dc machine practical

ELECTRICAL MACHINES LAB

DEPARTMENT OF ELECTRICAL ENGINEERINGMehran University of Engineering and Technology SZAB Campus.

For

Second Year( Department of Electrical Engineering)

ROLL No: 01, 03, 04, 05, 07, 09, 11, 12, 13, 14Batch : 12 ElectricalDepartment: Electrical EngineeringSubject: DC MachineClass Advisor: Ahsanullah Memon

Supervised by: Ahsanullah Memon (Lecturer)

Complied by: Group No. 1

INDEX

S.NO& DATE EXPERIMENTS

01.28-08-13

Introduction to Electrical Machines, and Transformer.

02.04-09-13

Verification of O.C.C of Seprately Excited DC Generator

03.23.09.13

Verification of O.C.C of Self Excited Series DC Generator.

04.23.09.13

To verify Internal Characteristics of Separately excited DC Generator (Verification of Armature reaction.)

05.24.09.13

Verification of Characteristics of DC Shunt Generator.

06.24.09.13

Verification of Characteristics of Over Compound Generator

07.27.09.13

Effect of Field current on back EMF & Speed of Separately excited DC Motor.

08.27.09.13

Effect of Line resistance on Back EMF of Separately excited DC Motor.

09.27.09.13

Effect of Load on Back EMF & Armature current of Separately excited DC Motor.

10.5.10.2013

Speed Control of DC Motor by Ward-Leonard System (Voltage Control).

11. 09.10.2013

Speed Control of DC Series motor by putting diverter in parallel to field winding.

12.15.10.2013

Speed Control of DC Series Motor by putting diverters in parallel to armature.

Practical # 01

OBJECT :-Introduction of Electrical Machines equipments and transformer

Equipment :-

1. DC Motor2. DC Generator3. Transformer4. YL-195 Motor, Electric traction and Electrical control trainer

1. DC Motor:

CIRCUIT DIAGRAM

THEORY :-Motors take electrical energy and produce mechanical energy. Mechanical energy can be supplied to

various types of load. The motor operating on DC supply are called DC motor. DC motors do not operate in protected locations, they face dust, moisture, fumes and tend to get mechanical damage. Hence, their construction is of closed type. The operation of motors is based on the principle – when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. The magnitude of this force is F = BIL, and its direction is determined by Fleming's left-hand rule.

DC Machine Practical Journal | 3

Working of D.C. Motor Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in Fig.(4.1). Since each armature conductor is carrying current and is placed in the magnetic field, mechanical force acts on it. Referring to Fig. (4.1) and applying Fleming’s left hand rule, it is clear that force on each conductor is tending to rotate the armature in anticlockwise direction. Speed is controlled by varying the rotor voltage and hence the rotor current, or by varying the magnetic flux in the air gap by changing the current in the field windings. With access to both the field and rotor windings, all DC motors offer the facility of simple speed and torque control.

Ratings and Figure of DC Motor

Below is One of type of DC Motor named as DC Separately Excited Motor in which The armature and field winding are electrically separate from each other and The field winding is excited by a separate DC source.


Schematic Diagram

Ratings of DC Motor (Seprately Excited)

1 Power (Pn) 120W

2 Frequency 1000 r/min

3 Voltage 110

4 Current (in amperes) 1.25A

Figure

2. DC Generator:Theory:

A generator is a machine that converts mechanical energy into electrical energy by using the Principle of magnetic induction which is done by rotating armature which contains conductors and move them through a magnetic field. The voltage generated by DC Generator varies from 0 to its maximum value twice each revolution of its loop.

The construction of a DC generator is very similar to the construction of a DC motor.The rotor consists of an electromagnet providing the field excitation. The construction of a DC generator is very similar to the construction of a DC motor


DC Generator

Basic Components of DC Generator

1. Armature: It is structure upon which are mounted are coils that which cuts magnetic lines of force.2. Commutator: It is that Component of DC Generator that rectifies generated alternating current to provide

direct current output and connects the stationary output terminal to rotating armature.3. Brushes: Brushes make contact with the commutator to collect the current generated by armature coil.4. Poles are required to produce magnetic field or flux which passes through conductor of armature coil.

