1
UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II Ia Ib Ic A B C N
YEAR II- SEMESTER III
THEORY
Version 1: December 2008
NATIONAL DIPLOMA IN
ELECTRICAL ENGINEERING TECHNOLOGY
ELECTRICAL MACHIENS I I
Table of Contents
Chapter 1: Basic Principles of electric machines: ...
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1.1 Introduction ...1
1.2 Electro- mechanical energy convertion ... 2
1.3 Alignment devices ... 6
1.4 Interaction devices ... 7
1.5 Induction devices ... 7
1.6 Work Examples ... 8
1.7Electromagnets ... 11
1.8 Faraday's Law ... 12
1.9Lenz's Law ... 13
1.10 Rotating magnetic field ...
Week2
1.11 Synchronous speed: ... 14
Chapter 2 Three phase induction motor ...
Week3
2.1 Introduction ... 17
2.2 Construction of Induction Motor ... 18
2.2 The principle of operation of the Induction Motor ...
Week4
2.3 The slip ... 20
2.4 Name Plate ... 22
Chapter 3: Synchronous Machine: ...
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3.1 Introduction ... 18
3.2 Stator construction ... 18
3.3 Rotor construction ... 18
3.4 Principle of operation of the synchronous generator ... 20
3.5 Principle of operation of the synchronous motor: ... 20
3.6 Excitation Methods ... 21
Chapter4:Control & protection of electric motors ...
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4.1 Need for Circuit Protection ... 22
4.2 Types of Overcurrent Protective Devices ...
Week7
4.2.0 Protection and control devices...Week8
4.2.1 Fuses ... 25
4.2.2 Circuit Breakers ... 27
4.3 Control devices ...
Week9
4.3.1 Relay
...
30
4.3.2 Contactors
...
31
4.3.3Pushbuttons
...
Week10
4.3.4 Selector Switches
...
35
4.3.5 Indicator Lights
...
35
Chapter 5: Energy convertion ...
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5.1 Electro-mechanical energy convertion ... 37
5.1.1Major energy convertion principle ... 38
5.3 Linked energy system ... 40
5.4 Energy storage ... 41
5.5 Energy ballance ...
Week12
5..5.1 Block diagram of energy balance equation ... 43
5.6 Magnetic field energy and force ...
Week13
5.6.1 Magnetic field ... 45
5.6.2 Magnetic circuit ... 46
5.7 Magnetic field energy ... 47
5.8 Maxwell stress ... 48
5.9 Energy density ...
Week14
5.10 Faraday's Lenz Law ... 49
5.11 Energy Convertion ... 50
5.11.1 Constant current ... 51
5.11.2 Constant flux ... 52
5.11.3 General Condition ... 53
5.11.4 Diffirencial foam ... 54
5.12 Alignment force and torque; single excitation ...
Week15
5.11 Work examples ... 56
1. Basic Principles of Electric Machines
Week 1
1
1.1 Introduction
It may be necessary to define what we mean by the term electrical machines. A machine is a device that does useful work in a predictable way according to some physical laws. It acts as a transducer, or convertors, accepting an input of energy in one physical from and transforming it, more or less effectively, into another.
An electromagnetic machine, in the essential conversion process, uses energy in an intermediate magnetic form. As a motor the machine takes in electrical energy and converts it into mechanical work, such as driving a machine tool or a lift, or operating a loudspeaker. An electro-magnetic machine is usually reversible and cab, as a generator, producer electrical energy form some other kind, such as the mechanical energy of prime- movers or the a caustic energy of microphones and gramophone pickups.
Electrical energy is versatile and controllable. Its special lie in that can be transfer continuously and economically from to place (Transmission), made widely available as a services (distribution), used in conveying intelligence (Telecommunication and data processing), and applied to indicate an supervise production systems (control, instrumentation and computation). It is readily converted into sound, light, heat and useful forms of energy. In particular it is easily converted to or from mechanical energy in the electromagnetic machines
1. Basic Principles of Electric Machines
Week 1
2
1.2 Electro-mechanical energy conversion
1.2.1 Principal of electrical machines
The operation of electromagnetic mechanical devices can be explained in terms of basic principals concerned with
i The development of magneto-mechanical forces and
ii. The induction of emf (electromotive force) by the rate of change of the linkage. Thus, electromagnetic energy conversion is based on three bask principles namely (i) induction (ii) interaction and (iii) alignment
1.
Principle of induction
It is known that when electrons are in motion, they produce a magnetic field. Conversely, when, a magnetic field embracing a conductor moves relative to the conductor, it produce a flow of electrons in the conductor.
The phenomenon whereby on e.m.f and hence current (i.e flow of electrons) is induced in any conductor which is cut across or is cut by a magnetic flux is known as electromagnetic induction
The induced emf E is given by
(a) E = NdØ OR (B) e = Bluw ……..volts dt
Where N = number of turns of the coil Ø = flux in webers linking the coil T = time in seconds
1. Basic Principles of Electric Machines
Week 1
3 This is the equation for the induced emf when the magnetic flux moves relatively to the conductor.
In equation 1.1(b), B = flux density in wb/m2
L = effective length of conductor in metre , U = velocity of the conductor in m/s
And this is the equation for the induced emf when the conductor moves relatively to the flux. The induction principle is employed in devices such as induction motors generators, transformers, controlling instrument etc.
1.2.2 Sketchmatic explanation of the induction principle
Fig:1.1a. Voltage & Current induced in the secondary Fig: 1.1b Conductor stationary, while the circuit due to flux linkage with the primary winding. field moves (current will be induced on the galvanometer)
Fig: 1.1c Conductor moves, while the field stationary (current will be induced on the galvanometer)
1. Basic Principles of Electric Machines
Week 1
4 Fig. 1.1 (a) shows an irobn- cored solenoid with a permanent magnetic place adjascent ot it. If the magnet‟s position is changed from position CD to position AB, the flux linking with the coils of the solenoid will change, leading to an induced emf in the coil which can be
detected by the sensitive galvanometer G. in this arrangement, the conductor (coil) i stationary whilst the filed (magnet) moves as in alternators i.e a.c generators.
