An AC induction machine has six poles and is designed for 60-Hz, 240-V (rms) operation. When the machine operates with 10 percent slip, it produces 60 N-m of torque.
a. The machine is now used in conjunction with an 800-W constant power load. Determine the speed and slip of the machine when used with the above-mentioned load.
b. If the machine has an efficiency of 89 percent, what minimum rms current is required for operation with the load of part a?
(Hint: You may assume that the speed torque curve is approximately linear in the region of interest.)
Analysis:
a) Synchronous speed of six-pole induction motor at 60-Hz:
Assume that it is linear around the original operating point and ideal synchronous speed point. We can use a line to approximate the torque-speed curve of the motor, and then use the intersection of such line and the constant power curve to decide the operating point.
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