# What power relationship (between active and reactive powers) should I expect from a CNC machine?

I have collected loads of power data from a 3-phase CNC machine and it all looks like this:

The 3-phase active and reactive powers can be seen above, as well as the Q (reactive) - P (active) difference.

My question is, does the relationship between the 2 powers make sense? They seem to be highly correlated and the reactive seems to have a higher increase when the cutting is happening (which is correct I guess, since that's when the spindle motor is spinning, so a magnetic field is generated.)

But I would like someone with more experience to explain if we always expect to see this kind of correlation in powers when a resistive and an inductive load are present or there should be a higher fluctuation of the reactive because of constant changes in the magnetic field.

Edit: This is the previous picture with the addition of the absolute apparent power |S| and a scaled (x10000) power factor that I calculated.

Can it be that the power factor increases when the cutting is happening?

• You don't do a difference between P and Qsince the relation is hypothenuse with S. You can do a division, to see the ratio. It may even turn out more leveled. – a concerned citizen Mar 15 '17 at 14:54
• See now that I have included the apparent power and the cosθ. – DimP Mar 15 '17 at 15:18
• It's going to depend a lot on the kind of actuators in the machine. Motors will probably all act similarly. So a mill or a drill or a lathe will have similar properties (except a lathe will not be stopped and started so often). But a hydraulic sheet-metal brake or punch will probably be different. – The Photon Mar 15 '17 at 16:08
• @ThePhoton But does what you see above make sense to you? It is from a milling operation. – DimP Mar 15 '17 at 16:36
• It's running the same motor every time, right? Why would you expect the power factor to be different from one operation of the motor to the next? Does your CNC program include widely different cutter sizes and feed rates? Then you might be able to detect some differences, for example if the motor moving the table has different characteristics from the motor turning the tools. – The Photon Mar 15 '17 at 16:40