Fig 1.1 (b) shows a filed arrangement that is stationary while the conduct a-b is free to move about the vertical axis. An emf, detectable by galvanometer G, will be induced in the conductor as it cuts through the flux through the flux. This principle is employed in the construction of d.c. generator,.
F ig 1.1(c) when a coil (Ni) is made to carry an alternative current (ii) it produces an alternative flux (g). if a second coil (N2) is now placed in a region whereby the alternative flux produced by
the first coil links with the second coil, an emf ( usually of the some frequency) will be induced in the second coil. This is the principle of the transformer and the induction motors
2. Principle of interaction
An electric current flowing in a direction making an angle (preferably a right-angle) with a magnetic filed produced by another current ( or a magnet) experience a force fe, the relative direction being shown in Fig 1.2.
1. Basic Principles of Electric Machines
Week 1
5 The force, fe, arises from the interaction of the flux (created by the current l‟ flowing in the conductor with the flux produced by a second current or magnet. Since lines of flux do not cross, the two fluxes will realign. Resulting in a stronger fie ld one side. The conductor and weaker filed on t6he other side. The conductor then tends to move from the region of stronger field to the region of weaker filed. Employed in electric motors.
3. Principle of alignment
A pieces of ferromagnetic materials in a magnetic field experience of force urging it towards a region where the field is stronger, or tending to align it so as to shorten the magnetic flux path as shown in fig: 1.3
Fig: 1.3a Moving coil meter Fig: 1.3b The force „fe‟ on shaped high permeability pieces in a field
1. Basic Principles of Electric Machines
Week 1
6 (a) Lifting magnet (f) Actuator
(b) Relay (g) Electromagnetic pump
( c) Telephone (h) Loudspeaker
(d) Moving-iron indicator (i) Moving –coil indicator
(e) Reluctance motor (k) Industrial rotating machine
1.3 Alignment devices
(a) The lifting magnet: Attract ferromagnetic loads such as beams, plates, and scrap-iron.
(b) The relay: the coil current causes the armature to be attracted towards the cover against a spring load: Millions of such relays do useful work in automatic telephone exchanges, traffic light installation and simple control systems,.
(c) The telephone receivers: has a ferromagnetic diaphragm attracted by a permanent magnet, the field is caused to fluctuated by the speech currents in the coil, so varying the deflection of the claptrap and producing sound waves in the air.
(d) The moving- iron indicator, uses the force between the fixed and moving irons to deflect a pointed against a spring.
(e) The Reluctance motor- the forces urge a displaced rotor in alignment with the magnetized stator.
(f) The actuator-the current-carrying coil “suck” a displaced ferromagnetic plunger into a position of symmetry: this is a useful and forceful device.
1. Basic Principles of Electric Machines
Week 1
7
1.4 Interaction devices
(g) Electromagnetic pump: current passed through a conducting liquid in an enclosed channel forces the liquid to move by interaction with a magnetic cross field; liquid sodium-potassium or lithium can be pumped in thy way for the extraction of heat from a nuclear reactor.
(h) Loudspeaker: alternating current in the coil flow in the radial magnet filed of the port magnet,. And the consequent movement of the attached diaphragm sets up sound waves. This is the same essential arrangement as a “generator‟ of mechanical vibrations
(i) Moving –coil indicator-current (normally direct) in the coil of the indirect develops a force in the radial permanent- magnet filed to move pointed against a control spring. (k) Industrial rotating machines: Current caused to flow in conductors the surface of a
rotor, mounted within a magnetic stator develop interaction forces tending to turn the rotor.
1.5
Induced voltage devices
Recalling that a conductor moving or cutting magnetic lines of flux or that the flux moves relative to the conductor will proan induced voltage, the following devices employ the induced voltage arrangement.
(i) The transformer- an alternating current flowing in the primary coil (winding) set up an alternating flux that links with the secondary coil inducing a voltage in the latter. (m) The generatopr-usually constructed like (k) but with the rotor mechanical energy (via
the prime –mover) will have emf induced in the stator coils. ( the stator is slotted to house conductors)
1. Basic Principles of Electric Machines
Week 1
8 (n) The induction motor- the stator usually carried one or htree –phawindings whilst the rotor may have a similar arrangement of coil as the stator or just carry or alirmium bars. Electrical energy supplied to the stator windings produced a rotating magnetic field with cuts the rotor conductors and hence induced voltages in them. A complete rotor circult will have current flowing in the rotor conductors (caused by the induced voltage) and by interaction forces produced motion of the rotor.
1.6 Work Examples
Examples 1A conductor carries a current of 800 A at right- angle to magnetic field having a density of 0.5wb/m2 calculated the force on a metre length of the conductor.
Solution
The force F is given by F = Bli
= 0.5 X 1 X 800 = 400N
Example 2
A four –pole generator has a magnetic flux of 12 mnb /pole calculated the average value of the emf generated in one of the armature conductors while it is moving through the magnetic flux of one pole, if armature is driven at 900 r.p.m
Solution
When a conductor moves through the magnetic field of one pole, it cuts a magnetic flux of 12 x 10-3 wb.
Time taken for a conductor to move through one revolution = 60 = 1 second
1. Basic Principles of Electric Machines
Week 1
9 900 15
Since the machine has 4 poles, time taken for a conductor to move through the field of one pole = ¼ x 1/15 = 1/60s
:. Average emf generated in one conductor rate of change of flux
= Ø = 12 x 10-3 1/60
t
= 0.012 – 0.01667 = 0.72v
= 0.72v
Example 4
A magnetic flux of 400 uwb passing through a coil of 1200 turns is reversed in 0.1s calculate the average emf induced in the coil.
Solution
The magnetic flux has to decrease form 400 uwb to zero and then increase to 400wwb in the reverse direction, hence the increase of flux is 400 (-400-400) uwb = -800 x 10-6 wb.
:. Average emf induced in coil
= (change in flux x No of turns) = NdØ Time taken. Dt
1. Basic Principles of Electric Machines
Week 1
10
1.7 Electromagnets
Anything with an electrical current running through it has a magnetic field. Figure1 shows different sources of magnetic field.
Figure1.4: Different sources of magnetic field
The most common forms of electromagnets are the Solenoids .When the wire is shaped into a coil as shown in Figure1.1, all the individual flux lines produced by each section of wire join together to form one large magnetic field around the total coil.
