Question E6D07:
What is the current that flows in the primary winding of a transformer when there is no load on the secondary winding?  

Choices:  
A: Stabilizing current  
B: Direct current  
C: Excitation current  
D: Magnetizing current    

Correct Answer is  D: Magnetizing current  

Correct answer: D — Magnetizing current

1) Why D is correct
- With the secondary open (no load), the primary still draws a small AC current to establish the alternating magnetic flux in the core. This is the magnetizing current.
- It is mostly reactive (lags the voltage by nearly 90°) and is typically a small fraction of rated current (often 2–10%).
- Useful relations:
  - Magnetizing reactance: $X_m = 2\pi f L_m$
  - Magnetizing current (approx.): $I_m \approx \dfrac{V_1}{X_m}$
  - Core flux from applied voltage: $V_1 \approx N_1 \dfrac{d\Phi}{dt}$, so for sine waves, $E \approx 4.44 f N_1 \Phi_{\text{max}}$
  - No-load current components: $I_0^2 = I_m^2 + I_w^2$, where $I_w$ is the in-phase component supplying core losses (hysteresis and eddy currents)

2) Why the other choices are incorrect
- A: Stabilizing current  
  Not a standard transformer term in this context. It doesn’t describe the no-load current in a transformer.

- B: Direct current  
  The primary is powered by AC, so the no-load current is AC. DC in a transformer primary would drive the core toward saturation and is not normal operation.

- C: Excitation current  
  In some texts, “exciting/excitation current” is used for the total no-load current, which includes both the magnetizing (reactive) component and the core-loss (in-phase) component. However, the question specifically points to the current that creates the flux, i.e., the magnetizing current. On the exam, “magnetizing current” is the accepted term.

3) Rules of thumb
- No-load current is small (a few percent of rated current) and largely reactive.
- Power factor at no load is low because the magnetizing current dominates.
- In the transformer’s equivalent circuit, the magnetizing branch is modeled by $X_m$ (reactive) in parallel with $R_c$ (core-loss resistance).