Ratings of DC Generator

1 Power (Pn) 120W

2 RPM 1000 r/min

3 Voltage 110



3. Transformer:Theory:

A transformer is a device having no movable parts that transfer power from one circuit to another circuit usually with changes values of both current and voltage. As current vary in primary coil will creates a varying magnetic flux in the transformer's core as a result magnetic flux vary in secondary coil. The variation in secondary coil will induces emf or voltage in secondary coil. Transformers can link two or more electric circuits. In its simple form two electric circuits can be linked by a magnetic circuit, one of the electric coils is used for the creation induced voltage in the same.

Transformers have good applications in High voltage Power Transmission.

Ratings of Transformer

4. YL-195 Motor, Electric traction and Electrical control trainer: This type of panel is used in practical such as of dc generators, motors, transformers etc. It contains equipments such as millimeter, power supply rheostats, switches etc. A general View of this panel is shown below.

Components:


1 Primary Voltage 127V

2 Frequency 50 hertz

3 Secondary Voltage 50V


http://en.wikipedia.org/wiki/Magnetic_core

http://en.wikipedia.org/wiki/Magnetic_flux

3-Phase AC DC power supply

1-Phase AC power supply and Knife Switch

AC DC voltmeters and ammeters

Electricity parameter tester


Practical # 02

Objective:

Verification of O.C.C (Open Circuit Characteristics) of Separately Excited DC Generator.

Equipments:Power SupplyDC VoltmeterDC AmmeterDC Supply3-Phase Induction Motor

The separately excited motor has independent voltage supplies to the field and rotor windings allowing more control over the motor performance.

CharacteristicsThe voltage on either the field or the rotor windings can be used to control the speed and torque of a separately excited motor.

This curve shows the relation between the generated e.m.f. at no-load (E0) andthe field current (If) at constant speed. It is also known as magnetic characteristic or no-load saturation curve. Its shape is practically the same for all generators whether separately or self-excited. The data for O.C.C. curve are obtained experimentally by operating the generator at no load and constant speed and recording the change in terminal voltage as the field current is varied.

The O.C.C is also known as no load magnetizing characteristics. It gives relation between generated emf on no-load and the field current when machine is driven at rated speed. The field winding of generator is provided with separate DC Source. The Ammeter is connected in series that will be used to read excitation current through field winding while voltmeter is connected with armature to read generated emf.

DC Machine Practical Journal 9

Procedure: The O.C.C. for a d.c. generator is determined and verified as follows

1. Connections are made as per the circuit diagram.2. Disconnect the field winding of DC Generator from the machine and is separately excited from an

external d.c. source as shown in figure above.3. Connect Ammeter with series with field winding that will read excitation current through the field

winding. 4. Connect the voltmeter across the armature that will read the emf generated by the machine5. The generator is run at fixed speed (i.e., normal speed). The field current (If) is increased from zero

in steps and the corresponding values of generated e.m.f. (E0) read off on a voltmeter connected across the armature terminals. On plotting the relation between E0 and If, we get the open circuit characteristic as shown in Fig.

Connections:


Readings:

V 15 31 48 74 97 124 153 183

If 0.02 0.05 0.07 0.10 0.13 0.17 0.21 0.24

Eo 26 54 84 121 141 154 163 172

Conclusion: We Observe from practical that on no loaf Generated emf was still 8V, This was due to residual flux. And there was one point after (183V )which No load voltage (Eo) stopped increasing, that point is called saturation point. To sum up, as the voltage was increased the generated e.m.f. (Eo) was increases.

Open Characteristic Curve


Practical # 03

Object: Verification of O.C.C of Self Excited Series DC Generator.

