SUMMARY: Artificial lighting on outdoor motion picture sets is essential for both day and night photography. In most cases an electrical distribution system is not available at the selected location, and power must be supplied by electric generators driven by internal-combustion engines. Because standard, commercially available, engine-generator sets are not suitable for the special performance requirements encountered in motion picture photography it is necessary to design and construct special equipment having the required features. This paper describes and evaluates the engineering factors involved, and illustrates how each of the desired characteristics was attained in equipment recently constructed.

BASIC REQUIREMENTS

THE ENGINEERING FACTORS which must be considered in the design of engine-generator equipment for supplying electric power for lighting on motion picture locations are as follows:

1. Electric Power. The generated electric power should be 120-v, d-c, to supply satisfactorily both arc lamps, which require direct current, and incandescent lamps, which operate on either alternating or direct current. Experience has shown that engine-generator sets having a capacity of between 750 and 1,400 amp¹ will currently satisfy the load requirement in practically all cases, with those at or near the higher rating being more in demand. The increase in the number of color pictures being made on locations indicates a possible future demand for engine-generator sets having capacities above 1,400 amp, and a few sets capable of producing 2,000 to 2,500 amp¹ have been constructed. However, in considering the feasibility of these larger units the advantage of increased capacity must be weighed against the disadvantage of decreased portability due to added size and weight. Also the relatively fewer occasions on which they can be used should be considered. To date, it has been generally accepted that it is more feasible to use two or more smaller, lighter, and hence more maneuverable, engine-generator sets to supply the higher current demands on location.
A three-wire d-c system is superior to a two-wire system for distributing large amounts of power.² A three-wire system produced by a single three-wire generator is not well suited for motion picture work because of the inherent variation in voltage caused by load unbalance, and such a system is best obtained by two generators connected in series. Driving two generators from one engine presents mechanical problems. When deciding upon the system to be used, the electrical advantages of the three-wire system should be weighed against the necessary mechanical complications required to produce it, and the decision is usually determined by the power rating. For engine-generator sets of capacities up to 1,400 amp which are to be driven by a single engine, experience has shown that it is better to use one two-wire generator and accept the disadvantages of the resulting two-wire distribution system. However, when considering larger loads up to 2,500 amp, the electrical advantage of the three-wire system becomes the governing factor, and a construction having two 120-v generators in series is to be preferred. Such plants should have two engines, one for each generator, thereby gaining an advantage in the form of flexibility, in that only one engine need be operated when the plant is supplying half-load or less.
Good, consistent lighting is dependent upon close control of the voltage at the load-end of feeder cables. The lighting load encountered on locations varies from a small fraction of the generator rating up to its maximum capacity. Feeder cables may be short or relatively long, dependent upon the conditions at the location. An appreciable increase in voltage cannot be tolerated because of the danger of damaging incandescent lamp filaments. Automatic voltage regulation to meet these operating conditions is essential.
Commutator ripple in the d-c voltage causes noise emission from carbon arcs which can be objectionable on sound sets. Generators for motion picture work should be so designed that their ripple voltage does not exceed ± 1/2 of 1% of rated voltage. Even then the ripple is, on many occasions, further reduced by means of filter circuits using choke coils and capacitors.³
Since the duty cycle of an engine-generator set is of an intermittent nature, a generator which will produce the required maximum amount of power for approximately one-half hour without injurious heating is considered to be adequate. Hence, much smaller and lighter weight generators can be used than would be the case if it were necessary to give them a continuous rating at the maximum output.
2. Prime Mover. Either a gasoline or diesel engine can be used to drive the generator and both types have been successfully employed. The speed-power curve of the engine should match the generator requirements, bearing in mind that, as a protective measure, engine horsepower should be somewhat below that capable of driving the generator at an injurious load. The engine should be equipped with a governor capable of manual adjustment to maintain automatically any desired speed within the generator operating range.
3. Control. The controls for both the engine and generator should be conveniently grouped so a single operator can quickly perform all operating functions required.
4. Noise. On sound locations the engine-generator set must be positioned so that its operation noise does not interfere with production. A design which effectively reduces the noise level saves setup time and permits the use of shorter feeder cables since the plant can be located closer to the action.
5. Portability. The engine-generator set should be as small and light in weight as possible. Its dimensions should allow passage through door-openings in railway cars, and be suitable for mounting on a truck or trailer. Maneuverability in and out of "tight spots" on locations is essential.
6. Dependability. More often than not, engine-generator sets operate in remote locations where supply parts and the repair-shop type of maintenance service are not available. A breakdown on such locations would hold up an entire company and increase production costs. For that reason the construction should be as foolproof as possible, using the minimum number of parts to satisfy operational requirements. The components employed should be of a standard commercial type which have proven their reliability under service conditions.
7. Protective Devices. Automatic-operating safety devices should be incorporated to protect the engine-generator set against damage caused by abnormal conditions.
8. Maintenance. The use of standard, readily available, commercially proven components greatly reduces the maintenance problem. In addition, the engine-generator set should be designed so that those parts which require periodic maintenance are readily accessible.

