Power measurement VSD

[maypot]

Hi,

Th power measured by a Fluke power meter on the input side of my VSD 400 k W capacity is around 300 k W. However, the VSD is only reading below 200 k W on both the VSD display and the 4-20 m A analogue output of the VSD.
I understand that the a 2 % loss is reasonable but 100 k W seems to be too much. I also understand that the capacitor bank is storing energy but that much.
Any advice ?

Bob

 

[torslum]

Sounds strange, I agree....

Guess you've double checked the ratios and settings of the Fluke measurement already? If we assume that the 300kW measurement is definitely, I would check out how the VSD is doing it's power calculation. Is it giving 200kW to the display directly, or is it given as percentage from which you calculate the kW (some drives do...)?

Some drives differs between output power and motor power; the first one usually the real power measured by currents and voltages. The latter one is a calculation based on the motor-model, which is strongly dependent on the parameters. Did you check the parameter settings for nominal voltage, nominal power, nominal current, cos fi et.c? This could effect the calculation being done in the drive. I guess the drive is giving you currents and voltages in the display as well? In that case, what are those and do they agree with the 200kW or 300kW (assuming a reasonable cos fi)?

tm

 

[electricuwe]

I guess the drive is displaying active power whereas the Fluke at the input is displaying apparent power.

Apparent power might be significantly higher than active power due to phase shift at fundamental frequency and harmonic distortion of the input current.

 

[aolalde]

Certainly something is wrong. The 100 kW difference is too much.
First ensure that the measurements are done properly.
Are you using three voltages and three current signals to measure three-phase power?
Can you provide the input Volts, Amps and PF readings?
The input voltage is a sinus wave; I am not sure what the current is. Can you see the wave form of voltage and currents?
Output from the driver is not a sinus wave and proper averages must be taken to get the equivalent RMS Power.
Can you estimate the mechanical power demand to the motor shaft at the selected speed?
What capacitor bank? I understand that capacitor banks and VFDs are not a good combination, may be you should disconnect that from the circuit.

 

[maypot]

Thanks for your valuable inputs. I have queried the manufacturer and it seems that the drive is taking into account the magnetising current. They suggest to play with the I mag to achieve the correct power.

Bob

 

[torslum]

maypot; to me, that sounds like a very strange explaination? What brand is this?

Did you read the power as kW directly from the display, or did you calculate it from some other readings? No way a drive should show 100kW less just because you didn't play any magnetizing current right?

Good luck, lpease let us know the end of the story as well

 

[electricuwe]

I agree to torslum

 

[Skogsgurra]

Agree. Sounds like salesman done the talk.

Gunnar Englund

http://www.gke.org

 

[maypot]

They are finally requesting me to change the gain. On each analogue output, the gain and offset may be adjusted to fit a particular application.The drive is Robicon.

Bob

 

[DickDV]

What does the analog output gain and offset have to do with the displayed KW? This sounds like pure nonsense unless the drive KW is being measured thru the analog output. But then it isn't displayed KW, is it?

It might be worth reading the fine print in the instruction manual concerning the displayed KW. It very well might be the drive's estimate of motor shaft power and not drive input power. In that case, larger error could be expected since you are comparing two different things. Even then, 100kw seems a bit too much of a difference even given the above.