Why is there an upper sideband component of current at +2sf above the supply component at f when there are broken rotor bars? What parameters influence the magnitude of the +2sf component of current?...

Why is there an upper sideband component of current at +2sf above the supply component at f when there are broken rotor bars? What parameters influence the magnitude of the +2sf component of current? Why is it not possible via MCSA to distinguish between numerous faulty joints (not broken), broken rotor bars, or a broken end ring and also discuss why it is impossible to determine which actual bars are broken?