Equipment Used:

3-Phase AC Power Supply 3-Phase Induction Motor Self excited series connected DC Generator Fixed & Variable resistive loads DC Voltmeter DC Ammeter

Theory:Figure below describes the characteristics of dc series generator. The curve (a) shows open circuit

characteristics, curve (b) shows internal and curve 3 indicate external characteristics of dc series generator. When Load current is 0 the generated and terminal armature V are same, both being due to residual magnetic field. When when load is applied, Automatic build up of V takes place from the point when load current flows through series field winding producing additional flux aiding residual flux.as further increase in load beyond the maximum voltage point produces sharply drooping characteristics as shows below. The reason being the increased voltage drops and the increased armature reaction decreased the load voltage at a much faster than increase in generated voltage taking place by the increased load current.

Procedure:


1. Connect the circuit as shown in connection below. (We need to connect the feld winding across the armature instead of connecting it to a separate supply).2. Slowly increase the input to the prime-mover. Speed of the set will increase. Observe the voltmeter connected across the terminals of the dc generator. Above a certain speed the voltmeter reading starts increasing (If this does not happen reduce the prime-mover input and switch o® the supply. Interchange the feld terminals of the generator and repeat the same procedure). By controllingthe input to the prime mover adjust the speed to the rated speed of the machine.3. Close the main switch S. Load the generator in steps by switching ON the load and for eachcase adjust the prime mover input such that the speed remains constant. Note down the loadvoltage and current, feld current of the generator, armature current and voltage.Repeat this procedure till the load current is equal to the rated current of the generator.4. Put off all the load, open the main switch S, reduce the prime mover input and put off the ACsupply to the controller.

Discription:

Connections:


Load Resistance Field Current Generated VoltageAt No load Eo ∞ 0 10

At Load 1 200 0.06 13At Load 2 300 0.76 78

At Load 3 400 1.54 79

Characteristic Curve:


3 Phase AC Supply

3 Phase Induction Motor

Load 1

Load 2

Self Excited Series Motor

DC Ammeter & Voltmeter

Switch 2Switch 1

Result:

We Observe from the curve that there is certain field current after which generated voltage remain constant even after increasing the load that point is called satuaration point.

Practical # 04Object: To verify Internal Characteristics of Separately excited DC Generator (Verification of Armature reaction.)

Equipments: 3-Phase AC Power Supply


Due to Satuaration Point

Eo Load 1 Load 2 Load 3

3-Phase Induction Motor Regulated AC & DC Supply DC Separately excited Generator Variable & Fixed Resistive Loads DC Voltmeter DC Ammeter

THEORY :-The load or external characteristic of a generator is the relation between the terminal voltage and load current. The characteristic expressed the manner in which the voltage across the load varies with I, the value of load current. The internal or total characteristic of a generator is the relation between the e.m.f actually induced in the generator Ea and the armature current Ia. The internal characteristic of the generator, which is separately excited, can be obtained as below:

Let:Vt = Terminal voltageIa = Armature currentRa = Armature resistanceThen,Ea = Vt + IaRa

Ia = ILTherefore if we add drop of armature (IaRa) to terminal voltage Vt we get actually induced e.m.f (Ea).

PROCEDURE:-1. Make the connections according to the circuit diagram.2. Run the generator at a constant speed and the exciting current.3. Connect the load across the generator armature.4. Increase the load current gradually by the help of load rheostat and note the readings of ammeters and voltmeter.

Discription (Readings):VDC= 100Ia= IL

Load Eg Ia=ILAt No Load 142 0At Load 1 (200Ω+100Ω variable)

126 0.61

At Load 2 (250Ω+150Ω variable)

111 0.85

At Load 3 (100Ω+0Ω variable)

85 1.58

Connections:


Characteristic Curve:

Practical # 05


3 phase induction motor

3 phase Power supply

Internal Characteristic Curve

Object: Verification of Characteristics of DC Shunt Generator.

