Voltage Regulation
The voltage regulation of an alternator is defined as the change in terminal voltage
from no-load to full-load (the speed and field excitation being constant) divided by
full-load voltage.
Note that E0 V is the arithmetic difference and not the phasor difference. The
factors affecting the voltage regulation of an alternator are:
(i) IaRa drop in armature winding
(ii) IaXL drop in armature winding
(iii) Voltage change due to armature reaction
Determination of Voltage Regulation
The kVA ratings of commercial alternators are very high (e.g. 500 MVA). It is
neither convenient nor practicable to determine the voltage regulation by direct
loading. There are several indirect methods of determining the voltage regulation
of an alternator. These methods require only a small amount of power as compared
to the power required for direct loading method. Two such methods are:
1. Synchronous impedance or E.M.F. method
2. Ampere-turn or M.M.F. method
For either method, the following data are required:
(i) Armature resistance
(ii) Open-circuit characteristic (O.C.C.)
(iii) Short-Circuit characteristic (S.C.C.)
(i) Armature resistance
The armature resistance Ra per phase is determined by using direct current and the
voltmeter-ammeter method. This is the d.c. value. The effective armature
resistance (a.c. resistance) is greater than this value due to skin effect. It is a
usual practice to take the effective resistance 1.5 times the d.c. value (Ra = 1.5
Rdc).
(ii) Open-circuit characteristic (O.C.C)
Like the magnetization curve for a d.c. machine, the (Open-circuit characteristic
of an alternator is the curve between armature terminal voltage (phase value) on
open circuit and the field current when the alternator is running at rated speed. Fig.
(10.20) shows the circuit for determining the O.C.C. of an alternator. The
alternator is run on no-load at the rated speed. The field current If is gradually
increased from zero (by adjusting field rheostat) until open-circuit voltage E0
(phase value) is about 50% greater than the rated phase voltage. The graph is
drawn between open-circuit voltage values and the corresponding values of If as
shown
Short-circuit characteristic (S.C.C.)
In a short-circuit test, the alternator is run at rated speed and the armature terminals
are short-circuited through identical ammeters [See Fig. (10.22)]. Only one
ammeter need be read; but three are used for balance. The field current If is
gradually increased from zero until the short-circuit armature current ISC is about
twice the rated current. The graph between short-circuit armature current and
field current gives the short-circuit characteristic (S.C.C.) as shown
There is no need to take more than one reading because S.C.C. is a straight line
passing through the origin. The reason is simple. Since armature resistance is much
smaller than the synchronous reactance, the short-circuit armature current lags the
induced voltage by very nearly 90°. Consequently, the armature flux and field flux
are in direct opposition and the resultant flux is small. Since the resultant flux is
small, the saturation effects will be negligible and the shortcircuit armature current,
therefore, is directly proportional to the field current over the range from zero to
well above the rated armature current
Synchronous Impedance Method
In this method of finding the voltage regulation of an alternator, we find the
synchronous impedance Zs (and hence synchronous reactance Xs) of the alternator
from the O.C.C. and S.S.C. For this reason, it is called synchronous impedance
method. The method involves the following steps: (i) Plot the O.C.C. and S.S.C. on
the same field current base as shown in Fig. (10.24).
(ii) Consider a field current If. The open-circuit voltage corresponding to this
field current is E1. The short-circuit armature current corresponding to field current
If is I1. On short-circuit p.d. = 0 and voltage E1 is being used to circulate the snortcircuit armature current I1 against the synchronous impedance Zs. This is illustrated
in Fig. (10.25)
Drawback
This method if easy but it gives approximate results. The reason is simple. The
combined effect of XL (armature leakage reactance) and XAR (reactance of armature
reaction) is measured on short-circuit. Since the current in this condition is almost
lagging 90°, the armature reaction will provide its worst demagnetizing effect. It
follows that under any normal operation at, say 0.8 or 0.9 lagging power factors
will produce error in calculations. This method gives a value higher than the value
obtained from an actual load test. For this reason, it is called pessimistic method.
Ampere-Turn Method
This method of finding voltage regulation considers the opposite view to the
synchronous impedance method. It assumes the armature leakage reactance to be
additional armature reaction. Neglecting armature resistance (always small), this
method assumes that change in terminal p.d. on load is due entirely to armature
reaction. The same two tests (viz open-circuit and short-circuit test) are required as
for synchronous reactance determination; the interpretation of the results only is
different. Under short-circuit, the current lags by 90° (Ra considered zero) and the
power factor is zero. Hence the armature reaction is entirely demagnetizing. Since
the terminal p.d. is zero, all the field AT (ampereturns) are neutralized by armature
AT produced by the short circuit armature current.
(i) Suppose the alternator is supplying full-load current at normal voltage V
(i.e., operating load voltage) and zero p.f. lagging. Then d.c. field AT
required will be those needed to produce
normal voltage V (or if Ra is to be taken into account, then V + IaRa cos ) on
no-load plus those to overcome the armature reaction, Let AO = field AT required
to produce the normal voltage V (or V + IaRa cos ) at no-load OB1 = fielder
required to neutralize the armature reaction Then total field AT required are the
phasor sum of AO and OB1 Total field AT, AB1 = AO + OB1
The AO can be found from O.C.C. and OB1 can be determined from S.C.C.
Note that the use of a d.c. quantity (field AT) as a phasor is perfectly valid in
this case because the d.c. field is rotating at the same speed as the a.c. phasors
i.e., = 2 f.
For a full-load current of zero p.f. leading, the armature AT are unchanged. Since
they aid the main field, less field AT are required to produce the given e.m.f.
Total field AT, AB2 =AO B2O
where B2O = fielder required to neutralize armature reaction This is illustrated in
Fig. (10.27 (ii)). Note that again AO are determined from O.C.C. and B2O from
S.C.C. (iii) Between zero lagging and zero leading power factors, the armature
m.m.f. rotates through 180°. At unity p.f., armature reaction is cross-magnetizing
only. Therefore, OB3 is drawn perpendicular to AO [See Fig. (10.27 (iii))].
Now AB3 shows the required AT in magnitude and direction
Procedure for at Method
Suppose the alternator is supplying full-load current Ia at operating voltage V and
p.f. cos lagging. The procedure for finding voltage regulation for AT method is
as under:
(i) From the O.C.C., field current OA required to produce the operating load
voltage V (or V + IaRa cos ) is determined [See Fig. (10.30)]. The field
current OA is laid off horizontally as shown in Fig. (10.31).
From S.C.C., the field current OC required for producing full-load current
Ia on short-circuit is determined. The phasor AB (= OC) is drawn at an angle of
(90° + ) i.e., OAB = (90° + ) as shown in Fig. (10.31). (iii) The phasor sum of
OA and AB gives the total field current OB required. This method gives a
regulation lower than the actual performance of the machine. For this reason, it is
known as Optimistic Method.