Sizing a diesel generator for its application

Sizing a diesel generator is incredibly important, and so often—especially in the rental market—we see contractors sizing generators incorrectly, thinking they need more power than what is actually necessary.

Typically, the biggest issue when sizing a diesel generator is motor starting. Motor loads can be difficult to estimate because it takes a lot to start an electric motor. Traditionally, generators are sized based on the starting requirements of a motor. This often means it is oversized for the motor running load. What we are finding with the Tier 4 Interim and some of the latest emission engines is that they really need heat to work well. All diesel generators prefer to be running hard and be loaded heavier as opposed to carrying lighter loads. The trouble with this is, by procuring a generator that is larger than needed for the application at hand, when the motor is running, it’s not creating enough load to keep the engine running well.

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What we are finding in some instances is people guessing the sizing and going for the next size up. For example, a construction contractor may go into a rental store thinking they need a 30 kW (37.5 kVA) generator, when a 20 kW (25 kVA) generator is actually better suited for their application. If the rental store then only has a 50 kW (62.5 kVA) generator available, but offers it at the same price as a 30 kW (37.5 kVA) generator, there is further chance of oversizing. The pitfall of this is that the generator will not produce enough heat to work efficiently and effectively.

Generators are only going to create as much power as is required by the load, so essentially, someone may have ample power available but will end up using only up to what is required by the application. If they size it bigger than the generator set, the engine is not reaching its maximum potential; it’s just running lightly loaded, not building any heat or any load against it. Diesel engines run better when loaded, and it is recommended to always keep the generator at 50 percent of maximum load or higher,with a minimum 30 percent load. 50 percent load is better, and, if the operator can manage it,maintaining loads at the 70 percent range would be ideal, as the generator is working hard and producing plenty of heat, doing exactly what it should be doing.

One recommendation that will allow users to start the diesel generator with increased ease is to install a soft start or variable frequency drive on the motor, which limits the starting, or inrush current. There are machines that come standard with AREP excitation whereas others have shunt or self-excited generators. AREP excitation gives 300 percent of max current output for motor starting while a shunt or self-excited generator will typically give around 180 percent max current for motor starting. A generator can be sized more appropriately by utilising variable speed drives, variable frequency drives or soft starts on the motor.

There are many things to consider when sizing power to the application. For starters, do not oversize the generator more than what is necessary for the application. So many people that guess what size they need might actually want to side with the smaller size in order to keep the generator working harder. Sizing appropriately will help in other areas like fuel consumption as well.

Another tip is to look for a diesel generator that feature a good starting ability – anything that comes with auxiliary windings or a permanent magnet synchronous generator (PMG). Also, keep in mind the type of automatic voltage regulator. There are many kinds of voltage regulators and some are better than others at motor starting, so when purchasing or renting, make sure to see what kind of regulator the generator has equipped.

A final tip to keep the generator working hard: look at sequencing the load steps if the total load is made up of several pieces of equipment. It is important to not energize all the loads at the same time. Start with the biggest loads first and work your way down to the smallest load. Finally—and this this goes along with guessing the size—always ask a certified electrician or generator expert for help in sizing to ensure the correct generator is selected for a job.

Sizing power to the application can be difficult, but it is best to stay on the conservative side and not go too much over in size for what you need out on the jobsite. Generators are built to work hard and the more accurate the sizing is to the application, the greater results you’ll have.