S YNOPSIS Introducing the Basic types of Electric Machines a) A.C. Motors - Induction and Synchronous Motors b) Ideal and Practical Transformers c) D.C. Motors and Generators d) Self and Separately Exited Motors e) Stepper Motors and their characteristics f) Assessment of Electric Motors i. Efficiency ii. Energy losses iii. Motor load analysis iv. Energy efficiency opportunity analysis v. Improve power quality vi. Rewinding vii. Power factor viii. Speed control
I NTRODUCTION TO E LECTRICAL M ACHINES Essentially all electric energy is generated in a rotating machine: the synchronous generator, and most of it is consumed by: electric motors. In many ways, the world’s entire technology is based on these devices. The study of the behavior of electric machines is based on three fundamental principles: Ampere’s law, Faraday’s law and Newton’s Law.
I NTRODUCTION TO E LECTRICAL M ACHINES Various conﬁgurations result and are classiﬁed generally by the type of electrical system to which the machine is connected: direct current (dc) machines or alternating current (ac) machines.
I NTRODUCTION TO E LECTRICAL M ACHINES Machines with a dc supply are further divided into permanent magnet and wound ﬁeld types, as shown in Figure 4.1.
I NTRODUCTION TO E LECTRICAL M ACHINES The wound motors are further classiﬁed according to the connections used: The ﬁeld and armature may have separate sources (separately excited), they may be connected in parallel (shunt connected), or they may be series (series connected). (figure follows)
I NTRODUCTION TO E LECTRICAL M ACHINES AC machines are usually single-phase or three-phase machines and may be synchronous or asynchronous. See figure next page.
I NTRODUCTION TO E LECTRICAL M ACHINES Several variations are shown in Figure 4.2.
1.1 - BASIC ELECTROMAGNETIC LAWS: AMPERE`S LAW AND FARADAY`S LAW The two principles that describe the electromagnetic behavior of electric machines are Ampere’s Law and Faraday’s Law. These are two of Maxwell’s equations. Most electric machines operate by attraction or repulsion of electromagnets and/or permanent magnets.
AMPERE`S LAW Ampere’s law describes the magnetic ﬁeld that can be produced by currents or magnets. In an electric machine, there will always be at least one set of coils with currents. A motor cannot be produced with permanent magnets alone.
AMPERE`S LAW Ampere’s law states that the line integral of the component of the magnetic ﬁeld along the path of integration is equal to the current enclosed by the path. This is exactly true for static ﬁelds and is a very good approximation for the low-frequency ﬁelds dealt with in electric machines: The right-hand side of the equation represents the current enclosed by the integration path and is called the magnetomotive force (MMF).
AMPERE`S LAW In electric machines, currents are frequently placed in slots surrounded by ferromagnetic teeth. The MMF corresponding to each path is the total current enclosed by the path. If the slots contain currents that are approximately sinusoidally distributed, then the MMF will be cosinusoidally distributed in space. In this way, the magnetic ﬁeld or ﬂux density in the air gaps of the machine will often have a sinusoidal or cosinusoidal distribution. An example illustrating the determination of the MMF is shown in Figure 4.3, where different integration paths are shown by dotted lines.
