If we increase the excitation voltage, the generator terminal voltage will also increase because we are creating stronger magnetism. The greater the excitation voltage, the greater magnetism, the greater output voltage. If your generator has no load, definitely there will be no current flowing on the generator terminal. If you have load, of course the current will decrease as you increase the voltage. Please refer to ohms law.
Ok thats fine. Ohm's law explains it. But I came across a case where Gen increasing winding temp. was a problem so it was advised to keep MVARs up to a certain limit. Intention was obviously that lower the excitation voltage(lower MVARSs), lower the voltage at Gen. terminals & hence lower the current in Gen. winding? Please comment
> Ok thats fine. Ohm's law explains it.
> But I came across a case where Gen
> increasing winding temp. was a problem
> so it was advised to keep MVARs up to a
> certain limit. Intention was obviously
> that lower the excitation voltage (lower
> MVARSs), lower the voltage at Gen.
> terminals & hence lower the current in
> Gen. winding? Please comment
Controlling Var will control pf which in turn will control Ampere, which results in Winding temp control
Depends on your situation. Are you connected to the grid? in that case you are NOT decreasing the voltage as this one is fixed by the grid itself.
As you are talking about decreasing voltage at your generator terminals it seems that you are with isolated generator supplying single or multiple load(s)and in that case you are not the "master" of the VAR (the load is fixing the var on the line) and the only possibility you have to decrease the winding temp is by decreasing the voltage but this is not a very good practice as you should supply nominal voltage to your loads
If you have some pbs with your windings temp with isolated gen and load seems to say that you gen is underated to supply such a load
If you are on a grid of course you should not exceed var rating of the gen but you should be able to go up to the maximum rated var if not that's mean you gen also have a problem
by the way most of the power (and therefore winding temp) should be more on the Watt than on the Var. What about watts on your gen?
No controlling var control only part of the PF what about the Watts? they are also part of the equation!
To be able to control var you need to be on infinite grid but vassy is talking about decrease of gen voltage which is not possible on grid therefore I think he is with isolated gen and load and so you cannot control var in that situation, watt and var are fixed by the load!
I want to weigh in with my point of view...large hydrogenerator connected to grid...With a generator online you can control your terminal voltage with your exciter, this changes Var output...frequency is fixed with a synchronous machine to the grid...real power output is controlled by the prime mover (gov.).
The capability curve of a gen has three parts...MWs only (zero vars) limiting factor is prime mover capability or overheating of the stator winding by pumping too much current.
Negative Vars/Under excited the end windings will overheat but you will probably run into the under excitation limiter first (and could start slipping poles).
Overexcited/positive Vars you will overheat the rotor
Vassy... there is no "one size fits all" answer to your question. The terminal-voltage and armature-current response (remember the ONLY TWO electrical variables a generator can deliver) depends on the nature of the load, i.e, its equivalent circuit representation. It could be resistive, inductive, constant impedance, constant kVA, or any coincident combination!
Another factor is the configuration of the transmission system between the generator terminals and the "load" it supplies.
To put it simply, there is no way to give you an adequate answer until you provide generation and load details!
An analogy to your query's answer is the question, "What did one elephant say to the other elephant, while floating on their backs?"
> Ohms law would says exactly the opposite: voltage increase current
> increase for the same load
The Ohms law formula you are referring to only applies to purely resistive loads, and does not account for Inductive reactance XL or Capacitive reactance XC.
Unfortunately, in AC power production and AC generators the formulas have more variables, and become a little more complex.
