January 2009

Harness the Power What to Know Before You Decide to Rent or Buy a Generator By Dan Thomsen

Most people take electricity for granted — just flip the switch and the power clicks on. When it comes to actually generating power (as opposed to just utilizing it), professional contractors need to ensure they use energy safely and efficiently. When finding the ideal generator, the first thing to know is that a unit can’t be too large or too small for a job — it must be perfectly sized for the application.

Think of mobile generators as mini electric substations on wheels. You will find generators utilized to power special events, fairs, outdoor sporting events, concerts and even used in television broadcasting. In construction, mobile generators are indispensable because a construction site may not have access to utility power.

Many times utility power lines have yet to be installed by the power company or cannot be installed because of the remote location of the jobsite. Even when there is utility power provided, you will typically find a generator on the jobsite. For instance, a generator can power anything from the office trailer, to the hand tools used by the tradesman, to the elevator on a high rise and even
a crane.

Rent vs. Own

Contractors may own a generator because their work is specific and power needs don’t change from job to job. However, more often than not, generators are rented based on the requirements and specifications of the job. Most general rental companies have a fleet of generators of varying capacities. Specialty rental businesses can provide a power generation specialist onsite.

A generator is a versatile piece of equipment, and it takes exactly the right size of generator to give optimal power for the applications. Too small of a generator will overload; too large of a generator will cause wet stacking.

Anatomy of a Generator

Simply put, a generator converts fuel into electricity. It works by the use of an engine (typically diesel) driving an alternator. Within the alternator is the main rotor that creates a rotating magnetic field, and the stator, which is an enclosed housing with copper windings. As the electromagnetic field rotates, an alternating electrical current is created in the stator. The electric power is delivered to the receptacles and connection points on the generator. The amount of voltage is controlled by the voltage regulator switch.

Most mobile generators output both single- and three-phase electric power. Single phase generally can handle low power applications such as a light bulb, and three phase operates higher-power appliances such as an air conditioner.

Understanding Load Requirements

Anything that requires electrical energy will have load requirements. To ensure you have a generator that meets all the needs of the construction site, it is imperative that the contractor clearly understands the wattage of all applications running from the generator. For instance, to power a jobsite trailer, the contractor will need to know the BTUs of the furnace and air conditioning units to determine the load on the generator. Common jobsite equipment such as air compressors, pumps, concrete drills and light towers all have running wattage and additional start-up wattage, which all need to be counted into the total power draw on a generator.

Anything that requires electrical energy will have load requirements. To ensure you have a generator that meets all the needs of the construction site, it is imperative that the contractor clearly understands the wattage of all applications running from the generator.

The total amount of power load should be 70 to 90 percent of the load capacity of the generator. For instance, if the total amount of application loads equals 80 kilowatts, then a generator with a maximum capacity of 100 kilowatts is a good fit.
If the applied loads running on one generator exceed its maximum load capacity, the unit will fail and all power will be lost.
If the generator’s load capacity is much larger than what is needed — underloading — the problem of wet stacking will occur.

Wet Stacking

Wet stacking is a common issue with diesel engines. A diesel engine needs to operate at least 70 percent capacity in order to maintain optimum temperature. When the proper temperature is not maintained, unburned fuel begins to gather in the exhaust, clogging the valves and fuel injectors. Wet stacking doesn’t do any permanent damage to the generator, but the engine’s torque is reduced and it cannot generate the correct kilowatts needed. To repair a wet-stacked generator it must be placed on a load bank to gradually increase the load on the generator and burn off the excess fuel. How long the generator operated underload will determine how long it will need to be on a load bank.

Proper Maintenance

Besides meeting a generator’s proper load requirements, daily and hourly maintenance checks will keep the generator at peak performance. The life of a generator can be anywhere from 15,000 to 20,000 hours, as long as the unit is properly maintained. A daily maintenance schedule needs to include oil and radiator checks, and visual inspection of all wiring. Anywhere from 250 to 500 hours, depending on the engine manufacturer recommendations, oil and fuel filters should be replaced.

As an extra safety precaution, it is smart to run a grounding rod into the land and wire the grounding rod to the generator. With proper planning and diligence, power to the jobsite will run trouble-free.

Dan Thomsen is a regional manager for Ingersoll Rand PowerSource mobile generators at Doosan Infracore Portable Power, based in Statesville, N.C.