emlab 2

STUDY OF INDUCTION STARTERS

AIM

To study different types of Induction based starters used widely in industries to start induction motors

THEORY

A Three-phase induction motor is theoretically self starting. The stator of an induction motor consists of 3-phase windings, which when connected to a 3-phase supply creates a rotating magnetic field. This will link and cut the rotor conductors which in turn will induce a current in the rotor conductors and create a rotor magnetic field. The magnetic field created by the rotor will interact with the rotating magnetic field in the stator and produce rotation. Therefore, 3-phase induction motors employ a starting method not to provide a starting torque at the rotor, but because of the following reasons,

1) Reduce heavy starting currents and prevent motor from overheating.

2) Provide overload and no-voltage protection.

There are many methods in use to start 3-phase induction motors. Some of the common methods are,

1. Auto Transformer Starter2. Rotor Impedance Starter3. Direct On-Line Starter (DOL)4. Star-Delta Starter

Auto Transformer Starter

The operation principle of auto transformer method is similar to the star delta starter method. The starting current is limited by reducing the initial stator applied voltage. The auto transformer starter is more expensive, more complicated in operation and bulkier in construction when compared with the star – delta starter method. But an auto transformer starter is suitable for both star and delta connected motors, and the starting current and torque can be adjusted to a desired value by taking the correct tapping from the auto transformer. When the star delta method is considered, voltage can be adjusted only by factor of 1/3. The advantage is only connection conductors are required between stator and motor.

Rotor Impedance Starter

When there is a need of starting an three phase induction motor on load, usually wound rotor machine will be selected. This method allows external resistance to be connected to the rotor through slip rings and brushes. Initially, the rotor resistance is set to maximum and is then gradually decreased as the motor speed increases, until it becomes zero. The rotor impedance starting mechanism is usually very bulky and expensive when compared with

other methods. It also has very high maintenance costs. Also, a considerable amount of heat is generated through the resistors when current runs through them. The starting frequency is also limited in this method. However, the rotor impedance method allows the motor to be started while on load.

Direct On-Line Starter (DOL)

The Direct On-Line (DOL) starter is the simplest and the most inexpensive of all starting methods and is usually used for squirrel cage induction motors. It directly connects the contacts of the motor to the full supply voltage. The starting current is very large, normally 6 to 8 times the rated current. The starting torque is likely to be 0.75 to 2 times the full load torque. In order to avoid excessive voltage drops in the supply line due to high starting currents, the DOL starter is used only for motors with a rating of less than 5KW. There are safety mechanisms inside the DOL starter which provides protection to the motor as well as the operator of the motor. The power and control circuits of induction motor with DOL starter are shown in figure. The DOL starter consists of a coil operated contactor K1M controlled by start and stop push buttons. On pressing the start push button S1, the contactor coil K1M is energized from line L1. The three mains contacts (1-2), (3-4), and (5-6) in the figure are closed. The motor is thus connected to the supply. When the stop push button S2 is pressed, the supply through the contactor K1M is disconnected. Since the K1M is de-energized, the main contacts (1-2), (3-4), and (5-6) are opened. The supply to motor is disconnected and the motor stops.

Star-Delta Starter

The star delta starting is a very common type of starter and extensively used, compared to the other types of the starters. This method used reduced supply voltage in starting. Figure(2) shows the connection of a 3phase induction motor with a star – delta starter. The method achieved low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw switch changes the winding arrangements from star to delta configuration. By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are start connected, each stator phase gets voltage VL/3 , where VL is the line voltage. Since the torque developed by an induction motor is proportional to the square of the applied voltage, star- delta starting reduced the starting torque to one – third that obtainable by direct delta starting.

K2M - Main Contactor K3M - Delta Contactor K1M -Star Contactor F1 - Thermal Overload Relay

Figures

Induction Motor with Auto Transformer starter

Induction Motor with Rotor impedance starter

Induction Motor with DOL starter

Induction Motor with Star Delta Starter

Experiment No.1

LOAD TEST ON SINGLE PHASE INDUCTION MOTOR

AIM

To conduct the load test on the given single phase induction motor and plot the following characteristics as a function of output.

• Efficiency

• Load current

• Power factor

• Speed

MACHINE DETAILS

Power: 1.5 H.P / 1.1kW

Voltage: 190/ 240 V

Current : 8.8 A

Speed: 1440 rpm

COMPONENTS REQUIRED

Items Rating Quantity

1. Ammeter 0-10 A,MI 1 Nos2. Voltmeter 0-300 V, MI 1 Nos3. Wattmeter 230 V, 10A, UPF 1 Nos4. Tachometer 1 Nos

INITIAL SETTING

Motor should be on no load.

