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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Electric welding. How to calculate a welding transformer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / welding equipment Transformer - this is the very first static device that allows you to convert AC electrical energy. transformer used:
The purpose of this section is to provide a methodology for calculating the transformer without going beyond the knowledge gained in the volume of the physics course for high school. Consider a variant of a transformer containing two windings - primary and secondary. The ratio of the number of turns W1 primary winding to the number of turns W2 secondary winding is called transformer transformation ratio KT:
where U1, SHE IS2 - voltage of the primary and secondary windings, V; I1, The2 - current of primary and secondary windings, A. The electromotive force (EMF) of one turn of the winding is directly proportional to the rate of change of the magnetic flux Ф penetrating this turn:
Since the transformer winding is wound on a ferromagnetic core, which has a magnetic permeability thousands of times greater than the surrounding space, almost the entire flux is concentrated in the core with a cross section Sc. If at the same time the induction in the core changes from -Bm up to +Vm with frequency Bmthen average coil voltage equals:
where Kф- form factor, taking into account the ratio of the effective and average voltage values, for a sinusoidal voltage Kф = 1,11; INm - maximum induction in the core, T; F - AC voltage frequency, Hz; Sc - cross-sectional area of the core, cm2; Toc - core fill factor. Despite the possibly different number of turns, the windings of the transformer have the same power, equal to its power, and equally share the area of the core window:
where so - core window area, cm2; Too - window filling factor; J is the current density in the transformer windings, A/mm2. Using (18.3) and (18.4), we determine the overall power of the transformer:
From formula (18.5) we find the dimensions of the transformer core:
To select B, J, K valuesc, Ko recommendations for transformers can be used (Table 18.5). For aluminum wire, the current density should be reduced by a factor of 1,6. Table 18.5. Core parameters
Although the most common type of transformer is double winding transformer, it happens that an amateur developer is faced with the problem of constructive calculation multi-winding transformer. Possible at least two cases multi-winding transformer: Case 1. The transformer has two main windings occupying more than 95% of the core window area, as well as one or more additional low-power windings occupying the rest of the window area. Choosing a smaller value of Ko from the table. 18.5, you can calculate the transformer as a two-winding. Most likely this assumption will not cause problems with the placement of additional windings. Case 2. The transformer has several windings, each of which occupies more than 5% of the area of the core window. The transformer should already be designed as a multi-winding one, otherwise there may be problems with the placement of the windings in the core window. The number of windings does not have any effect on the laws of electromagnetic induction, and therefore, when calculating a multi-winding transformer, it is enough to solve the problem of constructive placement of many windings in the core window. As we noted earlier (18.4), the windings of a transformer occupy a window area proportional to their power. This is not difficult to verify. Let us assume that all transformer windings are made of a similar winding material and the same current density J is chosen for them, taken from Table. 18.5. Since all windings are wound on the same core, therefore, one turn of any winding generates a similar voltage Eв, which can be determined by formula (18.3). In order to obtain the required voltage U at the terminals of the Nth windingN, it is necessary that this winding contains WN = UN / EB turns. If current I flows through the Nth windingN, then it must be wound with a wire that has a cross section SETC =IN / J. Knowing the cross section of the winding wire and the number of turns, you can determine the area that this winding will occupy in the core window:
where
It can be seen from the expression that the cross section of the winding is equal to the product of the winding power and the coefficient KEJ. In turn, the coefficient KEJ is determined by the parameters of the transformer core and has a similar value for all transformer windings, regardless of their number and power. Therefore, an arbitrary number of windings can be placed in the core window, provided that their total power does not exceed the value:
Of course, the resulting expression is also valid for a two-winding transformer, which makes it possible to select the dimensions of the core of a multi-winding transformer according to the method used for a two-winding transformer. To do this, it is only necessary to determine the overall power of the multi-winding transformer:
Example 1. Let's calculate the transformer T2 220/27 V with an overall power of 200 W. A similar transformer is used to power the feeder and control circuits of the semi-automatic welding machine. The transformer will be wound on a standard core type ShL. From Table. 18.5 for a transformer with a power of 200 W wound on a SL core, we select the values \u1,5b\u2,5bB \u2d XNUMX T, J \uXNUMXd XNUMX A / mmXNUMX and Ko = 0,32. For a strip core, we take the value Kc = 0,95. Now let's find the overall dimensions of the transformer core:
We choose the core ШЛ25x40, having Sc = 10 cm2 and So = 16 cm2. Having decided on the cross section of the core, according to the formula (18.3), we determine the EMF of one turn of the transformer:
Find the number of turns of the primary winding of the transformer:
Find the number of turns of the secondary winding of the transformer:
To find the wire diameter of the primary and secondary windings, you must first determine the currents flowing in these windings:
Now, knowing the current density in the windings J = 2,5 A / mm2, we can determine the diameter of the winding wire for primary winding:
и secondary winding:
We select the nearest standard diameters of the winding wire:
Therefore, the T2 transformer is wound on a standard W-shaped tape core of the SHL25x40 type, the primary winding contains 696 turns of copper wire with a diameter of 0,69 mm, the secondary winding contains 85 turns of copper wire with a diameter of 1,95 mm. Example 2. Let's calculate a three-winding transformer, which is used in an uninterruptible power supply. A sinusoidal alternating voltage with an amplitude of 10 V and a frequency of 50 Hz is supplied to the first winding from the output of a transistor DC-AC converter. The maximum effective current that the converter is able to provide is equal to
Since the amplitude value of the sinusoidal voltage is 1,414 times greater than the effective voltage, then the effective voltage will be applied to the first winding of the transformer:
To increase the voltage to U2 \u220d XNUMX V serves as the second winding, which is designed for current I2 = 1,36 A. To charge the battery, the third winding is used, which has a voltage U3 = 20 V and rated for current I3 = 6 A. According to formula (18.9), we determine the overall power of the transformer:
Suppose, as in the previous case, the transformer will be wound on a standard core of the SHL type. From Table. 18.5 for a transformer with a power of 360 W wound on a SL core, we select the values \u1,47b\u2bB \u2d XNUMX T, J \uXNUMXd XNUMXA / mmXNUMX and Ko = 0,33. For a strip core, we take the value Kс = 0,95. Now let's find the overall dimensions of the transformer core:
Let's choose the core ШЛ32х50 having Sc=16 cm2 and So=26 cm2. Having decided on the cross section of the core, according to the formula (18.3), we determine the EMF of one turn of the transformer:
Find the number of turns of the first winding of the transformer:
Find the number of turns of the second winding of the transformer:
Find the number of turns of the third winding of the transformer:
Determine the diameter of the winding wire for the first winding:
Most likely it will be quite problematic to find a winding wire of such a large diameter. Therefore, it is better to wind the first winding with a copper rectangular busbar with a cross section:
Determine the diameter of the winding wire for the second winding:
Determine the diameter of the winding wire for the third winding:
We choose the standard diameters of the winding wire for the second and third windings:
Author: Koryakin-Chernyak S.L.
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