1. Voltage Regulation

2. Armature Reaction Effect of the armature flux on the main flux.
Three phase current in a three - phase winding - a rotating magnetic field produced (MMF = 1.5 Im Tph).
UPF cross magnetizing.
Lag PF - demagnetizing.
Lead PF - magnetizing.

3. Armature Leakage Reactance(XL)
Three major components -Slot leakage
reactance, end winding leakage reactance
and tooth tip leakage reactance.
Synchronous reactance/phase
Xs = XL + Xar, where Xar is the fictitious
armature reaction reactance.
Synchronous impedance/phase
Zs = (Ra + jXs ).

4. Open-circuit test The generator is turned at the rated speed
The terminals are disconnected from all loads, and the field current is set to zero.
Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information +
Vdc If Vt

5. Short-circuit test
Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters.
Record the armature current Isc as the field current is increased.
Such a plot is called short-circuit characteristic. A +
Vdc If
Isc

6. DC Test
The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals.
The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings
then
If the stator is Y-connected, the per phase stator resistance is
If the stator is delta-connected, the per phase stator resistance is

7. Determination of Xs
For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc.
Then the synchronous reactance Xs could be obtained using
IfA
Ef or Vt (V)
Air-gap line
OCC
Isc (A)
SCC
If (A)
Vrated VA
Isc,B
Isc, A
IfB
: Ra is known from the DC test.
Since Xs,unsat>>Ra,

8. Xs under saturated condition
IfA
Ef or Vt (V)
Air-gap line
OCC
Isc (A)
SCC
If (A)
Vrated VA
Isc,B
Isc, A
IfB
At V = Vrated,
: Ra is known from the DC test.
Equivalent circuit and phasor diagram under condition Ia Ef
Vt=0
jXs Ra + Ef
Vt=0
jIaXs
IaRa Ia

9. Short-circuit Ratio
Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
Ef or Vt (V)
Air-gap line
OCC
Isc (A)
SCC
If (A)
Vrated
Isc,rated
If_V rated
If_Isc rated

10. Voltage Regulation of Alternators

11. Methods Of Predetermination Of Regulation
Synchronous impedance method (EMF
method)
Magneto Motive Force method (MMF
Zero Power Factor method (ZPF method)
American Standards Association method
(ASA method)

12. Synchronous Impedance Method (EMF Method)
OC and SC tests conducted.
Zs is calculated.
Ra measured and Xs obtained.
For a given armature current and power
factor, Eph determined - regulation is
calculated.

13. Magneto Motive Force Method (MMF Method)
OC & SC tests conducted.
field currents If1 (field current required to produce a voltage of(Vph + IaphRacos Φ) on OC) and If2 (field current required to produce the given armature current on SC) are added at an angle of (90± Φ).
For this total field current, Eph found from OCC and regulation calculated.

14. Zero Power Factor Method (ZPF Method)
OC test and ZPF test is conducted – characteristics are drawn . This is Potier triangle method
From this triangle the potier reactance (leakage reactance of the alternator), XLph is obtained.
The terminal voltage and the leakage reactance drop added vectorially - load induced EMF found.

15. Zero Power Factor Method (ZPF Method)
For this load induced emf, the corresponding field current If1 obtained from OCC.
The field current If2 required to balance armature reaction obtained from potier triangle.
If1 and If2 are added at an angle of (90± Φ). For this total field current, Eph found from OCC - regulation calculated.

16. American Standards Association Method (ASA Method)
The field currents If1 (field current required
to produce the rated voltage of Vph from the
air gap line).
If2 (field current required to produce the
given armature current on short circuit)
added at an angle of (90± Φ).

17. American Standards Association Method (ASA Method)
Load induced EMF calculated as was done in the ZPF method - Corresponding to this EMF, the additional field current (If3) due to saturation obtained from OCC and air gap line - If3 added to the resultant of If1 and If2 -For this total field current, Eph found from OCC and regulation calculated.

18. Example 1
A 200 kVA, 480-V, 60-Hz, 4-pole, Y-Connected synchronous generator with a rated field current of 5 A was tested and the following data was taken.
from OC test – terminal voltage = 540 V at rated field current
from SC test – line current = 300A at rated field current
from Dc test – DC voltage of 10 V applied to two terminals, a current of 25 A was measured.
1. Calculate the speed of rotation in r/min
2. Calculate the generated emf and saturated equivalent circuit parameters (armature resistance and synchronous reactance)