ALL polymer insulations are porous. The capacitance is not due to these. That makes no sense at all because where is the other conductor? I think this is being confused with the idea of eddy currents which describes where circulating currents kind of like eddy currents in a stream create self inductance in a cable.
Simply put, a capacitor is created whenever we have two conductors separated by an insulator. Distributed capacitance exists between phases as well as between all phases and the nearest grounded surface.
We dont normally worry about this at low frequencies like 60 Hz. We can calculate the wavelength at 60 Hz which is about 800 kilometers. So at around 80 kilometers the electrical power line will start to act like a transmission line but for most power distribution cases we can ignore cable inductance and capacitance. However VFD pulses and more specifically rise times are fast enough that starting at around 3 meters (10 feet) it's no longer just a cable, it's a transmission line and these minor distributed capacitances and inductances matter.
Taken together with cable resistance we can calculate a characteristic impedance of the cable. Motor characteristic impedance however is very different from cable impedance so when a pulse from the drive hits it a portion of the pulse, particularly at high frequency, reflects back along the cable. It reflects again back at the drive, back and forth as the wave dissipates.
http://alignment.hep.brandeis.edu/Lab/XLine/XLine.html
PVC and XHHW do have slightly different characteristic impedances but both will have reflected waves at about the same footage. Both must pass a high pot test using 200% of rated voltage plus 1,000 V, or 2,200 V. XHHW is slightly thicker compared to THHN. Actual testing by NEMA has shown that #14 THHN from various manufacturers fails (arcs) at around 2800-2900 V based on several samples from different manufacturers. XHHW fails at a little over 3200 V. VFD cable is rated usually for 2,000 V so must pass high potting at 5,000 V. Realistically it is probably much closer to the test compared to the 600 V cables.
However unrated motors without phase papers are only good for 1,000 V. Typical standard motors today pass up to 1200 V. Inverter duty motors following NEMA MG-1 go up to 1400 V. Most manufacturers however exceed this rating and hit around 1700-1800 V. However this is less than the test voltage for 600 V cable, never mind actual test results even on much maligned THHN. Simply put cable doesn't really matter because by the time the motors start showing fluting in the bearings and the first couple turns are burned up. Sure we can use VFD cable but so what...The motors are still shredded.
One of the final arguments for VFD cable is that since it is balanced, cable capacitance is minimized, etc., that it can improve distance limits and many VFD manufacturers include this in their motor cable tables. For instance with most Allen Bradley VFDs, it goes from 50 to 80 feet. But a simple dv/dt filter increases the distance to hundreds of feet. However no provisions are made for XHHW increasing distance over THHN.
Sent from my SM-T350 using Tapatalk
