There has been much written about the cables and motors for PWM drives. If you want to indulge into some really technical stuff, see "Selection of Power Cables for PWM AC Adjustable-Speed Drives" by C. David Mercier and David A. Cooper at
http://www.southwire.com/tech/library/papers/pwmpaper.htm I read it, but it is a little over my head, although I did get some things out of it.
Another source is "Cable Alternatives for PWM AC Drive Applications" by Eric Bulington, Scott ABney and Gary L. Skibinski at
http://bwcecom.belden.com/college/techpprs/capwmtp.htm They compared several cables and seems to conclude that shielded cable is best, with the shield grounded at "both ends". If you are having a problem with AM radio interference, then shielded cable on the drive input is recommended.
Shielded cables is what we specify for our designs here at my company.
