Asynchronous machines

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Asynchronous machines

Single Phase Induction Motor

Working principle
Basically, a single-phase induction motor comprises a squirrel cage rotor similar to that of three phase motors and a stator which has a winding single-phase ac-powered. Usually constryen with powers less than 1 hp and are therefore also called fractional motors.
By introducing an alternating current in the stator windings produces a magnetomotive force in the gap. This produces a magnetic field proportional in entrehierrro, which in turn induces currents in the rotor, as if the secondary circuit of a transformer, so that the torques generated by the interaction of the intensities of the two halves of the rotor winding inductor with the stator field are opposite each other and therefore the resultant torque acting on the rotor at rest is zero. The absence of initial boot pair represents the characteristic feature of single-phase and so this machine can not boot by itself.
If a three-phase motor phase is disconnected, it would have an operation similar to that described as the machine would work as a single phase motor.

Start
The single-phase motor has no starting torque and therefore can not start up by itself.
The procedures for starting of single phase induction motors are:
a) Engine starting phase
In this motor is located two windings on the stator electrical 90th date in space. The first winding, called the main cover 2 / 3 of the slots and has many turns of heavy wire so it offers a low resistance and reactance and is directly connected to the network, while the other, called auxiliary or starting covers the rest of the stator and has few turns of wire so thin and offers high resistance and low reactance is connected in series with a centrifugal switch located on the motor shaft.
b) Motor with capacitor
This type of engine isto consist of a main winding which covers 2 / 3 of the slots and has many turns of heavy wire so it offers great reactance and low resistance and is directly connected to the network, while the auxiliary winding covers the rest of the stator and has few turns of wire so thin and offers high resistance and low reactance, and is connected in series with a capacitor of sufficient capacity to advance the current phase of about 90 º off for the main winding. Sometimes, in order to improve torque characteristics and the pdf of the machine, capacitors are used with oil-impregnated paper that work continuously.


Introduction
The synchronous machines are electrical machines whose speed of rotation n (rpm) is linked tightly with the frequency f of the ac network with which you work: n = 60f / p where p is the number of pole pairs of the machine.
The synchronous machines can operate under either scheme generator and motor. However, in practice for the electrical installation is more frequent use as generators to produce electricity from alternating current (alternators). On the other hand, when work converting electrical into mechanical energy, leads to the system running as a synchronous motor. These engines are used in industrial drives those constant speeds, also taking the advantage over asynchronous motors can regulate both the FDP with which he works. When working with fdp synchronous capacitor is said to function as compensatory or synchronous condenser.

Constructive aspects
The synchronous machines, like other types of electrical machines, consisting of two separate windings:
a) A field winding, constructed in the form of distributed or concentrated windings in slots, fed by direct current, which leads to the poles of the machine.
b) A distributed armature winding formed a winding three-phase alternating current tour.
On small machines, for powers that do not exceed 10 kVA, the field winding is normally placed in the stator, in concentrated form, lying at the armature rotor generally forming three phases.
In large synchronous machines, which in the case of alternators can reach 1000-1500 MVA, the placement of the windings is opposite to the previous, so that the poles are located on the rotor and the stator phase winding . In this situation the rotor structure produced in two different versions, either salient pole or poles as smooth; in the first case the windings of the poles are concentrated, while for the cylindrical rotor winding is placed on the poles estdistributed to slots. The power inductor winding is performed by two rings placed on the moving part of the machine by introducing a continuous stream outside. There are two types of armatures: first, a rotating armature requires three rings while a fixed induced not need rings. It should be noted also that it is more difficult to isolate the conductors in a rotating armature in an armature fixed.

Excitation Systems
The windings which form the poles of a synchronous machine is fed with direct current, this cc comes in the traditional system of a generator exciter that is mounted on the shaft of the group and whose output is applied to the alternator rotor through slip rings with corresponding brushes. The exciter is a conventional DC generator, which sometimes replaces all or part of its excitation by a pilot exciter to improve speed of response. The synchronous machines often have smaller pilot exciter and the main exciter works as a shunt inductor or directly feeding the alternator field.
In modern times it uses a brushless excitation system. In this case the phase winding of the exciter is mounted on the rotor and the stator field winding. The ac output of the dc exciter is converted by rectifiers mounted on the shaft and directly feeding the alternator rotor without rings or brushes (rotating rectifiers).
In modern alternators used on generators to supply electricity to isolated installations is resorted to self-excitation of the alternator, which is necessary to obtain the dc poles generator output itself, which is then rectified.

