F Demagnetizing - Raytech WR14 Instruction Manual

F 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
Page 77 of 85
Type WR50, WR14, Version 2.01