why syn generator is slow down and finally stop in overloaded state? i think overload make load current large so stator magnetic field due to stator current would increase. and a force to follow moving rotor magnetic field would grows high.

i cant understand why it is 'slow down'.

sorry for English. i just want you to understand what i'm saying.

thanks

 

A generator is a device for converting torque into amps which are then transmitted over wires and reconverted into torque by electric motors to drive pumps and fans and such (and lights and computers (which I like to refer to as "virtual torque"). So, in a sense you can say that a generator is powering the pumps or fans or air conditioners or refrigerators that are driven by the electric motors, in addition to the lights and computers.

A bicycle is a device for converting torque into forward motion. Consider that you are asked to ride that bicycle at a constant speed. If it's just yourself on that bicycle, that's probably pretty easy. But, if a buddy were to hop on or you were asked to carry some baggage it would require more torque to maintain that same constant speed. If weight (load) kept being added to the bicycle, it would eventually slow down, even coming to a stop; you just may not have enough energy in your body to keep moving the pedals to keep the bicycle moving forward.

An AC (Alternating Current) system is very much like the analogy above. It is (supposed) to operate at a constant frequency (50 Hz in most parts of the world; 60 Hz in others). When the amount of load exceeds the ability of the prime mover (the reciprocating engines, the turbines (gas, wind, steam, hydro), etc.) then they will begin to slow down because they just can't maintain the rated frequency.

The frequency of an AC system is related to the speed at which the rotor of the AC generators are being turned by the prime movers. The speed is directly proportional to the frequency; as the speed goes up, the frequency goes up. As the speed goes down, the frequency goes down. It's as as simple as that.

So, try to think of an AC system as something that has to keep moving at a constant speed. And something has to be providing that energy (in the form of torque) to maintain that speed (frequency). If the load exceeds the ability of the prime mover driving the generator then it will begin to slow down. It's as simple as that.

The amount of load on an AC system is NOT a function of the number and capacity of the generators connected to the system, it's a function of the motors and lights and computers connected to the system. The amount of torque being produced by the prime movers driving the generators must be EXACTLY equal to the load on the system in order for the frequency of the system to remain constant.

That's quite a balancing act, isn't it?

Now, you need to use your Internet search engine of choice and start researching synchronous generators and prime movers and governors (the old-fashioned name for the prime mover control systems).

Hope this helps!