Equipments

3-Phase AC Power Supply 3-Phase Induction Motor DC Self excited Compound Generator DC Voltmeter DC Ammeter Regulated AC & DC Supply Variable & Fixed Resistive Loads

Theory:A generator when operated for the general operation its speed practically is constant under

such conditions it gives relationship among excitation load current and voltage. These relation when shown by the graphs then they are called characteristics which are actually three curves.

1. Open Circuit Chacteristics2. Internal Characteristics3. External Characteristics

OCC show relationship between no load generated emf and field current.when generator builds up small voltage generated emf remains constant coz it is not applied to any load. Internal Characteristics shows relationship between on load generated emf and armature current. When generator is put on load the emf decreases due to reduction of flux at per pole. So as obserbed in practical emf generated on no load condition is greater than the emf generated on load. The external characteristics curve show relation between terminal voltage and load current. As we put generator on load, the load current was decreased when calucalated this was because of because of amount of ohmic drop.

Procedure: 1. Make connection as per given. 2. Adjust field regulator to the motor so that generator runs at its rated speed.3. Adjust generator field regulator so that rated voltage is obtained at its terminals.4. Gradually apply load in steps and note down readings of the load. Keep the speed constant

at its rated value by adjusting motor field regulator throughout experiment.5. Plot the external load characteristics from the experiment.6. Determine the armature resistance experimentally7. Take readings of voltmeter and armature by varying the load resistance and determine

average value of armature resistance. Precautions:

While sharting the the motor the field regulator (rhostate) must be in minimum position

Connections:


Seprately Excited On No load.

Seprately Excited On load.


Regulator

Self Compund Generator


3 Phase DC Supply

DC Ammeter and Voltmeter


Load 1Load 2

Regulator3 Phase Induction Motor

DC Ammeter and VoltmeterSelf Compund Generator

3 Phase DC Supply

Practical # 06

Object: Verification of Characteristics of Over Compound Generator

Equipments: 3-Phase AC Power Supply DC Voltmeter DC Ammeter Regulated AC & DC Supply 3-Phase Induction Motor DC Self excited Compound Generator Variable & Fixed Resistive Loads

Theory:If the series winding turns are so adjusted that with the increase in load current the terminal

voltage increases it is called over compound. In such case as the load current increases the series field mmf increaes and tends to increase the flux and hence the generated voltage. The increase in generated voltage is greater than the IaRa drop so that instead of decreasing the terminal voltage increaes. The series excitation aids the shunt excitation. The degree of compounding depends upon the increase in series excitation with the increase in load current.

Discription:First we connected series winding, we observed that on no load voltage is 8V due to residual flux as the

load increases the voltage as well as current increases. Then we will observe parameters for Shunt connected also.

Incase of Series Windings Only.

Load Eg ILNo load 8 0Load 1 78 0.62

Load 2 94 0.88


On No Load: Eg is 8V due to residual Flux, Ia=0

When Load 1 is Connected as seen in connection: Putting L1 (using series winding only) Eg Increases as well as Ia.

When Load 2 is connected as seen in connection: Putting L2 (using series winding only) Eg increases as well as Ia.

When Parallel Component is not added



Self Compund Genrator

Load 1

Ammeter

Load 2

Voltmeter and Ammeter

3 Phase Supply


Incase of Shunt Also connected with Series Windings.

Load Eg ILNo load 8 0Load 1 28 0.21

Load 2 89 0.83

On No load: While using both shunt ans series winding, on no load Eg increases as well as Ia.

When Load 1 is connected: Eg increases as well as Ia.

When Load 2 is connected: Eg further increases as well as Ia.


When Parallel Component is added


3 Phase Induction Motor Self Compund Genrator

Load 2

Load 1

Voltmeter and Ammeter3 Phase Supply

Practical # 07

Objective: Effect of Field current on back EMF & Speed of Separately excited DC Motor.

