This paper explains the reason for the difference in maximum torque-speed characteristics and power factor of IM between the motor and generator operation modes and investigates the influence of machine design parameters on the difference, such as stator resistance, rotor resistance and iron loss etc. An analytical mathematical model based on the dq reference frame is employed for investigation. It is shown that the difference in torque-speed characteristics between the motor and generator operation modes is mainly caused by the opposite direction of q-axis current and affected by stator and rotor resistance values. Finally, analytically calculated torque-speed characteristics of a 12 kW IM in the motor and generator operation modes are verified by finite element analyses. (C) 2016 Elsevier B.V. All rights reserved.