DC Machine Experiments Manual

LAB EXPERIMENT REPORTS OF “DC MACHINES (EE-223)”

Instructor:

Sir Zeeshan Arfeen

Performed By:

Muhammad Sarwar 10EL20 Electrical Engineering (A) 4th Semester

University College of Engineering & Technology The Islamia University of Bahawalpur

2

Index of Performed Experiments:

Sr #

Experiments Title

Page #

1.

Safety precaution

3

2.

To identify and study main parts of a DC machine

4

3.

Different Types of Connections in Dc Generators

8

4.

O.C.C of Separately Excited Dc Generator

10

5.

12

6.

External characteristics of Separately Excited Dc Generator Ta /Ia Characteristics of DC shunt motor

7.

N /Ia Characteristics of DC shunt motor

16

8.

N/Ta Characteristics of DC shunt motor

18

9.

Plotting Graph of Torque Speed Curve of a Shunt DC motor using Matlab Plotting Graph of Speed(n) Vs Field Resistance(RF) of a Shunt DC Motor Plotting Graph of Torque Speed Curve of a Shunt DC motor using Matlab

20

10. 11.

14

22 24

3

SAFETY PRECAUTIONS Nine rules for safe practice and to avoid electric shocks:

1. Be sure of the conditions of the equipment and the dangers present BEFORE working on a piece of equipment. Many sportsmen are killed by supposedly unloaded guns; many technicians are killed by supposedly “dead”. Circuits, 2. NEVER rely on safety devices such as fuses, relays and interlock systems to protect you. They may not be working and may fail to protect when most needed. 3. NEVER remove the grounding prong of a three wire input plug .this eliminates the grounding feature of the equipment making it a potential shock hazard. 4. Disorganized mess of connecting leads, components and tools only leads to careless thinking circuits, shocks and accidents.

5.

DO NOT WORK ON WET FLOOR

Do not work on wet floor or bare footed. Always work on a rubber mate or an insulated floor.

6.

DO NOT WORK ALONE.

It’s just good to have someone around to shot off the power give artificial respiration and to call a doctor.

7.

WORK WITH ONE HAND WHILE WORKING WITH ELCTRIC CIRCUITS

A current in between a hand’s crosses your heart and can be more let than a current from hand to foot .a wise technician always work with one hand .watch your service man

8.

NEVER TALK TO ANYONE WHILE WORKING

Do not let yourself distracted. Also do not talk to anyone, if he is working on dangerous equipment. Do not be the cause of an accident.

9.

ALWAYS MOVE SLOWLY

When work around electrical circuits. Violent and rapid movements lead to accidental shock and short circuits.

4

The Islamia University of Bahawalpur University College of Engineering & Technology Electrical Engineering (4th Direct Current Machines LAB EXPERIMENT:01

Name:

Roll No:

Lab Instructor Signature:

Date:

Identify and Study Main Parts of DC Generator Objective: To study main parts of a DC generator.

Apparatus: DC generator Yoke, Poles, Armature, Commutator, Brushes.

Theory: A DC generator is comprised of following main parts

1. 2. 3. 4. 5.

Field system Armature Core Armature Winding Commutator Carbon Brushes

1. Field System: The function of the field system is to produce uniform magnetic field within which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the d.c exciting current. The field coils are connected in such a way that adjacent pole shave opposite polarity.

5

The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature and the frame Practical d.c. machines have air gaps ranging from 0.5 mm to 1.5 mm. Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap. By reducing the length of air gap, we can reduce the size of field coils (i.e. number of turns).

2. Armature Core: The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in Fig. The laminations

are individually coated with a thin insulating film so that they do not come in electrical contact with each other. The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to cross between the pole face and the armature “teeth”.

6

3. Armature Winding: The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding. This is the winding in which “working” e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current. The armature winding of a d.c. machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.

4. Commutator: A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes. The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine (See Fig). The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking. The sparks may bum the brushes and overheat and carbonise the commutator.

5. Carbon Brushes: The purpose of brushes is to ensure electrical connections between the rotating commutator and stationary external load circuit. The brushes are made of carbonand rest on the commutator. The brush pressure is adjusted by means of adjustable springs (See Fig). If the brush pressure is very large, the friction produces heating of the commutator and

7

the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking. Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal.

8


Name:

Roll No:


Date:

Different Types of Connections in Dc Generators Objectives: To understand different types of DC Machines 1.

Separately Excited DC Generator

2.

Shunt Excited DC Generator

3.

Series Excited DC Generator

Apparatus: 1.

DC Generator SM 2641

2.

DC Power Supply

3.

