TRENDS In Fire Apparatus Electrical Systems
Applications of electronic research are increasing the utility and versatility of the vehicle ignition system
—Photos courtesy Electric Auto-Lite Co.
THE ELECTRICAL SYSTEM of any modern fire apparatus is presently a somewhat complex network. In addition to providing for practically foolproof ignition, it must supply the heavy demand of all lights, flashers, sirens, radios, power amplifiers and traffic control transmitters and still work reliably under all types of weather and fire service conditions. This combination of wires, relays, contacts and batteries, plus the working load, is one of the few parts of all fire fighting equipment which must function continuously from the time an alarm is received until the apparatus returns to quarters.
Add to this consideration the fact that the fire service is constantly mounting more electrically operated equipment on its apparatus to the point where flood-lighting and power tools are now being supplied current from the engine generators. At one time these units were called upon to provide only the ignition and signaling requirements of the apparatus itself.
This trend towards increasing the demands on the electrical system has had beneficial effects from the standpoint of progress. To meet the needs, manufacturers have shown constant improvement in their products and have introduced new methods to overcome previous shortcomings.
The radio manufacturers have taken a large lead in this respect. A modern mobile transmitting-receiving unit may have a stand-by current drain of as little as 400 ma. This is less current than many pilot lights require! Compare this to the 6 to 12-amp load of only a few short years ago. Such progress has become possible due to technical advances which permit circuit features unheard of 10 years ago.
One of the most spectacular additions to the electrical system of fire apparatus was the introduction of the alternator shortly after World War II. It was developed to permit efficient mobile radio operations in the police, utility and taxi services and quickly adopted by fire departments, as radio became more widely used. The advantage of providing sufficient output to keep batteries charged when the apparatus engine is running at idle, appealed to everyone concerned. Its capabilities of providing excellent efficiency at high speeds is sometimes overlooked, except by automotive engineers and ignition experts.
—Motorola photo
—G-E photo
—Leece-Neville photo
—Delco-Remy photos
Another benefit concerns battery life. The latter is definitely extended, due to the lower incidence of “cycling” or discharging and charging. In this respect, the alternator has practically revolutionized the battery-charging methods of the fire service. Using the charger at least once a week and sometimes more often to bring the apparatus battery up to 1250 has been a regular chore for drivers. Two years ago, FIRE ENGINEERING ran a survey on charging methods and use of alternators. Some of the answers were startling to one accustomed to the oldtime methods. One department reported that in a certain company the charger had been used only once in the previous 12-month period. This was necessary, due to the driver’s failing to switch the alternate bank of batteries into the charging position as a routine matter.
We have been getting better batteries for automotive duty in the last few years and recently a replaceable cell battery was announced. The idea on which it lays claim to merit is that most battery failures occur in a single cell. Unless the department maintenance shop is equipped to rebuild batteries (and there are very few in the country), it usually means discarding the entire unit and purchasing a new one.
With the new battery, the offending individual cell is merely removed, since it is a complete unit in itself, and a replacement added. In the long run, this may tend to lower battery maintenance costs and permit uninterrupted operations.
Alternators are somewhat of a study themselves. These units generally deliver 3-phase A.C. power to a bridge-type rectifier. The common rectifier has been a selenium stack. This is fairly large in size and must be mounted externally where a strong flow of cooling air can reach it. Recently the silicon rectifier was introduced and the small units are now being used to replace selenium. Mounted directly in the alternator case, they eliminate much external wiring and the sometime tricky mounting problem of the older style rectifier.
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ELECTRICAL SYSTEMS
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It was mentioned that the alternator is a 3-phase electrical unit. When operated for ignition purposes with the components supplied, this poses no problem. However, some fire departments have taken advantage of the fairly high power supplied, and by means of transformers, have adapted the systems to provide extra lighting as a supplement to the portable generator. This eventually led to the use of power tools and immediately some master mechanics were running into electrical problems.
All manufacturers of alternators will supply diagrams of the recommended methods of taking power from the alternator. When connecting transformers, remembers the 3-phase rating. Most troubles in this respect have been due to neglect of this factor and resulting incorrect wiring.
Nearly all manufacturers recommend that the A.C. output be rectified to D.C., and that either D.C. or A.C.-D.C. universal tools be used. In this manner, the cycle problem due to the varying speed of the driving engine is of no consequence. With most alternators presently manufactured, the cycle varies as approximately one-tenth of the engine speed. Maintaining 600 rpm in a pumper merely to keep a 60-cycle tool operating sounds ridiculous and may be—especially if the pumper is supplying lines on a fire at the time—and even more so since both D.C. and universal motors are readily available for all tools. For units without pumps this may be no problem.
There is one large alternator on the market designed specifically for the purpose of supplying heavy-duty tools and floodlights. This somewhat specialized unit supplies 115/230 volts, single-phase, 60-cycle A.C. at 1,800 rpms. It has a special meter built into the throttle control panel to indicate when the speed is proper for 60-cycle. The manufacturer warns that operating A.C. induction motors from this unit requires careful planning, due to the great starting load. The induction motor may require from three to six times as much current for starting as for running. This may be determined from the code letters on the motor.* Again, the universal motor does not require this additional current. Due to the limitations on the alternator, some departments prefer to use them with transformers strictly for lighting purposes, and operate tools by means of portable generators.
Trend to transistors
It is probable the fire service will soon be seeing the transistor introduced into the ignition system. The trend to higher engine compression and higher rotational speed imposes demands on ignition, which are difficult to meet with present methods.
The transistorized ignition system replaces the points in controlling the input current. It switches it on and off with no moving parts. The point contacts are used only to turn the transistor on and off. Therefore, the current passing through the points is of a low value.
This improvement is said to eliminate “blue” points or oxidation and resulting hard starting; point erosion will no longer be a problem along with the sometimes resulting timing changes; and it is said to increase ignition power at high speeds, producing nearly constant output across the full range.
The transistor is sensitive to high temperature and must be properly cooled. Units presently available for pleasure cars mount the transistor in the coil assembly with special cooling fins or a “heat sink” provided as an integral part of the form.
*Rp, Rl and U—three times; CP and KL— four times; KS—six times.
—Courtesy, Electric Controls, Inc.
The transistor has been further adapted to the voltage regulator, which is said to increase its durability and dependability. One style of this new device eliminates all mechanical contacts and promises a trouble-free regulator good for the life of the apparatus.
There is an old saying that “nothing is new under the sun.” This is very difficult to contradict in the automotive field. For example, automatic transmissions and torque converters are presently making strong inroads into the fire service. Yet, it is recorded that American LaFrance was one of the first manufacturers in this country to produce a genuine automatic transmission, the “Manly.” This was fitted on a hose wagon in 1916 or ’17. It wasn’t practical at the time, but it did work.
The old adage is somewhat disproved, however, when we look at our modern ignition systems. As we progress further into electronics, we will certainly use techniques which have never before been employed. Some of those mentioned here were unheard of a few years ago.
