Overloads on generators

[electrofelon]

I got into a discussion about what exactly happens when a generator is overloaded. Say your typical generac standby generator for example. Considering the breaker on the generator is usually next size up from the rated current, and the trip time curve of the breaker, I am not sure the breaker would ever trip. Do generators typically have protection built-in that will shut them down on overloads? If so is this based on current or frequency?

 

[drcampbell]

One limiting factor is the amount of torque the engine can deliver.

When the genset is overloaded, the engine will slow down. The reduced speed means reduced voltage, and reduced current, and a reduced torque demand. More torque is available at a lower speed and it will usually find a new equilibrium if the overload isn't severe. But if that continues to the left side of the torque peak, (also known as "breakdown torque") the engine will stop.

Magnetic saturation of the generator laminations may occur first, in which case the engine will keep running but the output voltage will plummet.

Either way, the available fault current is a small fraction of what's available from a utility transformer, even in the absence of any protective devices.

(this illustration is not of a small genset engine, so look at general trends, not specific numbers)

 

[electrofelon]

One limiting factor is the amount of torque the engine can deliver.

When the genset is overloaded, the engine will slow down. The reduced speed means reduced voltage, and reduced current, and a reduced torque demand. More torque is available at a lower speed and it will usually find a new equilibrium if the overload isn't severe. But if that continues to the left side of the torque peak, (also known as "breakdown torque") the engine will stop.

Magnetic saturation of the generator laminations may occur first, in which case the engine will keep running but the output voltage will plummet.

Either way, the available fault current is a small fraction of what's available from a utility transformer, even in the absence of any protective devices.

(this illustration is not of a small genset engine, so look at general trends, not specific numbers)
View attachment 2556728

True, but it is highly unlikely that the engine will max out at the exact rated current of the unit. Presumably there is some overhead built in to cover different environmental conditions, engine size increments, excetera. This value will certainly very from generator to generator but say it's 10%, will the generator controls let me run the generator at 110% power?

Also just to be clear I'm not talking about available fault current. The question is do most generators have controls to prevent operation above rated current?

 

[kwired]

Question is how long can generator take a certain amount of overload - which the trip curve of a device could still provide reasonable protection if they are coordinated properly, just like we often have a certain amount of tolerable overload with motors.