Exp. 4- Determination of v Curves and Inverted v Curves of a Synchronous Motor and Regulation of Alternator With EMF and MMF Methed.

Indian Institute of Technology Gandhinagar Department of Electrical Engineering EE 333 Electrical Machines and Power Electronics Lab.

B. Tech.: Electrical, Sem. : VI

EXPERIMENT 4: 1.“V” CURVES AND INVERTED “V” CURVES OF A SYNCHRONOUS MOTOR 2.REGULATION OF ALTERNATOR BY EMF AND MMF METHOD.

AIM 1. To obtain the ‘V’ curves and inverted ‘V’ curves of a synchronous motor.

2. To run the same machine as Alternator and determine its Regulation by EMF and MMF methods.

APPARATUS REQUIRED: Sr. No. 1.

Apparatus Rotating

Test bench

Specification

Quantity

Machines from 1. For V and inverted V curves Experiment

the machines from the Test Bench are to be configured for running the Synchronous

1

machine as motor with the DC machine as a Generator

2. For Regulation of Alternator experiment by

EMF

and

MMF

methods,

The

synchronous machine run as an Alternator 2

Extension Panel

by the DC motor.

Panel that facilitates having terminals

extended from rotating machinery, power supplies, Load banks and panel meters.

1

THEORY: The Synchronous motor is one type of three phase AC motors which operates at a constant speed from no load to full load. It is similar in construction to three phase A.C. generator in that it has a revolving field which must be separately excited, from a DC source. By Changing the DC

field excitation current the power factor of this type of motor can be varied over a wide range of lagging and leading values. This motor is used in many industrial applications because of its

fixed speed from no load to full load and its high efficiency. It is also used to improve the power factor of the three phase industrial power circuits. It runs either synchronous speed or not

runs at all. It is not inherently self starting. It has to run up to the supply frequency. The Synchronous motors have the following fields of application. 1.

Power House and Substations to improve the power factor.

3.

Constant speed equipments such as fans blowers and MG sets.

2.

Factories mills to improve the power factor.

V Curves of a Synchronous motor When the field current of (i.e. Excitation) of a synchronous motor is reduced, a lagging stator

current I is produced which exceeds the minimum current at unity power factor or at normal excitation. Similarly when the motor is over excited, the stator current also rises and exceeds

the current required at normal excitation to develop the necessary torque at any given load. By

applying a given constant load to the shaft of a synchronous motor and varying the field current from under excitation to over excitation, recording the armature current at each step,

the “ V” curves are obtained. The AC stator current is plotted against the DC field current for no

load, half full load, and full load respectively. The power factor for each value of stator and field current at any given load condition is also recorded. The power factor is plotted against the

field current for various given loads. It is worth noting that both curves show that a slightly

increased field current is required to produce normal excitation as the load current is increased.

At no load the stator current at unity power factor (Normal excitation) is not zero but small

value of current per phase necessary to produce torque to counter balance rotational losses. As

load is applied (neglecting the armature reaction) not only does the stator current rise but it is

also to increase the excitation to bring the armature current back in phase with the bus phase

voltage. Each of the curves in the family therefore will have a shift to the right as the load is increased, to provide the excitation required to obtain the same power factor (0.8 lagging,

unity, and 0.8 leading) at an increased load. Thus the V curves represent the phasor diagrams, and vice versa for various conditions of load and power factor. THEORY (REGULATION BY EMF AND MMF METHOD) By conducting the Open Circuit Characteristics (Generated voltage vs. Field current) and Short

circuit Characteristics (Short circuit stator (or Armature) current vs. Field current), it is

possible to predetermine the regulation for various load conditions. From the short circuit characteristics the synchronous impedance can be found out. The constituent of synchronous impedance the synchronous reactance can also be found out with the knowledge of armature resistance.

By considering different load levels (from 1/4 to full load) and different power factors (UPF and 0.8 lag/lead) the voltage drop from the rated open circuit voltage levels can be calculated with the knowledge of armature resistance and synchronous reactance. This Method is known as EMF method.

