Good day All.

KIndly help me to find that why slip (s) is subtracted from synchronous speed (Ns) to get value for actual speed of Induction motor in "Separate-Winding Motor" case( a method for providing multi-speed control)??
i.e. Speed = 120*Frequency/No. of poles - Slip

Ref : STEP series by Siemens, Basics of Control Components, Page # 56

Separate-winding Motors Basics of Control COmponents


Essentially a squirrel cage induction motor can never keep up with the synchronous speed due to a few physical factors. The main reasons being friction and drag from load. So the rotor always lags behind the rotating field in the stator. At no load the motor will turn closer to synchronous speed as there is little load on the rotor. The speed on the nameplate is the speed at full load. This applies to all induction motors, single, three, multi-speed, etc.

  • \$\begingroup\$ That isn't why an induction machine doesn't generate torque at Sync speed... IF the rotor was rotating in sync with the stator field there would be no magnetic coupling between the stator (primary) and the rotor (secondary). At the end of the day an induction machine is a rotating secondary transformer \$\endgroup\$ – JonRB Mar 4 '15 at 19:56

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