Induction Motor - Equivalent Circuit, Maximum Torque, Breakdown (PART 1) for the NCEES Power PE Exam

[@electricalpereview]

Table of Contents (PART 1): 0:00 - Problem Statement 1:31 - Induction Motor Equivalent Circuit 6:44 - Breakdown slip (s max T) 12:04 - Rotor current at breakdown (I2 = Ib) WATCH PART 2 - kzbin.info/www/bejne/bn-aZH2wf5l9gs0 WATCH PART 3 - kzbin.info/www/bejne/iH-ZiKKom5KljLM

[@EE_Overload]

Excellent video. Maybe you could explain more as to the "why" the breakdown slip is just the ratio of rotot resistance (referred to stator) to the stator impedance + rotor reactance (referred to stator).

[@electricalpereview]

Hi Alexander, good question. Motor torque is equal to the ratio of air gap power to synchronous speed: T = PAG/Wsync Since the synchronous speed of the motor does not change, the maximum value of torque is directly proportional to the air gap power (PAG). Air gap power is the power consumed in the rotor slip resistance (R2/s). The rotor slip resistance varies indirectly proportional to slip as the slip changes while the motor gets up to speed. The maximum value of air gap power, and the resulting torque, occurs when the rotor slip resistance is equal to the magnitude of the up stream stator resistance and leakage reactance (R1 + j(X1+X2)). (See the maximum power transfer theorem if you are curious as to why this is: en.wikipedia.org/wiki/Maximum_power_transfer_theorem). The rotor slip resistance (R2/s) will momentarily equal the magnitude of R1 + j(X1+X2) as slip decreases from 1 to rated slip while the motor starts from a standstill (motor speed n = 0 rpm) and gets to rated speed. Any other value for the rotor slip resistance (R2/2) will result in a smaller value for air gap power, and a resulting smaller value of torque.