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Current threats on our military forces have created a tremendous requirement for mobility as it relates to mission specifications. The objectives of onboard vehicle power (OBVP) are to develop, demonstrate, and transition electronic technologies that enable lighter, physically smaller, more efficient, and more reliable electronics, which are easier to operate and maintain. This in turn will enable current and emerging electronic systems to increase performance, have improved physical characteristics, be more reliable, and require significantly fewer lifecycle resources. Further development of these programs will also maximize the advantages of increased DC supply voltage inherent in military vehicles such as the High-Mobility Multi - purpose Wheeled Vehicle (HMMWV) for delivery of a power solution for multiple military platforms in excess of today’s available 10KW.

An example of today’s extreme power requirements in military vehicles.
Sometimes OBVP is referred to as “under the hood power,” but whatever the vernacular, it simply refers to providing an alternative source of AC power on the battlefield that is generated from the vehicle itself, as opposed to hauling a large, heavy diesel generator around on a trailer. What’s the big deal with this concept? Well, the military has been hauling generators around for decades, and has done it quite successfully. When the enemy was confronted at well-defined lines and occupied a specific theater of operation, that may have worked, but even then, it was just barely adequate enough to accomplish the mission.

So what’s changed? The truth is that the U.S. has come through a long dormant period — nearly 30 years following the end of the Cold War — in which we as a nation got complacent while the events in the world were changing rapidly around us. The events of 9/11 and in Iraq were a shock and a wake-up call. Since that time, there has been a major shift in military strategy. The primary emphasis is no longer on watching our borders for approaching missiles or bombers. We now are faced with a new enemy — terrorists who hide in the dark, and pop up anywhere and everywhere on a global basis.


Our military is now in a rapid transformation to become a lighter, more agile, mobile, and more lethal force. To accomplish this, many technical and logistical problems must be solved; however, simply throwing money at a problem doesn’t always produce results. Even today the old engineering adage, “the probability of obtaining an answer to an undefined problem is zero” still applies.

Within the realm of this “military transformation” problem definition, there are opposing, or divergent, forces; i.e., to become more lethal and betternetworked involves new weapons systems and more sophisticated electronic systems, both of which demand more DC and AC power. How do you get more power? With larger, heavier generators. That makes the “lighter” and “more agile” components of the “transformation” equation harder to achieve.

The demand for fuel on the battlefield has doubled in the past ten years, and the demand for tactical power has quadrupled during that same time period. Should that be a surprise to us? Isn’t it logical to conclude that the demands of the war effort have resulted in advanced technology in the form of sophisticated electronic systems, improved weapons systems, state-of-theart field medical equipment, and elaborate networked Tactical Operations Centers (TOCs)? These all put a strain on the power systems currently deployable to the battlefield. In looking forward at what might be another 20 to 40 years of anti-terrorist military engagement, it’s easy to say that the fuel and power requirements on the battlefield will experience the same levels of increase over the next ten years.

So how does this monumental problem get solved? The best way is to break it up into smaller pieces. Identify the chunks that can be addressed with today’s technology and begin to deploy not all, but some portion, to the battlefield. Then, as technology allows, focus on tomorrow’s solutions.


OBVP Requirements

The first thing that can be done to lead to tomorrow’s solutions is to focus current efforts on the largest installed base of wheeled vehicles — the different variants of HMMWVs in operation by the joint military services. The large majority of the rolling stock of HMMWVs today contains a 28 VDC electrical system.

There is a move to possibly increase this to 42 VDC, 100+ VDC, and beyond in new-to-market models currently being designed and tested. However, a nice, clean, simple retrofit could result in an upgrade of the existing fleet to include OBVP in the range of 8 to 10 KW. While we would prefer 30 KW, it has been estimated that between 70 and 80 percent of the diesel generators in the military inventory today are in the 10-KW power range. So addressing this segment of the HMMWV installed base provides an interim step on the way to the more powerful systems planned for tomorrow.

A power conversion electronics unit for an OBVP system.
By providing the HMMWV with a 7- to 8-KW OBVP system supplying continuous AC power, the need to trailer a diesel generator is reduced significantly. In addition, now the HMMWV can negotiate terrain that it could not previously engage while towing a generator, thus leading back to the more agile concept.

What kind of AC power is required? Since the missions vary between the branches of service and even within the same branch of service, the AC power requirements vary considerably. In many cases, a simple conversion from the vehicle’s 28 VDC source to single-phase 120 VAC, 60 Hz is adequate. In that regard, a number of companies are providing this inverter technology; however, the demands for power on the battlefield often exceed this level of sophistication. Therefore, the challenge is designing an OBVP system that meets multiple mission demands, regardless of the branch of service. This is by no means a minor task. There are simply times when a “beefier” 120/208, 3-phase VAC is needed.


Regardless of whether the soldier needs single-phase 120 VAC or 3-phase 208 VAC in Europe where 50hz is prevalent, or if he’s on the flight line or a radar installation where power at 400 Hz is the frequency of choice, the OBVP system should be able to provide all of the frequency options. With sophisticated communications, computers, and other specialty electronic systems, Total Har monic Distortion (THD) becomes important since harmonic distortion emitted by the AC power source can render this type of equipment inoperable. Therefore, a low THD is essential to completing the desired operation.

