Reason: losses depend on frequency

Both losses are directly tied to how often the flux reverses, so they increase with the supply frequency.

Hysteresis loss

• In every cycle the core material is taken round one complete B–H loop. The energy lost per cycle per unit volume equals the area of the loop (a fixed amount for given B_max).
• If the supply has f cycles per second, the loop is traversed f times a second. So the loss per second is (area × volume × f).
• Pₕ = ηₕ B_max^1.6 f V, so Pₕ ∝ f.

Eddy current loss

• The emf induced in the core is proportional to the rate of change of flux, e ∝ dφ/dt. For a sinusoidal flux, E ∝ f B_max.
• The eddy current is proportional to this emf: I ∝ f B_max.
• The power loss is I²R, so it varies as the square of the emf.
• Pₑ = kₑ B_max² f² t² V, so Pₑ ∝ f².

Example: if the frequency of the same transformer is doubled at constant B_max, hysteresis loss doubles and eddy current loss becomes four times. This is why 50 Hz transformers are not suitable for operation at high frequency without redesign, while 400 Hz equipment uses thinner laminations.