LOAD CHARACTERISTICS OF A D.C.COMPOUND GENERATOR

SPEED CONTROL OF D.C. SHUNT MOTORBY FIELD CONTROL

SPEED CONTROL OF D.C. SHUNT MOTORBY ARMATURE CONTROL

MAGNETIZATION CHARACTERISTICS OF D.C.SHUNT GENERATOR

LOAD CHARACTERISTICS OF A D.C.SHUNT GENERATOR

OPEN-CIRCUIT & SHORT-CIRCUIT TEST ON A THREE-PHASE ALTERNATOR

EFFICIENCY OF A SINGLE-TRANSFORMER BY SUMPNER,S TEST

3-PHASE TO TWO-PHASE CONVERSION BYSCOTT-CONNECTION

TEE 451ELECTROMECHANICAL ENERGY CONVERSION-1 LAB

P LT

0 0Note: Minimum eight experiments are to be performed from the following list.

1. To obtain magnetization characteristics of a D.C. shunt generator.2. To obtain load characteristics of a D.C. compound generator (a)

Cumulatively compounded (b) Differentially compounded.3. To obtain load characteristics of a D.C. shunt generator.4. To obtain load characteristics of a D.C. series generator.5. To obtain efficiency of a D.C. shunt machine using Swinburn test.6. To performed Hopkinton’s test and determine losses and efficiency of

D.C. machine.7. To obtain speed torque characteristics of a D.C. shunt motor.8. To obtain speed controls of D.C. shunt motor using (a) Armature

control (b) Field control method.9. To obtain speed control of D.C. separately excited motor using Ward

Leonard method.10.To study polarity and ratio test of single phase and three phase

transformers.11.To obtain efficiency and voltage regulation of a single phase

transformer by Sumner’s test.12.To obtain 3- phase to 2- phase conversion by Scott connection.13.To perform open circuit and short circuit tests on a three-phase

transformer and determine parameters of equivalent circuit.

ELECTRICAL MACHINES LABORATORY

TEE 455 L T P0 0 3

Note: - Minimum nine experiments are to be performed from the following list.1. To obtain magnetization characteristics of a D.C. shunt generator.2. To obtain load characteristics of a D.C. shunt generator.3. To obtain load characteristics of a D.C. series motor.4. To obtain load characteristics of a D.C. compound generator (a0

cumulatively compounded (b) differentially compounded.5. To obtain speed-torque characteristics of a D.C. shunt motor.6. To obtain efficiency & voltage regulation of a single phase transformer

by Sumner’s (back to back) test.7. To perform no load & block rotor test on a three phase induction motor

and determine parameters of equivalent circuit and efficiency.8. To perform no load and block rotor test on a single phase induction

motor and determine efficiency.9. To obtain variation of stator current with excitation current (v-curve) of a

three phase synchronous motor at no load, half load and full load.10.To perform open circuit and short circuit tests on a three phase alternator

and determine voltage regulation by synchronous impedance method at a power factor of unity, 0.8 lagging and 0.8 leading.

11.To study operation of a 2-phase a.c. servomotor and a stepper motor.12.To study parallel operation of three phase alternators.

ENERGY CONVERSION LAB

TEE- 456 L T P 0 0 2Note: - Minimum 8 experiments are to be performed from the following.1. To obtain speed-torque characteristics of a D.C. shunt motor.2. To obtain speed-torque characteristics of a D.C. series motor.3. To run a D.C. shunt motor clockwise as well as anticlockwise.4. To control the speed of a D.C. shunt motor by field control method.5. To control the speed of a D.C. shunt motor by armature voltage control. 6. To obtain the running speed –torque characteristics of an induction motor.7. To obtain the v-curves of a synchronous motor.8. To obtain characteristics of S.C.R.9. To study a U.J.T. triggering of an S.C.R.10. To study commutation of an S.C.R. by any commutation circuit.11. Speed control of a single phase motor using TRIAC.12. Organizing a chopper circuit using a power transistor.13. Speed control of D.C. motor using a phase controlled converter.14. Speed control of D.C. motor using chopper.15. Speed control of an induction motor using an inverter.

