A permanent magnet generator creates $4.25$ MW at 15 rpm. How much rotational torque is required to turn the shaft at 15 rpm when the generator load is at $4.25$ MW?
Let us suppose for simplicity that the magnet generator is a cylinder and its radius is R = 1m. This will not affect the generalization of the result.
The speed of a point on the exterior of the magnet cylinder is
$V = \omega*R$, where omega is the pulsation of the movement and
$\omega = 2*\pi*frequency = 2*\pi*F$
$frequency = 15rpm =15/60 =0.25$ Hz
Hence $v = 2*\pi*F*R = 2*\pi*0.25 = 1.57$ rad/sec
The power generated by the applied force must equal at least the power generated.
$P = F*v$, where F is the applied force $F = P/v = 4.25*10^6/1.57 = 2.70*10^6$ N
The rotational torque of this force is simply
$M = F*R = 45092.8*1=2.70*10^6 (N*m)$
One can see from here that the choosing of a 1 m cylinder radius does not affect the result.