Why is the sum of rotor core loss and friction and windage losses of a 3-phase induction motor nearly constant at all loads?

This is explained as under :

(i) At no load, the rotor speed is maximum. The frequency and hence the rotor core loss is practically zero. However, the friction and windage losses are maximum.

(ii) When the load is increased, the rotor speed decreases and hence rotor frequency in­creases. Consequently, the rotor core loss increases while friction and Windage losses decrease.

It is found that at all loads, the rotor core loss plus friction and windage losses remain nearly constant.