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Synchronous impedance method requires
O.C.C.
S.C.C.
Both of above
None of above
Both of above
The Synchronous Impedance Method, often called the EMF method, is used to determine the voltage regulation of an alternator. It requires both the Open Circuit Characteristics (OCC) and the Short Circuit Characteristics (SCC) to calculate the synchronous impedance (ZsтАЛ) of the machine.
The Synchronous Impedance Method, often called the EMF method, is used to determine the voltage regulation of an alternator. It requires both the Open Circuit Characteristics (OCC) and the Short Circuit Characteristics (SCC) to calculate the synchronous impedance (ZsтАЛ) of the machine.
ZsтАЛ=Ra2тАЛ+Xs2тАЛтАЛ тАФ Total synchronous impedance
ZsтАЛ=IscтАЛEphтАЛтАЛтИгIfтАЛ=constantтАЛ тАФ Calculation using OCC and SCC data
The method determines ZsтАЛ by taking the ratio of open-circuit voltage to the short-circuit current at a specific excitation current (IfтАЛ). Since the machine is assumed to operate at constant synchronous speed, the OCC provides the internal induced EMF for a given field current, while the SCC provides the short-circuit current for the same field current. Using these values, the synchronous impedance is calculated as ZsтАЛ=IscтАЛEphтАЛтАЛ.
The method is known as the 'Pessimistic Method' because it provides a regulation value higher than the actual value.
O.C.C. is plotted between E0тАЛ (per phase) and field current IfтАЛ at rated speed.
S.C.C. is plotted between IscтАЛ and IfтАЛ at rated speed with terminals short-circuited.
Synchronous reactance XsтАЛ is derived from ZsтАЛ and RaтАЛ (armature resistance).
Simple to perform and calculate
Provides a conservative (safe) estimate for voltage regulation
Does not account for saturation effects at high loads
Overestimates the voltage drop, leading to less accurate results for salient pole machines
Estimation of voltage regulation for cylindrical rotor alternators
Used in preliminary design and testing of synchronous machines
The method assumes that the armature resistance RaтАЛ is negligible or known from independent measurement.
If only one of the characteristics (OCC or SCC) were available, the impedance could not be isolated from the saturation-affected EMF.
C is correct тАФ The Synchronous Impedance method relies on the ratio of induced EMF from O.C.C. to the short-circuit current from S.C.C. at the same field excitation.
Always remember that the Synchronous Impedance method is called 'pessimistic' because it predicts higher voltage regulation than what actually occurs in practice.