Working principle of an alternator
Running on Empty
By rotating the rotor speed n fems are induced in the windings of the three phases of the stator, which are outdated in the 120th time. If we consider the N turns of each phase are concentrated, and the concatenated flow for the same varies between the limits + ôm - o m, the average value of f.em. induced in each phase, during half period of the alternating current is: E med = 4fNôm
The effective emf E will have a magnitude: E = f Phnom 4K.
The vacuum curve is an important feature of the vacuum operation of the synchronous machine as it expresses the emf terminal of the machine being disconnected depending on the load current.

Performance under load. Armature reaction
If while running a load generator, with a given excitation current, closing the armature circuit by connecting a load impedance to the terminals, we obtain a voltage V across the machine below the value it had in vacuum Eo. The reduction in the output voltage of the generator is due to the appearance of a current in the armature which causes a voltage drop on this circuit at the same time produces an mmf which reacts with the inductor air-gap flux by modifying the machine . It should also consider the reactance of the armature, which is due to the stator leakage flux which does not interact with the rotor flux. This leakage flux can define an inductance Lor, that multiplied by the pulse of current, resulting in the stator leakage reactance X ù = ó = Ló Ló2pif. The effect that causes the armature mmf mmf on the inductor, modify the air-gap flux of the machine. This phenomenon is known as armature reaction.
Fmms Membership of the inductor and the armature as the resistive load, inductive or capacitive:
a) Resistive load
If the load is pure resistive, the FDP is unity, and if one ignores the impedance of the armature may be considered that the gap between the emf and the current is ö = 0. To calculate the direction and magnitude of induced fems drivers will have to apply Faraday's law in the form: e = L (VXB), where v indicates a velocity vector, contrary to the direction of rotation of the rotor speed equal to peripheral, which is the result of taking into account the relative motion between the two circuits. The fems are highest when the sides of the coils are located directly across from the centers of the poles. As the gap between the emf and the current is zero, this moment coincides with the maximum of intensity. It is noted that for a resistive load, the armature reaction is transverse, ie is displaced 90 degrees from the mmf
b) Inductive load
When the load is purely inductive, the gap between the emf and the current is about 90 degrees. In this case, the maximum flow will be moved in space from a peak of fems at an angle of 90 degrees in opposite direction of rotation of the rotor, the Whatever f.em.s are highest when the sides of the turns are in the center of the poles, the current will be highest when the north pole of the rotor are advanced 90 ° electrical connection to the position of maximum mmf mmf is noted that the armature reaction mmf opposes the inductor, which means that a pure inductive load produces a demagnetizing reaction, which tends to reduce the resultant mmf, reducing the flow in the gap, thus causing a reduction of the induced emf.

c) Capacitive Load
When the load is purely capacitive, the stator current will peak, 90 ° electrical pole before the driver faces forming the turns of the armature, which is the time when the maximum emf This time, there is Reinforcing the mmf of the inductor, which means that capacitive loads help the action of the field at the poles causing a magnetizing effect on them.
When loads are not pure, presents a difference of between -90 º and +90 º.
Thus, in synchronous machines, both as a salient-pole cylindrical rotor, the armature reaction causes a resultant change in the mmf acting on the magnetic circuit which changes in turn the magnitude of the air-gap flux and hence the value of the emf obtained in the armature.

Phasor diagram of an alternator. Voltage regulation
The phasor diagram of an alternator determines graphically the relationship between emf and voltage in different operating regimes of the machine.
In principle, to analyze the phasor diagram is considered a synchronous machine with uniform air gap (rotor cylinder) because then the armature reaction does not depend on the position of the rotor reluctance to be the same in all positions. It is assumed that the leakage reactance Xor is constant and can be neglected hysteresis losses in iron. This last condition is equivalent to saying that the resultant mmf is in phase with the flow it produces.
Onsider a machine is operating under synchronous generator with a phase voltage V, which carries a inductive current induced in a gap or degrees. To determine the resulting emf will be added to the terminal voltage of the voltage drops produced in the resistance and leakage reactance, resulting in:
E r = V + RI + jX or I. The flow is needed to produce the previous emf will advance to 90 º for E r, and if hysteresis is neglected, the direction of flow will also be corresponding to the resultant mmf F r. F r is the sum of the mmf of excitation or inducing Fe and for F-induced reactioni, ie:
F r = F e + F i. If this excitation mmf represented by Fe, the machine is left empty in the absence of armature reaction, ie it is F i = 0, the mmf becomes excited Fe resulting F e = F r and the flow placing in the gap increases in phase with F e and determine the curve of synchronous vacuum. The above process is the general method to calculate the mmf required in the excitement when the machine delivers a current I at a given voltage V.
Define voltage regulation of a synchronous machine to the quotient:
A = [(E 0-V) / V] · 100%, which expresses the change in terminal voltage generator load to full load for a particular excitement at the poles. With resistive loads and inductive loads, especially due to the demagnetizing effect of the armature mmf, resulting in a decrease in blood flow as output grows, leading to values of positive regulation. For capacitive loads, having the armature mmf magnetizing effect, the load voltage is higher than the vacuum, leading to a value of negative regulation.



