by **Yellowjacket** » Wed Feb 20, 2019 1:52 am

Electric motors are "torque" machines. That is, for a given amount of windings and a given current you get a commensurate torque. What this means is that for a given hp (torque x rpm/5252) the highest speed motor that will give you the torque will be the lightest solution. That is, if you want a the lightest electric motor, you want higher rotating speeds. For instance, an electric motor at 400 amps produces the same torque at 2500 rpm as at 5000 rpm. But the 5000 rpm motor will be delivering twice the power and have the same weight as the 2500 rpm motor.. Now the voltage required to drive that current through the motor at higher speed is higher so there is no free lunch, but faster turning electric motors are lighter for a given hp. The voltage required to drive the current is a function of the "back EMF" and the resistance of the motor windings. Back EMF is just a function of how many windings are in series or parallel. More windings in series gives a higher back EMF, more in parallel give a lower back EMF. The amount of current you can drive is based on the voltage supplied and the back EMF which is a direct function of winding number and rpm... That is, a back EMF of 20 volts with a 32 volt supply means that you really have only 12 volts with of potential "driving voltage" and the current limit is the driving voltage divided by the motor resistance. The real limit is what the rotor can handle in terms of speed. For a given current losses are pretty much the same (losses are current squared x resistance plus the magnetic losses) so I guess I'm surprised that they would pick a motor that spins at that low a speed. Aircraft starter generators generally turn 12000 rpm and are pretty efficient machines.