Paper 4 Transformers Drills

Q1 Measuring Transformer Efficiency

1. State the measurements needed to determine the efficiency of a low-voltage transformer when both loads are resistive. [2]
2. State one additional condition or measurement needed if the load is not purely resistive. [1]
3. State two safety precautions. [2]

Q2 Investigating Turns Ratio

A student applies a constant primary rms voltage $V_p=12.0\mathrm{V}$ to a transformer and uses secondary coils with different numbers of turns $N_s$. The secondary is unloaded while each voltage is measured.

$N_s$	200	300	400	500	600	700
$V_s$/V	2.9	4.6	6.0	7.6	8.9	10.6

1. Plot a graph of $V_s$ against $N_s$ and draw a best-fit straight line. [5]
2. Determine the gradient and state its unit. [2]
3. Use $V_s/V_p=N_s/N_p$ to determine the primary turns $N_p$. [2]
4. Explain whether the results support the ideal turns-ratio relation. [2]
5. State one reason why the measured points may not lie exactly on the ideal line. [1]