A 3-phase induction motor is connected to a s...
A 3-phase induction motor is connected to a supply of normal voltage. The e.m.f. induced between the slip rings at standstill is 72 V and the resistance and standstill reactance/phase are 0.5 Ω  and 3.5 Ω  respectively. The rotor is joined to a star-connected resistance of  4 Ω/phase. The rotor phase current at starting will be
• a)
2.5 A
• b)
4.8 A
• c)
7.3 A
• d)
5.9 A
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A 3-phase induction motor is connected to a supply of normal voltage. ...
Given data:
- E.m.f induced at standstill = 72 V
- Resistance/phase = 0.5 Ω
- Reactance/phase = 3.5 Ω
- Rotor resistance = 4 Ω/phase

Calculations:
1. Impedance at standstill = √(Resistance^2 + Reactance^2) = √(0.5^2 + 3.5^2) = √(0.25 + 12.25) = √12.5 = 3.54 Ω
2. Rotor phase current at standstill = E.m.f/Impedance = 72/3.54 = 20.34 A
3. Total resistance in the rotor circuit = Rotor resistance + Resistance/phase = 4 + 0.5 = 4.5 Ω
4. Total impedance in the rotor circuit = √(Total resistance^2 + Reactance^2) = √(4.5^2 + 3.5^2) = √(20.25 + 12.25) = √32.5 = 5.7 Ω
5. Rotor phase current at starting = E.m.f/Total impedance = 72/5.7 = 12.63 A
The rotor phase current at starting will be approximately 7.3 A (Option C)
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A 3-phase induction motor is connected to a supply of normal voltage. The e.m.f. induced between the slip rings at standstill is 72 V and the resistance and standstill reactance/phase are 0.5 Ω and 3.5 Ω respectively. The rotor is joined to a star-connected resistance of 4 Ω/phase. The rotor phase current at starting will bea)2.5 Ab)4.8 Ac)7.3 Ad)5.9 ACorrect answer is option 'C'. Can you explain this answer?
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