H Demagnetizing
Energizing magnetized transformers
Problems can be created when the core of a transformer becomes magnetized prior to being
energized for use. Damage to the insulation and deformation of the windings are two of the most
common side effects.
Generally, Transformer cores, during operation are magnetized, demagnetized and then
magnetized and demagnetized in the opposite direction for each sine wave cycle.
It is not easy to determine the magnitude the core is magnetized at the moment the Alternating
Current source is switched off. The larger the hysteresis of the iron in the core material, the greater
the magnetization level. The magnitude of magnetization depends upon the moment the
transformer is switched off and the applied voltage level during the power on state.
The highest magnetization in this scenario occurs when a transformer is switched off right at the
moment the voltage source sine wave crosses the zero-line.
Transformer cores are also magnetized after having a Direct Current source flow in a winding. For
example, the application of Direct Current when performing a winding resistance test. After
measuring the Winding Resistance the core is magnetized to its’ maximum Hysteresis.
Both of these scenarios can leave the core in an unpredictable state which can cause damage
when energized. These problems can be eliminated if the transformer is demagnetized prior to
being energized for use.
Conclusion
• A transformer is magnetized when power is switched off. Even if a transformer is completely de-
energized, it can still be in a magnetized state!
• After performing DC tests on a transformer winding, the core is magnetized.
Consequences of energizing a magnetized transformer
When a magnetized transformer is switched back on line, it is quite possible, depending upon the
position of the sine wave and other variables, that inrush current can exceed 8 times the nominal
current flow. This hazardous situation may cause tripping of over-current relays, damage to the
insulation medium and deformation of the transformer windings.
Instruction Manual Winding Resistance Meter
Type WR100 / WR50 / WR14
Page 130 of 139
