Answer :
A squirrel cage induction motor with the following nameplate data 125 hp, 3-phase, 440 V, 60 Hz, 6 pole, 0.8 pf was subjected to certain performance tests. The full load current was 187 A and the full load torque was 588.9 lb.ft. Here's how to solve the percentage slip and its rotor frequency
:The formula for torque in an induction motor is: Torque = (3V² * R2)/(ωs * R2 + R1) * ((s * R2)/(ωs * R2 + R1))Where V is the voltage, R1 is the stator resistance, R2 is the rotor resistance,s is the slip, andωs is the synchronous speed.
The full load torque is 588.9 lb.ft.125 hp = 92.97 kW6 pole motor: n = 120f/p= 120(60)/6= 1200 rpmSynchronous speed ωs = 2π * n/60 = 125.6 rad/sThe current is given as 187 A.Power factor = 0.8For 3 phase power = √3 * V * I * p.f. * 0.746125 hp = 92.97 kW = 92.97 × 1000 W = 93200 Wp.f. = 0.8P = √3 * V * I * p.f. * 0.746V * I * p.f. = P/(√3 * 0.8 * 0.746)V * I * p.f. = 93200/(√3 * 0.8 * 0.746)V * I * p.f. = 79148.06VA (Volt-Amps)V = 440 VCurrent = 187 APower = 92.97 KWPower factor = 0.8Applying the formula for torque in an induction motor we get,588.9 = (3*440²*R2)/(125.6*R2+R1)*((s*R2)/(125.6*R2+R1))Now, we have R1, which can be found using the nameplate data and the power factor.P = √3 * V * I * p.f. * 0.74692.97 * 1000 W = √3 * 440 V * I * 0.8 * 0.746I = 198.5 AR1 = V/I = 440/198.5 = 2.215 ΩSubstituting the values of R1, torque, voltage, and current in the above equation we get the value of R2 as 0.276 Ωs = (1200 - n)/1200 = (1200 - 1256.6)/1200s = 0.046The percentage slip is given by s*100s*100 = 0.046 * 100s*100 = 4.6%The rotor frequency fr is given by fr = s * f = 4.6% * 60 Hzfr = 2.76 HzHence, the percentage slip and the rotor frequency of the motor is 4.6% and 2.76 Hz respectively.''
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