UNIT5

E3106/05/18

ELECTRICAL MACHINERY & CONTROL                                                                                                          

UNIT 5

 

ALTERNATING CURRENT (AC) MACHINERY (PART I)

 

 

                            OBJECTIVES

 

 

General Objective

 

To understand the alternator

 

 

Specific Objectives

 

By the end of this unit, you would be able to:

 

• explain the basic construction of alternator
• summarize the concept of synchronous based on relation of speed, drive; generate frequency and number of pole
• write the formulae of voltage generator for alternator

 

 

 

 

INPUT

 

 

 

5.0              INTRODUCTION TO ALTERNATING CURRENT GENERATOR

 

 

             

                 lternators or generators operate on the same fundamental principle of

electromagnetic induction as a D.C generator. They also consist of an armature

 

Figure 5.1: The construction of an alternator

(Source: A Text Book of Electrical Technolog; B.L Theraja & A.K

Theraja)

winding and magnetic field. But there is in one

important difference between the two. Whereas

in D.C generators, the armature rotates and the

field system is stationary, the arrangement in

alternators is just the reverse of it. In their case,

standard construction consists of armature

winding mounted on a stationary element called

stator and field windings on a rotating element

called rotor. The details of construction are

shown in Figure 5.1. In the other words, based

on the  explanation given, we should be able to

understand the concept of synchronous and the              

generate voltage in alternators.

 

 

 

 

 

 

1. BASIC CONSTRUCTION OF ALTERNATOR

 

 

1. Stator

 

The stator consists of a cast iron frame which supports the armature core. The armature core has slots on its inner periphery for housing the armature conductors. It has two parts which are a stator frame and the stator core (shown in Figure 5.2.).

 

1. Stator frame

 

Figure 5.2: A typical of stator

(Source: A Text Book of Electrical Technolog; B.L Theraja & A.K Theraja)

 

The stator frame is used for holding the armature stampings and windings in position. Low speed large diameter alternators have facture, are cast in sections. Ventilations is maintained with the help of holes cast in the frame itself. The

provision of redial ventilating spaces in the

stampings assists in cooling the machine. But

these days, instead of using castings, frame are

generally fabricated from mild steel plates welded

together in such  a way as to form a frame having a

a box type section.

 

2. Stator Core

 

The armature core is supported by the stator frame and is built up of laminations special magnetic iron or steel alloy. The core is laminated to minimize loss due to eddy currents.

 

 

 

 

 

 

2. Rotor

 

The rotor is like a flywheel having alternate North (N) and South (S) poles fixed to its outer rim. The types of rotors are used in alternators are a salient pole type and smooth cylindrical type. Figure 5.3 show us the types of rotor in an alternator.

 

(a) salient pole	(b) smooth cylindrical
Figure 5.3: Types of rotor (Source: A Text Book of Electrical Technolog; B.L Theraja & A.K Theraja)

 

 

 

 

Which types of rotor are used for steam turbine driven alternators?

 

Answer: smooth cylindrical

 

 

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 5A
 

 

 

 

 

 

1. What is the different between an alternator and D.C generator?

 

2. From the figure given, complete the diagram. Next, explain the diagram.

 

(a) ……………….

(b) ……………….

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 5A

 

 

 

1. The armature of the alternator can not make a rotating. It will be a static.

 

2.  

 

• The stator consists of a cast iron frame which supports the armature core.
• It has two parts which is a stator frame and the stator core.
• The stator frame is used for holding the armature stampings and windings in position.
• The armature core is supported by the stator frame and is built up of laminations special magnetic iron or steel alloy.

 

 

 

 

 

 

 

 

 

INPUT

 

 

 

2. CONCEPT OF SYNCHRONOUS BASED ON RELATION OF

SPEED, DRIVE; GENERATE FREQUENCY AND NUMBER OF POLE

 

 

Figure5.4: Relationship between speed, frequency

and pole

(Source: A Text Book of Electrical Technolog; B.L Theraja & A.K Theraja)

 

In an alternator, there exist a definite relationship between the rotational speed (N) of the rotor, the frequency (f) of the generated E.M.F and the number of poles (P). To understand the relationship, we refer to the Figure 5.4.

 

Consider the armature conductor marked

‘X’ (refer to Figure 5.4) situated at the

centre of a N-pole rotating in clockwise

direction. The conductor being situated at

the place of maximum flux density will

have maximum E.M.F induced in it.

 

When the conductor is in the inter-polar gap as at ‘A’ (refer to Figure 5.4), it has minimum E.M.F induced in it, because flux density is minimum there. Again, when it is at the centre of a S-pole, it has maximum E.M.F when conductor is over s N-pole is opposite to that when it is over a S-pole.  Obviously, one cycle of E.M.F is induced in a conductor when one pair of poles passes over it.

