Iron losses of transformer are measured by Low pf wattmeter. Let's understand why.

Iron Losses in Transformer
Iron losses in a transformer are caused by the alternating magnetic field that produces eddy currents and hysteresis losses in the core material. These losses are constant and independent of the load conditions of the transformer.

Measurement of Iron Losses
To measure the iron losses of a transformer, a low power factor (pf) wattmeter is used. The low pf wattmeter can measure the small amount of power consumed by the transformer due to iron losses.

Working of Low pf Wattmeter
A low pf wattmeter works on the principle of the induction motor. It consists of a fixed and a moving coil. The fixed coil is connected in series with the load, and the moving coil is placed in the magnetic field produced by the fixed coil.

When the load current flows through the fixed coil, it produces a magnetic field that interacts with the magnetic field produced by the moving coil. This interaction causes the moving coil to rotate, and the pointer attached to it indicates the power consumed by the load.

Since the iron losses in the transformer are independent of the load conditions, the power consumed by the transformer due to iron losses can be measured using a low pf wattmeter. The low pf wattmeter is capable of measuring the small amount of power consumed by the transformer due to iron losses.

Conclusion
In conclusion, iron losses of a transformer are measured by a low pf wattmeter because it is capable of measuring the small amount of power consumed by the transformer due to iron losses.