We're measuring Volts, Amps, Power, and Frequency being supplied to a 5hp motor driven by a Delta VFD under zero load, normal load, and locked rotor conditions.
We want to make sure any filters used do not delay, attenuate, or amplify any one of these parameters differently than any of the others as they are being used to characterize and subsequently model the motor under test. This is where the inductance of the scavanged reactor investigated by my predecessor appeared to be shifting a couple of these measurements relative to one another. Thus far our data seems to correspond to the theoretical induction motor behavior within 5% or so. Unfortunately this doesn't involve a high volume product and this bit of analysis understandably has a pretty low priority. That is, I've only done a couple of motors at 20, 50, 60, and 90 Hz.
Measuring Amps and Frequency by themselves is not problem but as soon as Voltage and Power/Power Factor are introduced the 16 KHz switching frequency of the drive finds its way into everything. The three shunts and three dividers are arranged in a Delta configuration and the six RC filters have an impedency of about 150 KΩ at 2.0 KHz. and 85 KΩ at 16 KHz. The outputs of the RC filters are connected to 10 KHz 5B40-series modules with sampling being done at 4 KHz. In spite of the trouble we've taken, a small amount of the voltage signal still appears on the amperage inputs. Additional 1st order filtering in software and a little curve fitting to match the readings from some old analog meters we still keep in the calibration program has gotten us what we're looking for, reasonable estimates of torque and efficiency from 10Hz to 110 Hz.
BTW, given my work environment, I either have to do this cheaply or justify the cost of another reactor with an inability to get what we need without it.
Am I going too far to do this cheaply? Can the appropriate reactor between the drive and the motor potentially get me another 3-5% accuracy?
Of course I'd then need to justify the need for another 3-5% accuracy.