<P> Larger single phase motors are split - phase motors and have a second stator winding fed with out - of - phase current; such currents may be created by feeding the winding through a capacitor or having it receive different values of inductance and resistance from the main winding . In capacitor - start designs, the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a thermistor which heats up and increases its resistance, reducing the current through the second winding to an insignificant level . The capacitor - run designs keep the second winding on when running, improving torque . A resistance start design uses a starter inserted in series with the startup winding, creating reactance . </P> <P> Self - starting polyphase induction motors produce torque even at standstill . Available squirrel cage induction motor starting methods include direct - on - line starting, reduced - voltage reactor or auto - transformer starting, star - delta starting or, increasingly, new solid - state soft assemblies and, of course, VFDs . </P> <P> Polyphase motors have rotor bars shaped to give different speed - torque characteristics . The current distribution within the rotor bars varies depending on the frequency of the induced current . At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the cage rotor bars (by skin effect). The different bar shapes can give usefully different speed - torque characteristics as well as some control over the inrush current at startup . </P> <P> Although polyphase motors are inherently self - starting, their starting and pull - up torque design limits must be high enough to overcome actual load conditions . </P>

Explain the principle of operation of a three-phase induction motor