In one line
A transformer loses power as iron (core) loss, made of hysteresis and eddy-current losses that stay almost constant, and copper loss, the \( I^2R \) heating of windings that varies with the square of load.
Explained simply
A transformer has no moving parts, so there is no friction or windage loss. All its losses become heat in two places: the iron core and the copper windings. Think of the core loss as a fixed "entry fee" paid whenever the transformer is switched on, and the copper loss as a "per-kilometre charge" that grows rapidly with how hard you drive it.
Technically, hysteresis loss is energy used to repeatedly reverse the magnetic domains of the core every cycle. Eddy-current loss is \( I^2R \) heating from small circulating currents induced in the core itself. Copper loss is \( I_1^2R_1 + I_2^2R_2 \) in the windings.
How it works
| Loss | Depends on | Reduced by |
|---|---|---|
| Hysteresis | \( B_m^{1.6} \), \( f \), core volume, material | silicon steel (narrow hysteresis loop), CRGO steel |
| Eddy current | \( B_m^2 \), \( f^2 \), \( t^2 \) (lamination thickness) | thin insulated laminations, high-resistivity (silicon) steel |
| Copper | load current squared | thicker conductors, good-conductivity copper |
- With \( V_1 \) and \( f \) fixed, flux is nearly constant, so iron loss is a constant loss, measured by the open-circuit test.
- Copper loss is a variable loss: at half load it is one quarter of full-load copper loss. Full-load copper loss is measured by the short-circuit test.
- Small stray and dielectric losses also exist but are usually neglected.
Key formula
\( P_h = \eta B_m^{1.6}fV, \quad P_e = K_eB_m^2f^2t^2V, \quad P_{cu} = I_1^2R_1 + I_2^2R_2, \quad P_{cu}(x) = x^2P_{cu,FL} \)
\( \eta \) = Steinmetz hysteresis coefficient (here not efficiency), \( B_m \) = maximum flux density (T), \( f \) = frequency (Hz), \( V \) = core volume (m³), \( K_e \) = eddy-current constant, \( t \) = lamination thickness (m), \( x \) = fraction of full load.
Quick example
Distribution transformers use CRGO (cold-rolled grain-oriented) silicon-steel laminations of a fraction of a millimetre thickness to keep iron losses low, since they stay energised 24 hours a day.
Common mistakes
- Saying laminations reduce hysteresis loss; they reduce eddy-current loss.
- Saying iron loss changes with load; it is practically constant.
- Including friction or windage loss for a transformer.
How MU asks it
MU states numericals are not expected in this module, so questions are theory. Typical: "Explain the various losses in a transformer and how they are minimised" (5 marks) or "Why is the transformer core laminated?" (3 marks). For full marks classify into constant (iron) and variable (copper) losses, write the dependence of each, give the methods of reduction, and mention OC and SC tests as how they are measured.