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Difference between an over speed governor and a constant speed governor?

If within a governor, an over speed detection/actuation mechanism is provided to trip the 
engine in case of over speeding and or rapid increase of speed then the governor is called an  over speed governor. These were the obsolete governors and commonly had fly or bob 
weights restrained by spring. When the engine exceeds a predetermined speed, the weight 
moves out to strike some form of fuel cutoff. 
Governors designed to maintain the engine speed at the set point are called constant speed 
governors. These are also referred to as isochronous governors like the ones usually installed  on COPT and main engine

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