EXPERIMENT NO:-

SPEED-TORQUE (SERIES MOTOR)

OBJECT: - To obtain speed- torque characteristics of a D.C. series motor.

APPARATUS REQUIRED: - 1. D.C.series motor (2H.P, 8A, 230V, 1500 r.p.m.) 2. Starter (2H.P, 230V) 3. D.P.I.C.switch (16A, 240V) 4. Water cooled drum pulley with mechanical load arrangements.EQUIPMENT REQUIRED: - 1. Voltmeter –M.C. type (0-300V) -----1No. 2. Ammeter - M.C. type (0- 5/10A) -----1No. 3. Digital tachometer ---------------------1.No. THEORY: - The direct load test on a D.C. series motor is performed to obtain its speed-torque characteristics. The motor can be loaded by a belt and pulley arrangement as shown in figure. If W1 and W2 be the tension in Kg indicated by the two spring balances provided on the two sides of the belt, then the load torque on the motor is given by, Load torque T = (W1-W2) ×R -------Kg-Meter T = (W1-W2) ×R × 9.81 Nw-MeterMechanical power output of the motor Pm = 2πNT/60×.102 –wattsBut input power of the motor = VI --- watts Hence, efficiency of the motor (η℅) = (Pm/Pi)×100The speed of the D.C. motor is given by, N= K (V- IaRa)/ΦWhere, the applied voltage V is constant, more over the flux Φ is nearly constant for shunt motor thus the speed of the shunt motor will decrease as load on the motor increase, because of increases in the armature voltage drop IaRa. The drop in speed from no load to full load operation is hardly 4 to 5 percent of the rated speed. As such D.C. motor is regarded as constant speed motor for all practical purpose.

PROCEDURE: - 1. Make the connection as per the circuit diagram as shown.

2. Loaded the motor lightly by tightening the belt on the pulley through the wheels.3. Switch on the D.C. supply through D.P.I.C. switch and start the motor by starter.4. Now increase the load on the motor by tightening the belt gradually.5. Before loading water is field in the cavity of the drum pulley for cooling purpose during loading.6. Note the readings voltmeter, ammeter, and weighting balance.7. Load should not be increased above the motor rated current.

CIRCUIT DIAGRAM: -

OBESERVATION TABLE: - Radius of the pulley =------ (in meter)

S.No.

Voltage (V)

Current (I)

W1 (Kg)

W2 (Kg)

Speed (N)

Torque (T)

RESULT:-

PRECAUTIONS: -

EXPERIMENT NO:-

WARD LEONARD - METHOD

Object: - To obtain speed control of D.C. separately excited motor using Ward Leonard method.

Apparatus required: - (1) Motor-Generator set: - D.C. shunt motor (3 H.P, 230V, 1500 r.p.m, class-B insulated)(2) D.C. shunt generator (1.5 Kw, 230V, 1500r.p.m, class B insulated.) (3) Variable speed D.C. motor: - (Shunt wound, 2H.P, 230V, 1500 r.p.m, class- B insulated)

Equipment required: - 1. Wire wound rheostat (1.4 A, 260 Ω) - - -1No. 2. D.C. starter (2 H.P. 230 V) - - - - - - - - 1No. 3. D.P.I.C. Switch (16A, 240V) - - - - - - - 1No. 4. Wire wound rheostat (1.1A, 800Ω) - - -1No. 5. Volt meter M.C. type ()-300V) - - - - - - 1No. 6. Ammeter M.C. type (0-1/2A) - - - - - - -1 No. 7. Digital tachometer - - - - - - -- - - - - - - -1No.

Theory: - This method is commonly used where very delicate speed control over the whole range from zero to full rated speed is required. The method consists simply in working the motor with a constant excitation and applying to its armature sufficient voltage to give the speed required. A variable voltage supply is there fore required and it is obtain from a motor-generator set. The variable voltage of the generator is obtained by varying its excitation by means of the shunt field rheostat. Speeds in the reverse direction are obtained by reversing the generator excitation.