THEORY

Constructionally this motor is more or less similar to polyphase induction motor except that its rotor is provided with single phase winding. A centrifugal switch is used in some type of motor to cut out the winding. This is used for starting purpose. It has distributed stator winding and a squirrel cage induction rotor. When a single phase supply is given to the stator

winding , it produces alternating flux which is not rotating. Hence the motor is not self starting. However if rotor of such a machine is accelerated in either direction to its final speed, it tends to rotate by two ways:

• By two field or double field revolving theory.

• By cross field revolving theory.

PROCEDURE

• Keeping the autotransformer in the minimum position on the supply.

• Bring motor at rated voltage by adjusting autotransformer.

• Load is applied & all the motor readings are taken. Speed & spring balance reading are also noted.

• Machine is loaded upto rated current.

RESULT

The load test on single phase induction motor is conducted & the following graphs were plotted.

• Efficiency Vs Output

• Load current Vs Output

• Power factor Vs Output

• Speed Vs Output

SL No

Voltage Current Power Speed Weight balance Torque Output Efficiency Power (V) (I) (P) (N) W1 W2 W T W ƞ % Factor

volts Ampere Watts RPM Kg Kg Kg Nm Watts

Circumference of brake drum 2 πR = ------ mRadius of brake drum R = ----- / 2 π m

Useful Formulae(1) Torque T = (W )*9.81*R N-m

(2) Input power P = (Wattmeter reading) watts (5) Power factor = Input power / VI

(3) Output power Pm = 2π NT / 60 watts(4) Efficiency ƞ = (Pm / P) x 100 %

Circuit Diagram

Expected Graph

Experiment No.2

BRAKE TEST ON SQUIRREL CAGE INDUCTION MOTOR

AIM

To conduct the load test on the given three phase squirrel cage induction motor and plot the performance characteristics curves.

• Output Vs Efficiency

• Output Vs Torque

• Output Vs Power factor

• Output Vs Slip

• Output Vs Line current

• Output Vs Speed

MACHINE DETAILS

Power: 3.7kW

Voltage: 415 V

Current : 7.5 A

Speed: 1430 rpm

COMPONENTS REQUIRED

Items Rating Quantity

1. Ammeter 0-10 A,MI 1 Nos2. Voltmeter 0-600 V, MI 1 Nos3. Wattmeter 600 V, 10A, UPF 1 Nos4. Tachometer 1 Nos

THEORY

The three phase squirrel cage induction motors has normal starting torque and adjusted speed so that speed control can be achieved easily.

Normally DOL starter and auto transformer starter are used to start the motor. This motor may sometime show a tendency to run at a very slow speed. This is because of harmonics in sinusoidal wave produced by the stator mmf known as crawling. This motor

may exhibit a peculiar behaviour in starting for certain relationship between the number of stator slots equal to integral multiple of rotor slots.

The variation of reluctance as a function of space will be introduced. This in turn creates an aligning torque stronger than accelerating torque with consequent failure of motor start. The phenomenon is known as cogging.

PROCEDURE

Connections are made as per the circuit diagram. Ensure motor is on no load. Keep star delta switches on star with the supply. When the motor reaches about 70-80 % of normal speed through switch to delta position & take no load reading of voltmeter, ammeter, wattmeter & speed. Load the machine in steps & at each step note the readings. The experiment was conducted up to full load current.

RESULT

The load test on three phase induction motor is conducted & the following graphs were plotted.

• Output Vs Efficiency

• Output Vs Torque

• Output Vs Power factor

• Output Vs Slip

• Output Vs Line current

• Output Vs Speed

SL No

Line Voltage

Line Current

Input Power Speed Weight balance Slip Torque Output Efficienc

y Power

(V) (I) (P) (N) W1 W2 W S T W ƞ % Factorvolts Ampere Watts RPM Kg Kg Kg Nm Watts

Circumference of brake drum 2 πR = ------ mRadius of brake drum R = ----- / 2 π m

Useful Formulae(1) Torque T = (W )*9.81*R N-m

(2) Input power P = (Wattmeter reading) watts

(3) Output power Pm = 2π NT / 60 watts

(4) Efficiency ƞ = (Pm / P) x 100 %(5) % Slip s = (NS – N)/NS x100 (6) Power factor = Input power /3VI

Circuit Diagram

Expected Graph

Experiment No. 3

REGULATION OF SALIENT POLE ALTERNATOR

AIM

• To determine direct axis synchronous reactance Xd & Xq of the given alternator by slip, hence predetermine the full load regulation at upf, 0.8 pf lag & 0.8 pf lead.

• Conduct load test at upf on the given alternator & obtain the regulation at 0.5 Full load.

MACHINE DETAILS

• DC Motor

Power: 5.2 kW

Voltage: 220 V

Current : 29 A

Speed: 1500 rpm

• AC Generator

Power: 5 kVA

Voltage: 415 V

Current : 6.95 A

Power factor: 0.8

COMPONENTS REQUIRED

Sl No.

