Casper Walraven A decrease in rotational velocity means reduced moment (torque) but it does not mean negative moment. A negative value of moment simply implies an opposite direction of rotational velocity.

The curves shown in the figures are correct where the moment reduces/increases whenever the rotational speed is reduced/increased. The relation between the moment and rotational velocity is thus proportional. BTW, what turbines are you simulating—wind, hydro, or tidal?

The moment stays positive because it still acts in the same direction as the rotation. Even if the rotational speed is decreasing, the torque is not reversed . it's just slowing down the motion, not changing its direction. The moment would turn negative only if the rotation direction also changed.

Maybe I am misunderstanding, but for something to decelerate there must be a negative force

Casper Walraven The idea of negative force and torque that you mean is not quite exactly same as reduced force and rotational speed. The deceleration is due to

• external damping or resistive force which acts in the opposite sense to the accelerating force of rotor.

So,

• When the forces are POSITIVE the torque is positive, rotor turns in CLOCKWISE direction
• When the forces are NEGATIVE the torque is negative which makes the rotor turn in COUNTERCLOCKWISE direction.

However, in your case, FORCE is first increasing (ACCELERATING) causing rotor speed to increase and then its decreasing (due to RESISTIVE force) causing rotor speed to decrease (still in SAME direction sense) but resultant or net force is still POSITIVE but never a NEGATIVE value.

(This is because accelerating force is greater than resistive , damping force)