume that extra heating capacity provided by the SF in the design. So motor manufacturers will put that on their nameplates.

Note the way they put it on this "Severe Duty" motor, in the box on the lower right;

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Gr8blu]

compguy The motor rating is for when it is operated on "true" sinusoidal power. When operated on a drive, the chances are VERY good that the drive is putting out a bunch of harmonic content in both the voltage and current waveforms. What that means is that the thermal effect on the motor is worse for a VFD source than it is for a pure sinewave source - i.e., it gets hotter for the same "current" level.

Unless the DRIVE is ALSO rated for the continuous overload (in your case 20 percent above the "nameplate" power output), it just hasn't got it. And if the drive doesn't have it, the motor can't get it (and use it). Given the power rating you've named for your project, I suspect going to a drive rating that would handle the overload is a step change in drive cost as well.

As to the "Y" connection on the motor nameplate: that refers to how the winding of the motor is connected internally, not to how it connects to the power source (utility or drive). The source doesn't care what the load looks like: all it sees are three points to connect to. How those points interact just defines the relationship between volts, amps, and measured impedance between phases.

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