How to determine the condition of the electromagnetic system of transformers and electrical machines

A generally accepted method for determining the condition of the magnetic cores of electromagnets and their windings is the measurement of the current at idle speed or the characterization of magnetization.

Checking the magnetic circuits of power and measuring transformers

For power transformers and voltage transformers, the no-load current is measured by applying rated voltage (for voltage transformers — to the secondary winding) and measuring current (in all phases — for three-phase transformers) when the load is disconnected.

The measured current is compared to the nameplate or experimental data for the type of equipment being tested. Exceeding it, more significantly, is a sign of damage to the magnetic circuit (damage of the insulation between steel sheets, short-circuiting of the packages) or short-circuiting of part of the turns of the coils.

To measure current transformers, the characteristic of the dependence of the magnetizing current in the coil on the voltage supplied to it is taken. The nature of the current change magnetization allows you to assess the presence of damage (short circuit) in the current transformer.

In this case, a sharp decrease in the magnetization characteristic in its initial part is explained by a significant demagnetization of the magnetic circuit at low magnetic flux values. With a small number of closed turns, the number of magnetization characteristics changes only in the initial part, when significant and in the saturated zone.

The resulting magnetization characteristics of the current transformers are compared with a typical or experimental one. Significant deviations from typical or experimental characteristics are a sign of damage.

Checking the magnetic cores of electrical machines

The condition of the magnetic circuits of electrical machines is checked by taking no-load and short-circuit characteristics (for synchronous generators), as well as load characteristics (for direct current machines) and comparing the obtained characteristics with the factory ones available in the accompanying documentation.

According to these characteristics, additional parameters necessary for setting up excitation control devices and further calculations made under operating conditions are determined.