Rotational speed in DC machine

[paola21]

Homework Statement

rotational speed in DC machine

Relevant Equations

I tried to do the last point but I can't find the result. I calculated, knowing the new value of Vexc, the new value of Iexc and from this the new value of the flux. then with the inverse formula of the twisting torque I obtained the new speed which however comes out about 1920 rpm
I don't understand where I went wrong

 

[berkeman]

Welcome to PF.

Can you show your calculations so that we can check them? Thanks.

 

[paola21]

 

[paola21]

I solved the exercise, I had the wrong approach. I had to first calculate Kt which varies with Vexc and then from this new value of Kt, knowing that the torque remains the same, calculate the new rotation speed. Thanks anyway