Circuit diagram: -

Procedure: - 1. Make the connection according to the circuit diagram given in figure.2. Switch on the D.C. supply through D.P.I.C. switch and start the motor by starter.3. All the three machine are given separate D.C. field excitation. The variable speed motor (M1) is given constant field excitation which is same as applied to the prime mover motor. The generator (G) is having variable excitation obtain through field rheostat (1.1A, 800 Ω). Thus the variable voltage of the generator (G) is applied to armature terminals of motor (M1) and hens getting the variable speed corresponding to the change the excitation in generator. 4. For different values of field excitation and note the readings ammeter, voltmeter, and speed measure with the help of tachometer.5. To stop the motor from the switch of the D.C. supply.6. Plot the graph between armature voltages & speed.

Observation table: -

S. No.

Armature voltage(V)

Speed(N)

Result: -

Precautions: -

EXPERIMENT NO:-

SCOTT- CONNECTION

Object: To obtain 3-phase to 2-phase conversion by Scott-connection.

Apparatus required: Two nos 1-phase transformers (1Kva or 2 Kva) having tapping at 28.8%, 50%& 86.6% on primary winding while other transformer have tapping 50% and 86.6% on primary winding.Instrument required: 1. Voltmeter M.I. type - - - -(0-600V) - - - 2No. 2. Voltmeter M.I. type - - - - - - - (()-300V)- - - - -2No. 3. Ammeter M.I. type - - - - - - - (0-5A) - - - - - - -3No. 4. Lamp load (250V, 2KW) - - - - - - - - - - - - - - - 2No. 5. 3-phase autotransformer (0-400V, 8A) - - - - - - 1No.

Theory: The Scott-connection is the most common method of connecting two 1-phase identical transformers to perform the 3-phase to 2- phase conversion and vice-versa. The two transformers are connected electrically but not magnetically.One transformer is called main transformer and the other transformer known as auxiliary or teaser transformer. Tow 1-phase transformers of identical rating with suitable ratings provided on both are required for the Scott-connection.

The two transformer used for this conversion must have the following tapings on there primary windings. 1. Transformer A- 50% tapings and is called the main transformer.2. Transformer B- 28.8%, 50% & 86.6% tapings is called the teaser transformer.

The voltage across the primary CO of the teaser transformer will be 86.6% of voltage across the primary AB of main transformer. The neutral point of the three phase system will be on the teaser transformer, such that the voltage between O and N is 28.8 % of the applied voltage. Thus the neutral point divided 1:2.

1. Equal loadings on the two secondary at unity power factor (Resistive load) The current flowing in the primary windings on three-phase side will also be equal and that too a unity power factor.2. Equal loading on the two secondary at 0.8 lagging power factor: Load the two secondary with equal current but with inductive load at 0.8 P.F. lagging. Then the current of the primary side will also be balanced and that too at 0.8 P.F.lagging3. Un equal loading on the to secondary with different power factor: If both the current and power factor is different in the two secondary of the used for the Scott-connection, then the current on the primary side will also be UN balanced.

Circuit diagram:

Procedure: 1. Connected the circuit as per circuit diagram. 2. Ensure that the switches S1 and S2 are open. 3. Adjust the three-phase baric at zero voltage. 4. Switch on the 3-phase A.C. supply and apply rated voltage across the primary side by three-phase variac. 5. Record voltage V1, V2 and V3 and verify that the out put is a balanced two-phase voltage. 6. Switch off the A.C. supply and remove the dotted connection of two secondary and voltmeter V3. Adjust variac at zero position. 7. Switch on the A.C. supply again; adjust the rated voltage across the primary side of the transformer again. 8. Now switch on the load and adjust equal loading of the both secondary. Record the readings of all meters connect in the circuit. 9. Repeat step 8 for various equal unequal loading conditions on the two secondary. 10. Switch off the load & main A.C. supply.