As with the permanent magnet, these flux lines leave the north of the coil and re-enter the coil at its south pole. The magnetic field of a wire coil is much greater and more localized than the magnetic field around the plain conductor before being formed into a coil. This magnetic field around the coil can be strengthened even more by placing a core of iron or similar metal in the center of the core. The metal core presents less resistance to the lines of flux than the air, thereby causing the field strength to increase. (This is exactly how a stator coil is made; a coil of wire with a steel core.) The advantage of a magnetic field which is produced by a current carrying coil of wire is that when the current is reversed in direction the poles of the magnetic-as shown in Figure1.2- field will switch positions since the lines of flux have changed direction. Without this magnetic phenomenon existing, the AC motor as we know it today would not exist.
1. Basic Principles of Electric Machines
Week 1
11 Battery
Iron Core
S
N
Magnetic field LinesN
S
The Current
Figure1.5: Reversing the polarity of the solenoid by reversing the current direction
1.8 Faraday's Law
Faradays law states whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it, or Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor. The phenomenon of inducing a current by changing the magnetic field in a coil of wire is known as electromagnetic induction.
Figure1.3 shows an electromagnet which is connected to an AC power source. Another electromagnet is placed above it. The second electromagnet is in a separate circuit. There is no physical connection between the two circuits. Voltage and current are zero in both circuits at Time1. At Time2 voltage and current are increasing in the bottom circuit
1. Basic Principles of Electric Machines
Week 1
12
0 0 0
Time1 Time2 Time3
Figure 1.6: Experment showing the electromagnetic induction phenomena
A magnetic field builds up in the bottom electromagnet. Lines of flux from the magnetic field building up in the bottom electromagnet cut across the top electromagnet. A voltage is induced in the top electromagnet and current flows through it. At Time 3 current flow has reached its peak. Maximum current is flowing in both circuits. The magnetic field around the coil continues to build up and collapse as the alternating current continues to increase and decrease. As the magnetic field moves through space, moving out from the coil as it builds up and back towards the coil as it collapses, lines of flux cut across the top coil. As current flows in the top electromagnet it creates its own magnetic field.
1.9 Lenz's Law
Lenz's law enables us to determine the direction of the induced current: "The direction of the induced current is such as to oppose the change causing it." The Figure 1.4a shows the north pole of a bar magnet approaching a solenoid. According to Lenz's law, the current which is thereby generated in the coil must cause an effect which opposes the approaching magnetic field.
1. Basic Principles of Electric Machines
Week 1
13 Iron Core N Ammeter N N S N a bFigure1.7: Experiment demonstrating Lenz's law
This is achieved if the direction of the induced current creates a north pole at the end of the solenoid closest to the approaching magnet, as the induced north pole tends to repel the approaching north pole. The Figure1.4b shows the north pole of a bar magnet withdrawing from a solenoid. According to Lenz's law, the current which is thereby generated in the coil must cause an effect which opposes the departing magnetic field. This is achieved if the direction of the induced current creates a south pole at the end of the solenoid closest to the departing magnet, as the induced south pole tends to attract the departing north pole
Basic Principles of Electric Machines Week Two
5
1.
1.4 Rotating magnetic field
The three-phase induction motor also operates on the principle of a rotating magnetic field. The following discussion shows how the stator windings can be connected to a three-phase ac input and have a resultant magnetic field that rotates.
Figure 1.5 shows how the three phases are tied together in a Y-connected stator. The dot in each diagram indicates the common point of the Y-connection. You can see that the individual phase windings are equally spaced around the stator. This places the windings 120° apart. Ia Ib Ic A B C N
Figure 1.5:- Three-phase, Y-connected stator.
Using the left-hand rule the electromagnetic polarity of the poles can be determined at any given instant.
Basic Principles of Electric Machines Week Two
6
1.
The results of this analysis are shown for voltage points 1 through 7 in figure 2. At point 1, the magnetic field in coils A is maximum with polarities as shown. At the same time, negative voltages are being felt in the B and C windings. These create weaker magnetic fields, which tend to aid the A field. At point 2, maximum negative voltage is being felt in the C windings. This creates a strong magnetic field which, in turn, is aided by the weaker fields in A and B. As each point on the voltage graph is analyzed, it can be seen that the resultant magnetic field is rotating in a clockwise direction. When the three-phase voltage completes one full cycle (point 7), the magnetic field has rotated through360°.
Basic Principles of Electric Machines Week Two
5
1.
10 5 -10 -5 1 2 3 4 5 6 7 Point6 Point7Point1 Point2 Point3
Point4
Point5
1. Basic Principles of Electric Machines
Week 2
6
1.5 Synchronous speed:
The speed of the rotating magnetic field is referred to as synchronous speed (Ns). Synchronous speed is equal to 120 times the frequency (f), divided by the number of poles (P).
p
f
Ns
120
If the frequency of the applied power supply for the two-pole stator used in the previous example is 50 Hz, synchronous speed is 3000 RPM. RPM Ns 3000 2 50 120
The synchronous speed decreases as the number of poles increase. The following table shows the synchronous speed at 50 Hz for the corresponding number of poles.
No of poles Synchronous speed
2 3000rpm
4 1500rpm
6 1000rpm
8 750rpm
Table 1.1: Different speeds for different number of poles
2. Three phase induction motor
Week 3
1
2.1 Introduction
Three-phase AC induction motors are widely used in industrial and commercial applications. They are classified either as squirrel cage or wound-rotor motors.
These motors are self-starting and use no capacitor, start winding, centrifugal switch or other starting device.
They produce medium to high degrees of starting torque. The power capabilities and efficiency in these motors range from medium to high compared to their single-phase counterparts.
Popular applications include grinders, lathes, drill presses, pumps, compressors, conveyors, also printing equipment, farm equipment, electronic cooling and other mechanical duty applications.
Simple and rugged construction Low cost and minimum maintenance
High reliability and sufficiently high efficiency since there is no losses in brush contacts or mechanical friction
Needs no extra starting motor and need not be synchronized Need only one source of power
2.2 Construction of Induction Motor:
An Induction motor has basically two parts, Stator and Rotor. Also some of other parts were acknowledged in the following section
2. Three phase induction motor
Week 3
2
2.2.1 Stator construction:
Figure2.1: Diagram construction of the stator
The stator is made up of several thin laminations of aluminum or cast iron. They are punched and clamped together to form a hollow cylinder (stator core) with slots as shown in Figure 2.1. Coils of insulated wires are inserted into these slots.
