become 12-5 A and 0-2. Assuming R1 = R2', (a) Estimate the maximum torque and the total external resistance, expressed in terms of the rotor resistance, required to get this torque at standstill with full voltage applied. (b) What would be the line current if started from full voltage without externai rotor resistance? (c) How much external rotor resistance would be required to double the naturai starting torque?
A 3-phase, 4-pole, 440-V, 50-Hz slip-ring induction motor is started at full voltage uncoupled from its load. When a fixed external rotor resistance of value four times the rotor resistance itself is inserted at starting, the starting current and power factor are respectively 200 A and 0-6. The motor runs up to speed and the slip rings are then short circuited. On steady conditions being achieved the current and power factor become 12-5 A and 0-2. Assuming R1 = R2', (a) Estimate the maximum torque and the total external resistance, expressed in terms of the rotor resistance, required to get this torque at standstill with full voltage applied. (b) What would be the line current if started from full voltage without externai rotor resistance? (c) How much external rotor resistance would be required to double the naturai starting torque?
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