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Why does the Short Circuit Characteristic of a Synchronous Generator not saturate? 
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#1
Oct2008, 10:56 AM

P: 1,780

Hi everyone
Books mention the Open Circuit and Short Circuit tests on a synchronous generator in order to determine its synchronous impedance. One observes that the Short Circuit Characteristic (plot of short circuit armature current versus field current) is a straight line all the way, whereas the Open Circuit Characteristic (plot of open circuit terminal voltage versus field current) droops due to magnetic saturation. My question is: why does the Short Circuit Characteristic (SCC) not exhibit saturation? I would be grateful if someone could offer a detailed explanation. I suspect this is due to armature reaction, but I am not very clear about this. Thanks in advance. Cheers Vivek. 


#2
Oct2008, 08:01 PM

Sci Advisor
PF Gold
P: 2,719

I think it has to do with the fact that the generator still has inertia when the short occurs.



#3
Nov1608, 01:18 AM

P: 1,780

Thanks.



#4
Nov1608, 08:04 AM

P: 219

Why does the Short Circuit Characteristic of a Synchronous Generator not saturate?
Excuse me;
I think the question is about open circuit and short circuit characteristics of generators not short circuit fault current phenomenon. The opencircuit saturation curve is obtained when driving the SG at rated speed, on open circuit, and acquiring the SG terminal voltage, frequency, and field current. The aggregated core, friction, and windage losses may be measured as the input power for each opencircuit voltage level reading. As the speed is kept constant, the windage and friction losses are constant. Only the core losses increase approximately with voltage squared. The ShortCircuit Saturation Curve is obtained when the SG is driven at rated speed with shortcircuited armature, while acquiring the stator and field currents values should be read at rated 25%, 50%, 75%, and 100%. Data at 125% rated current should be given by the manufacturer, to avoid overheating the stator. The high current points should be taken first so that the temperature during testing stays almost constant. The shortcircuit saturation curve is a rather straight line; because the machine is unsaturated during steadystate shortcircuit. Indeed, in open circuit test, the primary mmf of generator (exciting flux) shouldn't be compensated by any armature fluxes, so saturation of core due to over exciting (saturation) can be occur, but in short circuit test, armature fluxes is operating against rotor fluxes and level of flux density can be stay constant approximately.  Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electricalriddles.com 


#5
Nov1608, 08:31 AM

Sci Advisor
P: 2,751

The reason why the open circuit voltage "saturates" as field current increases is simply because the magnetic field strength is a nonlinear function of the field current (due to the nonlinear B/H characteristics of the iron). This part I'm pretty sure you already understand, the voltage is just proportional to the rate of change of magnetic flux (and hence rotational speed times flux density) so it's fairly straight forward that it should follow the same nonlinearity as the flux density. When considering the short circuit current however you should note that (neglecting resistance) if v=0 then the rate of change of total flux must also be zero. So the short circuit current must in fact rise to whatever is required to exactly oppose (that is, to demagnetize) the main field. When you think of it this way it's acting rather like a current transformer, the current required to demagnetize is simply equal to the main field winding current multiplied by the effective turns ratio. Thus its a very simple linear relationship. 


#6
Nov1608, 08:33 AM

Sci Advisor
P: 2,751

ok m.s.j beat me to it while I was typing :)



#7
Nov2308, 01:09 PM

P: 1,780

Thanks msj and uart.



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