ELECTRICAL MACHINES
EEC-207
1
Syllabus as per GGSIPU
2
Content
Unit 1: Principles of Electromechanical Energy Conversion
Unit 2: Transformers
Unit 3:Induction Motors
Unit 4:Synchronous Machine
Department of Instrumentation and control Engineering, BVCOE, New Delhi
SubjectELECTRICAL MACHINES, Instructor:MANISH TALWAR
3
Unit 1: Principles of Electromechanical Energy Conversion
4
Topics Covered:
DC MACHINES:
PMDC MACHINE
5
Electromechanical Energy Conversion
6
DC MACHINES
What is dc machine?
The DC machine can be classified into two types namely DC motors as well as DC generators .Most of the DC machines are equivalent to AC machines because they include AC currents as well as AC voltages in them. The output of the DC machine is DC output because they convert AC voltage to DC voltage. The conversion of this mechanism is known as the commutator, thus these machines are also named as commutating machines. DC machine is most frequently used for a motor. The main benefits of this machine include torque regulation as well as easy speed. The applications of the DC machine is limited to trains, mills, and mines. For example, underground subway cars, as well as trolleys, may utilize DC motors. In the past, automobiles were designed with DC dynamos for charging their battery.
7
A DC machine is an electromechanical energy alteration device. The working principle of a DC machine is when electric current flows through a coil within a magnetic field, and then the magnetic force generates a torque that rotates the dc motor. The DC machines are classified into two types such as DC generator as well as DC motor.
The main function of the DC generator is to convert mechanical power to DC electrical power, whereas a DC motor converts DC power to mechanical power. The AC motor is frequently used in industrial applications for altering electrical energy to mechanical energy. However, a DC motor is applicable where good speed regulation & an ample range of speeds are necessary like in electric-transaction systems.
8
9
Construction:
The construction of the DC machine can be done using some of the essential parts like Yoke, Pole core & pole shoes, Pole coil & field coil, Armature core, Armature winding otherwise conductor, commutator, brushes & bearings. Some of the parts of the DC machine is discussed below.
10
Yoke
Another name of a yoke is the frame. The main function of the yoke in the machine is to offer mechanical support intended for poles and protects the entire machine from moisture, dust, etc. The materials used in the yoke are designed with cast iron, cast steel otherwise rolled steel.
Pole and Pole Core
The pole of the DC machine is an electromagnet and the field winding is winding among pole. Whenever field winding is energized then the pole gives magnetic flux. The materials used for this are cast steel, cast iron otherwise pole core. It can be built with the annealed steel laminations for reducing the power drop because of the eddy currents.
11
Pole Shoe
Pole shoe in the DC machine is an extensive part as well as to enlarge the region of the pole. Because of this region, flux can be spread out within the air-gap as well as extra flux can be passed through the air space toward armature. The materials used to build pole shoe is cast iron otherwise cast steed, and also used annealed steel lamination to reduce the loss of power because of eddy currents.
Field Windings
In this, the windings are wounded in the region of pole core & named as field coil. Whenever current is supplied through field winding than it electromagnetics the poles which generate required flux. The material used for field windings is copper.
Armature Core
Armature core includes a huge number of slots within its edge. The armature conductor is located in these slots. It provides the low-reluctance path toward the flux generated with field winding. The materials used in this core are permeability low-reluctance materials like iron otherwise cast. The lamination is used to decrease the loss because of the eddy current.
12
Separately Excited
In Separately Excited DC Machine, a separate DC source is utilized for activating the field coils.
Shunt Wound
In Shunt wound DC Machines, the field coils are allied in parallel through the armature. As the shunt field gets the complete o/p voltage of a generator otherwise a motor supply voltage, it is normally made of a huge number of twists of fine wire with a small field current carrying.
Series Wound
In series-wound D.C. Machines, the field coils are allied in series through the armature. As series field winding gets the armature current, as well as the armature current is huge, due to this the series field winding includes few twists of wire of big cross-sectional region.
