Many sinusoidal components different from power system frequency in instantaneous power occur in three phase induction motors during faults and unbalanced conditions. The amplitude and frequency of these components give an idea about faults and their level. Theoretical analysis indicates that side band components that changes with slip around twice of the power system frequency occur during broken rotor bars faults. The new detection algorithm that introduced is based on power spectral density changes between 90-110 Hz frequency band in three phase instantaneous power. Experimental results have demonstrated that the new algorithm detects the faults more reliable and effective than the other methods.