The constant torque zone is when the current in the coil is limited by the pot on the driver, meaning its working how its supposed to be. Now in an inductor, when the voltage is applied the current does not appear immediatelly, but instead it grows slowly from zero, sort of speaking its current needs time to grow untill it reaches the peak level set (driver). Its commonly said inductors are open circuit in short time and short circuit on long time (while capacitors are opposite). With increasing rpm, the current has less and less time to rise, untill after some rpm it will not reach its setopoint fast enough so driver will shut it down before that. So at a point the current simply does not have enough time to grow at setpoint level. Meaning the coils dont have enough time to energize. The average current goes down and output torque goes down as a result.
The geko page you mentioned is sort of full of bad choice of words probably just like myself. I would never say that driver is a current source because the coil IS the load. And the driver always puts the full psu voltage across the coil - just changes the duty cycle as triggered by coil current. So at all times its like a voltage source, but one that monitors the current in the load (coil). When it functions properly and peak is reached the driver just interrupts the voltage for longer, in order to limit the current. But at high rpm the current never reaches the setpoint, so the source voltage is on all time because driver does not need to interrupt it. So the average voltage over time can vary, as in an average value, because the cycle on-off times varies, but at any particular time its either 100% or 0% voltage. The driver just uses h-bridges linked to the input so only thing driver can do is turn on or off that voltage. Driver does not burn off the excess voltage like a linear voltage regulator because that would be an incredible waste. I would not say the driver changes the voltage, just that it changes the "average". So i would not say in one situation is current source and other voltage source. As choice of words i think that can be misunderstood. Like someone would think the coil itself does not matter because if is put on an ideal current source it will get the set current no matter what. And if it was like that we would not have this problem in the first place.
The geko page you mentioned is sort of full of bad choice of words probably just like myself. I would never say that driver is a current source because the coil IS the load. And the driver always puts the full psu voltage across the coil - just changes the duty cycle as triggered by coil current. So at all times its like a voltage source, but one that monitors the current in the load (coil). When it functions properly and peak is reached the driver just interrupts the voltage for longer, in order to limit the current. But at high rpm the current never reaches the setpoint, so the source voltage is on all time because driver does not need to interrupt it. So the average voltage over time can vary, as in an average value, because the cycle on-off times varies, but at any particular time its either 100% or 0% voltage. The driver just uses h-bridges linked to the input so only thing driver can do is turn on or off that voltage. Driver does not burn off the excess voltage like a linear voltage regulator because that would be an incredible waste. I would not say the driver changes the voltage, just that it changes the "average". So i would not say in one situation is current source and other voltage source. As choice of words i think that can be misunderstood. Like someone would think the coil itself does not matter because if is put on an ideal current source it will get the set current no matter what. And if it was like that we would not have this problem in the first place.