The generator stator core loss test, also known as the stator core magnetization test, is one of the most important large-scale test items in generator installation, maintenance, and preventive testing. This test simulates the magnetic state of the core during normal operation to detect the insulation condition between core laminations and evaluate the overall quality and loss characteristics of the core. This article systematically expounds the core purpose of the generator core loss test, comprehensively reviews the current main technical standards and regulations in China, and analyzes the key technical requirements and qualification criteria of the test, providing a reference for engineering practice.
The generator stator core is composed of a large number of thin silicon steel sheets stacked together, with an insulating layer applied between the sheets to reduce eddy current losses. During the manufacturing, transportation, installation, or long-term operation of the generator, the core may suffer from insulation damage between the sheets due to mechanical stress, local overheating, winding short-circuit faults, etc. Once the insulation between the sheets is damaged, a local eddy current loop will be formed, causing a sharp increase in temperature in that area, which may develop into core melting, winding insulation burning, or even ground short-circuit and other serious accidents. Therefore, after the generator handover, major overhaul, or core structure change, the core loss test must be conducted to ensure that the core quality meets the operation requirements.
Test Objectives
The core purpose of the generator core loss test can be summarized in the following four aspects:
Detecting the insulation condition between core sheets
The primary purpose of the core loss test is to check whether there are defects in the insulation between the silicon steel sheets. By winding an excitation winding on the core and passing in an industrial frequency alternating current, an alternating magnetic flux close to saturation is generated in the core. At this time, the eddy current loss in the core mainly depends on the integrity of the insulation between the sheets. If the insulation between the sheets in a certain area is damaged, an eddy current short-circuit loop will be formed at that location, resulting in a significant increase in local loss. By measuring the total active power loss and comparing it with the theoretical value or factory value, the overall insulation condition of the core can be determined.
Locating local overheating defects
Simple power measurement can only reflect the overall loss level of the core but cannot determine the specific location of the defect. Therefore, during the core loss test, an infrared thermal imager or embedded thermocouple is usually used to monitor the surface temperature of the core in real time. When a certain point is found to be significantly higher than the surrounding area and the temperature difference exceeds the specified limit, the fault point of the sheet short circuit can be accurately located. This function is crucial for subsequent defect repair work and can significantly improve the efficiency and targeting of maintenance.
Verifying the core assembly quality
For on-site laminated generators (such as hydroelectric generators), the core lamination process directly affects the electromagnetic performance and mechanical stability of the core. The core loss test can verify the uniformity of the pressure on the core by components such as tooth pressure plates, through bolts, and ventilation slot sheets, as well as whether there is any loosening or abnormal vibration in the core as a whole. At the same time, by comparing the measured unit core loss value with the factory guarantee value of the silicon steel sheet, the material performance and lamination process of the core can be verified to meet the design requirements.
Preventing the expansion of faults during operation
During normal operation of the generator, the short-circuit points between the sheets in the core will continuously generate local high temperatures. This thermal effect may initially only manifest as local discoloration of the core, but with the extension of operation time, the accumulation of heat will cause further aging of the insulation and gradual melting of the core, eventually possibly burning through the winding insulation and causing phase-to-phase short circuits or ground faults. Discovering and handling these hidden dangers through the core loss test before the unit is put into operation or after a major overhaul is an effective preventive measure to avoid major operation accidents.
Test Standards and Regulations
In the power industry, the generator stator core loss test is mainly carried out in accordance with the following standards and regulations, while meeting the corresponding test conditions:
Main standard documents 1. GB/T 20835-2017 “Guidelines for Magnetization Test of Generator Stator Core”: This standard is the most direct technical basis for iron loss tests, stipulating the test purpose, test conditions, test methods, measuring instruments, calculation methods, and qualification criteria. It is applicable to the magnetization tests of stator cores of all types of synchronous generators.
2. DL/T 596-2021 “Regulations for Preventive Tests of Electrical Equipment”: This regulation, from the perspective of preventive tests, specifies the test cycle, test requirements, and judgment criteria for generator iron loss tests, and serves as an important execution basis for on-site tests.