ATTAINING THE REQUIREMENTS

In order to illustrate methods which may be employed to meet the foregoing basic requirements, a particular design of a 150-kw engine-generator set recently manufactured by the Mole-Richardson Co. is (described. The completed unit (Figs. 1 and 2) consists of a cubicle

Fig. 1. 150Kw engine-generator set.

54.in wide by 72 in. high by 118 in. long and weighs approximately 10,000 lb.
The enclosure forms the outside walls of three separate internal compartments: the generator compartment at the radiator end of the plant, the engine compartment at the rear of the plant and the muffler compartment at the top. All outer surfaces of the housing are polished, stainless steel. The design features directly related to the basic requirements are described as follows:
1. Electric Power. When contemplating the construction of a power-package, the electrical requirements and the engine must be simultaneously considered. Obviously, a special engine cannot be designed and built for this particular application, and one which will drive a generator of the approximate desired rating must be selected from those commercially available.

Fig. 2. Oblique view, off-side and rear.

Generator performance should conform to the speed-horsepower characteristics of the chosen engine, and modification of standard generator design is generally required.
In this instance, after consideration of available engines, it was decided to construct equipment capable of delivering 1,200 amp at 125 v, or 150 kw. To meet this requirement, a generator (Fig. 3) was chosen which has an intermittent duty rating of 1,400, amp, 125 v at 1,800 rpm, and a continuous rating of 1,000 amp, 125 v at 1,400 rpm. It will pick up its voltage at 1,350 rpm without exceeding rated field current. The voltage rating allows for a 5-v line drop when maintaining 120 v at the load-end of feeder cables. The generator is two-wire, self-excited, flat compounded, Class B insulated, and weighs approximately 2,500 lb. Its voltage ripple characteristic is within the allowable limit of ± 1/2 of 1% of rated voltage. An automatic

Fig. 3. Engine-generator assembly.

voltage regulator, located as shown in Fig. 2, maintains the proper voltage setting over the speed range of the generator. A field control rheostat is provided so that voltage can be manually controlled in case trouble should develop with automatic voltage regulation. A field control switch at the control panel permits the operator to select the type of voltage regulation desired.

2. Prime Mover. The engine (Fig. 4) selected for this application develops 275 hp at 1,800 rpm as shown by its speed-horsepower curve (Fig. 5). After allowance is made for the power consumed by auxiliary drives, the engine is nearly fully loaded by 150-kw generator output at 1,800 rpm. Likewise, the engine power-input conforms to generator power-output at 1,400 rpm. The load current can therefore be increased from 1,000 amp at 1,400 rpm to 1,200 amp at 1,800 rpm at a rate of about 50 amp for each additional 100 rpm. This is a desirable feature since there is no need to operate the engine at a speed higher than necessary to develop the required horsepower.

Fig. 4. Engine-generator assembly.

The engine is of the industrial type having six cylinders with a bore of 5 3/4 in., a 7-in. stroke, a displacement of 1,090 cu. in., a compression ratio of 5.7 to 1, and a weight of approximately 2,600 lb. It is designed to consume ethyl gasoline fuel having an octane rating of approximately 80.
This engine is equipped at the factory with a governor adjusted to maintain an engine-operating speed of 1,400 rpm. The tension of an added external governor spring is varied by a knob at the control panel so engine speed can be adjusted above that which would otherwise be maintained by the governor proper. This allows the operator manually to adjust the automatic governor over the rated speed range of 1,400 to 1,800 rpm.
The generator end bell is designed to flange mount with a rabbet fit to the flywheel bell housing of the engine. The special resilient-type coupling (Fig. 6) is designed for connecting the generator shaft to the engine flywheel. The flywheel is modified to accommodate its portion of the coupling This type of construction results minimum of runout between the axis of rotation of the engine crankshaft and that of the generator armature and, therefore, insures long trouble-free coupling life. Mounting the generator direct to the* engine bell housing results in a minimum over-all length of the coupled units.
The engine and generator coupled to form one integral unit are mounted to the main-base frame primarily by means of the industrial