The iron core on the figure has paper liner insulation placed in some of the slots.
Each grouping of coils, together with the core it surrounds, forms an electromagnet (a pair of poles) on the application of AC supply. The number of poles of an AC induction motor depends on the internal connection of the stator windings. The stator windings are connected directly to the power source. Internally they are connected in such a way, that on applying AC supply, a rotating magnetic field is created
2.2.2 Rotor construction:
There are two main types2. Three phase induction motor
Week 3
3 Squirrel cage type
Wound rotor 2.2.2.1 Squirrel cage rotor:
This rotor has a laminated iron core with slots(Figure2.2), and is mounted on a shaft. Aluminum bars are molded in the slots and the bars are short circuited with two end rings. The bars are skewed on a small rotor to reduce audible noise. Fins are placed on the ring that shorts the bars. These fins also work as a fan and improve cooling.
Most motors use the squirrel-cage rotor because There are no commutators, slip rings or brushes. Hence this is a most rugged and maintenance-free construction.
Rotor bars ) slightly skewed(
End ring
Figure2.2:Squriell cage rotor 2.2.2.2 Wound rotor:
2. Three phase induction motor
Week 3
4 Wound Rotor Brush Slip Rings External Rotor ResistancesFigure 2.3: Schemtic diagram showing Induction motor, wound rotor type
The wound rotor or slip-ring induction motor differs from the squirrel-cage motor only in the rotor winding. The rotor winding consists of insulated coils, grouped to form definite polar areas of magnetic force having the same number of poles as the stator. The ends of these coils are brought out to slip-rings. By means of brushes, a variable resistance is placed across the rotor winding (Fig.2.3). By varying this resistance, the speed and torque of the motor is varied. The wound rotor motor is an excellent motor for use on applications that require an adjustable-varying speed (an adjustable speed that varies with load) and high starting torque.
Shaft Coils
Fan Slip rings
Bearings Laminated core
Terminals
Figure 2.4:Wound rotor
2.2.3 Enclosure
Enclosure or the frame (Figure2.5) ,its main application to hold the parts together. also it Helps with heat dissipation. In some cases, protects internal
2. Three phase induction motor
Week 3
5 components from the environment. A cooling fan is attached to the shaft at the left-hand side. This fan blows air over the ribbed stator frame
2.2.3 Bearings:
There are two main types, the sleeve bearings and ball bearings .Ball (Roller) Bearings (Figure2.6a) Support shaft in any position. Its Grease lubricated and required no maintenance
The Sleeve Bearings(figure2.6b) are Standard on most motors. They are only used with horizontal shafts and its oil lubricated.
2.2.5 Conduit Box
Point of connection of electrical power to the motor’s stator windings.
2.2.6 Eye Bolt
Used to lift heavy motors with a hoist or crane to prevent motor damage, as it can be seen in the following figure.
Figure2.5:Induction Motor enclosure
Figure2.6: a)Ball Bearings b)Sleeve bearings
(a) (b)
2. Three phase induction motor
Week 3
6
2. Three phase induction motor
Week 4
1
2.2 The principle of operation of the Induction Motor:
The three-phase current with which the motor is supplied establishes a rotating magnetic field in the stator. This rotating magnetic field cuts the conductors in the rotor inducing voltages and causing currents to flow. These currents set up an opposite polarity field in the rotor(Lenz's law). The attraction between these opposite stator and rotor fields produces the torque which causes the rotor to rotate. This simply is how the squirrel-cage motor works
2.3 The Slip:
There must be a relative difference in speed between the rotor and the rotating magnetic field. If the rotor and the rotating magnetic field were turning at the same speed no relative motion would exist between the two, therefore no lines of flux would be cut, and no voltage would be induced in the rotor. The difference in speed is called slip. Slip is necessary to produce torque. Slip is dependent on load. An increase in load will cause the rotor to slow down or increase slip. A decrease in load will cause the rotor to speed up or decrease slip. Slip is expressed as a percentage and can be determined with the following formula.
Ns
Nr
Ns
Slip
(%)
(
)
100
2. Three phase induction motor
Week 4
2 Where Nr is the actual speed of the rotor
For Example
, a four-pole motor operated at 60Hz has a synchronous speed (Ns) of
1800 RPM. If the rotor speed at full load is 1765 RPM (Nr), then the slip will be
calculated
%
9
.
1
1800
100
)
1765
1800
(
(%)
Slip
2.4 Name Plate:
It is essential that all motors have nameplates with certain information useful in the
identification of the type of motor, The following Table explain the indication of each code used on the shown nameplate
Name Of Manufacturer ORD. No. TYPE H.P. AMPS R.P.M DUTY CLASS INSUL. FRAME SERVICE FACTOR VOLTS HERTZ DATE NEMA NOM. EFF. IN123456789 HIGH EFFICIENCY 42 40 1790 CONT F B 286T 1.10 415 60 01/15/2003 95 3PH Y 4POLE NEMA Design Address Of Manufacturer
Term
Description
Volts Rated Supply voltage HP Rated motor output Amps Rated full load current
RPM Rated full load speed of the motor
2. Three phase induction motor
Week 4
3 Hertz Rated supply frequency
Frame External dimensions based on NEMA Regulations
Duty Motor load condition ,either its continuities load, short time,etc Date Date of manufacturing
Class Insulation Specifies the max. limit temperature of the winding NEMA Design Types of NEMA design, A,B,C etc
Service factor Factor by which the motor can be overloaded beyond the full load. NEMA Nom
Efficiency
Motor efficiency at rated load
PH Number of phases Pole Number of poles
Motor safety standard
Y The connection either star or delta
3. Synchronous machine
Week 5
1
3.1 Introduction:
Synchronous motors are motors that always run at the same speed regardless of load.
