In order to explore the characteristics of heat transfer for motors supplied by pulse width modulation (PWM)converter, and further reveal the influence characteristics of frequency converter parameters (modulation ratio) on the motor thermal distribution, a 55 kW induction motor supplied by PWM converter power is studied. Based on the computational fluid dynamics and the heat transfer theory, combined with the feature of motor ventilation structure, a three-dimensional flui d-thermal coupled model with motor encapsulated in air region is established. And the temperature field of asynchronous driving motor is simulated numerically by using the finite volume method. Therefore, characteristics of heat transfer for the whole motor domain are compared and analyzed under the control with two different modulation ratios. The motor temperature distribution trend is substantially the same in the solve region under the control with two different modulation ratios. In radial direction, rotor has the higher temperature. However, the curve of temperature-rise in the air gap has step-change and the lowest temperature-rise occurs in stator. In axial direction, the temperature-rise of each part of motor near the fan is lower, and that of each part of motor far away from the fan is higher. In circumferential direction, the curve of stator yoke temperature-rise has undulate change.