“how can I control the speed in torque mode”
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.
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