Transformers — Question 7

Question 7

An unloaded real core can draw both magnetizing and loss current

Use a no-load primary model consisting of an ideal magnetizing inductance Lm in parallel with core-loss resistance Rc. Let Lm=.800 H, Rc=2400 ohms, V=120 V rms, and f=50.0 Hz.

1. Find in-phase current, quadrature magnetizing current, and total rms current.

2. Find no-load real power and power factor.

Question 7 - Solution

An unloaded real core can draw both magnetizing and loss current

(a) Method and calculation

$$I_c=V/R_c=.0500\,A,\quad I_m=V/(2\pi fL_m)=.477465\,A.$$$$\boxed{I_0=\sqrt{I_c^2+I_m^2}=.480076\,A}.$$

(b) Interpretation and further calculation

$$P_0=V^2/R_c=6.00\,W.$$$$\boxed{\mathrm{pf}_0=I_c/I_0=.104150\quad\text{lagging}}.$$

Physical check

Open secondary terminals do not eliminate the current needed for the supplied nonideal core model.