ETO ETO Exam papers 📅 Aug 2024

Exam Question

(a) How does change to frequency affect the operation of the transformer? What makes this ratio different from the ratio of transformer (6)

(b) The coils A and B are connected in series to 50Hz mains. The current is 1A and the voltage across each coil is measured to be 45V and 70V respectively. When the coils are connected in a d.c. supply, the current is also 1A, but the voltages across the coils are now 20V and 40V respectively. Find the impedance, reactance and resistance of each coil, the total circuit impedance, the applied a.c. voltage and the power factor of the complete circuit. (10)

Reference Answer

(a) Effect of frequency on transformer:
The primary and secondary windings of a transformer exhibit reactance, which is in opposition to the flow of alternating current (AC). This reactance is directly proportional to frequency (f) and inductance (L), as shown in the formula
$$X_{L}=2\pi fL$$
Therefore, an increase in frequency leads to a higher reactance, reducing the current flowing through the windings. Conversely, a decrease in frequency lowers the reactance, increasing the current flow. This effect is particularly noticeable at higher frequencies where the reactance becomes dominant over the resistance of the windings.

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