Transformers and Transformer Investigations

Alternating primary current creates a changing magnetic flux in a low-reluctance core. This links the secondary turns and induces an alternating emf. More secondary turns give a larger emf because each turn experiences the same rate of flux change.

A steady dc current gives no sustained secondary emf once its flux is constant, although switching transients can occur. An iron core couples the flux efficiently; thin insulated laminations restrict circulating eddy currents. The physical link is magnetic induction rather than charge flowing directly from one winding to the other.

For an ideal transformer, input and output powers are equal. Raising secondary voltage therefore lowers secondary current for a given output power. The current ratio is the inverse of the turns ratio, not the same ratio.

A real transformer loses energy through coil resistance, eddy-current heating, repeated magnetisation of the core and imperfect flux coupling. Efficiency is useful secondary output power divided by primary input power. Core materials reduce hysteresis losses; laminations reduce eddy currents; low-resistance windings reduce I²R heating.

For fixed transmitted power P, current is P/V. Cable heating is I²R, so raising voltage reduces resistive loss strongly. If voltage increases tenfold for the same power and cable resistance, current falls tenfold and heating falls by a factor of 100.

High voltage has insulation and infrastructure costs, so transformers allow different voltages for transmission and local use. A statement that high voltage “saves energy” should specify the mechanism: lower current causes a smaller resistive heating rate in the cables.

Use a suitable isolated low-voltage ac supply and a demountable laminated-core transformer in the supervised laboratory setup. Keep primary turns and supply amplitude fixed, change secondary turns and measure both voltages with suitable ac meters. Plot secondary voltage against secondary turns; the ideal gradient is Vp/Np.

Record whether the secondary is unloaded or connected to a specified load. Measure primary and secondary currents under load to test the power/current ratio within meter and transformer uncertainties. Explain losses through winding resistance, eddy currents and imperfect coupling. Never connect a homemade transformer directly to mains.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.