As one reviews the specifications for an OBVP system to be installed on 28- VDC vehicles, there are certain performance minimums that become evident, as they relate to the output power, output voltage, input voltage, THD, and output frequency. Physical characteristics and mechanical dimensions tend to vary from vendor to vendor such as length, width, height, weight, and efficiency (heat dissipation) of the chassis.

The following is a list of these parameter requirements:

  • Power Level: 10 kw peak, 7 Kw continuous
  • Output Voltage: 120/208 VAC (singlephase and 3-phase)
  • Input Voltage: 24-28 VDC
  • THD: Less than 5%
  • Output Frequency: 50, 60, or 400 Hz (selectable)
  • Dimensions: As small as possible
  • Weight: As light as possible
  • Efficiency: As high as possible

Among the major challenges in developing a 10-KW OBVP system — especially a 208-VAC 3-phase version from a vehicle’s 28-VDC electrical system — is the internal power dissipation of the converter itself, without creating a heat problem within the enclosure. There fore, cooling is a major challenge, especially if the goal is to conductively cool the system so as not to require forced air through fans.

To obtain the necessary 28 VDC 400 amps, the HMMWV must either already be equipped with a 400-amp alternator or be retrofitted with one. While most HMMWVs today are equipped with 200- amp alternators, this is easily resolved since the Army has 400-amp, retrofittable alternators in their inventory. Once the 28-VDC 400-amp alternator power source is installed in conjunction with the 10-KW OBVP power converter system, there is the problem of keeping the alternator running at an rpm that is adequate to supply the current to the power converter system, which in turn will satisfy the AC electrical load as well as provide a continuous positive charge to the array of batteries. To accomplish this, an engine speed control system must become an integral part of the overall OBVP solution by increasing the engine rpm as the output load demand increases. Likewise, as the load is removed, the engine rpm must return to idle.


Once the alternator, battery package, and speed control have been defined, the actual design concept or method of converting the 28 VDC to 120/208 VAC (single-phase or 3-phase), 50-, 60-, or 400-hz selectable power can be chosen. The most common form of switch-mode DC to AC conversion is characterized by a DC link; however, the use of the DC link has limitations imposed by the DC link itself.

Another method involves high-frequency AC links that directly convert a higher frequency AC voltage to a lower frequency AC voltage. This involves the rectification and impulse excitation of a low-pass filter with integral half-cycle sinusoidal or quasi-sinusoidal voltages originating from a source of high-frequency AC voltage. While both represent solutions, the optimum solution to the 28-VDC to high AC voltage multiphase requirements of today’s OBVP system involves a variant of these existing technologies.

Through a series of events that spanned a number of years of research in the field of OBVP, Diversified Technology developed an OBVP system concept that meets the specifications addressed in this article. The system includes a power conversion electronics chassis that is shock-mounted inside a ruggedized, waterproof enclosure.

Developing an OBVP System

The original system definition for the OBVP program targeted the creation of compact, mobile power generation to be used for the Sentinel Radar System in stationary mode applications. As our development continued, additional applications were realized due to the system’s functionality and multiple mounting platforms. Mobile power became even more of a desire; therefore, the ability to produce power on the move was incorporated into the unit. More urgent applications arose for OBVP with the recent aftermaths of Hurricane Katrina and this summer’s Hurricane Gustav. These include the capability of supplying power to ravished areas in dire need as it relates to first responders, health care facilities, water purification centers, communications centers, and gas/fueling centers. Using the capabilities of the OBVP system, instant power could be realized in any area where a HMMWV could maneuver. National Guard first respond ers would have the tools necessary to provide instant relief to those needing it most.


On-Board Vehicle Power

More and more programs require additional power generation capabilities. Future development of the OBVP program will target higher-power- generation capabilities. Using the 10-KW system as the framework, higher power levels can be achieved by utilizing additional DC generating techniques for tactical wheeled vehicles. The focus is to make this technology available for already established vehicles currently in use, as well as for new programs under development as part of the vehicle integration package to power more advanced technology for immediate requirements in the war against terrorism.

The development work for our program focused all its efforts in providing a new solution never before seen by the armed forces, and supplying a modular approach of power generation in a smaller, smarter, and durable concept. This method of power generation provides the capability of achieving power levels only seen in the past by the utilization of a trailer-towed approach. The compact nature of this design allows for installation that does not affect cargo space needed for mission requirements. Being waterproof and not trailer-towed, the HMMWV now can operate under preferred mission requirements without having to modify its mission due to limitations of equipment deployment. This provides the solider with more options in accomplishing objectives without having to jeopardize safety based on limits imposed by the previous solutions. Flexibility, mobility, portability, and durability were the main points of design during development.

The military hierarchy is moving to become a lighter, more agile, mobile, networked lethal force through a modernization effort within the Pentagon, known as “transformation.” The profile of our enemy has changed from the more traditional adversaries to those threats imposed by the far more elusive terrorist movement that is global in nature.

This article is an effort to break down the technological challenges of providing tactical OBVP to the battlefield into levels of difficulty, thereby resulting in a real-time solution for the 10-KW and under systems, while acknowledging that there is intense research being conducted to attain the higher power demands of the future.

This article was written by Keith Varner, OBVP Program Manager, at Diversified Technology, Inc., Ridgeland, MS. For more information, click here.