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8.3. Concept of rotating magnetic field                    167


                           Therefore, rotor current frequency,


                                                f r = sf                      (8.6)
                         where f is the supply frequency in Hertz and s is the slip

                         Rotor current frequency = Slip × Supply frequency to stator


                     8.3.3 Applications of three phase induction motors

                     Squirrel cage induction motors are used in centrifugal pumps, fans,
                     conveyors, compressors, reciprocating pumps, line shafting, lathe works,
                     line shafting, etc.
                     Slip ring induction motors are used in lifts, hoists, cranes, conveyors,
                     pumps, flour mills, etc.

                     Advantages of induction motors

                        1. Simple design.

                        2. Low cost compared to other motors of the same capacity.

                        3. High overload capacity.

                        4. Very rugged construction.

                        5. Requires little maintenance.
                        6. Have reasonable good efficiency and provides reliable operation.

                        7. Has good speed regulation and high starting torque.

                        8. Uses readily available power supply (three phase AC).



                     Disadvantages of induction motors

                        1. Wide variation in speed is not possible.

                        2. Its speed decreases with increase in load.



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