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If a coil draws 0.5 A from a 120 V, 60 Hz source at a 0.7 lagging power factor, then (a) Coil resistance is 240 Ω


76. If a coil draws 0.5 A from a 120 V, 60 Hz source at a 0.7 lagging power factor, then
  • (a) Coil resistance is 240 Ω
  • (b) Power factor angle is cos-1 (0.7)
  • (c) Coil impedance is 168 + j171.4 Ω
  • (d) Power factor angle is 32.38°
  • (e) Coil induction is 0.455 H
  • A. (a) and (b) only
  • B. (c) and (e) only
  • C. (d) and (e) only 
  • D. (a) and (e) only
Answer: Option B
cos(phi) = R / Z
or, 0.7 = R / 240      (120/0.5=240)
or, R = 168

XL = sqrt(Z^2 - R^2) = sqrt(240^2 - 168^2) = 171.5 ohm

L = XL/2-pi*f = XL/2*3.14*60 = 171.5/2*3.14*60 = 0.455 H
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