We have already discussed about losses in electrical machine in “Losses in Electrical Machine – Core Loss”. Under normal operating condition, the core loss in the Rotor of an Induction Motor is negligible. In this post we will discuss about this.
Core loss consists of Hysteresis Loss and Eddy Current Loss. Hysteresis Loss is directly proportional to Bxfand Eddy current loss is directly proportional to B2f2. At standstill condition the frequency of induced emf is equal to the source frequency connected to the stator of induction motor but as the motor speeds up, the rotor frequency decreases and at a slip s, the frequency of rotor emf becomes sf.
As the slip of induction motor is near to zero i.e. of the order of 0.02, hence the frequency of rotor will be 0.02×50 Hz = 1 Hz (Assuming the supply frequency of 50 Hz). Thus we see that under normal operating condition the frequency of rotor is very small and therefore the corresponding Hysteresis and Eddy current Loss will also be less.
That is why under normal operating condition the rotor core loss of Induction Motor is negligible.