Power Electronics Handbook 3rd ed

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Induction motors, and other a.c. machines which run up as induction
motors, draw a large starting current when connected directly to a mains
supply. This is due to the fact that there is considerable slip between the
stator field and the rotor speed, so that at full stator flux there is a large
induced rotor current, which is reflected to the stator windings by
transformer action. To decrease the starting current one must reduce
either the stator flux, by lowering the stator voltage, or the stator
frequency, by a reduction of the supply frequency. The stator voltage must
now also be changed in order to keep the machine flux constant. When flux
reduction is required the stator voltage can be decreased by a.c. line
control methods, as described in Chapter 8.