SATURATION

by Sam Belkin, MSEE

One of the biggest problems of the welding transformer is saturation. When the primary current increases, flux density into the magnetic core also increases. The more current flows through the coil the more flux density inside the core. For a small level of current its magnetizing force and flux density are linearly related. When the current increases there is a point where the increasing flux density slows down and then stops to change. After this no energy is transformed to the secondary. When the flux does not change it is equivalent to a drastic decrease of effective permeability of the core. The primary inductance drops approximately to the air coil value, therefore the impedance of the primary coil goes down almost to zero. This will lead to a very high primary current which is limited only by the primary circuit resistance. This is the case when the leakage inductance Ls1 may help since it will limit the current without giving off heat.

An experienced operator may note the saturation by a distinctive sound. However it is better to check it by taking the primary current measurement (see Figure 1). In case of saturation the primary current plots have a significant distortion such as shown in Figure 3.

Figure 3. The saturation of the primary current.

Note the stretched shape of the positive current pulses and that they are seriously distorted. The normal amplitude of the sinusoidal current for this transformer is 115 A, which can be seen on the negative half way part of the plot. In saturation mode the amplitude goes up to 368 A. Saturation is very dangerous for the welding transformer and the controller, as well as for the power lines. The main reason of saturation is an insufficient cross sectional area of the core and the number of primary turns, as well as the primary current build-up speed.

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