Observation table:

For balanced 2-phase supply

Under load connections Primary side Secondary side

S.No. V1 V2 V3 I1 I2 I3 V1 V2 I2t I2m

Result:

Precautions:

EXPERIMENT NO:-

PARALLEL OPERATION (3-PHASE ALTERNATOR)

OBJECT: To study parallel operation of 3-phase alternators.

APPARATUS REQUIRED: - 1. Synchronizing panel for parallel operation, with all accessories. 2. M-G set with specification - -- - - 2No. A.C. generator- (2K.V.A, 3-phase, 50 Hz, 415V, 1500R.P.M, 2.8Amp, D.C.140V, D.C.Amp- .85, P.F- 8)D.C. motor- 230V, 1500R.P.M, 11Amp, shunt wound.

THEORY: Synchronizing of generator: -Synchronous generator can be put the share the load it should be properly connected in parallel with the common bus-bar. Internal connection of the terminals of a generator with the terminals of an other or a bus-bar, to which large number of synchronous generator are already connected is called synchronizing.Conditions for parallel operation: Following three conditions must be fulfilled.

(1) The generated voltage of the incoming alternator to be connected in parallel with a bus-bar should be equal to the bus-bar voltage.(2) Frequency of the generated voltage of the incoming alternator should be equal to the bus-bar frequency.(3) Phase sequence of the voltage of the incoming alternator should be same as that of the bus-bar. Generated voltage of the incoming alternator can be adjusted by adjusting the field excitation. Frequency of the incoming alternator can be controlled and made equal to bus-bar frequency by controlling the speed of the prime mover driving the alternator. Phase sequence of the alternator and the bus-bar can be checked by a phase sequence indicator. Alternatively, three lamps can be used for checking of phase sequence- Synchronous generator driven at rated speed if all the lamps glow together and dark together then the phase sequence of the incoming alternator in the same as that of the bus-bar.

Method of synchronization: - 1. Synchronizing lamps method – (a) Three bright lamps method (b) Two bright and one dark amps method. 2. Synchroscope method.Two bright one dark lamp method: In the method of synchronizing an alternator three lamps or connected, two lamps are connected with the bus-bar. In this method the brightness of the lamps will vary in sequence. A particular in sequence will indicate if the incoming alternator is running too fast or too slow. Perfect synchronizing will occur when lamp one is dark while two lamps are equally bright. Synchroscope method: A synchroscope determine s the instant of synchronism more accurately than the three lamps method. A synchroscope consists of a rotor (moving coil) and stator (fixed coil). One of which is connected the incoming alternator and the other to the bus-bar. A pointer connected to the rotor will rotate, if there is difference in frequencies of the incoming alternator and bus-bar. Anticlockwise rotation of the rotor pointer indicates that the frequency of the incoming alternator is slower, whereas clockwise rotation of the pointer indicates that the frequency is higher than the bus-bar frequency. The speeds of the prime mover driving of the alternator will there fore have to be adjusted such that, when the frequencies are equal to the pointer is stationary. The alternator can be switched on the bus-bar by closing the switch.

CIRCUIT DIAGRAM:

PROCEDURE: 1. Connections are made as per circuit diagram.2. Insure that the synchronizing switch is open external resistance in the field circuit of the motor is zero and external resistance in the field circuit of alternator is maximum.3. Switch on the D.C. supply to the D.C. motor-1 & D.C. motor -2 and start by the starter.4. Adjust the speed of both the D.C. motor to rated speed of alternators, by varying the rheostat in its field circuit of respective motors.5. Switch on the D.C. supply to the field of alternator by switching on the M.C.B. and moving the variac knob in clockwise position, so that the generated voltages of both the alternators are equal. Check the the phase sequence of both alternators by using phase sequence indicator. The phase sequence of both alternators should be same. As per the connection of the set of lamps, one set which is directly connected between the same phases should dark and the same instant, the order two set of the lamps, which are cross connected should be bright.6. Switch on the T.P.S.T.knife switch in upward direction. Now three set of lamps will flicker, in case flickering is fast adjust slowly the speed of both the D.C. motors, so that frequency becomes equal. Check the quantity of two voltages of alternators. Under such a condition, the set of lamps will go IN and OUT very slowly. At this point switch on the M.C.B.for synchroscope and when its pointer is in the middle the two alternators are synchronized and the T.P.I.S.T.

