Are the Cobra motors not "just" the Premium Efficiency line in the 56 frame size? It's not easy to sort out the useful stuff from the BS, particularly from here in the UK where NEMA frame sizing and US mains supplies are a foreign language. I think the Cobra motors also have a fancy rotor-grounding system that means ground current does not pass through the bearings and spark-erode the bearing balls when run on a VFD, along with a stainless steel fan cowl and a few other details that tend to make the difference between a premium industrial motor and a meh industrial motor.
In most of "our" cases, the effective differences between a premium industrial motor and a meh industrial motor are cost and smugness factor. A meh industrial motor might be expected to last perhaps 5 years in an industrial application running 24/365 at rated load: 40,000 hours-plus, where a premium motor might be expected to last perhaps 8 years in the same application, 70,000 hours-plus, based on my day-job experience. If changing a motor costs thousands an hour in lost production, an extra hundred or two for the premium motor is a no-brainer for the industrial user, who will also see a payback in reduced energy costs because a 3% reduction in power consumption of a 3HP motor at full load for 8760 hrs/yr is around a 578 kWhr/yr saving.
For the one-man shop, the economics tend to be somewhat different. The 40,000 hour expected life of the meh motor corresponds to 40 years of operation at 20 hrs/week. Even at 40 hrs actual grinding per week, you'd still expect to have to wait 20 years before seeing any advantage to a premium motor in terms of longevity. Power saving for the premium efficiency motor would also be much less: 3% power saving on 40 hrs/week at half load (on average) is about 69 kWhr/yr.
KBAC drives are justifiably popular because they are reliable and sealed. I get the impression that some of the appeal comes from being "Hybrid" drives: settings are by switches and potentiometers, rather than a menu-driven display and I think this sits well with the hands-on, analog, mind-set that seems to go with actually making stuff. The longevity of the design means that it is pretty old hat now, and a KBDA might be a better option. The menu-driven setup allows a bit more flexibility and I think the KBDA offers Sensorless Vector operation: the only area I've found SV to make a noticeable improvement is in low-speed operation (below about 10 Hz), but it's worth it for that alone.
If you are going 3 HP, you'll want a properly high speed at the top end for hogging and the SV drive is probably the best way to get low speeds for sharpening, etc.
If you are building a design that uses a 56-frame-size, you may find you can only get 2-pole (3600 RPM) motors with a 3HP rating, so can't use a 4-pole motor (1800 RPM at 60 Hz) and double the maximum frequency to get 3600 RPM at 120 Hz. Maximum speed is 3600 RPM either way. The 4-pole will run down to about 300 RPM at 10 Hz, while the 2-pole will run down to 600 RPM at 10 Hz. My experience is that motors become much less smooth-running (they feel "coggy") below about 10 Hz on a V/Hz drive, but will run smoothly down to 1 or 2 Hz on an SV drive.
