page_365

< previous page | page_365 | next page >

Page 365

| from which, |

| The Wheatstone bridge may also be used with an ac source to measure capacitive reactance (Fig. 17-29B) and inductive reactance (Fig. 17-29C) in a similar fashion. The equations are as follows: |

| Phase Rotation Indicator |

| The rotation or sequencing of phases in a three-phase system is often desired, particularly when pieces of equipment are connected in parallel. Two methods may be used. In Fig. 17-30 two lamps and a highly reactive coil (such as the potential coil of a watt-hour meter) are needed. The bright lamp indicates the particular phase sequence (x bright indicates sequence 1, 2, 3; y bright indicates sequence 3, 2, 1). In Fig. 17-30B, a noninductive resistance and a reactive coil of equal impedance are used in conjunction with a lamp. The brightness of the lamp indicates the phase sequence (bright light indicates sequence 1, 2, 3; dim light indicates sequence 3, 2, 1). |

| Construction Features |

| Electrical instruments of all types of construction contain five basic elements: (1) the moving element, (2) the stationary element, (3) the controlling devices, (4) bearings and pivots, and (5) case, terminals, dial, and so forth. |

| Since the moving element is the most susceptible to damage from electrical and mechanical shocks and disturbances, it is built ruggedly. Moreover, since it has to carry not only the pointer, but other damping devices as well, its balance in its pivots is critical. |

| The stationary element presents few problems. The selection of permanent magnets and the insulation of coils and leads are easily accommodated. |

| Controlling devices usually consist of damping magnets or air dampers or vanes that keep the moving element from oscillating. In general, their speed of response involves the inertia of the moving element and the spring or restoring force. Springs are made of special alloys as they also serve as connections and are susceptible to any heat that may be produced. |

| Bearings and pivots are perhaps the most critical element since they must sustain mechanical shocks while maintaining extremely low values of friction. Consequently, pivots of very hard metal have points with only a slight |

< previous page | page_365 | next page >