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.