Meter Rating Quantity

1 Ammeter 0-10 A 12 Voltmeter 0-600 V 13 Voltmeter 0-300 V 14 Rheostat 40Ω, 10 A 1

5 Rheostat 240Ω, 1.5 A 1

THEORY

A Synchronous machine is an AC machine in which the rotor moves at a speed which bears a constant relationship to the frequency of currents in the armature winding. Large ac networks operating at constant frequency of 50 Hz relay almost exclusively on synchronous generators also called the alternators for the supply of electrical energy & may have synchronous compensators at key points for control of reactive power.

The operating principle of a synchronous machine is fundamentally the same as that of a DC machine, but unlike the latter, in the synchronous there is no need to rectify the time varying emf which is induced in the armature winding. Consequently a synchronous machine does not require a commutator, thus they are simple & easy in construction.

Voltage regulation of the alternator is defined as the increase in terminal voltage, when the load of a given power factor is thrown off with speed & field current remaining the same.

% Voltage regulation= (E0-V)/V * 100

PROCEDURE

• Make the connections as in figure.

• Drive the alternator in the proper direction by the dc motor at a speed slightly above or below the synchronous speed.

• Apply a low voltage ( above 20 % of normal voltage) to the armature by means of autotransformer.

• If the slip is small & rotor rotates in the same direction as the rotating magnetic field, the voltmeter connected across the alternator field winding shows zero rading.

• If not, reverse the phase sequence of the ac supply.

• When the slip is small, the pointer of ammeter & voltmeter will give the maximum & minimum reading owning to the variations in the reactance of the magnetic circuit.

• Note the readings of Vmax, Vmin, Imax & Imin.

• Take different set of readings by increasing the applied ac voltage.

RESULT

The direct axis & quadrature axis synchronous reactance of the given alternator has been determined.

Xd =

Xq =

Full load regulation at upf =

0.8 pf lag=

0.8 pf lead =

Regulation at 0.5 full load=

Regulation at 0.5 full load from load test=

Experiment No. 4

PREDETERMINATION OF FIELD CURENT OF SYNCHRONOUS INDUCTION MOTOR

AIM

To conduct no load test and turns ratio test on the given synchronous induction motor and to predetermine the field current & input current corresponding to half full load & 1/4th full load to 0.8pf load, also draw the current locus of synchronous motor.

MACHINE DETAILS

Output Power: 3.5 kW

Voltage: 415 V

Current : 5 A

Speed: 1500 rpm

COMPONENTS REQUIRED

THEORY

PROCEDURE

No load test:

Conduct no load test after short circuiting the winding terminals. Using the autotransformer, apply reduced voltage at starting & increase the voltage to rated valve (Vo). Note Vo, Io & Wo.

Turns Ratio test:

Wireup the circuit. Apply a rated voltage to the stator (about 50% normal) & note the the rotor induced emf.

Turns ratio= Vs/ Vr

RESULT

The no load test & turns ratio test on synchronous induction motor has been conducted & the field current & input current corresponding to half full load & 1/4th full load at 0.8 pf leading has been predetermined. The current locus of the synchronous induction motor has also been drawn.

Experiment No. 5

NO LOAD & BLOCK ROTOR TEST ON SLIP RING INDUCTION MOTOR

AIM

To conduct no load test and blocked rotor test on the given three phase slip ring induction motor, & draw the circle diagram and equivalent circuit. From circle diagram & equivalent circuit compare efficiency, power factor, slip, line current & output corresponding to

• Name plate speed

• Rated output

MACHINE DETAILS


Voltage: 415 V

Current : 7.5 A

Speed: 1410 rpm

COMPONENTS REQUIRED

THEORY

PROCEDURE

No load test:

Connections are made as per the connection diagram. Make sure the motor is on no load & start the motor using the autotransformer. By varying the auto transformer, apply rated voltage on the motor. Note down the reading Vo, Io & Wo.

Blocked rotor test:

Connections are made as per the connection diagram. Make the motor on blocked condition & start the motor using the autotransformer. By varying the auto transformer, apply rated current across the motor. Note down the reading Vsc, Isc & Wsc.

RESULT

No load test & blocked rotor test on given three phase slip ring induction motor has been conducted . The circle diagram & the approximate equivalent circuit are drawn.

