Synchronous generator 2

[Ryc92]

Here's one for you...

Consider a synchronous generator, rotating field, operating in stand-alone at a given load when a sudden increase in load on the armature occurs (say a large heater is turned on). Let's say the excitation voltage/current on the field winding is held constant and the prime mover torque input is held constant (example, diesel engine with a fixed fuel input).

So at that instant, the armature current will increase right? I know the generator will slow down, but I am having trouble visualising what would be going on electro-magnetically such as the load angle between field and resultant MMF etc.

Can somebody explain to me what is going on in this transient state (I am sorry I have many books on machine theory but none seem to address this situation)

Many thanks, Ryan

 

[waross]

By the numbers:
1> Generator stable.
2> Load increases with no corresponding increase in input torque.
3> Current increases, but the excitation doesn't increase.
4> With increased current the voltage drop across the generator armature increases and as a result the terminal voltage drops.
5> The voltage drop causes a drop in current.
6> The generator slows down. With constant torque this causes a decrease in input and output power.
7> Speed will drop causing a drop in output power, voltage will drop causing a drop in the load power. This will continue until the two effects are equal. This may not occur until either the terminal voltage, the torque response or the load characteristics become non-linear.
Remember that the rotational inertia of the set will allow a momentary increase in the generator torque to allow the initial increased current.
Generators are not usually operated in this mode.


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

 

[ScottyUK]

Depending on how far into the modelling you want to go, have a look at

http://www.ece.umn.edu/users/riaz/animations/smswing.html