So if you can detect your load, and predict the max Iq needed then you can achieve what you want by adjusting the max Iq on the motor, and adjusting your throttle control to work proportionally to the current max.
But the question is, how do you do the load detection / max Iq prediction? The normal way to handle this is with velocity control, where the velocity PID will detect the velocity error using the sensor, and adjust the current Iq according to the PID settings.
If you don’t want velocity control then you’ll have to think of another algorithm to measure the throttle response to determine the load…
You cannot. By definition, the act of attempting to control the speed means you are in “speed mode”, requesting a change in torque.
“speed can be reduced when i reducing the dc voltage.”
Certainly. This is actually the way that many hobby ESCs work. They attempt to commutate at near 100% duty cycle and then just vary the bus to adjust the voltage to the motor. With low inductance, high speed motors this actually works pretty well. But FOC can do it even better, giving better dynamic performance and higher efficiency (our customers tell us on their full suite of testing they are getting 10-15% more battery life using InstaSPIN-FOC).
This is one of answers given in TI forum website very long back, I follow this method. Link