Induction motors are the mostly used motors in industrial applications. Improving the efficiency of these motors will lead to significant energy savings, which is the recent trend to face the energy crisis. Based on mathematical analysis, this paper presents the steady state mathematical model of induction motor taking iron loss into account. A comparison analysis between two model that taken iron loss into account with that ignored it. The influence on efficiency, power factor, current and speed under different load conditions has been studied for three cases. The effect of iron loss cannot be ignored, especially on applications related to motor efficiency induction motors. High efficiency occurs due to a favorable balance between copper and iron losses. When induction motors run closed to rated torque and speed, however there are many application required variable speed and torque. Motor efficiency drops off dramatically when they are subject to partial loading conditions due to unbalance between copper and iron losses. This paper investigates the optimal operation of a three-phase induction motor through maximizing the efficiency and improving the motor power factor. Performance characteristics at maximum efficiency are presented and compared with a constant V/F method for different load conditions. It is worth saying that the optimal V/F control approach for variable speed operation is the best option for all loading conditions and for any type of load, constant V/F is the best option for constant load torque that is equal to or close to the motor's rated full load.