Experiment No. 6

NO LOAD & BLOCK ROTOR TEST ON SQURRIEL CAGE INDUCTION MOTOR

AIM

To conduct no load test and blocked rotor test on the given three phase squirrel cage induction motor, & draw the circle diagram and equivalent circuit. From circle diagram & equivalent circuit compare efficiency, power factor, slip, line current & output corresponding to

• Name plate speed

• Rated output

MACHINE DETAILS


Voltage: 415 V

Current : 7.5 A

Speed: 1430 rpm

COMPONENTS REQUIRED

THEORY

PROCEDURE

No load test:


Blocked rotor test:

Connections are made as per the connection diagram. Make the motor on blocked condition & start the motor using the autotransformer. By varying the auto transformer, apply rated current across the motor. Note down the reading Vsc, Isc & Wsc.

RESULT

No load test & blocked rotor test on given three phase squirrel cage induction motor has been conducted . The circle diagram & the approximate equivalent circuit are drawn.

Experiment No. 7

STARTING TORQUE OF SLIP RING INDUCTION MOTOR

AIM

• To obtain the starting torque for different known values of resistances added externally in the rotor circuit.

• To plot the variation of starting torque with external resistance.

MACHINE DETAILS


Voltage: 415 V

Current : 7.5 A

Speed: 1410 rpm

COMPONENTS REQUIRED

THEORY

PROCEDURE

No load test:


Load test:

RESULT

Experiment No. 8

TEST ON POLE CHANGING INDUCTION MOTOR

AIM

To predetermine the starting torque, maximum torque & full load p.f for the two different speed settings ( 8 pole & 6 pole) of the pole changing induction motor.

MACHINE DETAILS

Rated Power: 3.7/ 2.2 kW

Voltage: 415 V

Current : 8.0/ 5.8 A

Speed: 1423/ 697 rpm

COMPONENTS REQUIRED

THEORY

The speed of an induction motor can be changed by changing the number of stator poles. This method is applicable to squrriel cage induction motors because it adopts itself to any number of stator poles.

Synchronous speed, Ns=120f/P, where P is the number of poles. This change of poles is achieved by having two or three entirely independent stator winding in the same slots. Each wining gives a different number of poles & hence different synchronous speed.

PROCEDURE

Conduct no load & blocked rotor test for the two different pole settings & draw the circle diagram in the same sheet to the same scale.

Note: Draw separate diagrams for no load & blocked rotor test for the 6 pole configuration. Fix the meter readings after noting the machine details.

No load test:

Connections are made as per the connection diagram for 8 pole setting. With the help of auto transformer, apply reduced at starting & increase the voltage to rated value (V). Note down the reading Vo, Io & Wo. Now change the connections to 6 pole setting & take no load readings.

Blocked rotor test:

Connect meters of appropriate readings. Block the rotor by belt. Apply a low voltage of approximately 20% of normal value to send rated current. Note Vs, Is & Ws. Repeat for 6 pole setting.

Measurement of Stator resistance

Measure stator resistance per phase using dc supply and calculate the effective ac resistance per phase as follows.

8 pole setting: measure the resistance between two terminals & calculate the resistance per phase.

6 pole setting: measure the resistance per phase by connecting dc supply.

RESULT

Predetermined the starting torque, maximum torque & full load p.f for the different speed setting in a pole changing induction motor.

Experiment No. 9

V AND INVERTED V CUREVES OF SALIENT POLE ALTERNATOR

AIM

To plot V & inverted V curves for the given salient pole machine & synchronize the machine using Dark lamp method.

MACHINE DETAILS

• DC motor


Voltage: 220 V

Rated current: 29 A

Speed: 1500 rpm

• A.C Generator

Output Power : 5 kVA

Voltage: 415 V

Current : 6.95 A

Speed: 1500 rpm

p.f: 0.8

COMPONENTS REQUIRED

Sl No.

Meter Rating Quantity

1 Ammeter 0-10 A 12 Voltmeter 0-300 V 13 Wattmeter 230 V, 10A, hpf 1

THEORY

PROCEDURE

• Connections are done as per the connection diagram. Motor armature rheostat is kept at maximum position & field rheostat at minimum positon, alternator field rheostat at minimum position.

• Keeping switch S2 in open position, supply is given to the motor.

• By adjusting the motor rheostats, set motor at rated speed.

• Field rheostat of alternator is adjusted to obtain the terminal voltage of alternator.

• Synchronizing using Dark lamp method.

• For correct phase sequence, all the three sets of lamps should flicker simultaneously. Otherwise the phase sequence is not correct. To correct it, any two terminals of alternator are interchanged.

• The frequency of alternating voltage made equal to the supply voltage frequency by adjusting the prime mover speed.

• After satisfying the necessary conditions the alternator is synchronized to the mains by closing the switch S2 in the middle of the dark period.

• To plot the V curves

• By varying the prime mover speed, active power is kept constant at 1kW.

• Reactive power is varied by varying the alternator side field rheostat & for various values of alternator field current, armature current & wattmeter readings are noted.

RESULT

The V & inverted V curves for the given salient pole machine using dark lamp method.

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