Hi Louis,
You are not right.
Iron loss depends not only on rpm, but also on the flux density. Things are complicated, but a simple formula (Steinmetz) describes this loss as:
W=C * f^a * B^b,
Where f is the frequency of the rotational magnetic field (in the case of 24N28P it is 14x the rotational speed of the rotor), B is the flux density through the iron core and a, b are exponents close to 1 and 2 respectively.
Your tests measure iron loss only on no load run, with very low current and flux density. As the load increases, the flux density increases too, as well as the iron loss.
