# Welding transformers - purpose and classification

In resistance welding, metal parts are heated to a plastic state by the heat generated by the electric current in the area between the electrodes. The total amount of thermal energy required directly for welding, according to the Joule-Lenz law, is determined by the relationship: W = I2nom ^ 2 * Ree * tw (1)
where I2nom - rated welding current, A; Ree is the active resistance of the parts to be welded at the electrode - electrode section, Ohm; tw - welding current flow time, sec. Each welding cycle begins with preliminary compression of parts to create mechanical contact between them. After switching on the current, the metal of the parts to be welded in the electrode - electrode zone heats up and the resistance Ree increases. After 0.02–0.03 s, when the parts are heated above the softening temperature (about 400 ° C), the contact area increases and Ree, having reached its maximum value, begins to decrease, and I2nom increases. The minimum value of Ree and the maximum value of I2nom, which occurs at the end of the welding process, are used as the numerical characteristics of these parameters for all types of resistance welding.
From expression (1) it follows that the heating of the parts is the greater, the greater the welding current, the active resistance of the parts and the welding time. However, the value of the final resistance Ree is always indirectly set, since it is determined by the physical properties of the metals of the welded parts of a certain thickness, the transition resistance between the surfaces and the type of welding transformer. The numerical characteristic of this parameter mainly determines the value of the welding current I2nom set in the technological charts and the welding time tw. With an increase in tw, the amount of released thermal energy, and, consequently, the dimensions and strength of the joints increase, but not infinitely. After a certain certain time tw, the quality of the welded joint deteriorates sharply due to splashes, increased useless heat loss. An increase in the welding current I2nom has the same effect on the quality of the welded joint. Therefore, obtaining welded joints of the same size and quality on the same parts is possible only with certain ratios of these parameters. A decrease in I2nom within certain limits can be compensated for by a corresponding increase in tbw and vice versa. Regimes characterized by relatively large I2nom and small tbw are usually called hard, and modes with small I2n and large tbw — soft. However, in modes of different rigidity, resistance welding can be carried out at different pressures on the electrodes Fw. Therefore, despite the variety of welding modes, all of them must be performed taking into account a number of requirements, which contributes to obtaining high and stable results in production conditions. The ratios I2nom, tcv, Fbw are the main parameters of the welding mode.
Although the resistance of the section electrode - electrode Ree is much higher than the resistance of any other section of the welding circuit, however, in absolute value it is very small and reaches several tens or hundreds of micro-ohms. In addition, due to the high thermal conductivity of the parts and electrodes being welded, the contact heating time tw should be very small (a few hundredths or tenths of a second). So, for example, for spot welding of products made of low-carbon steel, the duration of the flow of the welding current is set at the rate of 0.08-0.16 s per 1 mm of thickness of each of the parts to be welded. And when welding some metals and their alloys, even shorter periods of time are required.
Consequently, to ensure the necessary heating of the contact, a large welding current I2nom is required, which reaches several tens and hundreds of kiloamperes. Due to the low absolute resistance of all elements of the welding circuit, this current is provided by a low secondary voltage U20 - several volts (U20 is the secondary voltage of the transformer at no-load). In accordance with GOST 297-80, resistance welding machines must be manufactured for connection to industrial electrical networks with a rated voltage of 380 V and a frequency of 50 Hz. At the request of the consumer, the machines must be manufactured for a rated supply voltage of 660 V with a frequency of 50 Hz, and machines with the highest short-circuit power up to 60 kV-A - for a rated supply voltage of 220 V with a frequency of 50 Hz. Machines intended for export must be manufactured for a rated supply voltage of 50 or 60 Hz as ordered. The machines must be designed to operate with power quality in accordance with GOST 13109-87 and with deviations in the supply voltage from –10 to +5 % of the nominal value.
For elements of the secondary circuit of contact machines, the value of the rated continuous current in amperes is set in accordance with GOST 10594-80 with the following recommended series of numbers: 100, 112, 125, 140, 160, 180, 200, 220, 250, 280, 320, 360, 400, 450, 500, 560, 630, 700, 800, 900, 1000, etc. This parameter is one of the main quality indicators of the machine, since it characterizes its thermal or load power, at which this machine will work normally without overheating of individual structural elements of the secondary (power) circuit.
All modern contact electric welding machines use welding transformers as a power source. In each machine, the primary winding of the transformer is connected to the network using control equipment that provides the required duration of the welding current pulse, and the secondary winding of the transformer is electrically connected to the external circuit of the machine, on the electrodes of which contact welding of products is carried out. In special-purpose machines with complex power supply between the network and the primary winding of the welding transformer, in addition to the control equipment, an intermediate transformer, or a frequency converter, or a transformer with a rectifier, or a source for energy storage can be included.
Welding transformer is designed to convert electrical energy supplied to its primary winding into electrical energy with low secondary voltage and high current. The shape of the welding current pulse is completely predetermined by the circuit design of the power electrical intermediate part, from which the welding transformer is supplied or the welding circuit of the machine is supplied.
The figure shows a classification diagram of the main types of welding transformers used in modern contact machines, depending on the method of their power supply.According to this scheme, all welding transformers are divided into two main groups: transformers that convert electrical energy of alternating current with a frequency of 50 Hz, consumed from the network directly during welding, and transformers that convert energy previously accumulated by one of the considered methods.
Further subdivision within each group is carried out depending on the circuit design of the power electrical part, on the design features of the transformer, due to the requirements for the machine, the power source of which it is. Of the total number of welding transformers - power sources of contact machines - the main share (more than 90 %) falls on single phase ac transformers frequency of 50 Hz, therefore, consideration of issues related to these transformers is given the greatest attention.
Transformers with other circuit solutions of the power electrical part or using an intermediate power link will be discussed in other articles.

Classification diagram of transformers depending on the methods of their power supply.

Order and calculate the welding transformer

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