Thank you for your reply. But as you say above,as we increase the prime mover's input to increase generator's speed,it advances the rotor's position,causing eg, to lead vt.so my question is that, how eg lead vt as we increase the speed.

From Shvetang dave and karan ray

 

Sometimes it's very easy to forget, but the purpose of generating electricity is to convert torque into an easily distributed "medium" which can then be just as easily re-converted back into torque.

That's what a generator does--it converts torque into amps. The generator can be connected to a motor a great distance away using wire and the prime mover can convert it's input (fossil fuel, steam, water, wind, etc.) into torque which is converted to electricity which can power the motor which converts the electricity back into torque.

The "complicating" factor for many people is the concept of frequency and speed when talking about an AC generator, more rightly called an alternator. On an "infinite", or very large, AC grid there is so much inertia and so many generators that any one generator and its prime mover cannot have any appreciable affect on the grid frequency (unless we're talking about very large nuclear power plants or fossil power plants, which have generators on the order of 1,000 MW).

Because the frequency on an AC grid should be fairly constant, the speed of the generator rotors and the prime movers should be fairly constant. Therefore, increasing the energy input to the prime mover would TEND to cause the prime mover and generator rotor speed to increase--but the grid frequency is keeping the synchronous generator's speed constant. The increased torque results in an increases "twist" on the coupling shaft between the prime mover and the generator rotor, which results in a change in the relationship between the magnetic field of the rotor and the magnetic field of the armature. This twist is trying to force the generator rotor to turn faster than it is turning--trying to pull the rotor out of synchronism. As long as the excitation current is sufficient this will not happen, but the physical twisting and the interaction of the magnetic fields as a result of the twisting is one of the factors contributing to the phenomenon you are wishing to have described.

Any physics text or basic electricity text will cover these princicples and "armature reaction" in some detail. It's extremely difficult to explain these concepts in this forum because of the inability to draw diagrams or post pictures, so if you want more explanation you'll need to visit your library.

Also, since you have Internet access you can use any search engine to look for keywords such as armature reaction and overexcited and underexcited and synchronous generators and alternators, etc. www.wikipedia.com has links to other sites from many of their articles which are very useful.

markvguy

 

Responding to Shvetang's and Karan's Sep 2 query.... "one picture is worth a thousand words!"

Please send me your email address, so that I can forward a sketch of the vector relationships.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Read various explanations and noted it raised in turn more questions.

Please note following in case of synchronous alternators:

A lagging (Reactance) load current has demagnetising effect on main field. (you must have observed increase in field current with increase in reactive load).

A leading load (capacitance) has magnetising type armature reaction on main Field.

A pure resistive load neither has magnetizing or de-magnetizing effect but distorts the field (increase in MMF in one half of pole pitch and decrease in MMF in other half of pole pitch --Net increase or decrease in MMF being Zero).

If effect of capacitive load on MMF is understood above, it means if we increase capacitive loading on synchronous alternator will result in increase in voltage, for the set voltage on AVR thus AVR will reduce excitation.

Above means reduction in DC current in main Field with increase in capacitive loading-- end result is that main rotor turns in to mere piece of iron in stead of magnet.

Another aspect in synchronous machine is to note that the Three phase load current is equivalent to Single phase current rotating in direction of rotation of main field and same speed as synchronous speed.

Therefore in synchronous alternators there are two magnetic fields, one due to stator current and another due to main Field. Both rotating in same direction and same speed.

If we loose MMF in rotor due to capacitive type of load current, there will be practically only one MMF or magnetic field in air gap.

As a result of above, rotor is no more locked with imaginary stator MMF and starts hunting (slipping backward and forward. This results in induction of eddy currents in main pole and thus heating. As a result the load carrying capacity of machine is reduced - proportional to capacitive load. Roughly the capacitive loading capability of salient pole synchronous rotor is 30% of 0.8 PF nominal rating.

I am not sure but capacitive capability of Solid/single piece cylindrical rotors (typical of turbo alternators) must be lower than laminated salient pole with damper cage.

Thanks for asking this question on net, which brought my memories back of what I studied in my Engineering classes between 67 and 71.

Regards,
Ramesh

Power Services
Power solutions
India
[email protected]

 

Perhaps you can get the answer for this if you draw the vector diagrams. Draw the vector diagram for generator operating at lagging power factor. Now draw the flux vectors. Call phi as the flux and represent as F. Draw Fg(field flux) from origin which is perpendicular to Eg. Now draw Fa which is parallel to Ia from the end of Fg. Now draw a straight line from origin which is perpendicular to Vt and it intersects Fa. So you see that armature reaction flux is opposing the field flux. Similar approach is for the later part of your question.

 

In most of the text books it is described that a turbogenerator is absorbing MVARS from the system when operating in leading zone of the generator reactive capability region. Generator is designed to provide power to the system and not to take power from the system. If someone can kindly explain this phenomenon to me.

Again in this thread a lot is mentioned about under-excited operation of a turbogenerator. Most of the textbooks have a mention as many authors have explained that in the lagging region, the stator flux directly opposes the field flux whereas in the leading region the stator flux assists the rotor field flux. Hence the terminal voltage is increased. The vectorial diagrams prove what it is said in the text books. But what happens physically?

As one of the authors has explained like this :-When one reduces the excitation on the rotating magnetic field, the flux field of the rotor "shrinks". Which allows the flux field of the armature to "expand." The flux distributions get "disturbed" when the excitation is reduced too much, and this is what causes the concentrated heating which leads to the unwanted growth (things grow with heat!) which causes unwanted things to happen.

Agreed that the rotor flux "shrinks", but the magnitude of the flux on the stator depends on the field strength, so how does the armature flux "expand". If the author can kindly explain this I shall be grateful. The latter part will be clear then, that if there is expansion of flux, there will be core saturation, flux fringing and hence end winding heating.