Slip

Slip is the small difference in speed between the rotating magnetic field of an induction motor and its physical rotor. Think of a sheepdog chasing a flock: it can never quite reach the sheep, and that very gap is what keeps it running. In an induction motor, the rotor must always trail the field by a few percent, because only when it lags does the field sweep past the rotor bars, induce current, and create torque.

Slip is usually written as a fraction: a motor whose field rotates at 1800 rpm but whose shaft turns at 1764 rpm has a slip of 2%. Slip is the motor's automatic load sensor — load it heavily and it slows down slightly, increasing slip, which induces more rotor current and more torque to meet the demand. At zero slip there is no torque at all, and at standstill (slip = 1, or 100%) the motor draws its maximum, locked-rotor current.