The conventional calculation of rotor losses in cage induction motors by the time-stepping finite element method requires the full slip waves of the rotor flux densities and currents. The calculations of the full slip waves are extremely expensive in CPU time, memory, and hard disk space for the induction motors with skewed slots and pulsewidth modulated supply. This article proposes a technique that reproduces those full slip waves by utilizing both the time varying and the spatial information of the rotor electromagnetic quantities and, hence, significantly saves the calculation cost. The more rotor bars per pair of poles the induction motor has, the more calculation cost this technique saves. The calculation costs of the proposed and conventional methods are compared for the rotor losses of a 5.5-kW inverter-fed skewed induction motor under load condition, and the calculated losses are validated by tests.