Paper 4 Transformers Answers

Q1 Measuring Transformer Efficiency

1. Measure primary and secondary rms voltage and current under load; for the stated resistive conditions calculate $P_p=V_p{I}_p$, $P_s=V_s{I}_s$, then $\eta =P_s/P_p\times 100\%$. [2]
2. Use a wattmeter, or another suitable method that measures real input and output power directly; $V_{\mathrm{rms}}{I}_{\mathrm{rms}}$ alone is not sufficient for a non-resistive load. [1]
3. Use low-voltage isolated supplies, do not touch live terminals, avoid overheating, switch off before changing coils. [2]

Q2 Investigating Turns Ratio

1. Axes correctly oriented and labelled [1]; sensible scales [1]; six points plotted accurately [2]; best-fit straight line [1].

2. Gradient $\approx 1.51\times 10^{-2}\mathrm{V}\mathrm{tur}{\mathrm{n}}^{-1}$. Accept a graph-consistent value. [2]

3. The gradient is $V_p/N_p$, hence

   $$N_p=\frac{V_p}{\text{gradient}}\approx \frac{12.0}{1.51\times 10^{-2}}\approx 7.95\times 10^2,$$

   consistent with about $800$ turns. [2]

4. The points are close to a straight line whose intercept is consistent with zero, so $V_s\propto N_s$ at constant $V_p$ and $N_p$. [2]

5. Flux leakage, meter uncertainty, winding resistance, imperfect magnetic coupling, small primary-voltage variation or core loss. [1]