In a 15 HP induction-type motor with a squirrel-cage rotor, compared to the full load current at rated speed, how much starting current is required at standstill to produce successful starting torque?
β’ Induction motor starting characteristics β the relationship between starting current and starting torque for squirrel-cage rotors β’ Full-load current vs. locked-rotor current β how much current an AC motor typically draws at standstill compared to when running at rated speed β’ Effect of rotor design (squirrel-cage) on starting torque and current draw
β’ Think about what happens to back EMF (counter electromotive force) in an induction motor when the rotor is stationary versus running at rated speed. How does that affect current draw? β’ Consider typical industrial practice: if starting current were only equal to or less than full-load current, would that usually provide enough torque to start a 15 HP motor under load? β’ Compare the options: which ranges are realistically used in motor protection and sizing of circuit breakers for small-to-medium squirrel-cage induction motors?
β’ Recall the typical locked-rotor current multiple of full-load current for squirrel-cage induction motors (check common design data tables). β’ Verify whether half or equal to full-load current could ever generate adequate starting torque for a 15 HP induction motor under normal load. β’ Remember that motor starters and protection devices are sized assuming starting currents of several times the full-load current, not just slightly above it.
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