switch must be brought in down ward direction. Thus both the alternators are now supplying common voltage to the bus-bar. 7. Watch for the correct instant of synchronization which is denoted by synchroscope pointer in middle with the synchronizing switch in hand and close this switch in downward direction, when the directly connected set of lamps is dark and the other two set of lamps are equal bright, thus synchronizing the incoming alternator with the bus-bar.8. Switch OFF the synchronizing switch, bus-bar switch and then the D.C. mains to stop the D.C. motor and the other motor.

RESULT:

PRECAUTIONS: 1. Always switch on the M.C.B. for D.C. motor keeping in view that A.C. generator is not loaded. 2. Insure that the M.C.B. for D.C. exciter as should be in OFF position. 3. Insure that the T.P.D.T. switch is in middle position. 4. Insure that the M.C.B. for synchroscope should be in OFF position. 5. Insure that field rheostat is connected across F.R. terminals. 6. Never leaves synchroscope and phase sequence indicator permanently connected in the circuit i.e. M.C.B. meant for, Synchroscope should be turned OFF immediately once the synchronizing procedure is over.

EXPERIMENT NO:-

SWIN BURN, S - TEST

Object: To obtain efficiency of a D.C. shunt machine using Swin burn’s test.

Apparatus required: 1. D.C. motor (3 H.P, 230 V, 11A, 1500 R.P.M.)

Instruments required: 1. Lamp bank load - - - 1No. 2. Flexible wires -- - - - - L.S. 3. D.C. starter (3H.P, 230V) - - -1No. 4. D.P.I.C. switch (16A, 240V) - - - 1No. 5. D.C. voltmeter (0-300V) - - - - - - 1No. 6. D, C. ammeter (0-5A) - - - - - - - - -3No. 7. Tachometer - - - - - - - - - - - - - - - 1No.

Theory: Swin burn’s test is an indirect method (without loading) for finding out the efficiency of D.C. machine. Various losses occurring in a D.C. machine can be

classified as (I) constant losses and (II) Variable losses. Variable losses are directly proportional to the squire of armature current or approximately the load current, where as constant losses are independent of load conditions. In this method constant losses are determine experimentally by operating the D.C. machine as motor running at No-load. Variable losses in occurring on load are calculated from the known specification of the machine. Let the voltage applied to the shunt motor be V-volts and the current flowing in the armature and shunt winding under No-load running be Iao and Ish respectively. Then, Input power to the armature circuit = VIao - - watts Input power to the shunt field circuit = VIsh - - watts Total power input to the motor at No-load (W0) = V (Ia0+Ish) Armature copper losses at No-load = Ia

2 Ra - - wattsThus, The constant losses of the machine (Wc) = W0 - Ia0

2 Ra - - wattsNote: (The Swin burn’s test should be performed at rated voltage and rated speed)

Efficiency when machine is running as motor: Input power = VI - - - watts. Armature copper losses =Ia

2 Ra

= (I - Ish)2 Ra - - -watts Constant losses (Wc) = - - - watts Total losses = Wc + (I – Ish)2 Ra

(Input – losses)Percentage efficiency = --------------------- × 100 Input VI – Wc + (I – Ish)2 Ra

= ---------------------------- ×100 VIEfficiency when machine is running as Generator: Generator output = VI - - - watts Armature current (Ia) = (I + Ish)2 Ra - - - wattsConstant losses (Wc) = - - - wattsTotal losses = Wc + (I + Ish)2 Ra

Percentage efficiency = output×100 / (output + losses) VI

= ----------------×100 VI + Wc + ( I + Ish )2 Ra

(Let I be the load current at which efficiency is required)

Procedure: Swin burn’s test – 1. Connect the circuit as per circuit diagram No.12. Insure that the external resistance in field circuit as zero.3. Switch on the D.C.supply and start the motor help of the starter.4. Insure that the applied voltage to the motor must be rated voltage.Record the readings of all the meters connected in the circuit.5. To stop the motor switch OFF the D.C. supply.