13
Compound Wound
A compound machine includes both the series as well as shunt fields. The two windings are carried-out with every machine pole. The series winding of the machine includes few twists of a huge cross-sectional region, as well as the shunt windings, include several fine wire twists.
The connection of the compound machine can be done in two ways. If the shunt-field is allied in parallel by the armature only, then the machine can be named as the ‘short shunt compound machine’ & if the shunt-field is allied in parallel by both the armature as well as series field, then the machine is named as the ‘long shunt compound machine’.
14
EMF Equation of DC Machine
The DC machine e.m.f can be defined as when the armature in the dc machine rotates, the voltage can be generated within the coils. In a generator, the e.m.f of rotation can be called the generated emf, and Er=Eg. In the motor, the emf of rotation can be called as counter or back emf, and Er=Eb.
Let Φ is the useful flux for every pole within webers
P is the total number of poles
z is the total number of conductors within the armature
n is the rotation speed for an armature in the revolution for each second
A is the no. of parallel lane throughout the armature among the opposite polarity brushes.
15
Z/A is the no. of armature conductor within series for each parallel lane
As the flux for each pole is ‘Φ’, every conductor slashes a flux ‘PΦ’ within a single revolution.
The voltage produced for each conductor = flux slash for each revolution in WB / Time taken for a single revolution within seconds
As ‘n’ revolutions are completed within a single second and 1 revolution will be completed within a 1/n second. Thus the time for a single armature revolution is a 1/n sec.
The standard value of produced voltage for each conductor
p Φ/1/n = np Φ volts
The voltage produced (E) can be decided with the no.of armature conductors within series I any single lane among the brushes thus, the whole voltage produced
16
E = standard voltage for each conductor x no. of conductors within series for each lane
E = n.P.Φ x Z/A
The above equation is the e.m.f. the equation of the DC machine.
17
DC Machine vs AC Machine
The differences between AC Motor and DC Motor includes the following:
18
Losses in DC Machine
We know that the main function of a DC machine is to convert mechanical energy to electrical energy. .Throughout this conversion method, the whole input power cannot be changed into output power because of the power loss in different forms. The type of loss may change from one apparatus to another. These losses will decrease the apparatus efficiency as well as the temperature will be increased. The DC machine energy losses can be classified into Electrical otherwise Copper losses, Core losses otherwise Iron losses, Mechanical losses, Brush losses, and Stray load losses.
DC Machine Advantages
The advantages of this machine include the following.
19
Applications of DC Machine
At present, the generation of electrical energy can be done in bulk in the form of AC (an alternating current). Therefore, the utilization of DC machines like motors and generators DC generators are extremely limited because they are utilized mainly for providing excitation of tiny & middle range of alternators. In industries, DC machines are used for different processes like welding, electrolytic, etc.
Generally, the AC is generated and after that, it is changed into DC with the help of rectifiers. Therefore DC generator is suppressed through an AC supply which is rectified to use in several applications. DC motors are frequently used like variable speed drives & where changes in the severe torque occur.
The application of DC machine as a motor is used by dividing into three types like Series, Shunt &Compound whereas the application of dc machine as a generator is classified into separately excited, series, and shunt-wound generators.
Thus, this is all about DC machines. From the above information, finally, we can conclude that DC machines are dc generator & dc motor.The DC generator is mainly useful for supplying DC sources toward the DC machine in power stations. Whereas DC motor drives some devices like lathes, fans, centrifugal pumps, printing presses, electric locomotives, hoists, cranes, conveyors, rolling mills, auto-rickshaw, ice machines, etc.
20
Armature Windings
Basically armature winding of a DC machine is wound by one of the two methods, lap winding or wave winding. The difference between these two is merely due to the end connections and commutator connections of the conductor. To know how armature winding is done, it is essential to know the following terminologies -
21
l
22
Lap Winding Wave Winding
Lap Winding And Wave Winding
In lap winding, the successive coils overlap each other. In a simplex lap winding, the two ends of a coil are connected to adjacent commutator segments. The winding may be progressive or retrogressive. A progressive winding progresses in the direction in which the coil is wound. The opposite way is retrogressive. The following image shows progressive simplex lap winding.