F ARADAY ` S L AW & EMF Faraday’s law relates the induced voltage, or electromotive force (EMF), to the time rate of change of the magnetic ﬂux linkage: Electric circuit (loop of conducting material) Magnetic flux For voltage to be induced, there has to be a variation in time between the relative position of the magnetic flux and the electric circuit If the electric circuit is closed and current is allowed to flow, the current will produce a magnetic flux that opposes the increase of the applied flux = Lenz`s law
F ARADAY ` S L AW This law states that the voltage induced in a loop is equal to the time rate of change of the ﬂux linking the loop. The negative sign indicates that the voltage is induced such that the current would oppose the change in ﬂux linkage. The change in ﬂux linkage can be caused by a change in ﬂux density and/or a change in geometry. where E is the electric ﬁeld and B is the magnetic ﬂux density. or
MAGNETIC FORCE The change in or I I
BASIC CONCEPTS - REVIEW 1. MAGNETIC CIRCUITS
CONCEPTS REVIEW MAGNETIC CIRCUITS Lets consider first, the most basic ideal circuit: Some relevant parameters: r 0 0 A c N l c i
CONCEPTS REVIEW MAGNETIC CIRCUITS Ampère`s law applied over the typical mean-length (l c ) results in: The Magnetic Flux can be written as a function of B: The Magnetic Flux Density, B, in terms of the Magnetic Field, H, is: for magnetic circuits Substituting we find:
CONCEPTS REVIEW MAGNETIC CIRCUITS This found equation: Can be written in terms of the `Magnetomotive Force`: So that: But, Then, for magnetic circuits:
CONCEPTS REVIEW MAGNETIC CIRCUITS Also, if this equation: Then, the `Magnetic Reluctance` is given by: is equal to this:
FIRST: a voltage source is connected in the primary side and the secondary side is an open circuit; we want to find the voltage induced in the open secondary coil When the primary is energized: current in the primary coil magnetic flux in the core. CONCEPTS REVIEW TRANSFORMER BASICS
Flux generated by current 1 in coil 1: By Faraday`s law, the induced voltage is: Since there is no losses, the induced voltage is exactly the same as the applied voltage in coil 1: CONCEPTS REVIEW TRANSFORMER BASICS
The flux in coil 2, that was generated by current 1: And so, the voltage induced in coil 2 is given by the equation: CONCEPTS REVIEW TRANSFORMER BASICS
From the voltage equations: We get the Transformer Ratio Equation: CONCEPTS REVIEW TRANSFORMER BASICS
SECOND: there is now a load connected to the secondary coil, so i 2 (t) can flow. We want to find the new induced voltage. By applying Ampere`s law to the circuit, using the line of average path/length, we get the following expression: CONCEPTS REVIEW TRANSFORMER BASICS
For this expression: When i 2 (t) equal to zero: The flux in coil 1, produced by current 1, is: But, and So: CONCEPTS REVIEW TRANSFORMER BASICS
Substituting Into: We get the expression of the flux in coil 1 produced by current 1: where L 1 is the self inductance of coil 1; in this case given by: CONCEPTS REVIEW TRANSFORMER BASICS
For the general case, when i 2 0 and: CONCEPTS REVIEW TRANSFORMER BASICS
The flux in coil 1, produced by both currents, i1 and i2, is given by: The first term in parenthesis is the self-inductance of coil 1, the second term is the mutual inductance between coils 1 and 2; then: In a similar we may find the flux in coil 2: CONCEPTS REVIEW TRANSFORMER BASICS
The induced voltages in each coil are, then: Given that: and CONCEPTS REVIEW TRANSFORMER BASICS
Another equation very much used in transformers design and analysis is the following: This, however, is not an exact equation and can only be used when the magnetic permeability of the nucleus can be considered infinite. CONCEPTS REVIEW TRANSFORMER BASICS or
This relation comes from Ampere`s law, that for this case is: When we assume a very large r so that H 0. In this case: CONCEPTS REVIEW TRANSFORMER BASICS and The negative sign indicates that the currents produce magnetic fields with opposite polarities.
Another equation extensively employed in the design of transformers is the following: It is called “the Design Equation” and it encounters many practical usage. To deduce it we assume a sinusoidal voltage applied to the primary side when the secondary is open: CONCEPTS REVIEW TRANSFORMER BASICS
With primary voltage: The flux then will be: Such that: Resulting in, CONCEPTS REVIEW TRANSFORMER BASICS
BASIC CONCEPTS - REVIEW 2. ELECTROMECHANICAL ENERGY-CONVERSION ENERGY and FORCE
Energy storage in a system of current conductors Most of the important applications of electromagnetic fields are based in the capacity to store energy. ًًThe instantaneous input power given by the source is: So, the input energy is: CONCEPTS REVIEW ENERGY CONVERSION In this ideal magnetic circuito the energy must be stored in the system of conductors of current, made of a N turns winding and by currente i.
Input energy: Faraday`s law: Input energy: where is the linkage flux This integral equation gives the total energy stored in the system CONCEPTS REVIEW ENERGY CONVERSION
The processes of energizing the winding is seeing in the figure: CONCEPTS REVIEW ENERGY CONVERSION The area above the curve is numerically equal to the Stored Energy. There is no physical correspondence to the area below the curve, but it is called Co-Energy.
