Selecting a vehicle-mounted generator.
There are four ways to get vehicle-mounted electrical power:
* Generator sets with dedicated engines
* Under-hood power from a belted generator
* Hydraulic-powered generators that rely on the prime mover's power take-off (PTO).
On a traditional generator set, the engine has a separate fuel supply and maintenance requirements, such as oil changes. Large industrial units with self starters cost up to $5000. If they're vehicle-mounted, they may require a separate accessible compartment.
Inverters reconstruct DC (direct current) power into 60-hertz AC (alternating current) power by rapid switching using power electronics. Inverter systems work reasonably well for modest 1-to-2-kW requirements and are in broad use. For power outputs above about 2 kW, though, inverter systems may require extra battery and alternator capacity, says David Murray, director of business development for Raven Technology LLC, Brunswick, Maine.
Belt-driven generators under the hood are good for up to 5 kW. "It's not practical to mount (a generator) under the hood that's bigger than 5 kW," says Bob Fury, president of Fabco Power, Chester, N.Y. "Anything above 5 kW is too much power for the belt."
For power outputs ranging from 3 to 15 kW, you can specify a hydraulically powered generator that runs a hydraulic pump from your engine PTO. The pump in turn sends fluid to a hydraulic motor that powers the generator.
As for engine power and speed, utility trucks commonly have programmable logic control (PLC) modules that automatically set the proper engine speed when the generator is turned on. "The utility body builder or the truck dealer would program the PLC unit to control the engine speed when the generator comes on," says Leon Lease, president of Tendaire Industries Inc., Beresford, S.D.
Tendaire offers hydraulically driven generators of 2 to 15 kW in size, and belt-driven units in 1 1/2, 3, and 4 kW models. Mobile generator sets with dedicated engines can range up to sizes larger than 15 kW, but Lease says there's limited demand for them.
Raven's Murray recommends selecting a generator that produces frequency and voltage-stable output with pure sine-wave AC power. By contrast, some systems may allow frequency or voltage, or both, to vary as load or engine speed changes. Such variations are hard on motors and even harder on sensitive electronics, causing lights to dim and brighten.
RELATED ARTICLE: Hydraulic or belted units save space
Manufacturers of hydraulic- and belt-driven generators say they require less space than traditional generators with dedicated engines. Plus, hydraulic and belted units require little or no maintenance, don't need a separate fuel supply, and don't require a second emissions control system.
"The beauty of the belt- or hydraulic-driven generator is that you don't have to run a second engine," says Bob Fury, president of Fabco Power, a generator supplier.
A typical hydraulic generator is about 28 inches long by 12 inches high by 8 inches wide and "looks like a giant breadbox," says Fury. It can be mounted on the truck chassis, in a compartment inside or at the rear of the vehicle, or on the roof of the cab.
--Daniel C. Brown is a freelance writer in Des Plaines, Ill.