Three-phase induction (asynchronous) motors (IM) are industrial work-horses, making the protection of IM a very important topic. IM protection devices typically monitor the motor current and/or voltage to provide the motor protection functionalities like current overload, over/under voltage, etc. One of the interesting parameters to monitor is the operating power factor (PF) of the IM, which provides better underload protection compared to the motor current-based approaches. Traditionally, PF estimation would require both the voltage and the current measurements in order to apply the displacement method. In this paper, we present a method of determining the operating PF of the IM using only the measured current and the manufacturer data that are typically available from the nameplate and/or datasheet. Experimental results are presented to substantiate the feasibility of the proposed method, validated alongside the state of the art PF estimation methods. Operational PF can be effectively used for underload detection, like in a pump application, or PF compensation for improving the power quality.