Sizing Generators For Motor Starting: A Practical Guide to Understanding How Motor-Starting Loads Affect Generator Performance

Today’s standby power loads are more complex than ever before. In many industrial and commercial applications, standby generator must supply power to a mixture of linear and nonlinear loads in addition to large motor loads that cycle on and off. Of all the diverse loads a standby generator must supply, applications with motors present the most sizing issues. The dynamic interactions of motors and generators–along with the impact of motor starters, system inertia, motor loading, frequency dip, generator preload and nonlinear loading–make manual generator sizing difficult, if not impossible.

Not only is sizing an application with large motors complex, but different generator manufacturers have different approaches for specifying a standby power system that will function reliably. Each major generator manufacturer has created generator-sizing software to help with this complex task, but due to manufacturers’ differing approaches to motor starting, this software can yield quite different results—sometimes specifying a larger and more expensive generator or too small a generator than is necessary for reliable operation. The purpose of this article is to explain how motors affect generator performance and how sizing software, such as Kohler Power Solutions Center, deals with motor loads. Armed with this understanding, power system specifiers will be able to select the most cost-effective and reliable generator for motor-starting applications.

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