Synchronous motors are somewhat more complex than squirrel-cage and wound rotor motors and, hence, are more expensive. There is no slip in a synchronous motor, that is, the rotor always moves at exactly the same speed as the rotating stator field.. The machine consists of three main parts:
Stator, which carries the three phase winding, Rotor, with one DC winding or permanent magnets
Slip rings or excitation machine (exciter) (in case of electrical excitation).
Synchronous motors are used whenever exact speed must be maintained or for power factor correction. Synchronous motors are more expensive than other types for the lower horsepower ratings, but may possibly be more economical for 100 hp and larger ratings.
3.2 Stator construction:
The stator of a synchronous generator holds a three-phase winding where the individual phase windings are distributed 120° apart in space and is sometimes called the armature winding. The stator must be made of laminated iron sheets in order to reduce eddy currents.
3.3 Rotor construction:
The rotor holds a field winding,which is magnetized by a DC current( the field current). The rotating field winding can be energized through a set of slip rings and brushes (external excitation), or from a diode-bridge mounted on the rotor (self-excited). The rectifier-bridge is fed from a shaft-mounted alternator, which is itself excited by the pilot exciter. In externally fed fields, the source can be a
shaft-driven dc generator, a separately excited dc generator, or a solid-state rectifier. Several variations to these arrangements exist. There are two types of rotors:
Salient-pole rotor (Fig.2) for low-speed machines (e.g.hydro-generators) Cylindrical rotor (Fig.3) for high-speed machines (e.g. turbo-generators).
Laminated iron core with slots
Insulated copper bars are placed in the slots to form the three-phase winding Metal frame
3. Synchronous machine
Week 5
2 DC current terminals Shaft Steel retaining ring DC current terminals Wedges Shaft3-Phase Stator Winding Rotor Field
Winding
Brushes
Slip Rings Cylindrical
Pole Rotor Field current
+
-a- Schematic diagram showing a cylindrical rotor of a synchronous machine
b- cylindrical rotor of a synchronous machine Figure3.1: cylindrical rotor of a synchronous machine
3. Synchronous machine
Week 5
3 3-Phase Stator Winding
Rotor Field Winding
Brushes
Slip Rings Salient Pole
Rotor Field current
+
-a- Schematic diagram showing a salient-pole rotor of a synchronous machine
b- salient-pole rotor of a synchronous machine Figure3.2: Salient-Pole rotor of a synchronous machine
3.4 Principle of operation of the synchronous generator:
When the 3phase rotor is rotated (by an external prime-mover) the rotating magnetic flux(induced by DC current) induces voltages in the stator windings. These voltages are sinusoidal with a magnitude that depends on the field current, and also differ by 120° in time and have a frequency determined by the angular velocity of the rotation.
3.5 Principle of operation of the synchronous motor:
The stator is supplied with three phase supply in order to develop a rotating magnetic field. Also the rotor is supplied with DC supply to produce constant
magnetic field. As a result of the interaction of these two fields, the rotor will start to move. However, the synchronous motor is not self started. Consequently, it usually equipped with squirrel cage windings that mounted on the pole faces of the synchronous motor rotor. These rotor windings
Pole DC excitation winding Fan Slip rings
3. Synchronous machine
Week 5
4 are frequently referred to as damper or amortisseur windings. Thus, the synchronous motor starts as an induction motor. When the motor accelerates to near synchronizing speed (about 95% synchronous speed), DC current is introduced into the rotor field windings. This current creates constant polarity poles in the rotor, causing the motor to operate at synchronous speed as the rotor poles "lock" onto the rotating AC stator poles.
3.6 Excitation Methods
Two methods are commonly utilized for the application of the direct current (DC) field current to the rotor of a synchronous motor.
Brush-type systems apply the output of a separate DC generator (exciter) to the slip rings of the rotor.
Brushless excitation systems utilize an integral exciter and rotating rectifier assembly that eliminates the need for need for brushes and slip rings.
3.7 Method of Synchronization
There are three basic method of synchronizing two or more machine: . Bright lamp method
. Dark lamp method . automatic method
3. Protection & control of electric motors
Week 6
1
4.1 Need for Circuit Protection
Current flow in a conductor always generates heat(Figure1). The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Overcurrent protection devices, such as circuit breakers, are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.
Excessive current Flow Normal Current Flow
Figure4. 1:The effect of current flowing in conductors
overcurrent is defined as any current in excess of the rated current of equipment of a conductor. It may result from overload, short circuit, or ground fault
Overloads: An overload occurs when too many devices are operated on a single circuit, or a piece of electrical equipment is made to work harder than it is designed for. For example, a motor rated for 10 amps may draw 20, 30, or more amps in an overload condition.
3. Protection & control of electric motors
Week 6
2 Good Insulation
Damaged Insulation
Figure4. 2: Insulation of electric conductors
Conductor Insulation Motors, of course, are not the only devices that require circuit protection for an overload condition. Every circuit requires some form of protection against overcurrent. Heat is one of the major causes of insulation failure of any electrical component. High levels of heat can cause the insulation to breakdown and deteriorated, exposing conductors (Figure4.2).
Short Circuits When two bare conductors touch, a short circuit occurs (Figure4.3).
When a short circuit occurs, resistance drops to almost zero. Short circuit current can be thousands of times higher than normal operating current.
The heat generated by this current will cause extensive damage to connected equipment and conductors. This dangerous current must be interrupted immediately when a short circuit occurs.
3. Protection & control of electric motors
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Insulation
conductor
Short Circuit
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4.2 Types of Overcurrent Protective Devices
Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, overloads and short circuits do occur. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source. An overcurrent protection device must be able to recognize the difference between overcurrents and short circuits and respond in the proper way. Slight overcurrents can be allowed to continue for some period of time, but as the current magnitude increases, the protection device must open faster. Short circuits must be interrupted instantly. Several devices are available to accomplish this.