Connecting Leads

4.

Voltmeter

Circuit Diagram:

(Separately excited dc generator)

9

(Shunt dc-generator)

(Series dc generator)

Theory: DC Machines are classified according to manner in which armature circuit is connected to the field circuit. So there are following main types 1. Separate Excited DC Generator 2. Shunt Excited DC Generator 3. Series Excited DC Generator In a separate excited DC Generator the armature and field circuits are supplied by separate voltage sources. In a shunt excited DC Generator both circuits are connected in parallel to each other. In a series excited DC Machine both the field and armature circuits are connected in series to each other. These connections are shown in the circuit diagram. In this lab exercise our aim is to achieve above stated connections.

Procedure: Make connections according to the given circuit diagram for each type of machine separately. After that you will see that we got different values for different connections.

10


Name:

Roll No:


Date:

O.C.C of Separately Excited Dc Generator Objective: To determine open circuit characteristics of a Separate Excited DC Generator.

Apparatus: 1. Power Supply Unit(SM 2635) 2. Torque Measuring Unit (MV 1052) 3. Drive machine, DC Machine (SM 2641 ) 4. Test Generator, DC Machine (SM 2641) 5. Voltmeter 6. Ammeter

Circuit Diagram:

11

Theory:

Open circuit characteristics curve also sometimes called no-load

characteristic, is a graph showing the relation between induced e.m.f of a generator on noload and the field current. The e.m.f of the generator at no-load is given by: Eo α NΦ If the speed be kept constant while this characteristic is being drawn in that case Eo becomes proportional to flux Φ, but flux is proportional to field current If. The curve between E0 and If is known as open circuit characteristic.

Procedure: 1.

Achieve the connections as shown in circuit diagram.

2.

Turn on the DC Power Supply and increase the excitation voltage gradually from zero to full value while keeping speed of prime mover to be constant (prime mover not shown in the diagram).

3.

Record the value of output voltage against each value of field current.

4.

Turn off power to the machine after accomplishing the task.

5.

Draw the graph between E0 and If .

Observations: S. No.

If

Eo

01

0.06

81

02

0.09

96

03

0.12

115

04

0.16

124

05

0.213

127

06

0.22

128

Graph between E0 and If:

12


Name:

Roll No:


Date:

External characteristics of Separately Excited Dc Generator

Objective:

To determine external characteristics of a Separately Excited DC Generator.

Apparatus: 1.

Power Supply Unit (SM 2635)

2.

Torque Measuring Unit (MV 1052)

3.

Drive Machine, DC Machine SM 2641

4.

Test Generator, DC Machine SM 2641

5.

voltmeter

6.

Ammeter

7.

Resistive load (SM 2676)

Circuit Diagram:

13

Theory:

In External characteristics curve showing the relation between

terminal voltage of a generator and load current. The terminal voltage will be less then E due to voltage drop in the armature circuit .Therefore, this curve will lie below the internal characteristic. The formula of terminal voltage for external characteristic is

V = E- IaRa – armature reaction drop As the load increase, the terminal voltage falls due to armature reaction drop and voltage drop across armature resistance. Here load current and armature current is same because both are in series connection.

Procedure: 1.

Achieve the connections as shown in circuit diagram.

2.

Turn on the DC Power Supply and increase the excitation voltage gradually from zero to full value while keeping speed of prime mover to be constant .

3.

Record the value of output voltage against each value of load current.

4.

Turn off power to the machine after accomplishing the task.

5.

Draw the graph between V and IL.

Observations: S. No.

IL

VT

01

0.268

55.9

02

0.329

54.3

03

0.371

53

04

0.445

52.4

05

0.489

51.2

Graph between V and IL:

14


Name:

Roll No:


Date:

Ta /Ia Characteristics of DC shunt motor Objectives: To find the Ta /Ia Characteristics of self excited DC shunt motor

Apparatus: 1. 2. 3. 4. 5.

Power Supply Unit (SM 2635) Torque Measuring Unit (MV 1052) DC Machine (SM 2641) Voltmeter and Ammeters Resistive load (SM 2676)

Circuit Diagram:

Theory: It is the curve between armature torque Ta and armature current Ia of DC motor. It is also known as electrical characteristics of the motors .In shunt motors the field current I sh is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant. In a dc motors,

15

Ta

Ia 

As flux is constant in shunt motor .So, Ta Ia As both the armature current and the torque are directly proportional so their characteristic curve is straight line. And therefore large current is required to start a heavy load .

