E2063 Teknologi Elektrik 2 UNIT6

AC MACHINE (Part 1) E2063/ Unit 6/ 1

ALTERNATING CURRENT ELECTRIC MACHINES (Part 1)

OBJECTIVES

General Objective : To analyze the basic principles of operation of an AC generator and the differences between DC generator and AC generator by using commutator and slip ring.

Specific Objectives : At the end of the unit you will be able to:

Utilize the left-hand rule for generators.

Explain the concept of electromagnetic induction.

Draw the construction of DC generator and alternator.

Explain the principle of operation of an AC generator.

Differentiate between DC generators and AC generators.

UNIT 6


6.0 INTRODUCTION

An electric generator is a device used to convert mechanical energy into electrical energy.

The generator is based on the principle of "electromagnetic induction" discovered in 1831

by Michael Faraday, a British scientist. Faraday discovered that if an electric conductor, like

a copper wire, is moved through a magnetic field, electric current will flow (be induced) in

the conductor. So the mechanical energy of the moving wire is converted into the electric

energy of the current that flows in the wire.

The figure below shows a simple electric generator. In the animation, the mechanical

energy needed to turn the generator comes from the brown hand crank at the front of the

generator. In a hydroelectric power plant, the mechanical energy to turn the generator comes

from the water turbine, which is turned by the force of falling water. The hand crank in the

animation causes the red wire to spin inside a magnetic field (the blue lines). As Faraday

learned, moving the wire through the magnetic field causes electric current to flow in the

wire. The turning red wire is connected to a volt meter, which shows the amount of electric

current that is produced. In a hydroelectric plant, the generator is connected to transmission

lines that deliver the electricity to your home or business.

INPUT


Fig. 6.1 The simple electric generatorThe AC generator (alternator) is a device that converts mechanical energy into electrical

energy using the principle of electromagnetic induction. The amount of voltage generated depends on the following:

i. The strength of the magnetic field.ii. The angle at which the conductor cuts the magnetic field.iii. The speed at which the conductor is movediv. The length of the conductor within the magnetic field.

The polarity of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor. To determine the direction of current flow in a given situation, the left-hand rule for generators is used.

The rule is explained in the following manner. Extend the thumb, forefinger, and middle finger of your left hand at right angles to one another, as shown in Fig. 6.2. Point your thumb in the direction the conductor is being moved. Point your forefinger in the direction of the magnetic flux (from north to south). The middle finger then points in the direction of current flow in an external circuit to which the voltage is applied.

Source: Introduction to Electronics : DC/AC Circuits by Stephen C. Harsany

Fig 6.2The left-hand rule for generator

6.1 PRINCIPLE OF OPERATION OF AC GENERATOR (ALTERNATOR)

An elementary AC generator (Fig. 6.3) consists of a wire loop so that it can be rotated in a stationary magnetic field. This produces an induced e.m.f. in the loop. Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced e.m.f.


The pole pieces (marked N and S) provide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop. The loop of wire that rotates through the field is called the armature. The ends of the armature loop are connected to rings called slip rings. They rotate with the armature. The brushes, usually made of carbon with leads attached to them, ride against the rings. The generated voltage appears across these brushes.


Fig 6.3 An elementary generator

A voltage is produced in the following manner. The armature loop is rotated in a clockwise manner (Fig. 6.4). Its initial, or starting, position is shown in Fig. 6.4 (A). At 0 the armature loop is perpendicular to the magnetic field. The black-and-white conductors are moving parallel to the magnetic field; thus, they do not cut any lines of force. The induced e.m.f. is zero, as the meter shows at A. As the armature loop rotates from position A to B, the conductors cut through more and more lines of force at continually increasing angle. At 90 (position B), they are cutting through a maximum number of lines of force and at a maximum angle. The result is that between 0 and 90, the induced e.m.f. in the conductors builds up from zero to maximum value. Observe that from 0 to 90 the black conductor cuts down through the field. At the same time the white conductor cuts up through the field. The induced e.m.f. in the conductors are series-aiding, the result of the same relative motion between the armature and the field causing more flux lines to be cut. The meter at position B reads a maximum value.

As the armature loop continues rotating from position B (90) to position C (180), the conductors that were cutting through a maximum number of lines of force at position B now cut through fewer lines. At position C, they are again moving parallel to the magnetic field . Thus, from 90 to 180, the induced voltage decreases to zero. However, the polarity of the induced voltage has remained the same. This is shown by A through C on the graph.


As the loop starts rotating beyond 180, from position C through D, and back to position A, the voltage is in the direction opposite to that shown from A, B, and C. The magnitude of the voltage is the same as it was from A to C except for its reversed polarity (as shown by meter deflection in D). The voltage output waveform for one complete revolution of the loop is shown on the graph in Fig 6.4 and is a single-phase voltage.


Fig 6.4 Output voltage of an elementary generator during one revolution.


Activity 6A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

6.1 AC generator is also called ……………..6.2 The generator converts …………energy into………….energy.6.3 The ends of the armature loop are called ………… ……………6.4 What is the difference between a rotor and a stator?


