by Sam Belkin, MSEE


A transformer has two (sometimes more) electrical coils that are wound around an iron core. One coil is called the primary, and is part of the primary circuit. The other coils is called the secondary and is part of the secondary circuit. There is no electrical connection between the primary and the secondary coils. When an alternative current flows in the primary circuit, a magnetic field builds up and a magnetic flux flows inside the core. The magnetic flux changes its direction 120 times per second. These changes create a current through the secondary coils. The energy transforms from the primary to the secondary circuit, and the voltage converts according to turn ratio:

V1/ V2 = n1/n2 =n

when V1 and V2 are primary and secondary voltages accordingly,

n1 is primary coil's turns,

n2 is secondary coil's turns,

n is turn ratio.

Electrical current transforms according to reciprocal of the turn ratio:

I1/I2 = n2/n1 =1/n.

Resistance transforms according to square of the turn ratio:

R1/R2 = n2.

This is a very important correlation between the resistance in the primary and the secondary circuits. When the primary resistance is referred to the secondary circuit its value must be divided by the square of the turn ratio and when the secondary resistance is referred to the primary circuit its value must be multiplied by the square of the turn ratio.


Fig.2 Equivalent schematic of welding transformer


Now let us examine the welding machine's simplified equivalent schematic (Figure 2) and determine how its parts may affect the weld. For the sake of simplicity we do not include in this schematic some elements that are not very important for the resistance welding transformer. Despite these simplifications the formulae, derived from this schematic, provide good practical results and are easy to use. Notations on Figure 2 are: Rps is the resistance of power source line plus the resistance of any elements in the primary circuit (e.g. switching thyristor); R1 is the resistance of the primary winding; R 2 is the resistance of the secondary winding plus the resistance of any other parts in the secondary circuit R2 (e.g. electrodes, connectors etc.), referred to the primary circuit, R 2 = R2 x n2; R w is referred to the primary resistance of the welded parts Rw, R w = Rw x n2, and Ls1 is the leakage inductance of the primary winding. Vpe is the effective voltage applied to the primary winding, Vse is the effective secondary voltage applied to the welded parts, and T is the ideal transformer that has the same transformer ratio as the real one.

It is easy to see from this equivalent schematic that Rps, R1, R2 and Ls1 will decrease the voltage across the primary winding, therefore decreasing the efficiency of the machine. The bigger these values are the less welding current is available from the given transformer. The power line connection must be done by wires with appropriate AWG for a lower voltage drop between the source and the transformer. The same rule applies for the primary winding wire. In terms of the transformer efficiency leakage inductance Ls1 should be as small as possible. However, sometimes this inductance may serve as a protective means that reduces the effect of saturation and speed of the primary current changes. The leakage inductance of the secondary winding is usually negligible for the welding transformer due to small number of turns, thus it was not included in the schematic on the Figure 2.

Special attention must be devoted to the secondary circuit resistance. A voltage drop across this resistance is the direct loss of welding energy. The secondary must be wound by a wire that is as big as possible. All of the connections at the secondary circuit should be done from high conductive metals and with a high cross sectional area.

The equivalent schematic (Figure 2) allows to make some easy, yet helpful analysis. Suppose that the portable benchtop machine is operating from a 120 V single phase AC line. The welding rate is 4 welds 9 cycles each, and the duty cycle D.C. = 1 %. Assume that the resistance of the welded parts is 1 mOhm (10-3 Ohm), the turn ratio of transformer n = 22.2, the resistance of primary winding R1 = 0.4 Ohm, and the resistance of secondary winding plus resistance of the electrodes and connections R2 = 0.8 mOhm (0.8 x 10-3 Ohm). Also the welding machine is connected to a power line with a 6 ft long AWG 16 cord, and the leakage inductance Ls1 is negligible. Latter means that the impedance of Ls1 at 60 Hz much less than the primary circuit resistance (Rps + R1).


© Digiweld 1999