Faculty of Electrical Engineering and Computer Science

Smetanova 17

2000 Maribor

Device and testing procedure for determination of magnetic circuit quality

FIELD OF THE INVENTION

Electrical engineering; measurings; transformers; transformer cores TECHNICAL PROBLEMS

Technical problem solved by this invention is elimination of otherwise time-consuming process of transformer iron core non-linear characteristics evaluation, further reduction of otherwise time-consuming and demanding demagnetization of iron cores before testing, further elimination of otherwise time-consuming quality check of manufactured transfomer cores, further elimination of otherwise time-consuming individual check of transformer cores, further the complexity of comparison of transformer iron cores of different manufacturers, different series, different dimensions and/or different constructions, further the quick measuring procedure, further simplicity of the measuring, furthter the universality of measurings, further time- consuming quality check of the transformer iron core air gap.

STATE OF THE ART

The exsisting methods and mesuring instruments used by transformer core manufacturers are different and are usually based on one of the standards listed below: IEC 60404-2 cd3,l Consol. with ami (2008-06) Magnetic materials - Part 2: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of an Epstein frame.

IEC 60404-3 ed2.2 Consol. with aml&2 (2010-04) Magnetic materials - Part 3: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of a single sheet tester.

IEC 60404-4 ed2.2 Consol. with aml&2 (2008-11) Magnetic materials - Part 4: Methods of measurement of D.C. magnetic properties of magnetically soft materials.

IEC 60404-6 cd2.0 (2003-06) Magnetic materials - Part 6: Methods of measurement of the magnetic properties of magnetically soft metallic and powder materials at frequencies in the range 20 Hz to 200 kHz by the use of ring specimens.

IEC 60404-10 edl.O (1988-08) Magnetic materials. Part 10: Methods of measurement of magnetic properties of magnetic sheet and strip at medium frequencies.

IEC 60740-1 edl.O (2005-08) Laminations for transformers and inductors - Part 1 : Mechanical and electrical characteristics.

IEC 62044-1 edl.O (2002-05) Cores made of soft magnetic materials - Measuring methods - Part 1 : Generic specification.

IEC 62044-2 edl.O (2005-03) Cores made of soft magnetic materials - Measuring methods - Part 2: Magnetic properties at low excitation level.

IEC 62044-3 edl .O (2000-12) Cores made of soft magnetic materials - Measuring methods - Part 3: Magnetic properties at high excitation level.

IEEE-Std 393-1991

ASTM A772 A772M-00 (2005) and similar ones. There is a long list of standards, therefore, transformer core manufacturers can use various measuring methods, i.e. they apply sinusoidal voltage supply of certain frequency (i.e. 400 Hz), determine the maximum magnetic field density and measure active power P and apparent power S at the rated point (IEC 404-2). The sinusoidal change of the magnetic flux desity B inside the tested iron core must be ensured during such testing, which requires a special testing equipment. They can also use sinusoidal current excitation and measure induced voltage in the secondary winding (ASTM A772 A772M-00 (2005)). This method requires sinusoidal magneto motive force (mmf) in the tested core.

There are a lot of measuring devices on the market to determine the magnetic properties, such as BROCKHAUS MESSTECHNIK (C 510, MPG 100D, EBA), MAGNET-PHYSIK DR. STEINGROEVER GmbH (Remagraph C-500), e.t.c. These devices are based on previously mentioned standards and are used to determine the magnetic properties and to ensure quality in overall process of transformer core manufacturing.

Disadvantages of the measuring methods and devices based on the standards listed above are:

- previously demagnetization of cores is necessary,

- time-consuming process,

- expensive measuring equipment, and

- difficult comparison of the cores with different cross-sections area and lamination thicknesses.