Linear analysis of synchronous machine: the equivalent circuit
General
In the synchronous behavior is necessary to take into account the effect of armature reaction, which requires the simultaneous use of electrical quantities: emf, voltage and current magnitudes magnetic Nixon: fmms and flow. This analysis procedure is called a general method accurately reproduces the physical phenomena involved, but has the disadvantage that the handle two types of quantities have no choice but to resort to using phasor diagrams.

Behn-Eschenburg method. Synchronous impedance
This method is applied to cylindrical rotor machines working in linear regime, which means that the flows are proportional to the fmms and therefore can use the principle of superposition. The advantage of this method is that leads to an equivalent electrical circuit of the synchronous machine, with the analytical advantages that entails. It is known that there is actually a single stream in the air gap synchronous machine that is produced by the joint action of the excitation fmms Fe and F i reaction. However, it is more convenient to consider that each produces a flow mmf independent turn creates a corresponding induced emf. This will work only with fems and electrical variables, apart from the magnetic. This idea involves three streams:
a) The leakage flux W, which results in a voltage drop in the reactance of the same name X or: + jX or I, ie, the voltage drop caused by the leakage reactance is advanced 90 ° with the armature current.
b) The flow of excitation or E, which is responsible for the emf produced in vacuum E0.
c) The armature reaction flux or I, which results in an emf E p delayed 90 ° with the flow.
Finally, with new phasor diagram leads to the final expression: E0 = V + RI + jX + jX or I p I, indicating that gap induced emf E 0 due to the excitation mmf Fe can be considered as the result of adding the voltage V across the machine voltage drops Resistance: RI

Characteristics of vacuum and short of synchronous machine. Determination of the synchronous impedance
To study the behavior of this machine will be necessary to determine the parameters that are included in this circuit: E 0 and Z s. The value of E 0 may be determined by analyzing blank:
Empty: I = 0 => E 0 = V (empty)
That is, the emf E 0 is the terminal voltage of the machine when the armature current zero. Finally reach the vacuum feature: E0 = f (I e) is a curve.
The calculation of the synchronous impedance Z s required short essay:
Short circuit: V = 0 => E0 = (R + jX s) · I short = Z s · I short
Where is the value of modular synchronous impedance Z s = E 0 / I short, ie the synchronous impedance is the ratio between the voltage and current corocircuito. Having completed the steps .... The curve representing I cc = ö (I e) is called short-circuit characteristic and is virtually a straight line, because in these conditions the magnetic circuit is not saturated because both the excitation and the resulting flow are of a low value.
For small excitation synchronous impedance Z s is constant, since the vacuum feature coincides with the line of the gap and leads to the so-called unsaturated synchronous impedance: Zs (nosaturada) = Od / O'e
In the different proposals for standards and instructions electrotechnical committees of different countries it is customary to take the so-called impedance Sicrono saturated (or adjusted), which is from the rated voltage Od, which accounts for an exciting current and Ob produce a current in the armature O'f: Z s (saturated) = Z s = Od / O'f.
Finally and after several operations may arrive at the equation shows that the short-circuit ratio is the inverse of the saturated synchronous impedance values expressed in per unit: 1 / Z s (pu) = O'f / O'g = Ob / Oc = SRC (short-circuit ratio).

Nonlinear analysis of synchronous machine: Method fdp Potier or null. Calculation of regulating
Potier's method is applied to the cylindrical rotor synchronous machines operating in the saturation zone. In these machines the method saturated Behn-Eschenburg leads to appreciable errors, since the fems are not proportional to the fmms now due to the nonlinearity of the magnetic circuit area where she works.
Portier method determines the value of the decrease in the leakage reactance X or I and the mmf produced by the armature reaction, so that the calculation of the regulation is based on building general phasor. To calculate the adjustment by the method of Potier curve requires knowledge of the vacuum and is also need for testing with pure inductive shit, in a graph representing the output voltage curve with respect to the mmf of excitation, for constant armature current and equal to the rated current. By Potier triangle points are determined feature strip. Furthermore Potier reactance is greater than X or something.