If the system is LINEAR the ENERGY is EQUAL to the CO-ENERGY In linear systems: CONCEPTS REVIEW ENERGY CONVERSION Linear System
If the system is LINEAR the ENERGY is EQUAL to the CO-ENERGY In linear systems: CONCEPTS REVIEW ENERGY CONVERSION Non-Linear SystemLinear System
If the system is LINEAR the ENERGY is EQUAL to the CO-ENERGY It can also be shown that the Energy per Volume Unit is: CONCEPTS REVIEW ENERGY CONVERSION Linear System
Force Lets now consider a magneto-mechanic arrangement, to see the exchange of energy between the magnetic field and the mechanic system, and how the magnetic force can be derived: When the current flows in the coil the magnetic flux will produce a force on the iron-magnetic core pulling it to the coil nucleus. This is how the interaction occurs. CONCEPTS REVIEW ENERGY CONVERSION
The force and the magnetic flux are depended of current and position, that is: (i,x), Fm(i,x) Or, it is equally true to state that, the force and the current are depended of flux and position, that is: i(,x), Fm(,x) The law of energy conservation requires that any variation in the magnetic energy stored in the magnetic circuit should be balanced, either by a variation in the input energy from the voltage source or by a variation of energy in the mechanical system; the following equation describes this requirement: Since:, a small energy variation is given by: CONCEPTS REVIEW ENERGY CONVERTION
By substitution we arrive in the following equation: The magnetic force can, then, be found, as a function of current (i) and position (x or ) - by the equation: CONCEPTS REVIEW ENERGY CONVERTION
The magnetic force can, then, be found, as a function of current (i) and position (x or ) - by the equation: In lienar systems: ; for this case we can write the equations: CONCEPTS REVIEW MAGNETIC FORCE for rotating systems or for rotating systems
Alternatively, we can obtain the force as a function of flux ( ) and position (x or ), when and x/ are chosen as independent variables: And for linear systems: CONCEPTS REVIEW MAGNETIC FORCE or
The rotor in this example has four poles, as shown. The idea is to review qualitatively the behavior of this system after the energizing of each phase sequentially CONCEPTS REVIEW STEP MOTOR Now lets consider a machine with six poles in the stator (armature), arranged in three groups (phases) a-aa, b-bb, c-cc. Coils are wounded for the three phases but, for clarity's sake only the coils in phase a-aa are indicated.
Then, the current in coil a-aa is interrupted and coil b-bb is energized The position of minimum reluctance now is reached when b-bb is aligned with m- mm. CONCEPTS REVIEW STEP MOTOR When coil a-aa is energized, the rotor searches for a position of minimum reluctance, corresponding, in this case, to the alignment of the rotor in the position: a-aa with I-II So, the rotor moves clockwise by an angle of: 90º - 60º = 30º
As the windings become energized sequentially, one at each time, following from a-aa b-bb c-cc, the rotor moves clockwise in steps of 30º. CONCEPTS REVIEW STEP MOTOR This is a very useful and widely employed machine, known as the “Step Motor”. If the windings are energized in the sequence a-aa c-cc b-bb, the rotor will turn anticlockwise. The speed of the rotation is determined by the rate the current is switched from one winding to the next.
Consider now the case where windings a-aa and b-bb are energized simultaneously. The position of minimum reluctance is not reached by the alignment of a-aa with I-II or by aligning b-bb with m-mm. In fact, the rotor will stop in a position of partial alignment between poles a-I and b-m. This corresponds to a 15º rotation CONCEPTS REVIEW STEP MOTOR Step motors may be easily electronically controlled. They may be operated at low speeds and admit acceleration without difficulty.
Consider now the case where windings are present in both, the stator and the rotor part of the machine This is a more practical case. The energy stored in such systems can be described by the equation: CONCEPTS REVIEW MOTORS 1 and 2 are the total linkage flux in coils 1 and 2. The linkage flux in coil 1 is partialy due to curren i 1 and partialy to currente i 2 :
The linkage flux in coil 1 is partialy due to current i 1 and partialy to currente i 2 and is given by: CONCEPTS REVIEW MOTORS Where L 1 is the self inductance of coil 1 and M is the mutual inductance Similarly, the linkage flux in coil 2 is given by:
CONCEPTS REVIEW MOTORS Similarly, the linkage flux in coil 2 is given by: So that,
Substituting: CONCEPTS REVIEW MOTORS in this previously given equation: We obtain: and or In a linear system, so the torque in the rotor is obtained:
The presented developments (equations+ideas) are useful in the study of the behavior of electrical machines, and are used in the study of electromechanical energy conversion. Prof. Elisete Ternes Pereira, 2010