4.2.1 Fuses
A fuse is a one-shot device (Figure1). The heat produced by overcurrent causes the current carrying element to melt open, disconnecting the load from the source voltage. There are three types of fuses, namely
Semi-enclosed (Rewireable) fuse Cartridge fuses
High Breaking Capacity(HBC)
Fuse Cap Glass or Ceramic Body Open Element Good Element
Figure 4.4: Plug fuse
4.2.1.1 Cartridge
The cartridge type have fuses which look similar to those you would find in a standard household plug. This type is improvement of the rewirable fuse type. It is main advantages, is easy to replace, totally
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2 enclosed and its current rating is very accurate
4.2.1.2 HBC
HBC stands for "high blow current (sometimes described as HRC = high rupture current). HBC fuses are designed not to explode when failing under currents
many times their normal working current (e.g. 1500 amps in a 10
amp circuit). They are therefore to be preferred for the
protection of main voltage circuits where the power source may be capable of providing very high currents. HBC types can
usually be recognized by being sand filled though they may have
a thick ceramic body.
4.2.1.3 Semi-enclosed(Rewireable) fuses
As the name indicates, the rewireable type have a fuse wire held at both ends by a small retaining screw. Once the fuse is blown, the fuse wire is the only pieces to be replaced. It is cheap, but replacing a wrong size of element can cause catastrophic consequences.
Figure 4.5: A cartridge fuse and its holder
Figure 4.5: A HBC fuse
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Figure 4.7:Miniature circuit breakers with different poles
The problem with fuses is they only work once. Every time you blow a fuse, you have to replace it with a new one. A circuit breaker(Figure 4.7) does the same thing as a fuse .It opens a circuit as soon as current climbs to unsafe levels ,but you can use it over and over again.
The basic circuit breaker consists of a simple switch,(see figure 4.8) connected to either a bimetallic strip or an electromagnet. The diagram below shows a typical electromagnet design.
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circuit breakers generally employ a bimetal strip to sense overload conditions(Figure4.9b). When sufficient overcurrent flows through the circuit breaker’s current path, heat build up causes the bimetal strip to bend. After bending a predetermined distance the bimetal strip makes contact with the tripper bar activating the trip mechanism.
A bimetal strip
is made of two dissimilar metals bonded together(Figure4.8). The two metals have different thermal expansion characteristics, so the bimetal bends when heated. As current rises, heat also rises. The hotter the bimetal becomes the more it bends, until the mechanism is released.Material 1
Material2
Heat source
}
Bi-metalFigure 4.8:The effect of heat on a bimetal strip
Short circuit protection is accomplished with an electromagnet (Figure4.8a). The electromagnet is connected in series with the overload bimetal strip. During normal current flow, or an overload, the magnetic field created by the electromagnet is not strong enough to attract the armature. When a short circuit current flows in the circuit, the magnetic field caused by the electromagnet attracts the electromagnet’s armature. The armature hits the tripper bar rotating it up and to the right. This releases the trip mechanism and operating mechanism, opening the contacts. Once the circuit breaker is tripped current no longer flows through the electromagnet and the armature is released. (See figure 4.9).
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29 C u rr e n t in Latching mechanisim Current out Magnetic coil Electrical contacts Spring Normal Conditions Current in Latching mechanisim Current out Magnetic coil Electrical contacts Spring Overload ConditionsThermal-magnetic Circuit Breaker(b)
Current in Latching mechanisim Current out Magnetic coil Electrical contacts Spring
Overload Conditions
Magnetic Circuit Breaker(a)
Current in Latching mechanisim Current out Magnetic coil Electrical contacts Spring
Normal Conditions
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14.3 Control devices
4.3.1 Relay:
Battery Relay Coil Contacts To Power Circuit Iron Core Switch Relay Coil Contacts To Power Circuit Iron Core SwitchRelay Open Relay Close
Figure4.10:The principle of operation of the relay
The relay is a remotely controlled switch. In the diagram above, a power circuit contains a switch which is opened and closed by operation of a
relay. The relay is activated by a magnetic core which is energised when a controlling switch is closed. As the core is energised, it lifts and closes a pair of contacts in a second circuit - usually a power circuit. The current required for the relay is usually much lower than that used for the power circuit so it can be provided by a battery In the left hand of figure4.10,the diagram shows the
controlling switch is open, so the relay is de-energised and the power circuit contacts are open. If the controlling switch is
closed, as in the right hand diagram, the relay is therefore energised and its core magnet lifts to close the contacts in the power circuit.
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4.3.2 Contactors:
( a ) ( b)
Figure4.12a shows the interior of a basic contactor. There are two circuits involved with the operation of a contactor, the control circuit and the power circuit. The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the stationary contacts.
When the control circuit supplies power to the coil, a magnetic field is produced, magnetizing the electromagnet. The magnetic field attracts the armature to the magnet, which, in turn, closes the contacts. With the contacts closed, current flows through the power circuit from the line to the load. Figure(4.13)
When current no longer flows through the control circuit, the electromagnet's coil is de-energized, the magnetic field collapses, and the movable contacts open under spring pressure.
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4.3.2.1 Overload relays
Overload relays (Figure4.14)are designed to meet the special protective needs of motor control circuits. Overload relays allow harmless temporary overloads that occur when a motor starts.
Overload relays trip and disconnect power to the motor if an overload condition persists. Overload relays can be reset after the overload condition has been corrected.
Figure4.13:Principle of operation of the contactor
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4.3.2.2 Contactors and Overload Relays
Contactors are used to control power in a variety of applications. When used in motor-control applications, contactors can only start and stop the motors. Contactors cannot sense when the motor is being overloaded and provide no overload protection.
Most motor applications require overload protection, although some smaller motor, such as household garbage disposals, have overload protection built into the motor. Where overload protection is required, overload relays (such as the one shown here) provide such protection.
4.3.2.3 Motor Starter
Contactors and overload relays are separate control devices. When a contactor is combined with an overload relay, it is called a motor starter.Figure4.15
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4.3.3Pushbuttons
A pushbutton is a control device used to manually open and close a set of contacts. Pushbuttons may be illuminated or non-illuminated and are available in a variety of configurations and actuator colors.
Figure 4.16: Different types of pushbuttons
i) Normally Open Pushbuttons
Pushbuttons are used in control circuits to perform various functions such as starting and stopping a motor. A typical pushbutton uses an operating plunger, a return spring, and one set of contacts. This illustration shows a pushbutton with normally open contacts. Pressing the button causes the contacts to close (figure4.17). This pushbutton has momentary contacts which means that the contacts will open when the pushbutton is released.
ii) Normally Closed Pushbuttons
Pushbuttons with normally closed contacts, such as the one shown here, are also used in control circuits. The contacts remain in the closed position allowing current to flow through them until the pushbutton is pressed(figure4.17). Pressing the pushbutton opens the contacts and interrupts current flow. The pushbutton shown here also has momentary contacts; however, normally open and normally closed pushbuttons with maintained contacts are also available.
