The star-connected rotor of an induction motor has a stand-still resistance of 4.5 ohms/phase and a resistance of 0.5 ohm/phase. The motor has an induced emf of 50 V between the slip-rings at stand-still on open circuit when connected to its normal supply voltage. Find the current in each phase and the power factor at start when the slip-ring is short-circuited
Given
X = 4.5 ohm {Standstill resistance = Reactance}
R = 0.5 ohm
E = 50 V (Line voltage)
To find
1. Iph
2. cos φ
Solution
In star connection Eph = Eline / √3
= 50 / √3
= 28.87 V
Zph = √(R2+X2)
= √(0.52 + 0.452)
= 4.53 ohm
Iph = Vph / Zph
= 28.87 / 4.53
= 6.37 A
cos φ = R/Z
= 0.5 / 4.53
= 0.11
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