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1945 Theatre Catalog, 4th Edition, Page 314 (290)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 314
Page 314

1945 Theatre Catalog, 4th Edition, Page 314

to the projection lamp. While advantages are claimed in that this feature saves a small amount of original expense in installation, let us observe the other results. In the first place, bulb type rectifiers are installed in the projection room so that in case of failure the operator can quickly replace this unit. There are other cases in which it is necessary to have them so installed. Because of the small amounts of vibration transmitted into a building by nearby traffic bulbs may become loose in their sockets and break the circuit.

Also consider the amount of unnecessary heat in the projection room caused by installing any rectifying device directly in it. Take, for instance, a rectifier of the bulb type which is delivering 50 amperes at 33 volts. Let us assume an overall efficiency of 60 per cent. This means that the input power to the rectifier is 2750 watts and output power is 1650 watts. The difference between 2750 and 1650 represents the inefficiency of the rectifier which is dissipated as heat.

Anyone familiar with the ordinary type of radiant heater will know that constant dissipation of 1100 watts in the form of heat in the ordinary projection booth does not add to the comfort of the projectionist, especially in the summer time.


Much has been said and written about the efficiency of motor generator equipment compared to the other types of rectifying equipment. We believe that most people are familiar with the efficiency obtained from a well designed and proportioned loweloss motor generator set, and typical values of efficiency can be obtained from most any handbook.

TYPE LV>40-80 TRANSVERTER is typical of the products made for current-converting purposes. It is this type of motor generator with which the present article deals. The motor generator designed for arc proiection service has earned its place by usage and inherent proved characteristic of stable voltage, low


We note in the case of most rectifiers that claims are made for efficiency anywhere from 40 to 70 per cent, depending on the period of use of the rectifier. Tests which have been conducted by our company indicate that the efficiencies of motor-generator equipment and other rectifying equipment for arc service are comparable when the rectifier is new, but manufacturers admit that the efficiency of plate type rectifying devices decrease with age.

The eliiciency and output of the motorgenerator set do not decrease with age. In fact, after a period of several weeks, usage, it is a known fact that the brush friction decreases anl also in many cases the bearing friction will decrease slightly. In this event, the efficiency, of course, would increase during this period. In any later period, the efficiency would remain absolutely constant, barring no further change in brush friction or bearing friction during the normal life of the equipment. There are no parts in this rotating equipment which age, insofar as having any eEect on power or efiiciency.

High room temperatures have little effect on a properly designed motor generator. However, some types of dry plate rectifiers age far more rapidly, requiring the replacing of the rectifying element, when the room temperatures are higher than 98 degrees Fahrenheit, a not uncommon temperature in most projection rooms in the warm summer months.

In order to protect from overheating, a fan is usually provided in most types of rectifying devices including the motor generator. In the case of the motor generator the fan is an integral part of the rotating element and requires no attention or outside source of power to drive it. Some rectifiers use the 110-Volt light

ing circuit to operate a separate driven fan which, incidentally, is also a rotating device, and a failure of the 1-10-volt circuit on the fan itself could shut the rectifier down even though the normal power supply was available.

The windings and core of a motorgenerator set compare to the transformer section of a rectifier. These parts can normally be expected to last until obsolescence of the equipment. Bearings, if kept clean and lubricated according to instructions, should never need replacing. This leaves the commutator as the only other element, outside of the brushes previously mentioned, which has a slight amount of wear. This Wear is very slow.

As a motor generator set reaches the state where some repair or maintenance is required, it generally gives warning, similar to the condition existing in a car which needs new piston rings. In other words, in this comparison, your car serves notice by gradually consuming more oil. It is found likewise in any motor-generator set that you are generally warned. Failures of other types of rectifying equipment would compare favorably to a tire blowout on a car.

Telephone interference factor and radiated noise are receiving more and more attention by the manufacturer and by the public. The first element mentioned would not ordinarily exist in a motor generator set of this type while the latter could. However, radiated noise of this origin fades out very quickly as distance from source increases. For instance, at three feet from the generator, a sensitive radio receiver could barely detect it. In rectifiers, telephone interference factor may become troublesome due to the harmonics fed into the alternating current supply lines in some instances.

ripple, long life, low upkeep, no replacement of rectifying elements, overload capacity, independence from effects of a.c. line-voltage fluctuations, isolation of a.c. and d.c. circuits, constant efficiency, and no aging effect. Over-all efficiency is the combination of all these assets. (Hertner Electric Company photograph.)

1945 Theatre Catalog, 4th Edition, Page 314