Procedure: 1. Achieve the connections as shown in circuit diagram. 2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value. 3. Record the value of torque Ta and armature current Ia. 4. Turn off power to the machine after accomplishing the task. 5. Draw the graph between Ta & Ia

OBSERVATIONS: Sr. No. 1 2 3 4 5

IA (Amperes) 1.81 1.91 2.16 2.38 2.97

Ta (N-m) 0.09 0.11 0.24 0.33 0.48

Graph Between Torque (Ta) and speed (Ia):

16


Name:

Roll No:


Date:

N /Ia Characteristics of DC shunt motor Objectives: To find the N /Ia Characteristics of self excited DC shunt motor

Apparatus: 1. 2. 3. 4. 5.

Power Supply Unit (SM 2635) Torque Measuring Unit (MV 1052) DC Machine (SM 2641) Voltmeter and Ammeters Resistive laod (SM 2676)

Circuit Diagram:

Theory: It is the curve between speed N and armature current Ia of DC motor. .In shunt motors the field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant.

17

N

Eb/ 

Since flux is constant so we can say that speed is effected when Eb changes. And very small variation occurs in speed.

Procedure: 1. Achieve the connections as shown in circuit diagram. 2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value. 3. Record the value of torque N and armature current Ia. 4. Turn off power to the machine after accomplishing the task. 5. Draw the graph between N & Ia


IA(rpm) 1.81 1.91 2.16 2.38 2.97

N (r.p.m) 1200 1180 1160 1140 1120

Graph Between Torque (Ta) and speed (Ia):

18


Name:

Roll No:


Date:

N/Ta Characteristics of DC shunt motor Objectives: To find the N/Ta Characteristics of self excited DC shunt motor

Apparatus:     

Power Supply Unit SM 2631 Terminal Board SM 2635 Torque Measuring Unit MV 1052 DC Machine SM 2641 Volt and Ammeters

Circuit Diagram:

Theory: It is the curve between speed N and the armature torque Ta of DC motor. N/T a Characteristics is also known as mechanical characteristics. In a shunt motor the torque of an electric motor is not necessarily dependent on its speed but also on armature current. In this curve speed falls somewhat as the load torque increase. Increasing the load decreases the speed linearly. If the field current is varied within an appropriate range, constant speed can be maintained from no load to rated load. The Rotational Losses of a DC machine

19

includes all speed dependent losses, such as bearings and brushes friction losses, windage losses, and eddy current and hysteresis losses in the armature core. These losses are independent of the load (ignoring the armature reaction effect). The other losses are due to the resistance of the windings. Some depend on the load (copper losses in the armature and series field windings), others on the applied voltage (copper losses in the shunt field winding).

Procedure: 1. Achieve the connections as shown in circuit diagram. 2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value. 3. Record the value of torque Ta, Speed N and armature current Ia. 4. Turn off power to the machine after accomplishing the task. 5. Draw the graph between N & Ta


T (N-m) 0.09 0.11 0.24 0.33 0.48

N (r.p.m) 1200 1180 1133 1097 1026

Graph Between Torque (Ta) and speed (N) :

20


Name:

Roll No:


Date:

Plotting Graph of Torque Speed Curve of a Shunt DC motor using Matlab Matlab Script .M File: % M-file to create a plot of the torque-speed curve of the % the shunt dc motor with armature reaction in Example 9-2. % This is mat file for magnetization curve. load fig9_9.mat % First, initialize the values needed in this program. v_t = 250; % Terminal voltage (V) r_f = 50; % Field resistance (ohms) r_a = 0.06; % Armature resistance (ohms) i_l = 10:10:300; % Line currents (A) n_f = 1200; % Number of turns on field f_ar0 = 840; % Armature reaction @ 200 A (A-t/m) % Calculate the armature current for each load. i_a = i_l - v_t / r_f; % Now calculate the internal generated voltage for each armature current. e_a = v_t - i_a * r_a; % Calculate the armature reaction MMF for each armature current. f_ar = (i_a / 200) * f_ar0; % Calculate the effective field current. i_f = v_t / r_f - f_ar / n_f; % Calculate the resulting internal generated voltage at % 1200 r/min by interpolating the motor's magnetization curve. e_a0 = interp1(if_values,ea_values,i_f,'spline');

