kenkad wrote:Regarding the recent torque constant vs phase current plots (model5d), I keep wondering 'if the torque constant is higher at lower phase currents', why are motors not designed so they have a multiple of 3-phase groups so that each 3-phase coil set is operating in the max range of 15-20 amps? Certainly, it seems that six 3-phase groups at 15 amps (90 amp equivalent) is much greater torque that one 3-phase group operating at 90 amps (seems like about 6 Nm vs 4.9 Nm on the last graph). Would this not also mean less concern about coil saturation, etc. Can someone provide an explaination? I cannot imagine more electronic losses with six 3-phase group of drivers. Just trying to understand what the simulation is showing.
It's a function of saturation in the stator iron. As the current gets higher, it begins to require larger and larger incremental changes in current to get a certain incremental change in the B field, due to the non-linear magnetic properties, which in turn means larger incremental changes in current to get a certain incremental change in torque. Thus, the torque constant tends to drop off.
If you want to avoid that effect, as you say, just operate the motor at a lower current. But that means you'll have a much larger and heavier motor for a given power output. Picture an X5 limited to 500 W output. It doesn't matter how you have the coils configured, the more torque you want out of the motor the further into saturation you'll need to operate it.