No-load loss refers to the active power consumed when the secondary winding of a transformer is open, and the rated voltage of a sinusoidal waveform at the rated frequency is applied to the primary winding. This loss is primarily due to the magnetic core's hysteresis and eddy current losses, and it is calculated using the formula: No-load loss = no-load loss coefficient × unit loss × core weight. Load loss, on the other hand, occurs when the secondary winding is short-circuited (in steady state), and the primary winding carries the rated current. The active power consumed in this case is known as load loss, which includes resistance losses in the windings and additional losses such as eddy currents, loop losses, stray losses, and lead losses. Impedance voltage (Uz) is the voltage applied to the primary winding when the secondary is short-circuited and the rated current flows through the primary. It is typically expressed as a percentage of the rated voltage: Uz = (Uz/U1n) × 100%. The induced voltage per turn (ζ) can be calculated using the formula: u = 4.44 × f × B × At, where f is the frequency, B is the magnetic flux density, and At is the effective cross-sectional area of the core. For standard frequencies, this simplifies to: - When f = 50Hz: u = B × At / 450 × 10ⵠ- When f = 60Hz: u = B × At / 375 × 10ⵠIf you know the phase voltage and the number of turns, the ζ potential equals the phase voltage divided by the number of turns. Transformer no-load loss is mainly due to iron loss, which includes hysteresis and eddy current losses in the core. Copper loss from the primary winding’s resistance is negligible because the no-load current is very small. Iron loss depends on factors like core material, manufacturing quality, and operating voltage. Mathematically, it can be represented as: P = P_h + P_e = k_n × f × B_m^n + k_w × f² × B_m² Here, P_h is hysteresis loss, P_e is eddy current loss, and n is a constant depending on the core material. For silicon steel sheets, n ranges between 2 and 3.5. According to transformer theory, the induced voltage E1 is proportional to the frequency and magnetic flux density: E1 = K × f × B_m². Under normal operation, the primary voltage U1 is approximately equal to E1, so the no-load loss remains relatively constant. However, if the supply voltage fluctuates, the no-load loss will also change. Importantly, no-load loss is independent of the load and is determined by the core’s properties and design.
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