In wave winding, a conductor under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole which is of opposite polarity. In other words, all the coils which carry emf in the same direction are connected in series. The following diagram shows a part of simplex wave winding.
23
Torque Production
24
When a DC machine is loaded either as a motor or as a generator, the rotor conductors carry current. These conductors lie in the magnetic field of the air gap. Thus each conductor experiences a force. The conductors lie near the surface of the rotor at a common radius from its center. Hence torque is produced at the circumference of the rotor and rotor starts rotating. The term torque as best explained by Dr. Huge d Young is the quantitative measure of the tendency of a force to cause a rotational motion, or to bring about a change in rotational motion. It is in fact the moment of a force that produces or changes a rotational motion.
25
The equation of torque is given by,
Where, F is force in linear direction.
R is radius of the object being rotated,
and θ is the angle, the force F is making with R vector
The DC motor as we all know is a rotational machine, and torque of DC motor is a very important parameter in this concern, and it’s of utmost importance to understand the torque equation of DC motor for establishing its running characteristics.
26
To establish the torque equation, let us first consider the basic circuit diagram of a DC motor, and its voltage equation.
27
28
29
30
31
Magnetizing Curve
32
DC generator is that curve which gives the relation between field current and the armature terminal voltage on open circuit.
When the DC generator is driven by a prime mover then an emf is induced in the armature. The generated emf in the armature is given by an expression
33
34
35
Magnetization curve of a DC generator has great importance because it represents the saturation of the magnetic circuit. For this reason, this curve is also called saturation curve.
According to the molecular theory of magnetism the molecules of a magnetic material, which is not magnetized, are not arranged or aligned in definite order. When current passed through the magnetic material then its molecules are arranged in definite order. Up to a certain value of field current the maximum molecules are arranged. In this stage the flux established in the pole increased directly with the field current and the generated voltage is also increased. Here, in this curve, point B to point C is showing this phenomena and this portion of the magnetization curve is almost a straight line. Above a certain point (point C in this curve) the nu-magnetized molecules become very fewer and it became very difficult to further increase in pole flux.This point is called saturation point. Point C is also known as the knee of the magnetization curve. A small increase in magnetism requires very large field current above the saturation point. That is why upper portion of the curve (point C to point D) is bend as shown in figure.
Magnetization curve of a DC generator does not start from zero initially. It starts from a value of generated voltage due to residual magnetism.
36
Types of Generators and Motors
37
38
39
Characteristics of DC Generators & Motors
Internal & External
40
Commutation & Interpoles
41
Armature Reaction
42
Speed Control of dc Motor and Starting
43
Often we want to control the speed of a DC motor on demand. This intentional change of drive speed is known as speed control of a DC motor.
Speed control of a DC motor is either done manually by the operator or by means of an automatic control device. This is different to speed regulation – where the speed is trying to be maintained (or ‘regulated’) against the natural change in speed due to a change in the load on the shaft.
44
Terminal voltage and external resistance involve a change that affects the armature circuit, while flux involves a change in the magnetic field. Therefore speed control of DC motor can be classified into:
We will discuss how both of these methods control the speed of DC series motors and DC shunt motors.
Speed Control of DC Series Motor
Speed control methods for a DC series motor can be classified as:
Armature Controlled DC Series Motor
Speed adjustment of a DC series motor by armature control may be done by:
45
Armature Resistance Control Method
This is the most common method employed. Here the controlling resistance is connected directly in series with the supply of the motor as shown in the fig.
46
The power loss in the control resistance of DC series motor can be neglected because this control method is utilized for a large portion of time for reducing the speed under light load condition. This method of speed control is most economical for constant torque. This method of speed control is employed for DC series motor driving cranes, hoists, trains etc.
