v and Inverted v Curves of Synchronous Motor

MAHA HATMAGA GAND NDHII NSTI TUTEOFTECHN HNOLOGY GANDI PET,HYDE DERAB ABAD. EEEDe De par t me nt–EMMI ILAB AB 6 Exper i mentNo:

I I I / I VEEE( ISe me me s t e r )

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V AND INVERTED-V CURVES OF SYNCHRONOUS MOTOR Aim: Aim: To draw the V and inverted-V curve of three phase Synchronous Motor at different loads.

Corollaries: 1. Obtain the characteristics of field current verses armature current and field

current verses power po wer factor.

Theo Th eory ry :With With constant constant mechanical mechanical load on the synchronous motor the variation of fiel field d curr curren entt chan chan!e !ess the the arma armatur turee curr curren entt draw drawn n by the the moto motorr and and also also its its opera operatin tin! ! powe powerr fact factor or.. "s such such the the beha behavio viorr of the synchr synchrono onous us motor motor is described below under three different modes of field e#citation. 1. Normal excitatio: The armature current is minimum at a particular value of field current which is called the normal field e#citation. The operatin! power factor of the motor is unity at this e#citation and thus the motor is e$uivalent to a resistive type of load. !. U"er excitatio: When the field current is decreased !radually below the normal e#citation the arma armatur turee curr current ent incre increas ases es and and the the oper operat atin in! ! power power fact factor or of the the motor motor decreases. The power factor under this condition is la!!in!. Thus the synchronous motor draws a la!!in! current when it is under e#cited and is e$uivalent to an inductive load. #. O$er Excitatio: Excitatio: When the field current is increased !radually beyond the normal e#citation the armature current a!ain increases and the operatin! power factor decreases. %owever the power factor is leadin! under this condition. %ence the synchronous motor draws a leadin! current when it is over e#cited and is e$uivalent to a capacitive load. &f the above above variati variation on of field field curren currentt and the corres correspond pondin! in! armatu armature re current are plotted for a constant mechanical load a curve of the shape of 'V( is obtained as shown in fi! )1*.1*+. Such a characteristic of synchronous motor is common commonly ly called called as 'V( curve curve of the motor motor.. The charac character teristi isticc curve curve plotted plotted between input power factor and the field current for a constant mechanical load on the motor are of the shape of inverted 'V( and are ,nown as inverted 'V( curves. or or incre increase ased d const constan antt mech mechan anic ical al load load on the the moto motorr V curv curves es bodily bodily shift shift upwards as shown in fi! )1*.1*+. The curve oinin! the minimum current points of various 'V( curves plotted for different mechanical loads is normally called a unity power factor compoundin! curve. /////////////////////// /////////////////////////////////// /////////////////////// /////////////////////// /////////////////////// ///////////// // / De%artmet De%artmet o& Electrical a" Electroic' Electroic' E(ieeri(

MAHATMAGANDHII NSTI TUTEOFTECHNOLOGY GANDI PET,HYDERABAD. EEEDe par t me nt–EMI ILAB 6 Exper i mentNo:

I I I / I VEEE( ISe me s t e r )

Pa ge2of5

Name Plate Details: Type : Volts : re$uency : 0W%2 : 32M : "mps : 3atin! : 4onnection : &nsulation :

Specifications / Technical details of Equipment and instruments: a) E*+i%met: S.o 1 5 7 8

Name o& e*+i%met "lternator 64 shunt motor 3heostat 3heostat

Ra(e

,+atit 1 1 1 1

Mae

/) I'tr+met': S.o 1 5 7

Name o& i'tr+met "mmeter "mmeter "mmeter

Ra(e )*-5+ " )*-1*+ " )*-+ "

T%e M& M&

,+atit 1 5

Mae "9 "9

M4

1

"9

8

Voltmeter

)*-;**+ V

M&

1

"9

Circuit Diagram: -

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!ser"ation Ta!les: # cur"es:

Sl.
ield current )"+

"rmature current )"+

1. 5. 7. 8. . ;. =. >. ?. 1*.

Procedure: 1. 4onnect the circuit as per circuit dia!ram shown in fi! ;.1. /////////////////////////////////////////////////////////////////////// / De%artmet o& Electrical a" Electroic' E(ieeri(



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5. &nitially the motor should be at no load and the auto transformer is at @ero position. 7. Aive the supply closin! the M4B and start the motor by means of 7-C variac. 8. or !ettin! V curves allow the motor to run at one particular load. . or different values of field current note down the various meter readin!s. ;. 2lot the curves between field current verses armature current to !et V curves and field current verses power factor to !et inverted V curves. =. or different load settin!s different V and inverted V curves will be obtained.

Precautions: 0eeral: 1. There shouldn(t be any loose connections. 5. The readin!s must be ta,en without any paralla# error. Tecical: 1. The motor field rheostat should be in minimum resistance position. 5. The alternator field potential divider should be in the ma#imum volta!e position.

$odel %raphs:

&esult : The V and inverted V curves of a synchronous machine is obtained. #'#A #CE:




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1. What are the basic differences between a 7-phase synchronous motor and 7 phase induction motorD Sol. Synchronous motor is a constant speed machine rotates at synchronous speed where as induction motor is a variable speed machine rotates at asynchronous speed. 5. What are the various methods of startin! a 7-phase synchronous motorD Sol. 6amper windin! method pony motor method and 64 motor 7. What is the power factor of the motor at normal e#citationD Sol. E2 8. What is the nature of power factor when a synchronous motor is operated )i+ under e#cited )ii+ over e#citedD Sol. Fa!!in! power factor in case of under e#citation and leadin! power factor in case of over e#citation. . &s it possible to operate a synchronous motor on any other speed than the synchronous speedD Sol.

v and Inverted v Curves of Synchronous Motor

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