Professional Equipmen


AMA ' j down designs ranging from 15 va to 10 kva.

How transformer works

A constant-voltage transformer has a magnetic core structure different from conventional transformers, It has a magnetic shuiit with a fixed air gap interposed between the primary and secondary windings. The secondary winding is shunted by a fixed ac capacitor. ITpon application of primary voltage, the secondary voltage increases to the point at which that portion of the magnetic core directly under the secondary winding approaches saturation due to the capacitative load connected across the secondary winding.

As the core approaches saturation, it cannot carry much additional magnetic flux. The increase in secondary voltage is less than any proportional increase in primary voltage. Thus, a condition of relative stability of secondary voltage is reached. Over the range of specified primary voltage, the core under the secondary changes very little for this range of primary voltage. Due to the magnetic shunt between the primary and secondary windings* that part of the core under the primary is not saturated.

Vo equalize the small effect of increasing primary voltage on the secondary, a compensating coil is wound over the primary coil and is connected in series with the secondary load circuit—but out of phase with the secondary. Result is that as primary voltage increases beyond the design voltage, the voltage in the compensating coil also increases. Since it is out of phase with the secondary voltage, however , it subtracts (from the secondary voltage) an amount equal to the slight increase induced in the secondary winding by the increase of primary voltage. Likewise, when the primary voltage decreases, the compensating coil voltage decreases 111 proportion to the primary voltage, and subtracts from the secondary voltage. The design is such that the vector sum of the compensating coil voltage and the secondary voltage is practically constant throughout the design range of input voltage.

Using the CV

When the constant voltage transformer is overloaded in excess of its rated load, a point is reached where the output volt-

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