Shunted Armature Control
The combination of a rheostat shunting the armature and a rheostat in series with the armature is involved in this method of speed control. The voltage applied to the armature is varies by varying series rheostat R1. The exciting current can be varied by varying the armature shunting resistance R2. This method of speed control is not economical due to considerable power losses in speed controlling resistances. Here speed control is obtained over wide range but below normal speed.
47
Armature Terminal Voltage Control
The speed control of DC series motor can be accomplished by supplying the power to the motor from a separate variable voltage supply. This method involves high cost so it rarely used.
48
Field Controlled DC Series Motor
Speed adjustment of a DC series motor by field control may be done by:
Field Diverter Method
This method uses a diverter. Here the field flux can be reduced by shunting a portion of motor current around the series field. Lesser the diverter resistance less is the field current, less flux therefore more speed. This method gives speed above normal and the method is used in electric drives in which speed should rise sharply as soon as load is decreased.
49
50
Tapped Field Control
This is another method of increasing the speed by reducing the flux and it is done by lowering number of turns of field winding through which current flows. In this method a number of tapping from field winding are brought outside. This method is employed in electric traction.
51
Speed Control of DC Shunt Motor
The classification of speed control methods for a DC shunt motor are similar to those of a DC series motor. These two methods are:
Armature Controlled DC Shunt Motor
Armature controlled DC shunt motor can be performed in two ways:
52
Armature Resistance Control
In armature resistance control a variable resistance is added to the armature circuit. Field is directly connected across the supply so flux is not changed due to variation of series resistance. This is applied for DC shunt motor. This method is used in printing press, cranes, hoists where speeds lower than rated is used for a short period only.
Armature Voltage Control
This method of speed control needs a variable source of voltage separated from the source supplying the field current. This method avoids disadvantages of poor speed regulation and low efficiency of armature-resistance control methods.
The basic adjustable armature voltage control method of speed d control is accomplished by means of an adjustable voltage generator is called Ward Leonard System. This method involves using a motor-generator (M-G) set. This method is best suited for steel rolling mills, paper machines, elevators, mine hoists, etc. This method is known as Ward Leonard System.
Advantages of Armature Controlled DC Shunt Motor
53
Disadvantages of Armature Controlled DC Shunt Motor
Field Controlled DC Shunt Motor
By this method a DC Shunt motor’s speed is controlled through a field rheostat.
Field Rheostat Controlled DC Shunt Motor
In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field. An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed. This method of speed control is independent of load on the motor. Power wasted in controlling resistance is very less as field current is a small value. This method of speed control is also used in DC compound motor.
Disadvantages of Field Rheostat Controlled DC Shunt Motor
54
Solid State Speed Control
Static Ward Leonard drives are being used these days because of the drawbacks of the classical method. Rotating M-G sets are replaced by solid state converters to control DC motor speed. The converters used are choppers (in case of DC supply) or controlled rectifiers (in case of AC supply). This method is not suitable for intermittent loads.
DC Motor Speed Control Theory
To derive the speed of a DC motor, we start with the equation for the DC motor’s EMF (Electromagnetic Force). We know that the EMF equation of DC motor is equal to:
55
56
PMDC Machine
Introduction:
What is a Permanent Magnet DC Motor?
The permanent magnet dc motor can be defined as a motor which includes a permanent magnet pole is called Permanent Magnet DC Motor. In this motor, the magnet can be used to make the flux working within the air gap in its place of the field winding. The rotor structure is similar to the straight DC Motor. PMDC Motor’s rotor includes armature core, commutator, & armature winding. Normally, in a conventional DC motor, there are two kinds of winding such as armature as well as Filed.
57
The main function of field winding is to produce the functioning magnetic flux within the air gap as well as wound on the stator of the motor while armature winding can be wound on the rotor. Inactive carbon brushes are pushed on the commutator like in conventional DC motor. The operating voltage of the PMDC motor is 6 volts, 12 volts otherwise 24 volts DC supply attained from the voltage sources.
