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1948-49 Theatre Catalog, 7th Edition, Page 348 (335)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 348
Page 348

1948-49 Theatre Catalog, 7th Edition, Page 348

mer and after the Autrastat dimmer, for each branch circuit.

Connections for reactance type dimmers are not shown. However, they are usually connected in the feeder to each group of branch circuits. This type of dimmer is usually included in a completely wired switchboard so that the electrical contractor is not required to do more than connect the main feeders and the branch circuit wiring.

In the arrangement of the lighting panel controlling the circuits that are to be dimmed, it is always desirable to balance the total connected load on the two sides of a 3 wire single phase system or on the 3 phases of a 3 phase, 4 wire system. However, in order to bring the number of dimming units to a minimum, it is sometimes necessary to unbalance the connected load so that several branch circuits can be controlled from a single dimmer unit. Where this condition exists, the unbalance in the auditorium lighting should be compensated by an unbalance in the opposite direction on other panels in the building so that the total connected load of the building is within the limits of unbalance that may be set by the local power company.

It is not possible to place a single dimmer in the common neutral return of a 3 wire or 4 wire circuit as an examination of the diagram indicates that the current nowing in the common neutral will be zero if the branch circuits are equally loaded and no dimming effect would be obtained since there is no current to introduce a voltage drop in the dimmer. Also it is not possible to control both sides of a single phase 3 wire system with a single Autrastat dimmer. Thus, if some branch circuits must be fed from opposite sides of the 3 wire system or the various phases of a 3 phase, 4 wire system, a separate dimmer unit must be provided for each side of the line or each phase of the system. These dimmers may be mechanically connected so that they operate as a single unit, but they are electrically independent, each one carrying its own load on its own part of the system.

Non-Incandescent- Limitations

The dimming of electric light is almost as old as the electric lamp invented by Edison. All of the devices discussed previously were basically deSigned for use with incandescent lighting. There are available for the illumination in theatres other types of light sources such as the fluorescent lamp, both hot and cold cathode type. Unfortunately, the hot cathode type of fluorescent lamp has characteristics which prevent any type of dimming or intensity control. These lamps have an auxiliary equipment that produces a high potential to start the arc in the lamp and includes a starting device which proheats the filaments at each end of the tube until the arc ignites. Any attempt to reduce the voltage on the lamp will


cause the arc to drop out and the starter to attempt reignition.

On the other hand, fiuorescent lamps of the cold cathode type can be and are successfully controlled in intensity. These lamps have for auxiliary equipment a high potential transformer without any starting device and the electrodes in the ends of the lamp do not require preheating. There are several factors which affect the success of this type of lighting installation. Among these are the desirability of underloading the high voltage transformers required with the fluorescent tubing. In other words, a transformer normally suitable for sixty feet of tubing should have only forty or fifty feet of tubing connected to it. Furthermore, it has been found that the larger diameter tubes operating at lower pressures and higher voltages provide the widest range of intensity control. The gas used in the fluorescent tubing should contain some or all mercury vapor since it permits ignition at lower potentials than the other types of gases commonly used in fluorescent lamps.

At the other extreme, neon gas pros vides the poorest results in intensity control as the arc will drop out with only a slight reduction in intensity.

With any type of cold cathode lamps the intensity will reduce as the primary voltage on the high potential transformer is reduced by the dimmer. It will reach a point at which the gas is not completely ionized, causing dark lines to appear in the tubing. Shortly below this condition the arc will completely drop out to blackness. If this type of lighting is used for picture lighting it is desirable to install with the dimmer equipment some means to prevent the intensity being reduced to the point at which this flicker occurs. Further reduction to blackout would be performed by switching rather than further dimming. Such arrangements can readily be made on motor operated dimmer equipment and can also be furnished on the manual type of apparatus.

All three types of dimmer equipment have been used successfully with cold cathode lighting installations, however, the Autrastat provides the best control.

BELOW may be seen a typical pilot control panel for an electronic controlled reuctance type dimmer. Reoctance dimmers consist of magnetic elements connected in series with the lamp loads.




1948-49 Theatre Catalog, 7th Edition, Page 348