 

 

 

Other words…

The E.M.F in an armature conductor goes through one cycle in angular distance equal to twice the pole pitch as shown in Figure 5.4

 

 

 

 

 

 

 

 

Since one cycle of E.M.F is produced when a pair of poles passes past a conductor, number of cycles of E.M.F produced in one revolution of the rotor is equal to the number of pairs of poles.

(i) Number of cycles/revolution = P/2	(ii) Number of revolutions/second = N/60
So…                                        frequency = =

 

 

 

N is known as the synchronous speed because it is the speed at which an alternator must run in order to generate an E.M.F of the required frequency.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 5B
 

 

 

 

 

 

 

3. As you know in an alternator, there is a definite relationship between the rotational speed of the rotor, the frequency of the generated E.M.F and the number of poles. Could you explain about this comment by using a suitable figure?

 

4. From the table given, how do you think the relationship between speed and numbers of poles if the frequency is fixed?

 

Number of poles	2	4	6	12	24	36
Speed (rpm)	3600	1800	1200	600	300	200

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 5B

 

 

 

 

5. Refer to the notes 5.2

 

6. Based on equation P = 120f/N. It is because of slow rotative speeds engine-driven alternators, their number of poles is much greater as compared to that of the turbo generators which run at very high speed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INPUT

 

 

 

5.3              FORMULAE VOLTAGE GENERATOR

 

 

How can the voltage generator in alternator be determined? First, you must be familiar with terms that are usually used in the equation. So, let we identify all the terms below:

 

Z	:	Number of conductors or coil sides in series or phase Z = 2T ; where T is number of coil or turn per phase. G remember one turn or coil has two side.
P	:	Number of poles.
f	:	Frequency of induced E.M.F in Hz.
f	:	Flux pole in Weber.
kd	:	Distribution factor.
kc or kp	:	Pitch or coil span factor
kf	:	Form factor ; if E.M.F is assumed sinusoidal
N	:	Rotative speed of the rotor in rpm.

 

 

 

 

Actually available voltage phase (rms) is

 

If the number of coil or turn per phase (T) is given,

 

If you want to know the line of the voltage generator,

 

 

 

  Example 5.1

An alternator three phase, have two poles, 210 conductors inside the armature winding and value of flux per poles is 17.5mWb. The waveform for this machine is a sinusoidal and the frequency to generate the voltage is 50Hz. Assume that kd and kp is unity. Calculate the rms voltage generates in a single phase.

              Solution

                                          Given:

P	:	2
Z	:	210
f	:	17.5 x 10-3
f	:	50
kd	:	1
kp	:	1

 

                                          Recall:

 

\ The rms voltage generates in a single phase

 

\ The rms voltage line generates

                                                        Recall:

             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 5C
 

 

 

 

 

 

 

 

7. What is the formula of voltage generator?

 

8. A three phase, 12 poles, star connected alternator has 600 conductors. The distribution factor (kd) is 0.957 and coil span factor (kp) is 0.95. Determine the phase and line value of E.M.F if the alternator runs at 600rpm and the flux per pole is 0.06Wb distributed sinusoidal over the pole.

 

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 5C

 

 

 

5.5             

 

9. Ephase = 4360V

Eline = 7550V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SELF-ASSESMENT

 

If you face any problem, discuss it with your lecturer

You are approaching success. TRY all the questions ini this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.

 

 

 

Question 5-1

 

 

A               What is the difference between stator and rotor?

 

B               What does a stator consists of?                           

 

C              How many types of rotor are there?

 

 

Question 5-2

 

 

A               How can you explain the relationship between pole, synchronous speed, and

              frequency?

 

B              What should you do if you want to control the speed of an alternator?

 

 

 

Question 5-3

 

 

A              A three phase, four poles, 50Hz, star connected alternator has 80 conductors. The

distribution factor (kd) is 0.957 and coil span factor (kp) is 0.95. Determine the phase and line value of E.M.F if the flux per pole is 943mWb distributed sinusoidal over the pole.

 

B               A synchronous generator runs at 250rpm and generate voltage terminal E.M.F 1990V

              at 50Hz. There are 360 conductors in a winding for three phase star connection. The

distribution factor (kd) is 0.9598 and coil span factor (kp) is unity. Find:

1. number of poles.
2. E.M.F phase.
3. the flux per pole distributed sinusoidal over the pole.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO SELF-ASSESMENT

 

 

 

Question 5-1

 

 

A              The stator consists of a cast iron frame which supports the armature core having slots

              on its inner periphery for housing the armature conductors.

              The rotor is like a flywheel having alternate North (N) and South (S) poles fixed to

              its outer rim.

 

B              stator frame and the stator core.

 

C              Two. A salient pole type and smooth cylindrical type

 

 

Question 5-2

 

 

A              refer to the notes 5.2

 

B              pole and frequency.

 

 

Question 5-3

 

A              7613V; 13.185kV

 

B              (i)              24              (ii)              1.15kV                            (iii)              30mWb