Equipments

3-Phase AC Power Supply 3-Phase Induction Motor DC Self excited Compound Generator DC Voltmeter DC Ammeter Regulated AC & DC Supply Variable & Fixed Resistive Loads

Theory: The aim of this Practical is to see the effect of field current on back emf and to observe the speed of separately excited dc motor of DC motor speed control. The greatest advantage of DC motors may be speed control. Since speed is directly proportional to armature voltage and inversely proportional to the magnetic flux produced by the poles, adjusting the armature voltage and/or the field current will change the rotor speed.

The field current, If, is constant (and hence the flux density B is constant), and the armature voltage is varied. A constant field current is obtained by separately exciting the field from fixed dc source. The flux is produced by the field current, therefore, essentials constant. Thus the torque is proportional only to the armature current. As we increase the Vdc of separately excited motor If increases, flux increases and back emf increases but speed (N) decreases. Since we know that

Eb= V-IaRa,

Increasing resistance will decrease Eb &As load is increased, armature current also increases.

Connections:


Table:


LiResistance is kept constast that is 294 whereas applied voltage is158 Volts

Practical # 08Object:


Vdc 15 34 51 71 100 121 149If 0.02 0.05 0.07 0.10 0.14 0.17 0.20

Eb 63 96 107 117 121 122 123

Effect of Line resistance on Back EMF of Separately excited DC Motor.

Equipments: Single Phase Supply AC/DC Converter & Regulator DC Ammeter DC Voltmeter Variable Resistor DC regulated Power Supply Separately Excited DC Generator or Motor

Theory: In D.C motor, supply voltage V has to overcome back e.m.f which is opposing V and also various drops

as armature resistance drop, brush drop etc. Infact the electrical work done in overcoming the back e.m.f. gets converted into the mechanical energy developed in the armature. The voltage equation of a d.c. motor is,

Eb= V-IaRa,

It was observed from the practical as the resistance was increased back emf was decreased this was due to that the armature drops are inversely proportional to Baack emf.

Table:

Observed Effect of Line resistance on Eb.As seen Eb is decreasing on decrease of RL.

VDC = 149 Volts If = 0.2 amp.

R 294 394 494 594 694 794 894Eb 123 111 99 89 88 78 57

Connections:


Practical # 09

Object: Effect of Load on Back EMF & Armature current of Separately excited DC Motor.

Equipments: Single Phase Supply AC/DC Converter & Regulator DC Ammeter DC Voltmeter Fixed Resistance DC regulated Power Supply Separately Excited DC Generator or Motor Mechanical Loads

Theory: The principle that the back e.m.f. is proportional to speed, Eb α N. Initially When load is put on to the motor, motor slow down as a result speed of the motor and back emf reduces. The net voltage across the armature increases. As a result when load is put on motor, the motors carries more armature current. The motor speeds stops decreasing when the armature current is just enough to produce torque demand by the new load. When load on the motor is decreased, the speed of the motor tries to increase. Hence back e.m.f. increases. This causes (V- Eb) to reduce which eventually reduces the current drawn by the armature. The motor speed stops increasing when the armature current is just enough to produce the less torque required by the new load.

Table:

Table (Readings):

Armatur drops/Line resistance = 174 ohms, VDC = 148 Volts & V = 157 Volts

Load Eg IaAt No Load 138 0.13At Load 1 124 0.18At Load 2 111 0.25At Load 3 94 0.33

Connections:

At Load 1:


At 2 Load:


At Load 3


Practical # 10Object: Speed Control of DC Motor by Ward-Leonard System (Voltage Control).Equipments:

Single Phase Supply AC/DC Converter & Regulator DC Ammeter DC Voltmeter Fixed Resistance DC regulated Power Supply Separately Excited DC Generator Separately Excited DC Motor

Theory: This method of Ward Leonard system is one of favorable method for DC Motor speed control. In this process the speed of motor is controlled by armature from 0 to load base speed. On separately excited flux is constant and do not depend on load (N constant) (V constant). If you require higher motor speed Field current is lowered in this process as a result torque reduces. This system provides constant torque below base speed, rated horsepower (and torque) at base speed, and constant horsepower (hence variable torque) above base speed. In ward Leonard system speed of motor is controlled in both directions

Procedure: Give 3 Phase supply to induction motor.