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Normally Open Pushbutton
Normally Close Pushbutton
Figure 4.17: The mechanism of the pushbuttons
4.3.4 Selector Switches
Selector switches are another means to manually open and close contacts and are commonly used to select one of two or more circuit possibilities.
Selector switches may be maintained, spring return, or key operated and are available in two-position, three-two-position, and four-position types.
The basic difference between a pushbutton and a selector switch is the operator mechanism. A selector switch operator mechanism is rotated to open and close contacts.
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3 Figure 4.18: Different types of selector switches
4.3.5 Indicator Lights:
Indicator lights, often referred to as pilot lights, provide a visual indication of a circuit's operating condition. An indicator light may be wired to turn on for any predetermined condition. Indicator lights are available in round designs with 16 mm, 22 mm, or 30 mm mounting diameters as well as in square designs.
5. energy conversion
Week 11
5.1
Electro-mechanical energy conversion
Energy is converted to electrical form because of the advantages listed in the introductory part of the note. It is seldom available or used in electrical form, but converted into electrical form at the input to a system and back to non-electrical form at the output of a system. A typical example is the processing of energy from and hydro generating plant. It is converted into electrical form at the power plant. Transmitted through transmission lines and distribution lines, and converted to mechanical energy in an electric motor are the point use. A second example is in the conversion of the energy in sound pressure waves, and the transmission in electrical form from the taker to the listener in a telephone system. Few more energy conversion principles will be mentioned.
5.1.1 Major energy coversion principles
Energy conversion between electrical and non- electrical forms includes (i) Electrochemical eg battery
(ii) Electrothermal eg. Thermocouple (iii) Photo electrical eg photo cell
5.2
Energy coversion
Theoretically, only a sourceless current is needed to develop a mechanical force magnetically. But in a machine the production of force is hardly enough: something must move in order to do useful work done demands a corresponding energy supply form somewhere.
In a device energized only by a permanent magnet, the only energy source is the magnet itself. If the displacable part of the machine moves under force and does work, this can only be at the expense of the field energy of the permanent magnet, which must decrease. Such an
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arrangement has obvious limitations. It may also be inconvenient a permanent magnet” lifting magnet, for example would not e capable of releasing its load. Where the magnetic affected by the movement is produced by a current circuit, changes of field energy have to be supplied electrically from a source. This implies the appearance in the circuit of an electromotive force e, which, with the current I, represents the delivery or absorption by the source of energy at the rate ei consider the elementary system of fig 2.1 A sources of voltage is connected to a device (e.g a secondary battery or a machine) in which the energy-conversion process results in the appearance of an e.f.m.
The effective resistance of the circuit is represented by R. if current flows into the circuit form the positive terminal of the source, and the input power p = Vi Rl + el, has the direction as shown at (a). However¸ if e >, the current reverses and we can now call it –e. the power input from the now p = v (-i) = Rl2 + e (-l), which is negative, i.e it is an output from the device into the source, as at (b). to illustrate this simple but fundamental point, suppose that v = 10v d.c nad R = 1-2. then if e = 8vd.c.
The current o = v-e = 10-8 =2A R 1
And the source provides an input power p = 10 x 2 = 20w The converting device accepts 8 x 2 W as a motor And power loss due to plissipation = 1 x RT2 = 4W Conversely, if e 12V, The current again is 10 -12 -2A (I.e reversed) The device produces 12 x 2 = 24 W as a generator of which RT2 22 x 1 = 4W is dissipated in R, and 10 x 20W is delivery as an output to the source. In the case of the electromagnetic machine, the relationship between the emf and the magnetic field is obtained from the faraday induction law (which had been mentioned in 1.1)
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5.3
Linked energy systems
An electromechanical machine forms a coverting link between an electrical energy system ( such as a main power –supply network) and a mechanical one (such as a prime- mover or a train). In action a machine is not an isolated things, but has a behavious strongly influenced by its terminal systems. A relay, for instance, will be affected if its operating battery becomes discharged; a loudspeaker will behave very differently it enclosed in an evacuated vessels with the air loading thus removed; a hydro electric generator, suddenly short-circuited, will react severely on the turbine and pipe-line.
A machine can, of course, be studies initially in isolation, but the engineering interest begins in fact when the complete linked system is considered. Again, the steady-state behavious is informative up to a point, but operation in responses to change – i.e, the transient responses is fro move important and fundamental.
Fig 2.1 Electro–mechanical linked energy system
System analysis can be complicated. Fig 2.2 shows diagrammatically a typical electric supply system feeding a mechanical load through an electromechanical machine. In some cases we might simplify the analysis by assuming, say, that the terminal voltage and frequency of the machine were constant. This is good enough if the machine is a small contactor but if it is a 25MW motor the effects of its behaviour reach for back through even an extensive supply system. Methods are available for evaluating such a complex for any
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given stimulus, such as the occurrence of a transmission- line fault or starting of a large motor.
5.4 Energy storage
We now consider how a flux is established and energy is stored in simple toroidal magnetic Circuit of cross sectional area A, path length L, and of material of constant permeability u, The flux is to be established by a current i, in a uniformly wound coil of N-turns. In order To concentrate on energy storage we neglect the coil resistance. With i initially zero, let a Voltage V, be applied to the coil terminals, what happen thereafter depends on Faraday’s Law of electromagnetic induction.
