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Dcgenerator
1.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.1 Electric Motors and Generators  Introduction  A Simple AC Generator  A Simple DC Generator  DC Generators or Dynamos  AC Generators or Alternators  DC Motors  AC Motors  Universal Motors  Electrical Machines – A Summary Chapter 23
2.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.2 Introduction  In this lecture we consider various forms of rotating electrical machines  These can be divided into: – generators – which convert mechanical energy into electrical energy – motors – which convert electrical energy into mechanical energy  Both types operate through the interaction between a magnetic field and a set of windings 23.1
3.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.3 A Simple AC Generator  We noted earlier that Faraday’s law dictates that if a coil of N turns experiences a change in magnetic flux, then the induced voltage V is given by  If a coil of area A rotates with respect to a field B, and if at a particular time it is at an angle θ to the field, then the flux linking the coil is BAcosθ, and the rate of change of flux is given by 23.2 t Φ NV d d = ( ) θωθ θθ coscos d d d sind === tt BA dt dΦ
4.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.4  Thus for the arrangement shown below t Φ NV d d = ( ) θω θ cos d sind d d NBA t NBA t Φ NV ===
5.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.5  Therefore this arrangement produces a sinusoidal output as shown below
6.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.6  Wires connected to the rotating coil would get twisted  Therefore we use circular slip rings with sliding contacts called brushes
7.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.7 A Simple DC Generator  The alternating signal from the earlier AC generator could be converted to DC using a rectifier  A more efficient approach is to replace the two slip rings with a single split slip ring called a commutator – this is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity – hence the voltage across the brushes is of a single polarity – adding additional coils produces a more constant output 23.3
8.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.8  Use of a commutator
9.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.9  A simple generator with two coils
10.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.10  The ripple can be further reduced by the use of a cylindrical iron core and by shaping the pole pieces – this produces an approximately uniform field in the narrow air gap – the arrangement of coils and core is known as the armature
11.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.11 DC Generators or Dynamos  Practical DC generators or dynamos can take a number of forms depending on how the magnetic field is produced – can use a permanent magnet – more often it is generated electrically using field coils  current in the field coils can come from an external supply – this is known as a separately excited generator  but usually the field coils are driven from the generator output – this is called a self-excited generator – often use multiple poles held in place by a steel tube called the stator 23.4
12.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.12  A four-pole DC generator
13.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.13  Field coil excitation – sometimes the field coils are connected in series with the armature, sometimes in parallel (shunt) and sometimes a combination of the two (compound) – these different forms produce slightly different characteristics – diagram here shows a shunt-wound generator
14.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.14  DC generator characteristics – vary slightly between forms – examples shown here are for a shunt-wound generator
15.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.15 AC Generators or Alternators  Alternators do not require commutation – this allows a simpler construction – the field coils are made to rotate while the armature windings are stationary  Note: the armature windings are those that produce the output – thus the large heavy armature windings are in the stator – the lighter field coils are mounted on the rotor and direct current is fed to these by a set of slip rings 23.5
16.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.16  A four-pole alternator
17.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.17  As with DC generators multiple poles and sets of windings are used to improve efficiency – sometimes three sets of armature windings are spaced 120° apart around the stator to form a three-phase generator  The e.m.f. produced is in sync with rotation of the rotor so this is a synchronous generator – if the generator has a single set of poles the output frequency is equal to the rotation frequency – if additional pole-pairs are used the frequency is increased accordingly
18.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.18 Example – see Example 23.2 from course text A four-pole alternator is required to operate at 60 Hz. What is the required rotation speed? A four-pole alternator has two pole pairs. Therefore the output frequency is twice the rotation speed. Therefore to operate at 60Hz, the required speed must be 60/2 = 30Hz. This is equivalent to 30 × 60 = 1800 rpm.
19.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.19 DC Motors  When current flows in a conductor it produces a magnetic field about it - as shown in (a) below – when the current-carrying conductor is within an externally generated magnetic field, the fields interact and a force is exerted on the conductor - as in (b) 23.6
20.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.20  Therefore if a conductor lies within a magnetic field: – motion of the conductor produces an electric current – an electric current in the conductor will generate motion  The reciprocal nature of this relationship means that, for example, the DC generator above will function as a DC motor – although machines designed as motors are more efficient in this role  Thus the four-pole DC generator shown earlier could equally well be a four-pole DC motor
21.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.21  DC motor characteristics – many forms – each with slightly different characteristics – again can be permanent magnet, or series-wound, shunt-wound or compound wound – figure below shows a shunt-wound DC motor
22.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.22 AC Motors  AC motors can be divided into two main forms: – synchronous motors – induction motors  High-power versions of either type invariably operate from a three-phase supply, but single-phase versions of each are also widely used – particularly in a domestic setting 23.7
23.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.23  Synchronous motors – just as a DC generator can be used as a DC motor, so AC generators (or alternators) can be used as synchronous AC motors – three phase motors use three sets of stator coils  the rotating magnetic field drags the rotor around with it – single phase motors require some starting mechanism – torque is only produced when the rotor is in sync with the rotating magnetic field  not self-starting – may be configured as an induction motor until its gets up to speed, then becomes a synchronous motor
24.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.24  Induction motors – these are perhaps the most important form of AC motor – rather than use slip rings to pass current to the field coils in the rotor, current is induced in the rotor by transformer action – the stator is similar to that in a synchronous motor – the rotor is simply a set of parallel conductors shorted together at either end by two conducting rings
25.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.25  A squirrel-cage induction motor
26.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.26  In a three-phase induction motor the three phases produce a rotating magnetic field (as in a three-phase synchronous motor) – a stationary conductor will see a varying magnetic field and this will induce a current – current is induced in the field coils in the same way that current is induced in the secondary of a transformer – this current turns the rotor into an electromagnet which is dragged around by the rotating magnetic field – the rotor always goes slightly slower than the magnetic field – this is the slip of the motor
27.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.27  In single-phase induction motors other techniques must be used to produce the rotating magnetic field – various techniques are used leading to various forms of motor such as  capacitor motors  shaded-pole motors – such motors are inexpensive and are widely used in domestic applications
28.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.28 Universal Motors  While most motors operate from either AC or DC, some can operate from either  These are universal motors and resemble series- wound DC motors, but are designed for both AC and DC operation – typically operate at high speed (usually > 10,000 rpm) – offer high power-to-weight ratio – ideal for portable equipment such as hand drills and vacuum cleaners 23.8
29.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.29 Electrical Machines – A Summary  Power generation is dominated by AC machines – range from automotive alternators to the synchronous generators used in power stations – efficiency increases with size (up to 98%)  Both DC and AC motors are used – high-power motors are usually AC, three-phase – domestic applications often use single-phase induction motors – DC motors are useful in control applications 23.9
30.
Storey: Electrical &
Electronic Systems © Pearson Education Limited 2004 OHT 23.30 Key Points  Electrical machines include both generators and motors  Motors can usually function as generators, and vice versa  Electrical machines can be divided into AC and DC forms  The rotation of a coil in a uniform magnetic field produces a sinusoidal e.m.f. This is the basis of an AC generator  A commutator can be used to produce a DC generator  The magnetic field in an electrical machine is normally produced electrically using field coils  DC motors are often similar in form to DC generators  Some forms of AC generator can also be used as motors  The most widely used form of AC motor is the induction motor
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