Connect remaining terminal in star connection.

Attach Motor shaft to Dc generator.

Provide Separately Dc TO DC generator to be excited. (Sperately DC canbe achieved from AC/DC Regulated panel

One Ac supply is given to Ac/Dc regulated panel, which gives us regulated supply.

Connect Voltmeter to A1 and A2 to measure Eg.

Dc supply generated from generator is given to Dc motor and In between them ammeter is connected to measure IL.

Separately Dc is supplied to Dc motor to be separately excited.

Volt meter is connected across A1 and A2 of Dc motor to measure Eb.

Fixed resistor is connected in between that supply and B1/B2.

Ammeter is also connected between separately excited winding of Dc motor and Dc supply.

Dc voltage across B1 and B2 of Dc generator varied to see its effect on Eg , Eb, IL and If.

As Vdc increases, all Eg, Eb, IL and If and speed also increases.

Connections:


Table:VDC 0 12 29 46 61 76Eg

(Generator)5 20 49 78 102 121

IL

(Motor)0.09 0.10 0.11 0.11 0.12 0.12

If

(Generator)0 0.02 0.04 0.06 0.09 0.12

N(Speed)

50 75 100 150 175 200

Conclusion: Flux of series and shunt depend on load. it was seen speed increases with increase in Eg means supply of DC motor.

Practical # 11DC Machine Practical Journal 36

Object: Speed Control of DC Series motor by putting diverter in parallel to field winding.

Equipments: Equipments used in practical were: Series DC Motor Loads (DC Generator & 3-Ph Induction Motor) Variable Resistor DC Ammeter & Voltmeter

Theory: In this method variable resistance (called field diverter) is connected parallel with series field winding. The main reason to do this is to shunt some portion of the line current from the series field winding, as a result weakening the field and increasing the speed. The lowest speed obtainable is that corresponding to zero current in the diverter). The lowest speed obtainable is the normal speed of the motor. Consequently, this method can only provide speeds above the normal speed. The series field diverter method is often employed in traction work.

Procedure: Apply Vdc is applied across A1 and B2 (VDC Of motor is constant)

Connect Ammeter between supply and A1 B2

Connect A2 to B1 to connect field winding in series

Apply Load through the shaft of motor


Attach diverter across the field winding to control the speed of DC Motor. Here we used Variable resistor as a diverter Connections:

Conclusion:

It was seen that when the field diverter resistance was increased then the flux were increasing and due to increase in flux the speed was decreasing, as are inverse proportional to each other.


Practical # 12Object: Speed Control of DC Series Motor by putting diverters in parallel to armature.

Equipments:

Series DC Motor Loads (DC Generator & 3-Ph Induction Motor) DC Ammeter & Voltmeter Variable Resistor

Theory: We connect variable resistance or armature diverter is connected in parallel with the armature Line current is shunt by the diverter that results in reducing the armature current. Now for a given load, if Ia Is decreased, the flux must increase Since, the motor speed is decreased. By adjusting the armature diverter, any speed lower than the normal speed can be obtained.

Procedure: Give separately DC Supply to A1 & B2 terminals of DC Series connected Motor. Connect Ammeter in between them to measure Armature Current. Connect Variable resistor in Parallel/Shunt to Armature A1 & A2. Connect Loads to the shaft of DC Series Motor. Vary Resistance to see its effect on armature current.

Connections:


Conclusion: The speed of DC motor was decreasing with the increase of resistance in armature diverter. AS the resistance increase the speed will decrease because the current will then go through the armature & Ia is increased due to load then the flux increases & speed will decrease as they have inverse relation to each other.

The End

T h a n k y o u


dc machine practical

Engineering

dc motors

dc supply

dc motor2

dc generator3

type of dc motor

speed control of dc

dc shunt generator

component of dc generator