Construction of PMDC Motor
The PMDC motor’s permanent magnets are maintained with a cylindrical-steel stator and these supplies like a return lane for the magnetic flux. The rotor supplies like an armature, and it includes commutator segments, winding slots, & brushes like in conventional dc machines. The permanent magnets used in this motor are classified into three namely Alnico magnets, Ceramic (ferrite) magnets, and Rare-earth magnets.
58
59
Operation of PMDC Motor
In this motor, a permanent magnetic field can be generated with the permanent magnets which communicate by the perpendicular field stimulated by the flow of currents within the rotor windings; therefore a mechanical torque can be created.
When the rotor rotates in response to the created torque, then the position among the stator as well as rotor fields can be reduced, and the torque would be reversed in a 90-degree rotation. To maintain the torque performing on the rotor, PMDC motors include a commutator, set to the rotor shaft.
60
The commutator activates the current supply toward the stator thus as to continue a steady angle = 90, among two fields. As the flow of current is frequently activated among windings like the rotor twists, then the current within every stator winding is truly exchanging at a frequency comparative to the no.of motor magnetic poles as well as the speed.
Circuit Diagram of PMDC Motor
The circuit diagram of the PMDC motor is shown below. As in permanent magnet DC motor the field can be generated with the permanent magnet, then there is no requirement of drawing field coils within the PMDC motor equivalent circuit.
61
62
The voltage supply toward the armature will contain fall of armature resistance as well as break of the voltage supply can be countered with the motor’s back e.m.f. Therefore the voltage equation of the motor is given by,
63
The PMDC Motor characteristics are related to the dc shunt motor characteristic in terms of speed, torque, as well as armature current. But, the characteristics of speed-torque are more linear as well as conventional in these types of motors.
Advantages and Disadvantages of the PMDC Motor
The advantages and disadvantages of the PMDC motor include the following.
64
Applications of the PMDC Motor
The applications of the PMDC Motor include the following.
65
Need of Brushless Motors
66
Why is brushless motor needed?
Because computers control the electrical current, brushless DC motors can achieve much more precise motion control. Because of all these advantages, brushless DC motors are often used in modern devices where low noise and low heat are required, especially in devices that run continuously.
Why are Brushless Motors better?
The main difference between brushless or brushed drill motors is that the brushed variants are made of carbon while the brushless units use magnets to generate power. For this reason, brushless motors are better adapted, generate no friction, produce less heating and provide better performance.
How does a brushless motor work?
Like brushed DC motors, brushless motors work by alternating the polarity of the windings on the interior of the motor. The magnetic fields created when the coils are generated exert a push/pull force on the permanent magnets arranged around the outside of the casing.
67
References:
[1] Yang L, Li H, Shen M Y Speed control of BLDCM for industrial sewing machine Based on DSPACE.IEEE 2006 International Conference on Mechatronics and Automation, 2006 (11):2127-2132.
[2] Lin Xu;Yan Jiang;Jian hui Wang;Tong Gao; Xiang yi De, Design of Fuzzy-PID controller based on prediction model and its realization in PLC[J].Control and Decision Conference, 2008. CCDC. Chinese .2008:3808-3811.
[3] YXIA C L, LI Z Q, SHI T N.A control strategy for four-switch three-phase brushless DC motor using single current sensor [J].IEEE Transactons on industrial electronics, Vol. 56, NO. 6, June 2009:2058-2066.
[4] Lin X Q, Zhang H L, Han B. The Design of Fuzzy Control for Linear Motor Based on TMS320F2812.Chinese 2009 Control and Decision Conference,2009(1):5369-5373.
[5] Ren Hai Peng, Liu Ding. A novel digital position servo system using DSP and Fuzzy PID. In: Proceedings of the Fifth International Conference on ICEMS 2011. Shenyang, 2011, 722-725.
68