# Explanation of "Droop".



## KRE (Nov 29, 2011)

Just so everyone understands, a rotating generator set (no matter the size) has two (2) parameters that the word droop applies to. The first is speed droop which in rotating units is engineered between 5% max to 0.05% at a steady state. This does not apply to high inrush currents that suddenly load the set. Most all mechanical governors save very few mfg's are 5% droop governors. On small units mostly covered in/on this forum they are 5% at best. This means if you have a max high idle(no load) speed of 3750 rpm when 100% loaded the engine will droop to 3562.5 rpm. Now a engine running at 3600 rpm with a 2 pole exciter rotor attached the product hertz (frequency) will be 60 which is the target for most everything US an some other places in the world. So 3600/60 means for every 60 rpm of engine speed change you will get a single (1) hertz change. This is speed droop simplified. Now for Voltage Droop. This is the percentage of change, from a no load state at rated speed, to a full loaded state, at target speed. In small gen-sets talked about on this forum this percentage of change is based upon two(2) things. The load power factor and the unit's speed. Most all small units have a volts/hertz curve which means the voltage follows the speed in a steady state. There are different parameters for larger gen-sets but most covered here are as per above. One last thing if your engine is lacking HP for what ever reason the governor will only respond to it's design spec's. The engine has to produce design HP for the governor to do it's job, which then allows the Generator end (alternator) to do it's job correctly. Even if the gen-set is preforming at 100% applying the load wrong or over loading the set do to none education of power factor, it's control and how it effects the set will result in overloading or burning up the alternator end. HTH Kenneth


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