Ron, I read the whole paper you asked about and find it to be very responsibly written. In my view, there is no "snake oil".
My only observation is that the paper favors special duty inverter motors a bit more than general application experience indicates is appropriate. There have been significant design improvements in commodity grade AC motors in recent times that permit their use on an increasingly broad range of applications. Also, recent improvements in sensorless vector performance by a few leading manufacturers have widened even further the successful use of commodity motors.
Unfortunately, their use requires a large amount of judgement and this is hard to describe or teach with any precision. The result is overspecifying and that is not all bad. Some of us "old heads" can play it a little closer to the edge and be comfortable with it but most cannot and should not.
It is handy tho when I need to "steal" a job from the competition!!
TFC, you must be careful to distinquish between available motor torque and load torque. Load torque is whatever the load requires to do its work over its speed range plus whatever accel and decel torques are needed to change speed. Clearly, the drive-motor system must be able to supply at least that level of torque at all speeds to get the necessary performance in the load.
Available motor torque is another thing entirely. From motor base speed (either 50 or 60Hz normally) down to zero speed, available motor torque is defined by the ratio of the drive output voltage divided by the drive output frequency--thus the term "volts per Hz". Most drives give you the option of altering this linear ratio so it, for example, follows a "squared" curve from base speed down to zero speed. This would limit torque to the same curve over the speed range and is intended for loads that have such torque/speed curves like fans and centrifugal pumps. Otherwise, with a linear curve, the motor will have available the same torque from base speed down to zero or near zero.
You can also change the slope of the V/Hz curve by altering the voltage setting at base speed. Normally, if you enter 460V and 60Hz in the drive software for motor data, the drive will automatically interpret that as the highest voltage point on the curve. If, on the same motor, you were to change the software entry to 460V and 80Hz, the V/Hz curve would rise more slowly reaching 230V at 40Hz, 345V at 60Hz, and 460V at 80Hz. All of the above would alter the available torque from the motor.
Up to this point, I have only been referring to continuous torque available from the motor. Normally, short term overload torques also need to be examined. At least for NEMA Design B motors, short term overload torque is available up to about 220% of nameplate. To get this, somewhere around 250% overload current would need to be supplied by the drive. Depending on the size rating of the drive, you may only have 110% (usually called variable torque rated or normal duty rated) or 150% (usually called constant torque rated or heavy duty rated) overload current available thus limiting what you can get from the motor. You may, of course, deliberately oversize the drive to get all 220% overload torque from the motor but that is not normally done for cost reasons. You must also be mindful of thermal issues in the drive and motor when you do this.
Above motor base speed, assuming constant full voltage applied to the motor, the continuous and overload torques begin to drop off as a function of the inverse of the percent of overspeed. By this I mean that, at 110% of base speed available torque will be reduced by 100/110 or .91, at 125% it would be 100/125 or .8, at 150% it would be 100/150 or .67, and so on.
Hope this helps clarify some of these issues.
