Q10Electric Drives

Slip is defined as the fractional (or percentage) difference between an induction motor's synchronous speed Ns (determined by supply frequency and the number of stator poles, Ns=120f/P) and its actual rotor speed N: s=(Ns-N)/Ns. Slip is the fundamental variable governing induction motor operation, since it directly determines the frequency and magnitude of the EMF induced in the rotor circuit (rotor EMF and rotor frequency both scale directly with slip), and hence the resulting rotor current and developed torque, as derived in detail (via the equivalent circuit and torque expression) elsewhere in this paper. Motor performance varies substantially with slip: at very low slip (light load, near synchronous speed), the motor operates efficiently with small rotor losses (since rotor copper loss is proportional to slip times air-gap power) and in the nearly-linear, stable region of its torque-slip characteristic; as slip increases (with increasing load), torque and rotor current both increase (up to the maximum/breakdown-torque slip point), while rotor losses and motor operating temperature also increase; and at very high slip (heavy overload or stall conditions, s approaching 1), the motor draws very high current with poor torque-producing efficiency and is at risk of thermal damage if sustained, making a proper understanding of the slip-dependent behavior of torque, current, and losses essential to correct induction motor drive design, protection, and operation.