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Power is a general requirement on most job sites. From drills and saws to such support equipment as compressors and heaters, most jobsites need a little help from a portable generator. But with all the makes and models available, customers may struggle to know which is best for their site.


March 30, 2011
By Marc Leupi Product Manager Utility Wacker-Neuson Corporation

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When adding up total wattage for tools the generator will be powering, don’t forget to factor in such support equipment as light towers.


 

Power is a general requirement on most job sites. From drills and saws to such support equipment as compressors and heaters, most jobsites need a little help from a portable generator. But with all the makes and models available, customers may struggle to know which is best for their site.

Generator selection can be a bit tricky, so it is up to you, as the rental centre operator, to provide the necessary guidance and help the customer make the right choice. There is more to the task than just recommending the largest, most expensive model, as this often just ends up wasting money and fuel. On the other hand, if customers think they can always get by with the smallest and cheapest option, their experiences will likely end in frustration, inefficiency and, ultimately, unhappy customers.

Powering up
Your first step is to take a look at the generator’s use to determine size requirements. First, have the customer list all the equipment the generator is likely to power. Do not forget to factor in such supportive equipment as compressors and even computers and radios. Note which pieces will not need to run at all times – planning this out can help maximize the generator’s efficiency. It is also wise to list tools in the order they will be started, as this will further help maximize efficiency.

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Next, the customer will need to include each piece of equipment’s running wattage requirement. This number is usually located on a tool’s tag or nameplate. Some tools may list power requirements in volts and amps; if this is the case, wattage can easily be calculated by multiplying volts and amps. In addition to the running wattage, another number key to selecting an adequate-size generator needs to be considered: starting wattage.

Motor-powered tools often require more wattage to start than they do to run. Starting wattage can be double or even five times a tool’s running wattage. Many operators do not know if their tools even have a starting wattage requirement, but it is crucial to note this number and factor it in when looking at generator size in order to select a generator capable of handling this high, one-time surge of power. After all, if the generator doesn’t have the ability to start the tool, running it is irrelevant.

Keep in mind that all the tools will probably not need to run at the same time, nor will they all be started at the same time. Although it may take some thought and planning, taking the time to think through the process will save the customer from selecting an oversized generator.

It is not necessary to take the total of the equipment’s running wattages and starting wattages. Because each piece will be started separately, they will never all be drawing starting wattage at the same time.

The chart below shows how a typical contractor could apply this concept to his or her individual jobsite. In the example, each piece of equipment is listed in the order it will be started, with both starting and running wattage numbers.

chart

First, determine the continuous running load. Total up all running wattages, and arrive at 5,050 watts. Next, factor in 10 per cent to allow for error, or the occasional extra piece of support equipment. The total is 5,555 watts.

Next, starting wattage needs to be considered, and the worst-case starting load scenario must be determined. Because the tools will be started separately, only the tool with the highest starting wattage – the saw at 3,500 watts – needs to be factored in. Replace the saw’s running wattage with the starting wattage from the previous calculation, and arrive at a new total of 6,050 watts.

From here, the worst-case surge scenario can be determined. Most quality, non-inverter generators can surge to 1.5 times the continuous run rating to start a motor. So take that worst-case starting scenario, 6,050 watts, and divide the number by 1.5 to arrive at 4,033 watts.

Be sure to recommend a generator that meets both the continuous running load and surge start scenario. The continuous run number of 5,555 watts is higher than the surge starting number of 4,033 watts, so a generator that produces at least 5,555 watts will be the best choice. Coincidentally, the 5,000-watt (continuous output) generator is by far the most popular size with contractors.

For your consideration
Starting wattage often plays the biggest role in generator requirements and some models include features specifically designed to handle these surges in power. A high-quality alternator is the key to ensuring the generator can handle starting wattage requirements. A customer should look for a generator with an alternator featuring a separate excitation winding and an automatic voltage regulator (AVR). These features work in conjunction to deliver additional power to the rotor, which aids in starting a heavy load.

An AVR system offers an additional benefit, in that it allows a standard generator to deliver cleaner power – meaning consistent voltage and frequency (60 hertz). This is often crucial for such equipment with electronic controls as computers, battery chargers and tools with timers or integrated electronic motor controls. This system is usually found only on more expensive, high-quality generators due to the cost of these features.

Many jobsites, especially residential areas and sites operating at night, may have noise restrictions. In these cases, find out the maximum decibel level allowed and keep this in mind during selection, as certain features will contribute to quieter operation. For example, all generators will include a muffler, but many lower-quality models will offer simple stock mufflers. So-called low- or side-mount mufflers will provide significant sound reduction. Additionally, a low-mount air cleaner will further reduce noise levels. Both of these features are typically found on higher-quality models from reputable manufacturers.

It is also important to assess portability needs. Many construction-grade generators are equipped with large tires to enable easy transport, often including wheelbarrow-style handles as well. Some models incorporate a central lift point for enhanced transport capabilities, enabling the unit to be lifted via a sling or chain by a crane, telehandler or other mobile piece of equipment. Regardless of how the unit is moved, remind every customer that a generator must always be shut down and given ample time to cool off before it is touched or moved.

Beyond standard generators, another type of portable generator exists and offers its own unique features and benefits. Depending on individual requirements, an inverter generator may be the best choice.

Same but different
Inverter generators differ from traditional jobsite portable generators in several important ways. First, inverters are, by design, variable speed generators whereas standard generators run at a fixed rpm (usually 3600 rpm to create 60 hertz power). Inverter generators use electronics and a small flywheel generator closely coupled to the engine to create usable power. This gives them several big advantages: very clean power (precise frequency and voltage output), lower noise and better fuel economy when running variable loads (the engine only runs to the speed needed to run the load). Since the big, heavy alternator of a traditional generator is replaced by a much smaller, integrated flywheel generator, inverters are usually smaller and lighter. Without the large alternator however, inverters lack the impressive reserve power capacity of a high-quality standard generator necessary for starting such larger tools as compressors and pumps.

Inverters are especially suited for areas where decibels cannot exceed a particular level. They are often fully enclosed, further reducing sound emissions. While high-quality standard generators are able to produce clean power, inverters really shine here because they use microprocessors to convert the raw power output of the alternator into a nearly perfect sine wave and precise voltage. This gives them an edge in powering certain types of very sensitive electronic equipment. However, most types of construction grade electrical tools run just fine on quality, standard generators.

Finally, inverters are typically smaller and lighter than standard generators, and therefore great for contractors who must be very mobile or need to carry their power to remote locations – another reason why inverters are so popular with recreational users.

In the end, it doesn’t take an expert to properly select the right generator for a jobsite, but it does take a bit of planning, prior knowledge and thought to make the most economical decision. Educating a customer on how to properly select the right generator will ensure the best choice is made, benefiting both the customer and the rental center. After all, happy, satisfied customers are often repeat customers.


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