3. GB 50150-2016 “Code for Handover Test of Electrical Installation Projects – Electrical Equipment”: This standard, for the handover acceptance tests of newly installed generators, clearly states that the iron loss test is one of the mandatory inspection items and provides basic technical requirements.
Test Conditions Requirements
Before conducting the iron loss test, the following basic conditions must be met:
1. The stator windings of the generator are short-circuited and grounded in three phases, and the winding ends are securely fixed.
2. The interior of the core is clean, with no residual metal debris.
3. The ambient temperature is generally not lower than 5°C, and the temperature change during the test should not exceed 3K.
4. The excitation power supply should have sufficient capacity, with a frequency deviation not exceeding 0.5Hz and a voltage waveform distortion rate not greater than 5%.
Determination of Excitation Parameters
The number of turns in the excitation winding and the required excitation current are calculated based on the core’s geometric dimensions. For generators with a rated frequency of 50Hz, the commonly used test flux density is 1.0T. At this time, the number of turns in the excitation winding W₁ and the measurement winding W₂ are determined according to the following principle: when the induced voltage in the measurement winding reaches a certain set value, the magnetic flux density in the core yoke is exactly 1.0T. The magnitude of the excitation current needs to be estimated based on the core’s magnetization characteristics to ensure that the power supply can meet the reactive power demand.
Key Technical Requirements and Qualification Criteria
Test Flux Density and Duration
According to the provisions of GB/T 20835, the flux density for iron loss tests is typically 1.0T or 1.4T, with corresponding test durations of 90 minutes and 45 minutes, respectively. 1.0T is the most commonly used magnetic flux density level in on-site tests, mainly due to the following considerations: on one hand, at this flux density, the core has entered the saturation zone, which can effectively expose inter-lamination insulation defects; on the other hand, the excitation capacity requirement corresponding to 1.0T is moderate, facilitating on-site power supply configuration, and the test duration is controlled within 90 minutes, avoiding irreversible damage to the core due to prolonged overheating.
Limit Value of Unit Iron Loss
Unit iron loss refers to the loss per unit mass of the core, with the unit of W/kg. During the test, the total active power loss P₀ (after deducting the copper loss of the excitation winding) is divided by the total mass G of the core to obtain the measured unit iron loss p₀ = P₀ / G. The qualification criterion is that the measured unit iron loss should not exceed 1.3 times the manufacturer’s guaranteed value. If there is no manufacturer data, the standard unit iron loss value of the same grade silicon steel sheet can be used for comparison. It should be noted that the measured loss usually includes a certain proportion of edge leakage loss and structural component loss of the core, so the 1.3 times coefficient has taken this engineering margin into account.
Limit Values of Temperature Rise and Temperature Difference
Temperature characteristics are the most sensitive indicators for judging local defects in the core. Under the test conditions of 1.0T and 90 minutes, the main temperature criteria are as follows:
1. The temperature rise of the core teeth should not exceed 25K. The temperature rise is calculated as the difference between the core’s highest temperature at the end of the test and the ambient temperature at the beginning of the test. This limit ensures that the core does not damage the insulation due to overheating during the test.
2. The temperature difference of the core teeth should not exceed 15K. The temperature difference is defined as the difference between the highest and lowest temperatures of the core at the end of the test. This indicator is particularly crucial for detecting local inter-lamination short-circuit faults – the normal temperature distribution of the core should be basically uniform. If there is a short-circuit point, the temperature at that point will be significantly higher than the surrounding area, resulting in an excessive temperature difference.
Other judgment criteria
In addition to the above quantitative indicators, the following phenomena should also be observed during the test:
1. The core should have no obvious abnormal vibration or noise. If there is severe vibration or abnormal sound, the excitation current should be reduced and the cause investigated.
2. Fasteners such as the core-piercing bolts and tooth pressure plates should not show local overheating.
3. After the test, there should be no new discoloration, peeling or burn marks on the core surface.
Post time: May-07-2026