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Variable frequency drive into a motor then a screw - How to chk Power

Variable frequency drive into a motor then a screw - How to chk Power

Variable frequency drive into a motor then a screw - How to chk Power

(OP)
Hi

I am using a variable frequency drive to feed a motor which in turn rotates a screw.

Mains-> VFD(400V)-> Motor(400V,1380rpm)-> Gearbox-> Screw

The frequency range of the 400 volts inverter(VFD) is 0-65 Hz. The motor is also 400 volts and the current output from the inverter to the motor ranges from 2.1 to 1.4 A(according to the display). After about 20 Hz on the inverter the current remains constant at 1.4 A.

Being bad at electrics, I am a bit confused with regard to how much power will be required to move the screw. The screw will be used to convey material. Using the formula P=V.I.cos(phi), we have V for the motor but I remains same even when the the frequency and the screw speed increases.

I am thinking that the place that I need to check the current is

Mains-> VFD(400V)-> Motor(400V,1380rpm)->CHECK-> Gearbox-> Screw

instead of just reading the display from the inverter. A picture is also attached which hopefully will help.


Thanks

http://files.engineering.com/getfile.aspx?folder=71db8a83-beab-40a8-bf2e-5a78ed6b09e6&file=photo1.JPG

 

RE: Variable frequency drive into a motor then a screw - How to chk Power

The best thing you can do is to read power from the VFD's display. Modern VFDs are quite accurate and every attempt to read current and voltage and calculate power from there will not be as accurate. Also, you have no idea what the cos(phi) is. Especially not when the VFD is boosting voltage at low speeds, which it does if it is a U/f inverter and also if it is a vector drive.

A final note: If you try to calculate power (which is a bad idea) do not forget the sqrt(3) factor.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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