Originally Posted by modisc
I am aware that motor consumes current depending on the loading torque. But I also notice that when throttle is small, it is very easy to hold the rotor still; while in higher throttle, it can be very difficult to stop the rotor. However, for servo drive, even when the motor is running at low speed, the torque needed to stop the rotor can be very big; in other words, constant torque.
This may explain why high speed inrunner + gear to reduce the output speed is favoured. For example, the maximum torque of a inrunner (15000 rpm)+ gear (5:1) at 3000rpm, may be better than a motor directly running at 3000rpm, at 50% throttle.
I am also not sure, just curious. Thanks for the input.
Maybe I don't completely understand your point (it happens!), but at low throttle, you just need to think that you are applying a low voltage to the motor. Under those circumatances, even stalled only a small current can flow (I=V/R, when the motor is stalled). Small current =small torque..
Now if you increase the voltage (higher throttle), more current can flow, so the torque also can increase.
It isn't actually the load that pulls current and produces the torque, it is the rpm the motor is turning at. That rpm creates a back emf that fights the impressed voltage from the power supply.
So I guess you know all that, but that's my explanation for the low torque at low throttle.
Now for a servo motor, you have to ask what the controller is trying to do. My guess is that it is trying to move to a position, or to a velocity (rpm). If the motor can't move or move fast enough, the controller increases the applied voltage (or equivalently increases the PWM pulse width to the motor) until the motor starts accelerating to the target velocity.
Our RC motors aren't doing that, the controller is just producing a fixed PWM width (or equivalent voltage level), and not trying to hit a rpm. The rpm they get is just set by when the motor torque = load torque. At that point the motor stops accelerating.
The only time that isn't true is when you have a set rpm target in the motor--but then you are letting the controller find the throttle level to reach that rpm.