Inspired by this question and specifically this answer to it.
From my experience there's always some very specific limit to how much torque an electric motor can output. For example, an electric drill will often have a manually switched mechanical transmission – if one needs to drill some relatively weak materials (like wood) he will use the setting that outputs lets torque at higher RPM and if one want to drill steel or mix cement mortar he will use the settings for more torque at lower RPM.
The relation between RPM and torque is more or less clear if one imagines a set of two gears of different diameters and thinks that their radiuses are lever arms – three times more RPM automatically induces three times lower torque and vice versa.
But where does the limit to any given electric motor torque come from? Say I have some specific motor right now in front of me and it can output 40 Newton-meters at 500 RPM. Why exactly 40 and not more?
Best Answer
Let's imagine that we live in beautiful world of brush-less motors without friction.
Limitation must come from coil resistivity - the more power you pump into coils, the more losses you have due to their non-0 resistivity. At high RPM resistance also increases due to skin effect (reducing effective cross-section of the wire).
So if one want to have more force at higher RPM - he would need thicker wire in coils (copper, or even silver), and in case of high RPM - made of litz wire (this would be incredibly expensive).
But if you can make coils out of superconductor - problems solved, and your power is limited only by mechanics & coil commutation.