Fig. 5. Speed-horsepower curve for Hall-Scott model 400-0 engine.

base of the engine (Figs. 3 and 4). Thick rubber belting under the engine base and rubber bushings and washers at the holddown bolts prevent metal-to-metal contact between the engine and the main-base frame, minimizing the transmission of engine vibrations to the power plant structural members. Since the generator is an overhung weight mounted to the engine bell housing, secondary supports consisting of rubber tube-form mountings (Fig. 3), are positioned between the main-base frame and the ears of the generator with their upward thrust forces evenly adjusted for minimum strain on the engine bell housing. Here again, there is no metal-to-metal contact with the main-base frame.

Fig. 6. Coupling.

3. Control. All operating controls and adjustments of the engine-generator set are located on the control panel shown in Fig. 1 and illustrated in detail in Fig. 7. The controls and instruments are divided into two groups: the engine controls on the left and the electrical controls on the right.
4. Noise. The major portion of operational noise of an engine-generator set consists of engine mechanical noise, exhaust noise, radiator fan and cooling air noise, and carburetor-intake noise.
As described above, the engine and generator are resiliently mounted so there is no metal-to-metal contact with the structural members of the unit, thus minimizing the transmission of engine

Fig. 7. Control panel.

vibrations to the housing. The engine is completely enclosed by the engine compartment, the walls of which are insulated against sound transmission. The wall construction consists of the following' enumerated in order of position from outside to inside: stainless steel sheet, sound-deadening undercoating, air space, sound-deadening undercoating, asphalt compound, fiber glass insulation, fiber glass cloth and perforated stainless steel sheet. (Since the engine is completely enclosed, cooling is dependent entirely upon the water-cooling system.)
The exhaust noise is reduced by a large muffler 14 in. in diameter by 6 ft long, mounted in the muffler compartment above the engine compartment. The walls of the muffler compartment are also insulated against sound transmission in the manner described above.

Fig. 8. View through off-side generator compartment service door.

A right-angle exhaust stack (Figs. I arid 2) directs the exhaust gases in an upward direction.
The attainment of adequate engine cooling with a minimum noise from fan and airflow is primarily accomplished with the use of a radiator having an exceptionally large frontal area (Figs. 1 and 8). The radiator has approximately twice the capacity of those normally used in industrial applications of this engine thereby providing sufficient cooling with a relatively low air speed. This radiator also permits the use of a large 48-in. diameter, 8-blade fan (Fig. 8) which can provide the required rate of air flow at a low top-speed of 740 rpm. The fan is belt-driven from a sheave on a shaft extension at the commutator end of the generator.
A further reduction of fan and air noise is obtained by the use of a variable-fan-blade-pitch control system which automatically controls the pitch of the blades in accordance with the cooling-water temperature; thus no more air is passed through the radiator than is necessary. A manual control of the pitch of the fan blades permits the operator temporarily to feather the blades to zero pitch and stop the air flow, and the resultant noise, during a "take."
The cooling air, after being drawn through the radiator, passes through the generator compartment and is exhausted through the muffler compartment door, and top louver-door whose opening can be adjusted. On many occasions the power plant can be operated with the top louver-door closed, with all of the cooling air passing through the muffler compartment and exhausted upward at the rear of the plant. All walls of the housing are insulated, as explained for the engine compartment, and therefore are deadened against vibrations, caused by air flow.
The carburetor-intake noise cannot be neglected. In this application outside air is drawn in through a baffled cap (Fig. 1), then through a large industrial-type air cleaner (Fig. 9), and into the carburetor. The baffled cap and air cleaner function as silencers.
5. Portability. The minimum over-all engine-generator set dimensions result from the following considerations: The 54-in. width accommodates a radiator having the desired capacity and in addition provides adequate clearance between the inside walls of the housing and the internal components for service and maintenance accessibility. The minimum height is determined on one end by a radiator of sufficient capacity, and on the other end by the combined heights of the engine and muffler. To reduce the latter, the engine mounting platform, consisting of steel channels welded crosswise and longitudinally, is recessed below the top surface of the main-base channel structure. Thus the minimum height requirement for the engine and muffler is made to conform to that required for the radiator. The length of the power plant is considerably reduced by the design of the short-coupled arrangement between the engine and the generator.
Such engine-generator set cubicles are either carried on trucks or trailers. In the design of this plant, the 54in. width of the enclosure is reduced by a curved contour near its base to 33% in., which is slightly less than the standard 34-in. over-all width of the main frame of commercial trucks having the capacity to carry this power plant. The 9 1/4 in. height above the bottom surface of the main base to the point where the power plant enclosure becomes 54 in. wide provides clearance over the rear wheels of the standard commercial trucks.

