GANDHINAGAR INSTITUTE OF TECHNOLOGY

GROUP MEMBERS :-

Nayan Solanki (130120109055)

MITESH TANWANI (130120109056)

Het Trivedi (130120109060)

“ ACM Lab”

Topic : DETERMINATION OF VOLTAGE REGULATION METHOD OF SYNCHRONOUS

MACHINE

Guided By: Prof. Supraja Giddaluru

ContentsDefination Methods to determine voltage

regulation

1) Synchronous Impedance method

2) MMF Method

3) Zero Power Factor Method

• A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). The VR of a synchronous generator at a given load, power factor, and at rated speed is defined as

%V

VEVR

fl

flnl 100

• Where Vfl is the full-load terminal voltage, and Enl (equal to Ef) is

the no-load terminal voltage (internal voltage) at rated speed when the load is removed without changing the field current. For lagging power factor (PF), VR is fairly positive, for unity PF, VR is small positive and for leading PF, VR is negative.

What is Voltage Regulation.. ??

Voltage regulation by synchronous impedance method

To find the voltage regulation following data are required:

I. Resistance of armature or stator winding per phase

II. Open circuit characteristic

III.Short circuit characteristic.

Resistance of armature

• The resistance of armature or stator winding per phase can be determined by using voltmeter-ammeter method or by using wheastone bridge.

• The effective stator winding resistance is always greater than dc value due to skin effect.

• Ac resistance may be taken approximately 1.2 to 1.3 times the dc resistance measured.

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

+

Vdc

If

Vt

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

A+

Vdc

If Isc

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

scA

fAunsat,saunsat,s I

EVXRZ

22

22aunsat,sunsat,s RZX

scA

oc,t

scA

funsat,s I

V

I

EX

: Ra is known from the DC test.

Since Xs,unsat>>Ra,

Example 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.

a) from OC test – terminal voltage = 540 V at rated field current

b) from SC test – line current = 300A at rated field currentc) 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/min2. Calculate the generated emf and saturated equivalent

circuit parameters (armature resistance and synchronous reactance)

Solution 1. fe = electrical frequency = Pnm/120fe = 60Hz P = number of poles = 4

nm = mechanical speed of rotation in r/min. So, speed of rotation nm = 120 fe / P

= (120 x 60)/4 = 1800 r/min2. In open-circuit test, Ia = 0 and Ef =Vt

Ef = 540/1.732 = 311.8 V (as the machine is Y-connected)In short-circuit test, terminals are shorted, Vt = 0Ef = IaZs or Zs = Ef /Ia =311.8/300=1.04 ohmFrom the DC test, Ra=VDC/(2IDC)

= 10/(2X25) = 0.2 ohm

Synchronous reactance2,

2, satsasats XRZ

02.12.004.1 2222,, asatssats RZX

Ia

EfVt

j1.02

0.2

+

+

Voltage regulation by MMF (Magnetomotive Force) Method

• This method is also called Ampere-turn method• This method is based on the results of open & short circuit

tests on an alternator• For an alternator, the field MMF is required to produce the

normal voltage V on full load is the vector sum of the following

1) Field MMF required to produce rated terminal voltage on no-load

2) Field MMF required to neutralize the demagnetizing effect of armature reaction on full load

• On short circuit condition, the field MMF required to produce full load current balances the impedance drop & armature reaction.

• But since Ra & XL are very small due to S.C ,the impedance drop can be neglected.

• Hence Power factor on S.C is almost zero lagging & the field MMF are used completely to overcome the demagnetizing effect of armature reaction.

• Therefore the demagnetizing armature MMF on full load are equal & opposite to the field MMF required to produce Full load current on S.C.

•

Procedure to draw phasor diagram for finding voltage regulation

Field Current required to produce rated terminal voltage V obtained from O.C.C . This is denoted by Ifo.

Field Current required to circulate full load current obtained from S.C.C . This is denoted by Ifa.

If the alternator is supplying full load then the total MMF Is the vector sum of Ifo & Ifa.

OA= Ifo= field current required for rated voltage on load

AB= Ifa=fieald current required to circulate full load current on short circuit.

The phasor sum of Ifo & Ifa gives the total field current OB=

=

For a leading power factor load AB= Ifa is draw at angle of (90-α) and for unity power factor load, AB= Ifa is drawn at right angle.

Zero power factor method

• This method is based on the separation of armature leakage reactance drop & armature reaction effects.

• In this method, the armature leakage reactance XL is called potier reactance.

• • The following experimental data is required to

determine the regulation by this method• Resistance of armature or stator winding per phase• Open circuit characteristics (No load curve)• Zero power factor curve (Z.P.F full-load curve)

• Z.P.F is the curve between terminal voltage & excitation when the armature is delivering full-load current at zero power factor while the machine is running at synchronous speed.

• Z.P.F full load curve can be obtained:• By loading the alternator with highly inductive load &

the field current corresponding to full-load armature current at zero p.f and rated voltage is noted.

• Field current corresponding to rated current under short circuit test is noted.

• In this test, the alternator is connected to a purely inductive load through the ammeter.

• The speed of the alternator is maintained constant at its synchronous value.

• The load current delivered by an alternator to purely inductive load is maintained constant at its rated full load value by varying excitation & adjusting variable inductance of the inductive load.

• Fig shows the circuit diagram for performing Z.P.F full load test on 3-

Alternator.

• The graph relating the terminal voltage & the field current when the armature is delivering full-load current at zero p.f. is obtained

• 1)First of all, draw the o.c.c. (open circuit terminal voltage /phase versus filed current ) and also draw the air gape line, a line which is tangent to O.C.C.(no—load curve)

• 2) Draw the field current OA corresponding to full-load armature current on short circuit.

Point a is obtained from a short circuit test with full load armature current. hence OA represents the field current required to overcome demagnetizing effect of armature reaction and to balance leakage reactance drop at full-load.• 3) Draw the field current, If Ifzp at rated voltage (line XP) which corresponding to

full-load armature current at zero p.f., thus obtaining a point P on the Z.P.F full-load curve.

Point P is the field current corresponding to f.l. armature current at zero p.f Fig shows the Z.P.F full load characteristic of alternator.• 4) from point P.PQ is drawn equal to and characteristic of alternator. Q,QR is drawn

parallel to tangent (air gap line).hence point R is obtained on no-load curve, which corresponds to point P on full-load Z.P.F curve.

• Joint R to P. now PQR is a triangle. The triangle PQR is called potier triangle. This triangle is constant for a given armature current.

• Point p’ and p on the Z.P.F. full-load curve can be obtained by tracing the potier triangle as shown in fig.

• 5) draw RS perpendicular to PQ.• RS represents the armature leakage reactance drop(IXL).• PS gives field current necessary to overcome

demagnetizing effect of armature reaction at full load and

• SQ for balancing the armature leakage reactance drop RS.

•Thank you

Questions?