π Key Concepts
β’ Carefully compare the percent of rated torque and percent of rated current marked at key points on the curves (locked-rotor, pull-up, rated).
β’ Relate percent of synchronous speed to slip (slip is highest at standstill and very small at rated speed).
β’ Estimate numerical values from the graph (e.g., starting/locked-rotor current and torque) and compare them to the full-load (rated) values.
π Think About
β’ On the torque curve (Figure A), where does the pull-up torque sit relative to the dashed rated torque (100%) lineβwell below it, about equal, or well above?
β’ On the current curve (Figure B), approximately what percent of rated current is shown at locked-rotor (starting), and how many times the rated current does that represent?
β’ At about what percent of synchronous speed does the motor reach rated torque and rated current, and what does that imply about the slip at rated load?
β
Before You Answer
β’ Verify from Figure A whether the pull-up torque is less than, equal to, or greater than 100% of rated torque before judging choice A.
β’ Read from Figure B the approximate locked-rotor (starting) current in percent of rated current (it should be several hundred percent) before comparing B and D.
β’ Confirm from the horizontal axis that rated torque/current occur very close to 100% synchronous speed, meaning slip is only a few percent, before evaluating statement C.