It is also possible to determine the additional field current that is required to generate the load

current at the rated open circuit voltage. The Vector addition of the field currents give effective field current under load condition and its corresponding voltage on the OCC gives the total internal voltage E 0 generated at the load condition. The voltage regulation can now be calculated and this method is known as MMF method. CIRCUIT DIAGRAM (V Curves)

3Φ, 415VAC, 50 Hz

R

Y

A

M

V

C

V

R Y

C

B

V

A

L

F

FF

G

B

V

V _

Contro l

Variable DC Power Supply

Display T/I

1 φ Supply

1 φ Supply

CIRCUIT DIAGRAM FOR EMF AND MMF TEST

V _

F

A

M _

FF AA

A _

1 φ Supply

A

R B

Y

F

A _

FF

V _

Variable DC Power Supply

Control

V _

Variable DC Power Supply

Control

Variable DC Power Supply

A _

V

A

Sensors T&S

Variable DC Power Supply

Control

M

A

L

1 φ Supply

V

L O A D

PROCEDURE (V Curves) 1. Select the Synchronous Machine in Motoring mode and the DC machine in Generating mode. Keep the Field of Synchronous Machine and The DC machine off initially. Ensure Mechanical coupling between Synchronous machine and the DC machine.

2. Keep the synchronous machine field off. Start the synchronous machine as Induction motor

(with the built in Damper winding). Once the synchronous machine reaches the no load speeds close to synchronous speeds, turn on the field of the synchronous machine. Now the

rotor (the poles of the synchronous machine) gets locked with the rotating magnetic field and the machine now runs as synchronous motor driving the rotor of the DC machine.

3. Now to obtain ‘V’ curves on NO LOAD the field current is adjusted to a minimum value and the stator current of the synchronous motor is noted. The field current is varied from minimum to maximum and the corresponding values of the stator current are recorded.

4. Switch on the field excitation to the DC machine and bring up its generated voltage to its the

rated voltage by adjusting the DC machine field. Add the resistive load on the generator such that the load current of the Sync. Machine is half of its full load current.

5. Now again vary the field current of the alternator from minimum to maximum and note the armature/stator current.

6. The above step can be repeated for obtaining the ‘V’ curves for near full load by loading the DC machine (now acts as a generator) by resistive loads at different values.

TABULATION: Terminal voltage: ______________________ Sr. No.

Load (KW)

1

No Load

2

Half Full Load

3

Full Load

Field Current IF (Amp.)

Line Current IA (Amp.)

Power Per Phase (Watt)

Power Factor √𝟑𝑾�𝑽𝑰𝒂

MODEL GRAPHS Ia

Full Load

Pf

No Load

Full Load

No Load

If

0

0

If

PROCEDURE(Regulation by EMF and MMF Methods) •

OPEN CIRCUIT TEST

1. The Rotary Bench is configured for running the Synchronous machine as Alternator and the

DC machine as its prime mover. The DC machine and the Alternator are mechanically coupled. (Induction machine not used.)

2. The Synchronous machine to be selected as Alternator and the test selection is at OC.

3. The Alternator Field kept at minimum.

4. The DC machine to be selected for motoring mode. The DC Machine field is kept at maximum.

5. The DC machine is applied with armature voltage gradually and the rated voltage is reached. The Field current of the Dc machine is now adjusted to bring the DC motor to run at Synchronous speed of the Alternator.

6. Vary the field current of the Alternator in steps (of 0.5 A) and note down the corresponding generated voltage of the Alternator (it is the open circuit voltage).

7. Repeat the above steps till the generated voltage reaches 1.20 times the rated voltage, maintaining the speed constant. Record the Field current and the Generated voltage one phase voltage).

8. Reduce the field of the Alternator to the minimum. Turn of Field Supply. •

SHORT CIRCUIT TEST

1. Keeping the Alternator field at Minimum Select the Alternator test mode SC test. 2. Turn on Field Supply for the Alternator.

(any

3. Keep running the DC machine as Prime mover of the Alternator at its Synchronous Speed.

4. Gradually apply Alternator field current keeping a watch on the stator current. Increase it

to the rated current of the alternator. Record at least three readings of field current Vs Stator Current.

5. Reduce the field excitation to the minimum and switch off the Field.

6. Bring down the DC machine Armature voltage to Zero and turn of Armature supply and the field supply.

7. Switch of the Rotary Machine Bench supply and remove the coupling between Alternator and the DC machine. •

MEASUREMENT OF STATOR RESISTANCE PER PHASE

Armature resistance is measured with a Digital Multimeter TABULATION OPEN CIRCUIT TEST Sl. No.

I f (Amp.)

R a * ( Effecive resistance)

SHORT CIRCUIT TEST E G (Volt)

=

Sl. No.

I f (Amp.)

1.5 R ( R=measured resistance by multimeter)

PRE-DETERMINATION OF VOLTAGE REGULATION (Calculated on Phase Voltage) 1. EMF METHOD The steps involved in this method are

1. Plot OCC from the OC test data ( E G Vs I f )

2. Plot SCC from SC test data. ( I sc vs I f ) is a straight line passing through the origin

3. Both these curves are drawn on common field current base as shown in fig. From this Synchronous Impedance Z S is calculated as follows,

Synchronous Impedance

Z S = (E 1 /I 1 ) at I f = I fr and E 1 is the Rated voltage.

