Alternators for Fire Apparatus
Alternator reliability, a critical consideration on any vehicle, must be near perfect on fire apparatus. Correct selection of the alternator and its related equipment is, of course, a major factor in assuring good performance. Although good preventive maintenance of the system is also essential, it is the purpose of this article to explore some of the most significant considerations in selecting the correct alternator for the job.
Selection of the alternator, regulator and, where required, the 110-volt power supply is based primarily on current demands. The alternator must be sized to provide adequate current output to keep the batteries fully charged even under severe electrical demands.
Alternator output is a function of its rotational speed, which in turn is directly related to engine speed, taking into account the ratio of the belt drive between the engine and the alternator. Output of the alternator is normally presented as a curve, such as those in Figure 1. These curves, for three different alternators show ampere output at various alternator rpm.
To properly select an alternator, we must first determine the electrical load of the vehicle for a given period. For purpose of calculation, we may use any convenient period of time, such as a 24-hour day, week or, in the case of fire vehicle, a typical run. The problem involves determining how much total current will be used during the time period and then selecting an alternator capable of supplying this amount of current so that the batteries do not discharge.
The current in ampere hours consumed by each electrically operated device on the vehicle is calculated by the following formula:
A x T = E
A = Current draw of device in amps T = Time the device is operated in hours
E = Total power consumed by the device in ampere hours
For example: Low beam headlights may draw 7 amperes. If they are operated for 3 hours, 21 ampere hours will be consumed.
This calculation must be made for each electrical device on the vehicle, such as cranking motor, ignition, headlights, running lights, flashers, radio, 110-volt power pack, etc. The total ampere hour load for a given period of time is then obtained by adding the individual ampere hour figures together. We then add an additional 25 percent to our answer as a safety factor.
Now that the total electrical load for a given period of time has been determined, the proper alternator must be selected to supply this load under all conditions of vehicle operation. To do this, the alternator output under various operating conditions— over-the-road travel, at idle, while pumping, etc., must be determined. To do this, the alternator speed must be known.
Alternator speed is obtained by multiplying engine speed by the pulley ratio. Pulley ratio is obtained by dividing the diameter of the alternator pulley into the diameter of the drive pulley. For example, if the engine speed is 750 rpm and the pulley ratio is 2:1 (drive pulley twice the size of alternator pulley), then the alternator speed is 1500 rpm (750 x 2 = 1500). Output of the alternator at various speeds can then be determined from the alternator performance curve, Figure 2.
By multiplying the alternator output in amps by the length of time the alternator will operate at that speed, total output in amp-hours can be determined. Obviously, the output for a given period of time must be equal to, or greater than, the ampere-hour demand arrived at in the calculations.
Calculation of load
The following is an example of a typical calculation using the method described. In this case, a 20-hour period is used.
Total amp-hours charging capacity—1160
In this example, the alternator installation would be sufficient to handle the vehicle load.
Erratic use of apparatus
Unlike a typical over-the-highway truck, the duty cycle of a fire vehicle may be widely erratic. On one run, the apparatus may be out and back in a few minutes, operating virtually all the time at accelerated engine speeds. Another trip may involve extended periods of idle. On another occasion, the vehicle may be at a fire for many hours or even several days. However, since electrical current availability is so important on an emergency vehicle, the alternator capability must be based on the most severe conditions.
In reviewing the duty cycle, it will become apparent that certain situations are more difficult for the charging system than others. For example, operating the vehicle at idle for extended periods with all accessories and lights turned on imposes a heavy load on the system at a time when the alternator is running at its slowest speed. This situation will require an alternator with a high output at idle and the selection of the alternator will be based largely on the idle output rather than the rated output.
On apparatus with a 115-volt system for powering portable lamps, smoke ejectors, heaters and small tools, it may be this system which will dictate selection of the alternator size.
Typically, the 115-volt DC power pack furnished by Leece-Neville provides 1200 watts when powered by a 100-amp heavy-duty alternator. If a large number of additional accessories are operated while the power pack is supplying current, either the current draw on the 115-volt system must be reduced by the amount of the draw of the accessories or a larger alternator must be provided to furnish adequate current for both requirements.
Like a generator, an alternator requires a regulator which controls the output voltage to prevent overcharging the battery and to protect the vehicle load.
Therefore, selection of a regulator is another important consideration. The nearly universal use of silicon rectifiers with their extremely low-leakage current has eliminated the need for a load relay in the regulator, and self-limiting alternators have done away with the need for a current limiter. So the present regulator, in most cases, is a simple voltage regulator only. This greatly simplifies installation and servicing and makes possible the use of transistor, or solid state, regulators.
The output of an alternator is proportional to rotational speed and field excitation. The regulator must allow full excitation current at minimum speed and maximum load and decrease the excitation as the speed increases and/or the load decreases if voltage is to be held to a fixed value— usually 14 volts. Since the load is a lead-acid storage battery, the alternator output voltage must maintain the batteries in an almost fully charged condition.
Although vibrating contact regulators are still in use, many fire departments have converted to transistor regulators which have several important advantages. They have no moving parts, thus leading to greater reliability and longer life. They have no contacts as do mechanical regulators, which arc and burn and in time require replacement or repair. They also greatly reduce radio interference caused by contact arcing, and the solid state components have a long service life which can easily outlast the vehicle. As a result, transistorized voltage regulators can maintain their accuracy for longer periods without maintenance or adjustment.
As part of the transistor regulator package offered for fire apparatus, Leece-Neville supplies a special capacitor designed to reduce transients in the vehicle’s electrical system. Voltage transients may occasionally cause malfunction and failure of regulator transistors. Hooking this capacitor across the output and ground of the regulator effectively reduces these transients.
One other consideration in the specification of alternator charging systems is the question of location. The alternators normally used tend to be fairly bulky because of their high output100 amps as compared with 40 to 50 amps for the typical passenger car. Thus, care must be taken to assure that the alternator and its mounting brackets will fit in the available space. It is desirable to locate both the alternator and regulator where they will receive air circulation even when the vehicle is stationary. It is equally desirable to locate the alternator where it is protected from road splash and other contamination.
It should also be noted that the large alternators normally used on fire vehicles use dual belts to handle the substantial forces required to drive the unit when it is operating at full load. These heavy loads impose a severe strain on the alternator bearings, particularly the drive-end bearing, and it is important that an alternator with bearings adequate to meet these loads be selected.