Measurement of armature resistance: 1. Connect the circuit as per circuit diagram.2. Connect the measuring instruments externally meant for armature resistance calculation.3. Switch ON the D.P.I.C. switch and now switch ON some bulbs in the lamp bank load, so that the current flowing in the armature circuit is the rated full-load current of the D.C. motor, weight for few minutes with the full-load current flowing in the armature winding, so that the armature of the armature winding approximately becomes equivalent to that obtained under working conditions. 4. Record the readings of both meter connected in this circuit.5. Switch OFF the D.C. supply.

Observation table:

Swin burn’s test Armature Resistance testS.No. Vs Ia0 Ish S.No. Va Ia Ra

Result:

Precautions:

ELECTRICAL MACHINE – LAB(FOR IV SEMESTER, ME-BRANCH)

ENERGY CONVERSION – LAB(FOR IV SEMESTER, EC-BRANCH)

ELECTROMECHANICAL ENERGY CONVERSION LAB – I

(FOR IV SEMESTER, EN-BRANCH)

EXPERIMENT NO:-

V- CURVE (SYNCHRONOUS -MOTOR)

Object: - To obtain the v-curves of a synchronous motor.

Apparatus required: - (1) D.C.shunt generator (for loading) 1.5kw/2h.p 230V, 1500rpm.(2) Synchronous motor (3H.P. salient pole, 1500 r.p.m. 50 Hz, 415V, star connected, class-B insulation)(3) D.C. shunt generator (for excitation) separately, type- static type through rectifier with controlling arrangement.

Equipment required: - (1) Volt meter – M.I. type (0-500v) - - - - 1No. (2) Ammeter –M.I. type (0-5A) - - - - - - -1No. (3) Power factor meter - - - - - - - - -- -----1 No. (4) Volt meter M.C. type (0-300V) - - - - 2No. (5) Ammeter M.C. type (0-5A) - - - - - - 2No. (6) D.O.L. starter - - - - - - - - - - - - - -- 1No.

Theory: - With constant mechanical load on the synchronous motor the variation of field current change the armature current drawn by the motor and also its operating power factor. As such the behavior of the synchronous motor is described bellow under three different modes at field current.(1) Normal excitation: - The armature current is minimum at a particular value of field current (If) which is called the normal field excitation. The operation power factor is unity at this excitation and thus motor is equivalent to a resistive load.(2) Under excitation: - When the field current is decreased gradually bellow the normal excitation. The armature current increase and the operating power factor of the motor decreases. The power factor under this condition is lagging. Thus the synchronous motor draws a lagging current, when it is under excited and equivalent to an inductive load.

(3) Over excitation: - When the field current is increased gradually beyond the normal excitation, the armature current again increase the operating power factor increases. How ever the power factor is leading under this condition. Hence, the synchronous motor draws a leading current, when it is over excited and is equivalent to a capacitive load. It the above variation of field current and the corresponding armature current are plotted for a constant mechanical load, a curve of the shape of V- is obtained and the characteristic curve plotted between input power factor and the field current for a constant mechanical load on the motor are of the shape of inverted V- curves.

