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Thyratron Transformers
The average load current which may flow without overheating the transformer decreases as Θ increases.
High-voltage surges occur when capacitance input filters are used with grid-controlled rectifiers. To a degree these surges are likely to occur even when the load is nominally resistive, because of incidental capacitance in the transformer, wiring, and other components. If the load is a radio-frequency generator, the r-f bypass capacitor adds to this effect. In Fig. 191 (a) the total amount of external capacitance is designated C1. A half-wave anode transformer is shown for simplicity, but each half of a full-wave transformer, or each phase of a three-phase transformer, behaves similarly. When the thyratron firing angle is greater than zero, a steep voltage wave front occurs at the instant of firing tΘ, Fig. 191(b), as follows: Normal voltage induced at point A in the secondary winding is e1 volts above ground, just prior to tΘ. As soon as the thyratron fires, the external wiring and circuit capacitance C1 momentarily forms an effective short circuit from A to ground. A large surge current flows into this short circuit, but initially this current cannot be drawn from the primary because of the inevitable inductance of the windings. The initial current is therefore supplied by the secondary winding capacitance. Since point A is momentarily short-circuited, a surge voltage, equal and opposite to e1, is developed in the secondary winding. This voltage surge appears across the turns or layers of winding nearest to A. Unless precautions are taken in the design of the anode transformer the voltage may be high enough to damage the winding insulation. As is shown in Pulse and Video Transformers, with steep wave fronts in single-layer windings initial voltage distributes most equally between turns when ratio α = √(Cg/Cw)is small, Cg being the capacitance of the winding to ground and Cw the series capacitance across the winding. If the secondary of Fig. 191 were a single-layer winding of n turns, Cw would be CS/n. In multilayer coils, ratio α is not so readily defined. In general, small effective layer-to-layer capacitance means small effective Cg in relation to Cw, small α, and more linear initial distribution of voltage. Many layers are better than few layers in keeping capacitance Cg small. In the limit, a one-turn-per-layer coil would have small α and good initial voltage distribution. In practice this extreme is not necessary to avoid layer insulation breakdown. It is usually sufficient to split the secondary into part coils, like S1 and S2 in Fig. 59. This reduces Cg to a quarter of the corresponding capacitance of full-width coils. Ratio α is reduced, and voltage distribution improved. Even with part coils there is some non-linearity of voltage distribution, especially in the top layer. This non-linearity may be minimized by providing a static shield over the top layer and connecting it to point A, Fig. 191. The momentary voltage described above appears within the winding, and unless there are taps it may not be observable. If a surge suppressor circuit (usually a capacitor-resistor network across the secondary) is used, it does not appreciably diminish the internal winding voltage surge, but such a surge suppressor may be necessary to damp out oscillations in the external circuit due to firing of the thyratrons. Secondary windings of control transformers in thyratron grid circuits, like those of Fig. 187, should be insulated for the anode voltage. When thyratrons arc-back the grids may be subjected to full anode potential, which would damage lesser amounts of insulation.
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