Disadvantages of the measuring procedures are emphasized due to the large number of standards, which can be used when checking the transformer core quality. In such way, it is difficult for a buyer of iron cores to compare cores of different manufactures. The transformer cores are often not tested at the rated point in which the iron core operates in the final product. Also, the tests are mostly based on sinusoidal excitation, however, majority of transformers is already supplied by inverters, which give a pulse form of output voltage. Due to such difficulties, the buyer must provide his/her own testing equipment for comparison of different cores that can be very expensive. DETAILED DESCRIPTION OF THE INVENTION

The above mentioned technical problem is solved by Device and testing procedure for determining magnetic circuit cores quality, which is equiped with a hysteresis current controller on the primary side of the transformer in no-load (open secondary winding). Device or invention is for the patent application purposes reffered to as »Transformer Iron Core Tester«. Device that determines the quality of magnetic circuits comprises a pulse excitation and hysteresis control of the transformer primary current. The invention subject, i.e. Transformer Iron Core Tester, switches the supply voltage by the H-bridge inverter with the help of the hysteresis controller when the primary current reaches the defined maximum value, priority 4A. The Transformer Iron Core Tester evaluates the iron core quality and the magnetic characteristic of analyzed magnetic circuit on the basis of determined operating frequency /, core cross-section area ^ _e as well as the measured reactive power Q. As lower is the product of cross- section area and frequency Ay _e *f, as well as the reactive power Q is, the better is the quality of the core.

The invention is further described by means of drawingsrepresenting an integral part of this patent application as follows:

Figure 1 presents circuit drawing of transformer iron core tester core (1 ), transformer primary winding (2), primary current sensor (3), hysteresis current controller (4), control logic for H- bridge inverter switching (5), H-bridge inverter (6), DC bus (7) and primary voltage sensor (8).

Figure 2 presents transformer iron core with an air gap (9) and cross-section area ^Fe (10).

Transformer Iron Core Tester is carried out by means of a regular H-bridge inverter (6) which does not require any expensive measuring equipment. The operation point in tested iron core is defined with the maximum magneto motive force (mmf) N _p *I _max , which is determined by the number of turns N _p of primary winding (2) and current 7 _max , controlled by the hysteresis current controller (4). The primary current is measured by primary current sensor (3) while H-bridge inverter (6) is switched by the hysterysis current controller (4) using corresponding control logic (5). The quality of tested core (1) is defined with the maximum flux density B _max , which is reached at maximum mmf, as well as with the input reactive power Q, which value should be as low as possible. The value of input reactive power Q mostly depends on the size of the transformer air gap (9). The value of flux density B _max and the input reactive power Q can be calculated from the measured primary current (sensor (3)) and primary voltage (sensor (8)). For particular examples, where voltage of DC bus (7) and transformer core cross-section area A _?t (10) are constant, the lower operation frequency / means the higher quality of the tested transformer iron core. The before mentioned criterion of lower frequency / is replaced by the criterion of a lower product A _Fe *f, when iron cores with different cross-section areas A _?e are compared. Voltage £/DC of D.C. bus is constant. Iron cores with the different cross-section areas A _fe can be compared in that way. The transformer iron core tester, respectively the procedure of core quality evaluation, is preferentially adequate for input quality checking of iron cores before the asembling of transformers.

The Transformer Iron Core Tester frequency range can be adequately set by changing N _p and U _D c, in a way that there is no impact of eddy currents on the magnetic characteristic. Due to this the cores with differet lamination thicknesses can be also compared. In order to test transformer iron cores it is not previously necessary to demagnetize the tested iron cores. Already built-in cores can be also checked during the process of asembling. Determination of iron core quality is illustrated with the comparison of three core samples with the following measurement results:

_Fe */ [mm ² /s] Q [Var]

56142 15.07

56570 10.52

68429 14.79

The first core is of better quality than the third core. The value of product Af _e *f is considerably lower which shows that the value 5 _max of the first core is much higher than of the third core. The comparison of the first and second core shows that they riches an approximately equal B _max , although the air gap in the second core is treated much better in relation to the first core, hence the value of reaction power Q is lower. Therefore, the second core is the best choice among the three cores.