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5.5
Energy balance
Fig: 2.1 Electromechanical machine conventions
A machine accepts energy in a variety of forms from its attached terminal systems. By conversion we take energy input as positive, so that an output is regarded as a negative input. The machine internally electrical energy- mechanical energy is a motor mechanical energy to electrical energy is a generator converts some energy, stores some, and dissipates the rest: these energies are positive if they increase with time. As the prime object of a machine is conversion to useful output, one of the terminal inputs will normally be negative. Recalling the principle of conservation of energy which states that energy is neither created nor destroyed and combining it with the laws of electric and magnetic fields, electric circuits and Newtonian mechanics, the energy balance can be expressed as:-
Total terminal energy input internal energy + Dissipation 2.1 for an electromechanical machine using a magnetic field as the means of conversion, the balance can be stated in more specific terms as electrical energy input + mechanical energy input
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Reckoned from an initial condition of zero energy, w = o.A comparable relation must apply to energy changes dw, and also to energy rate dw/dt i.e to power, P. in corresponding symbols these relations are total energy wf + ws + w 2.2(a)
Energy change dwe + dwm = dwf + dws + dw 2.2(b)
Energy rate Pe + pm = dwf + dws + p 2.2(c)
dt dt
The rates of change of stored field energy wf and stored mechanical energy, ws, are left in
differential form because there is always a practical limit to storage. A magnetic field can not grow in strength indefinitely when ferromagnetic materials is employed; and if the kinetic energy in a flywheel is continually increased, the speed must rise and the wheel may burst under centrifugal force.
We shall now examine the electromechanical machine in more detail with fig 2.3. The machine links an electric source of voltages supplying a current; and a mechanical sources represented by a bar moving to positive directions, thus both vi and fmu are inputs ( The
mechanical source could alternatively be a shaft rotated at angular speed wr by a tongue mm to give an input power mnwr ). The electrical end of the machine is precisely that of fig 2.1
(a), with opposing v. the mechanical end has the magnetically developed force fe opposing fm > fm it can reverse speed w so that the mechanical system is driven and absorbs a
mechanical output.
The behavior can now be summarized. With the machine operating in the steady state as a motor, the applied voltage u drive +I against e to give a total electrical power input pe = u(+e), of which the part ei is converted. The outcome of conversion is the force fe which drives the bar against fm to develop the mechanical input pm = fm (-u) which, being
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negative, is actually an output. With the machine as a generator; the bar is driven at speed u by the force fm to provide the mechanical input pm = fm ( +u), as a result which e now exceeds u and reverse the current to provide the negative electrical input (i.e output (i.e output) pe = u (-i) the sum of the inputs (pe +pm) must be rate of rise of internal energy storage plus the rate of energy dissipation.
A real electromagnetic machine has fairly obvious points of attachment (e.g the electrical terminals and the shaft) by which it is connected to the electrical and mechanical sources to form a link between them. But it is very to concentrate source to from link between them. But it is very convenient attention on the conversion region enclosed by the chain- dotted line in fig 2.3, for it contains only the essential quantities e and i,. U and fe. Various losses, and the mechanical storage, are excluded so that attention can be directed on to the physical process if useful energy conversion by electromagnetic means outside the conversion region we can account for conduction and core losses associated with the electrical end and represented rough by the resistance R in fig 2.3, and friction and similar losses on the mechanical side. It is to be noted that the externally applied force fm is not necessarily equal to –fe because there may be force-absorbing components of inertial and elasticity in the mechanical working parts of the machine itself, as well as internal friction.
The machine has new been reduced to an analyzable form. Its behaviors under specified conditions involves the forces and movement of the mechanical parts, the voltages and current at the electrical terminals and processes of energy conversion and storage and dissipation going on inside. Evaluation is based on the well-established principles and laws summarized in the following table.
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Part of system Quantities Principles
Electrical Voltage, current Faraday-Lenz and Kirchhoff laws Conversion
Mechanical
E.M.F current magnetic field,. Force, displacement Force, displacement speed
Magneto- mechanical Principles induction and thermal laws Newton law
5.5.1 Block diagram for energy balance equation
The energy balance equation is given by equation 2.2 as electrical energy input mechanical energy input
= stored magnetic-=field energy + stored mechanical energy + dissipation The dissipation (energy lossess) arise from three main causes
(ii) Part of electrical energy is converted directly to heat in the resistance of current path. (ii) Part of mechanical energy developed with the device is absorbed in friction ad
windage and converted to heat.
(iii) Part of the energy absorbed by the coupling field is converted to heat in magnetic core losses (for magnetic coupling ) or dielectric loss) for electric coupling).
if we associate the various losses with the corresponding energies, equation 2.2 be written as Electrical energy mechanical energy increase in energy stored
Input minus = Output plus friction + in the coupling field Resistance losess and windage losses plus associated losses
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Equation 2.3 is obtained ( for a motor) with the mechanical energy transferred to the R.H.S of the equality sign and neglecting the energy mechanical stored energy ( for a machine without a flywheel and neglecting the mass of the shaft). If there is a flywheel, the stored mechanical energy is 1/2mu2 or ½ mr2w2
Where m = mass of flywheel
V = linear velocity of rotating wheel w = angular velocity of rotating wheel r = radius of flywheel
Equation 2.3 may be represented in the form of a block diagram as shown in fig 2.4 Fig 2.4 General representation of electromagnetic energy conversion. Fro a generator action, the positions of the electrical system and that or the mechanical system will be interchanged.
5. energy conversion
Week 13
5.6
Magnetic field energy and forces
In order to be able to analyse mathematically the electromechanical system that is completely described by the energy balance equation, we need to be able to determine qualitatively the energy of the magnetic field and the associated force.
5.6.1 Magnetic field
A magnetic field is a region of space in which certain physical effects occurs in particular the development of mechanical force. A pictorial model of the field can be made by drawing closed loops of magnetic flux, such that their direction and spacing at any point are a measure of the flux density. The magnetic circuit in the present context is composed partly of ferromagnetic material such as iron, and partly of an airgap. The iron serves to “guide” the flux in a desired path; the airgap is necessary to make useful magnetic effects readily accessible.
The lines in a flux plot have no real existence. In a given region a magnetic field may change direction, become weaker in some place and stronger in others.
5.6.2 Magnetic circuit n/a
Engineers look upon magnetic flux (Weber) as produced by electric current. A current I develops around any path that links it a magneto motive force (M.M.F) F = I (ampere). The effect of a current can be multiplied. By coiling the electric circuit into N turns so that around a path linking all N turns the m.m.f is N times as great, giving F =- ampere- turn. The m.m.f is distributed along the path, to give along a path element of length dx the magnetic field intensity h (ampere-turn/ metre). The summation of Hdx around a single loop closed with F i.e F = Hdx = m.m.d.