(Ratio of Open circuit voltage per phase to short circuit current per phase for the same field

current) Stator resistance R a is measured. This measured value of R a per phase is multiplied by 1.5 to take into account the skin effect.

OCC

ISC E0 Volts E1

SCC

I1

If in A

Knowing R a Synchronous reactance is calculated as X S = √ Z s 2 – R a 2. Now the induced

emf (E 0 ) can be calculated by drawing the phasor diagram for a given Load current and the rated output voltage, using these values of X s and R a . Phasor diagram for any load

and power factor can be drawn as follows. FOR UPF LOAD

Taking load current I as reference vector, phasor diagram for UPF load can be drawn as C

IXs

EO

V A

0 I

IaRa B

𝑂𝐶 = �𝑂𝐵2 + 𝐵𝐶 2 𝐸𝑂 = �[(𝑉 + 𝐼𝑅𝑎 )2 + (𝐼𝑋𝑆 )2 ]

Where E 0 is the no load induced emf per phase, V is the rated terminal voltage per phase and I is the load current.

FOR LAGGING PF LOADS

IXs EO IaRa V φ

FOR LEADING PF LOADS

I

In this case the induced emf is given by 𝐸𝑂 = �(𝑉 cos 𝜑 + 𝐼𝑎 𝑅𝐴 )2 + (𝑉 sin 𝜑 + 𝐼𝑋𝑆 )2 Where cos φ is the load power factor

I EO

V IXs

𝐸𝑂 = �(𝑉 cos 𝜑 + 𝐼𝑎 𝑅𝐴 )2 + (𝑉 sin 𝜑 − 𝐼𝑋𝑆 )2

IaRa

The % regulation in each case can be calculated as, % 𝑹𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 =

TABULATION EMF METHOD Terminal Voltage

Load Fraction 25%

Load Current (Amp.)

(𝑬𝑶 − 𝑽) 𝒙 𝟏𝟎𝟎 𝑽 Load Power Factor

Induced EMF (E O )

% Regulation

0 0.8(lag) 0.8(lead)

50%

0 0.8(lag) 0.8(lead)

75%

0 0.8(lag) 0.8(lead)

100%

0 0.8(lag) 0.8(lead)

% Regulation Vs Load Current for different power factors is plotted MMF METHOD OR AMPERE TURN METHOD

This method also utilizes OC and SC test results. In this method, the total field Ampere turns required to produce rated voltage on any given load is calculated.

The total field AT required is the vector sum of the field AT required to produce rated voltage

on No load and field AT required to produce a given load current on short circuit current. Since the number of turns is constant for a machine the current can be calculated and the corresponding EMF from the OCC can be observed.

If OA is the field current I f1 required to produce rated voltage on no load and AB is the I f2 is the

field current required to produce the rated current on short circuit, then the total field current I f required to produce the rated voltage on full load can be calculated as follows… FOR UPF LOAD The Total Field Current

B

If

If2

𝐼𝑓 = �𝐼𝑓1 2 + 𝐼𝑓2 2

A If1

FOR LAGGING PF LOAD

B

The Total Field Current

If If2

𝐼𝑓 = �𝐼𝑓1 2 + 𝐼𝑓2 2 + 2𝐼𝑓1 𝐼𝐹2 cos Φ

90 + φ

0 If1

A

FOR LEADING POWER FACTOR LOAD The Total Field Current

B

If2

If 0

A If1

𝐼𝑓 = �𝐼𝑓1 2 + 𝐼𝑓2 2 − 2𝐼𝑓1 𝐼𝐹2 cos Φ

After calculating the total field current I f required to produce rated voltage on full load in each case, the corresponding induced voltage E0 can be found from OCC. % 𝑹𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 =

(𝑬𝑶 − 𝑽) 𝒙 𝟏𝟎𝟎 𝑽

The Total field current can also found graphically as follows

OCC

EO

SCC Educed emf EO

B

90 + φ 0

If in A

A

MMF METHOD – FULL LOAD REGULATION Terminal Voltage

Load Fraction 25%

Load Current (Amp.)

Load Power Factor

Induced EMF (E O )

0 0.8(lag) 0.8(lead)

50%

0 0.8(lag) 0.8(lead)

75%

0 0.8(lag) 0.8(lead)

100%

0 0.8(lag) 0.8(lead)

% Regulation Vs Load Current for different power factors is plotted

RESULT AND INFERENCE

% Regulation