Eric,
The maximum torque output of the 3520 at low speeds is the same as the torque at the motor's "base" speed (the speed of the motor when operated on 60 Hz power, the normal power line frequency in the US). The reason for this is that motor torque output is approximately a linear function of motor current and, therefore, the controller must limit the maximum current to the motor's rated FLA (full load amps) to avoid overheating. This would be true for any lathe with electronic variable frequency drive (VFD). So much for the good news . . . . .
Now, the bad news . . . . If you compare the low speed torque of two lathes that have the same size AC induction motor, where one has a conventional stepped sheave design for speed control and the other has a modern high-tech electronic variable speed drive, the stepped sheave design will win the torque war. Why? In a mechanical drive system where the motor is always running at the same speed, the mechanical horsepower that it is capable of producing is constant so if the output spindle speed is less than the motor shaft speed, then the spindle torque capability is greater than the torque at the motor shaft. On the other hand, the variable speed motor is limited to producing less horsepower as the speed is decreased because horsepower is defined as the product of speed and torque.
It is for the reason that I briefly described above that you generally find larger motors on modern lathes with variable frequency drive. Manufacturers generally have two recourses to have decent low speed torque capability with VFD systems. The first is larger motors and the second is to have two or more speed ranges by using stepped sheaves. Hmmmm....it almost seems as if we have come full circle...we are back to stepped sheaves except that now we have a way to make incremental speed adjustments between the large steps.
Bill