Circuit diagram:

Procedure: - 1. Make the connection a per circuit diagram.2. Before starting the synchronous motor makes sure that the D.C. excitation switch is in off position.3. Switch on the A.C. supply and start 3- phase synchronous motor with the help of D.O.L. starter.4. Observe direction of rotation, incase it is rotating in opposite direction, stop the motor and reverse the sequence. Start the motor again using starter and insure motor is running at no load.5. Switch on the D.C. excitation switch.6. Now gradually increase the excitation by the rheostat a few step and note the corresponding decrease in armature current (Ia) and field excitation current (If) Vary excitation till the armature current is minimum. After the point increase excitation, the armature current will increases and note corresponding armature current and field current values. Vary the excitation up till the rated value of synchronous motor. This value with correspond to synchronous motor at no load.7. Now switch on the load on D.C. generator and various loads setting ½ loads, full load on D.C. generator repeat step no 2 to 5.8. Plot the graph between field current and armature current for V-curve and inverted curve between power factor and field current.9. Switch off the D.C. excitation switch, and then stop the motor, by press the red button of D.O.L. starter.

Observation table: - S.No. Field-current

(If)Armature current

(Ia)Power factor

(cosΦ)

Result: -

Precautions: -

EXPERIMENT NO:-

V- CURVE (SYNCHRONOUS -MOTOR)

Object: - To obtain the v-curves of a synchronous motor.

Apparatus required: - (1) D.C.shunt generator (for loading) 1.5kw/2h.p 230V, 1500rpm.(2) Synchronous motor (3H.P. salient pole, 1500 r.p.m. 50 Hz, 415V, star connected, class-B insulation)(3) D.C. shunt generator (for excitation) separately, type- static type through rectifier with controlling arrangement.

Equipment required: - (1) Volt meter – M.I. type (0-500v) - - - - 1No. (2) Ammeter –M.I. type (0-5A) - - - - - - -1No. (3) Power factor meter - - - - - - - - -- -----1 No. (4) Volt meter M.C. type (0-300V) - - - - 2No. (5) Ammeter M.C. type (0-5A) - - - - - - 2No.

(6) D.O.L. starter - - - - - - - - - - - - - -- 1No.

Theory: - With constant mechanical load on the synchronous motor the variation of field current change the armature current drawn by the motor and also its operating power factor. As such the behavior of the synchronous motor is described bellow under three different modes at field current.(1) Normal excitation: - The armature current is minimum at a particular value of field current (If) which is called the normal field excitation. The operation power factor is unity at this excitation and thus motor is equivalent to a resistive load.(2) Under excitation: - When the field current is decreased gradually bellow the normal excitation. The armature current increase and the operating power factor of the motor decreases. The power factor under this condition is lagging. Thus the synchronous motor draws a lagging current, when it is under excited and equivalent to an inductive load.

(3) Over excitation: - When the field current is increased gradually beyond the normal excitation, the armature current again increase the operating power factor increases. How ever the power factor is leading under this condition. Hence, the synchronous motor draws a leading current, when it is over excited and is equivalent to a capacitive load. It the above variation of field current and the corresponding armature current are plotted for a constant mechanical load, a curve of the shape of V- is obtained and the characteristic curve plotted between input power factor and the field current for a constant mechanical load on the motor are of the shape of inverted V- curves.

Circuit diagram:

Procedure: - 1. Make the connection a per circuit diagram.2. Before starting the synchronous motor makes sure that the D.C. excitation switch is in off position.3. Switch on the A.C. supply and start 3- phase synchronous motor with the help of D.O.L. starter.4. Observe direction of rotation, incase it is rotating in opposite direction, stop the motor and reverse the sequence. Start the motor again using starter and insure motor is running at no load.5. Switch on the D.C. excitation switch.6. Now gradually increase the excitation by the rheostat a few step and note the corresponding decrease in armature current (Ia) and field excitation current (If) Vary excitation till the armature current is minimum. After the point increase excitation, the armature current will increases and note corresponding armature current and field current values. Vary the excitation up till the rated value of synchronous motor. This value with correspond to synchronous motor at no load.7. Now switch on the load on D.C. generator and various loads setting ½ loads, full load on D.C. generator repeat step no 2 to 5.8. Plot the graph between field current and armature current for V-curve and inverted curve between power factor and field current.9. Switch off the D.C. excitation switch, and then stop the motor, by press the red button of D.O.L. starter.

Observation table: -

S. No. No-load test Half –load test Full-load test

If Ia If Ia If Ia

Result: -

Precautions: -