> > > >

1945 Theatre Catalog, 4th Edition, Page 312 (288)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 312
Page 312

1945 Theatre Catalog, 4th Edition, Page 312

Provider of D. C. Current: The Generator

Advantages of Motor Generators Set Forth In Relation to the Best Proiection Lighting

The carbon are used as the light source in most projection lamps for the motionpicture industry has a resistance characteristic requiring true direct current and constant voltage for stable operation. That is, while the most common loads applied to a direct-current generator are such that the current increases proportionally to the voltage, and vice versa, with an are this is not the case, as the resistance varies with the current.

Failure to provide true direct current results in arc hissing and hum which may be picked up in the sound system, and lack of constant voltage results in arc instability which shows up as reduced light or a bad color of light on the screen.

Practically speaking, there are two sources of true direct current power, the first being the storage battery and, the second, the homopolar generator. Neither of these, however, are suited for motionpicture arcs. Running a close second to these is the direct current generator designed for motion-picture service. Next


Chief Engineer and Sales Engineer, Respectively. Hertner Eleclric Company

on the list probably are various classes of rectifiers. But, without increasing very greatly the number of phases or adding a stabilizing coil in the output side, rectifiers do not run even close to motor-gem erator sets in this respect.


A suitably designed motor generator provides constant voltage independent of load in range well beyond the nominal rating of the generator. This feature provides an inherent overload capacity not found in other types of rectifying equipment. No damage is caused to the motor generator because of the loss of the arc. Breaking the direct-current power side of tube rectifiers may cause tube dash-over and consequent failure.

FIGURE l.-lnput-outpui voltage curves, showing the small amount of change in tic-operating volts with a large variation in the (LC. power-line voltage. Curve A shows actual cl.c. voltage change with an HI-SO-TOO TransVerteR carrying a normal single arc (50 amperes). Curve 8 shows the some data expressed in terms of the percentage of normal d.c. volts. (Hertner Electric Company graph, prepared for this publication.)




In 34C w $0 200 2.20 240 Lee 7.80

AC UNE VOLTS {7.7.0 Vows





90 CURVE B l i .- - .mn..n mm. 80 90 NO NO llQ BO


A great advantage in constant light output is gained in using a motor generator over other types of rectifying devices, because the motor speed varies 'only slightly with changes in alternating current line voltage, even with quite large fluctuations. This is because motor speed is dependent on ac power supply frequency rather than voltage. It is a known fact that the power supply frequency never varies as attested by the way electric clocks, also governed by the a.c. frequency, never gain nor lose.

This inherent characteristic of alternating current motors gives the particularly worthwhile feature to motor generators that cannot be claimed by other types of rectifying equipment. It means constant, unvarying light intensity on the screen regardless of the changes in ac voltage which cause other types of rectifying equipment to Vary the light output to the screen. The dc. voltage of rectifiers varies proportionally to the ac supply voltage, while in the case of the motor generator, the generator being mechanically connected to the motor will only change in dc. voltage proportionally to the change in speed of the driving motor, resulting in constant direct current voltage and hence, constant light output.

By referring to Figure 1 (Curve B), it will be noted that when the alternatingcurrent line volts drop to 80 per cent of the nominal value, the direct-current output voltage is maintained at 98.2 per cent of the normal voltage and that, at 125 per cent of nominal alternating-current line volts, the direct-current voltage has increased to 102.5 per cent of normal.

Very few power systems in the United States are so regulated that, during certain parts 0f the day, the ac. line voltage is regulated better than within 5 to 10 per cent. This is especially true later in the evening as the domestic, manufacturing, and transportation load drops, resulting in an ever-increasing voltage. Obviously, this cannot be corrected for by changing taps, inasmuch as during some other portion of the day the voltage will pass through the nominal value.

This means that, with rectifying equipment of the type in which the output varies proportionally to the ac. input, the output voltage will vary in the neighborhood of 5 to 10 per cent during this period. The deleterious effect in light resulting by these voltage changes is well known to most projectionists.

In Figure 2 are presented graphic curves showing the amount of disturbance transmitted by various types of converting equipment from the alternating current power supply line to the direct current are circuit.

When a motor generator is used, disturbances in the a.c. power supply are not transmitted into the dc. arc circuit apartially because there is no electrical


1945 Theatre Catalog, 4th Edition, Page 312