breaking of synchonous generator 1

[roemsen]

How is it possible to break (slow down) a synchronous generator ones he is not any longer connected to the grid? Why do some power plants have a possibility to break the synchronous generator and other don’t?

 

[dpc]

"Brake" not "break". Yes, it's possible, but what would be the purpose?

A mechanical brake could be used, but braking a large turbine-generator would generate a lot of heat. It will generally stop on its own (eventually).

 

[waross]

First remove the energy source from the prime mover. The generator will stop eventually. Quickly if it is diesel driven, slower if it is driven by a large turbine.
Some alternators will not be lubricated properly if they are left to coast for too long. Such machines will have brakes to stop them fairly quickly.
Some hydro powered machines are allowed to coast down to a fairly slow speed before the brakes are applied.
Another issue is small water leaks through the water control valve. This may cause the turbine/generator to revolve slowly without proper lubrication. The brake prevents the machine from turning slowly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

Only ones i know with a brake are generators with a clutch between the generator and prime mover (so you can run the generator as a synchronous compensator, with the prime mover shut down).

Somer I saw had a twin shaft gas turbine, and had a brake on the turbine side to prevent the power turbine windmilling due to oil drag across the clutch. These were only used at standstill, not to actually slow anything down.

 

[cranky108]

And to add to the confusion, some steam turbines must be kept turning until they are cold to prevent warping of the shaft.

 

[davidbeach]

Even beyond cold to counteract the pull of gravity. Many large generators are kept on turning gear when not running.

 

[kolo1]

During stand still, the vertical machines usually rests on the thrust bearing which has soft babbit material to prevent scratch/damage to the journals. During running, the rotor is slightly lifted above the thurst bearing due to centrifugal action. Now when the machine is to be brought under shout down, the speed gradully decreases and touches thrust bearing. To prevent thrust bearing from getting damage, the brakes are applied at 15% of the rated speed. Also when starting, the high pressure lubrication system is employed to lift the rotor before spinnig the rotor.
The brakes also serves as jacks during maintenance.