cerewa said:
According to ebikes.ca, I_motor = I_battery/D, so by naive inference, an infinitesimal D would result in a nearly infinite motor current thus nearly infinite torque!
Bad formula. They meant to say
I_motor = I_battery * D
Where D (duty cycle) is a number ranging from 0 to 1 and I_battery is the "current you'd have if 100% duty cycle", and I_motor is the current at the motor.
No this is not true, the formula listed ( I_motor = I_battery/D) is (to a first order) completely correct. And the explanation and equations that I gave in the simulator description is all that there is to it, there is no 'behind the scenes' trickery going on. If you had a motor with zero winding resistance, then yes at low duty cycle the current through the motor would be many many times more than the current from the battery pack. When the motor has a finite winding resistance, then the current that results through the motor is to a first order just the battery voltage, times the duty cycle, divided by the motor resistance
Imotor = Vbatt * D / Rmotor
In a 1 quadrant motor like the Crytslayte, it is actually less than this because of the voltage drop across the freewheeling diodes, which is accounted for in the simulator but we can ignore that here.
So at low values for D, then you also get a lower (not a higher) value for Imotor, and hence less torque. Once you have calculated Imotor this way, then you can go backwards to find the battery current, Ibatt = Imotor*D.
Justin