Fig. 9. View through engine compartment service door on operator's side.

This allows the power plant to be positioned low between the rear truck wheels, hence resulting in a minimum over-all height, and a minimum height of the center of gravity of the power plant above the ground. This same design feature is equally advantageous if the cubicle is to be mounted on a trailer.
Arrangements are provided for handling the cubicle as a unit. 'Heavy steel tubular members are welded crosswise through the main-base frame near the front and rear and other tubular openings are located at both ends. These tubular openings are for insertion of heavy steel handling bars or wheel axles.
6. Dependability. Dependability is necessary for trouble-free operation of all components. The engine chosen for this application is one which has proven its dependability in years of truck service, bus service,

Fig. 10. Bus bar panel.

in oil field locations and in other industrial applications. The generator is of a rugged design which has performed successfully in railway transportation equipment. The method used in coupling the generator to the engine insures long coupling life.
Wherever practical, automatic operating components are supplemented by manual controls to reduce the possibility of shut-down. The automatic voltage regulator is supplemented by manual field rheostat control. The pitch of the fan blades can be manually controlled in case of failure of automatic control. Should manual control become inoperative, the blades can be mechanically locked in their pitched position and operation continued.
There are, of course, inevitable possibilities of failure of equipment which could cause shut-down. The best insurance against such occurrences is the use of components which have by service proven their reliability.

Fig. 11. Generator on dolly fixtures.

7. Protective Devices. An overspeed governor, coupled to engine rotation, will open the ignition circuit and stop the engine before a damaging speed occurs. The engine cannot be restarted until the overspeed governor is manually reset by the operator.
A safety switch in the ignition circuit will stop the engine if a loss of oil pressure occurs, or if the cooling-water temperature becomes too high.
An overload relay, which will open the main line contactor, protects the generator against overloads.
The positive battery cable is grounded through a pin-plug arrangement so it can be conveniently and positively removed as a safety precaution during servicing, or when the power plant is idle. All battery circuits, except the starter motor circuit, are fused.
A Thyrite discharge resistor, permanently connected across the generator shunt field, protects the field insulation against high voltage breakdown by limiting the induced voltage which momentarily appears when the field circuit is opened.
The positive and negative bus bars for external feeder-cable lug connections (Fig. 10) are recessed behind an insulation panel, and an insulation barrier is located between them for protection against short circuits. The convenience outlets at the left of the bus bars are fused.
8. Maintenance. Door openings in the enclosure are such that all components are accessible for routine maintenance. The housing is constructed in sections to provide access for general overhaul. The top section, or muffler compartment, is removable as a unit. Each of the sides consists of two separate wall panels, and the rear is made up of one panel, all of which are individually removable. External stainless steel trim strips not only serve to cover the joints between sections for the sake of appearance, but also as mechanical members which tie the housing sections together.
During a major overhaul of the power plant it may become necessary to detach the generator from the engine bell housing and separate the coupling. Adjustable dolly fixtures are provided (Fig. 11) which can be secured to the ears of the generator in place of the secondary supports. With the dolly wheels resting on the main base channels, the generator may be rolled in or out of engagement with the engine.

CONCLUSION

In the making of motion pictures the production-time factor is of such importance that every precaution against possible power failure should be given the utmost consideration. The engine-generator set must be sufficiently rugged to withstand hard travel over rough roads, yet deliver the maximum of power with constant performance. In spite of the fact that it is a highly specialized piece of equipment, it should, so far as possible, be designed for, and constructed of, units which have been proven dependable in other fields.

REFERENCES

1. "Report of Studio-Lighting Committee," Jour. SMPE, vol. 51, pp. 431-436, Oct. 1948

2. "Report of Studio-Lighting Committee," Jour. SMPE, vol. 49, pp. 279-288, Sept. 1947

3. B.F. Miller, "A motion picture arc-lighting generator filter," Jour. SMPE, vol. 41, pp. 367-373, Nov. 1943

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