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| small. With large short-circuit current flowing, however, this counter voltage becomes relatively large and bucks the applied voltage. The resultant voltage, which will push current into the fault, is small, and the fault current (by Ohm's Law) will also be small. The isolation of the fault from the system is obtained by means of circuit breakers. Since reactors limit short-circuit currents, lower capacity circuit breakers may be installed. |

| When reactance is placed in series with the equipment under fault, the voltage applied to the fault is reduced, but the voltage on the remainder of the circuit or system is maintained while the fault is being cleared. |

| Construction |

| Reactors are of two types, the air-core type and the iron-core type. The first is simply a carefully constructed, mechanically well supported circular coil of suitable proportions for cooling. The second consists of a coil encircling an iron core that makes up a magnetic circuit in which air gaps may exist. Both types of reactors may be air cooled or immersed in oil or similar fluid (in much the same way as transformers). |

| Types of Reactors.. |

| Air-core reactors. An air-core reactor produces a large magnetic field external to the coil. In constructing supporting structures, therefore, it is necessary, to avoid employing metal loops or circuits in which stray currents can form and circulate. Such circuits produce undesirable heat and unnecessary losses. Reinforcing rods in concrete supports, for example, should be isolated, never in contact with each other. Care should be exercised in spacing adjacent reactors and in the fastening of support insulators and bracing units. |

| Mechanical stresses should be provided for by ample supports. The windings are usually a series of pancake coils, mounted one above another, that exhibit as nearly equal electrical characteristics as possible to avoid circulating currents. The complete reactor is normally mounted on a concrete disc supported on porcelain insulating feet (Fig. 16-1). |

| Iron-core reactors. Coils for these reactors consist of small wires, straps, or cables wound about an iron core. The air gap may consist of several small gaps to keep eddy currents to a minimum. |

| Saturable iron-core reactors. By saturating the core of an iron-core reactor by means of a controlled dc winding, the reactance of the unit can be made variable. If the core is magnetically saturated, an ac winding will produce only small changes in its magnetic field (or flux), and the inductive reactance of the coil will be small. To keep the ac winding from inducing large voltages of changing magnitudes in the dc winding, the winding is arranged |

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