Feedback to the Activity 6A

6.1 alternator6.2 mechanical, electrical6.3 slip rings6.4 rotor – rotating part of the machine

stator – stationary part of the machine


6.2 THE DIFFERENCES BETWEEN AC GENERATOR AND DC GENERATOR

The difference between AC and DC generator is that the DC generator results when you replace the slip rings of an elementary generator with two-piece commutator, changing the output from AC to pulsating DC as shown in Fig. 6.5.


Fig. 6.5 A Basic DC generator

The major difference is that the alternator has several design differences that compensate for the problems that are common in the DC generator. The first difference is that the part of the alternator that produces the large current is located in the stationary part of the machine called the stator, so that the brushes are needed to transfer the large amount of electrical energy that is produced. This means that the part of the alternator that uses a small amount of current to produce the magnetic field must be located in the rotating part of the machine, called the rotor. Since the motor needs a small amount of constant DC voltage, this voltage can be supplied through slip rings and two brushes rather than commutator segments. Fig. 6.6 shows the basic parts of the AC alternator. You should notice that slip rings are mounted on the rotor and they run completely around the armature shaft like a collar, instead of the commutator segments in the DC generator.

INPUT


Source: Industrial Electronics Application for Programmable Controllers, Instrumentation and Process Control, and Electrical Machines and Motor Controls by Thomas E. Kissel

Fig. 6.6 Basic parts of an alternator

Fig. 6.7 shows the general construction of DC generator. It has five principal components i.e. (i) field systems (ii) armature core (iii) armature windings (iv) commutator (v) brushes. While Fig. 6.8 shows the alternator (ac generator or synchronous generator). It has 3-phase winding on the stator and dc field winding on the rotor.

Source: Principles of Electrical Engineering and Electronics by V.K. Mehta

Fig 6.7 DC Generator


Source: Principles of Electrical Engineering and Electronics by V.K. Mehta

Fig 6.8 Alternator

6.3 E.M.F. equation of an Alternator

Let Z = No. of conductors or coil sides in series per phase = Flux per pole in webers P = Number of rotor poles N = Rotor speed in r.p.m

In one revolution (i.e., 60/N second), each stator conductor is cut by P webers,i.e.,d = P ; dt = 60/N

Average e.m.f. induced in one stator conductor

= = = volts

Since there are Z conductors in series per phase,

Average e.m.f. per phase = Z

=

= 2fZ volts

R.M.S value of e.m.f./ phase = Average value/ phase form factor=2fZ 1.11 = 2.22 f Z

Er.m.s / phase = 2.22 f Z volts.


Example 6.1

A 3-phase, 50 Hz star-connected alternator has 180 conductors per phase and flux per pole is 0.0543 wb. Find

(i) e.m.f. generated per phase(ii) e.m.f. between line terminals. Assume the winding to be full pitched and

distribution factor to be 0.96

Solution to Example 6.1

(i) Generated e.m.f./phase, Eph = 2.22 KpKdZf = 2.22 ×1×0.96×180×50×0.0543 = 1041.5 V

(ii) Line voltage = = 1803.19 V

Activity 6B


TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

6.5 State the differences between an AC generator and a DC generator.6.6 Draw the output that is produced by a DC generator and an AC generator.


Feedback to the Activity 6B

6.5 components used, output waveform6.6

(AC waveform) (DC waveform)


KEY FACTS

1. Basically the generator is the opposite of the motor.

2. An alternator is a device that converts mechanical energy to electrical energy using

the principle of electromagnetic induction.

3. An alternator uses slip rings to provide a means for the generated voltage to be taken

from the device.

4. To form a basic DC generator, the slip rings are replaced with a commutator.

5. The output of the commutator is a pulsating DC waveform.

6. In this case the rotation of the coil produces a current.

7. The current produced is an alternating one.

8. The split rings are replaced with two slip rings.


SELF-ASSESSMENT 6

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment 6 given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 6-1

a. Describe the components of an elementary AC generator.b. What component is used to convert an alternator to a DC generator?c. Synchronous generator or alternator is the other name for ……….……….d. AC systems have a number of advantages over DC systems. (Yes/No)e. A rectifier is used to convert AC to DC. (Yes/No)f. The operation of an alternator is not based on the concept of electromagnetism.

(Yes/No).g. For an alternator operation, the direction of induced e.m.f. can be determined by

Fleming’s right hand rule. (Yes/No)h. Find the number of armature conductors in series per phase required for the

armature of a 3-phase, 50Hz, 10-pole alternator. The winding is star-connected to give a line voltage of 11000. The flux per pole is 0.16 wb. Assume Kp = 1 and Kd = 0.96.

Question 6-2

a. A 3-phase, 50 Hz star-connected alternator has 100 conductors per phase and flux per pole is 0.05wb. Find(i) e.m.f. generated per phase(ii) e.m.f. between line terminals. Assume the winding to be full pitched and

distribution factor to be 0.96


FEEDBACK TO SELF-ASSESSMENT 6

Have you tried the questions????? If “YES”, check your answers now.

Solution of Question 6-1

a. pole pieces, brush, slip rings, armature loopb. commutatorc. AC generator.d. Yese. Yesf. Nog. Yesh. Z = 372.5

Solution of Question 6-2

a. (i) 532.8 V(ii) 922.84 V

CONGRATULATIONS!!!!…..May success be with you always….

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E2063 Teknologi Elektrik 2 UNIT6