21

% Calculate the resulting speed from Equation (9-13). n = ( e_a ./ e_a0 ) * n_0; % Calculate the induced torque corresponding to each speed from Equations (8-55) and (8-56). t_ind = e_a .* i_a ./ (n * 2 * pi / 60); % Plot the magnetization curve subplot(1,2,1); hold on; plot(if_values,ea_values,'color','k','linewidth',2); grid on; xlabel('I_F (Amperes)'); ylabel('E_A (Volts)'); title ('\bf Magnetization Curve of a typical DC motor at n=1200 rpm'); % Plot the torque-speed curve subplot(1,2,2); plot(t_ind,n,'Color','k','LineWidth',2.0); hold on; xlabel('\tau_{ind} (N-m)'); ylabel('n_m (r/min)'); title ('\bfShunt DC motor torque-speed characteristic (Example 9.2)'); axis([ 0 600 1100 1300]); grid on; hold off;

Output:

22


Name:

Roll No:


Date:

Plotting Graph of Speed(n) Vs Field Resistance(RF) of a Shunt DC Motor Script .M File: % M-file to create a plot of the speed of a shunt dc motor as a function of field resistance, assuming % a constant armature current (Example 9-3). % The magnetization curve of typical DC motor at n=1200 rpm. % It is same as for example 9.2 load fig9_9.mat % First, initialize the values needed in this program. v_t = 250; % Terminal voltage (V) r_f = 40:1:70; % Field resistance (ohms) r_a = 0.03; % Armature resistance (ohms) i_a = 120; % Armature currents (A) % Calculate the internal generated voltage at 1200 r/min for the reference field current (5 A) by % interpolating the motor's magnetization curve. The reference speed corresponding to this field % current is 1103 r/min. e_a0_ref = interp1(if_values,ea_values,5,'spline'); n_ref = 1103; % Calculate the field current for each value of field resistance. i_f = v_t ./ r_f; % Calculate the E_a0 for each field current by interpolating the motor's magnetization curve. e_a0 = interp1(if_values,ea_values,i_f,'spline'); % Calculate the resulting speed from Equation (9-17): % n2 = (phi1 / phi2) * n1 = (e_a0_1 / e_a0_2 ) * n1 n2 = ( e_a0_ref ./ e_a0 ) * n_ref; % Plot the magnetization curve for the motor.

23 subplot(1,2,1); hold on; plot(if_values,ea_values,'color','k','linewidth',2); grid on; xlabel('\bfI_F (Amperes)'); ylabel('\bfE_A (Volts)'); title ('\bf Magnetization Curve of a typical DC motor at n=1200 rpm'); % Plot the speed versus r_f curve. subplot(1,2,2); plot(r_f,n2,'Color','k','LineWidth',2.0); xlabel('Field resistance, \Omega','Fontweight','Bold'); ylabel('\itn_{m} \rm\bf(r/min)','Fontweight','Bold'); title ('Speed vs \itR_{F} \rm\bf Graph for a Shunt DC Motor','Fontweight','Bold'); axis([40 70 0 1400]); grid on; hold off;

Output:

24


Name:

Roll No:


Date:

Plotting Graph of Torque Speed Curve of a Shunt DC motor using Matlab Matlab Script .M File: % M-file create a plot of the torque-speed curve of the % the series dc motor with armature reaction in % Example 9-5. % The magnetization curve. load fig9_22.mat % First, initialize the values needed in this program. v_t = 250; % Terminal voltage (V) r_a = 0.08; % Armature + field resistance (ohms) i_a = 10:10:300; % Armature (line) currents (A) n_s = 25; % Number of series turns on field % Calculate the MMF for each load f = n_s * i_a; % Calculate the internal generated voltage e_a. e_a = v_t - i_a * r_a; % Calculate the resulting internal generated voltage at % 1200 r/min by interpolating the motor's magnetization curve. e_a0 = interp1(mmf_values,ea_values,f,'spline'); % Calculate the motor's speed from Equation (9-13). n = (e_a ./ e_a0) * n_0; % Calculate the induced torque corresponding to each % speed from Equations (8-55) and (8-56). t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

25 % Plot the magnetization Curve Used subplot(1,2,1); hold on; plot(mmf_values,ea_values,'color','k','linewidth',2.0); grid on; xlabel('\bfField Magneto-Motive Force, \bfF (A. Turns)'); ylabel('\bfInternal Generated Voltage, E_{A} (Volts)'); title('\bf Magnetization Curve of the motor used in Exp 9.5'); % Plot the torque-speed curve subplot(1,2,2); plot(t_ind,n,'Color','k','LineWidth',2.0); xlabel('\bfTorque Induced, \tau_{ind} (N-m)','Fontweight','Bold'); ylabel('\bfRotational Speed, n_m (r/min)','Fontweight','Bold'); title ('Series DC Motor Torque-Speed Characteristic (Example 9.5)','Fontweight','Bold'); axis([ 0 700 0 5000]); grid on; hold off;

Output:

DC Machine Experiments Manual

Recommend Documents