True/false: rotor core loss neglected

True.

Core loss consists of hysteresis loss (∝ f) and eddy-current loss (∝ f²), so it depends strongly on the frequency of the flux alternations in the iron.

• In the stator, the flux alternates at the supply frequency f (50 Hz), so stator core loss is significant. It is included in the equivalent circuit as the core-loss resistance Rc.
• In the rotor, the flux alternates at the slip frequency f₂ = s·f. At normal running slip (2 to 5 %), f₂ = 1 to 2.5 Hz for a 50 Hz supply. At this frequency the hysteresis and eddy-current losses are very small.

For example, at s = 0.04 and f = 50 Hz, f₂ = 2 Hz. The rotor eddy-current loss is about (2/50)² = 0.16 % of that at supply frequency, and the hysteresis loss is about 4 %.

So rotor core loss is negligible compared with stator core loss, copper losses and friction. It is neglected in the equivalent circuit, and the only core loss shown there is the stator core loss. (The statement is true only for the running condition. At standstill, f₂ = f and the rotor core loss is not small, but the motor does not stay in that state.)