Operation of an alternator on an isolated network
The behavior of a synchronous generator under load varies greatly depending on the power factor of load and if the generator works only or in parallel with other alternators. First we study analysis of the behavior of the machine running in isolation. There are two important controls: first voltage regulator, which is incorporated in the exciter and that varying the generator field current can control the output voltage and on the other hand, the primary engine that drives the alternator, which carries a speed regulator which acts on the water inlet, thereby allowing the group control the speed and hence frequency.
Assuming that the machine moves at constant speed, the frequency is a fixed parameter. As the load increases, increase the armature current and po aumnta both the armature reaction mmf F i, which results in a lower resultant mmf F r, r E minor emf and a lower output voltage.
The equation governing the electrical behavior of the machine will be: V = E 0 - jX s I
In definitva at a alternator that works in an isolated system we have:
1.La often depends entirely on the speed of the prime mover that drives the synchronous machine.
2.The pdf of the generator is the pdf of the load.
3.La output voltage depends on: a) speed b) of the exciting current, c) of the armature current, d) the pdf of the load.

Coupling an alternator to the grid
In today's world is very rare that there is a unique alternative in isolation that feeds your own load, this situation only occurs in some applications such as generators. General rule is that the alternators in power stations are located next to where are the primary energy sources.
In order to increase performance and reliability of the system, the central dioferentes are connected together in parallel, through transmission and distribution lines. The network so formed is a huge generator in which virtually the voltage and frequency remain constant.
For example, in Spain toral installed electrical power of the whole country is about 65,000 MW, but the maximum unit power of alternatives does not reach 1000 MW. In electrical terminology, we say then that it has a network of infinite power (voltage and frequency) which are connected to various generators in the country. The parallel connection of an alternator to the grid involves a series of complex transactions that constitute the so-called synchronization of the machine. For such a connection is made without any disturbance it is necessary that the instantaneous value of the generator voltage has equal magnitude and phase to the instantaneous value of mains voltage. This requirement leads to the following conditions, necessary to attach an alternator in parallel to the network:
1.The sequence of phases of the alternator and the network must be equal.
2.La voltage generator must have an effective value equal to the voltage of the network and its phases must match.
3.The frequency of the two voltages must be equal.

To verify these conditions are used in a practice called synchroscopes devices, which in the simplest case consist of three incandescent lamps. The operation begins booting the machine through the primary engine to a speed approaching that of synchronism: n) 60f / p. Excitation is then introduced into the inductor of the alternator and it gradually rises up until the terminal voltage of the generator matches the mains.
In practice, in large alternator has been replaced by another lamp synchroscope needle. The position of the needle shows the phase angle between the tensions of the generator and the network. When the frequencies are equal to the needle and when the frequencies differ from the needle rotates either direction depending on whether the generator goes faster or slower than the network.

Active and reactive power developed by a synchronous machine coupled to infinite power network
It is considered a cylindrical rotor synchronous machine in which unsaturated can depreciate induced resistance against the synchronous reactance whose magnitude is assumed constant. The active and reactive power the machine will be delivered po:
P = 3E 0 Vše? / X s = P max · Seine, Q = 3 [(E 0 VCOs-V 2) / X s]
Where the angle is called the power angle and load angle. The maximum active power is better: P max = 3E 0 V / X s
If a> 0, the active power developed by the machine is positive and corresponds to the operation as a synchronous generator or alternator. If a <0, the active power is negative, ie it receives active power from the network and therefore works as a synchronous motor delivering mechanical power at the shaft.
If E0cosä> V, the synchronous inductive reactive power delivered to the REDM or what is the same, it receives power capacitive network. Aue then said the machine is overexcited. In the case of enforcing inequality E0cosä <V, the reactive power supplied by the generator is negative, ie, capacitive, or an equivalent way, it receives inductive power of the network. We then say that the generator works underexcited.

Operation of a synchronous machine connected to an infinite power grid
When an alternator is connected to a network of infinite power, it becomes part of a system that includes hundreds of other alternators feeding among all the millions of cargo. Unlike a generator working on an isolated network, where the load is properly specified, it is now impossible to know the nature of the cargo (large or small, resistive or inductive) connected to the terminals of a specific alternator. It is known that the group has two controls: a) the voltage regulation system that controls the alternator field current and in the case of isolated generator was used to regulate the output voltage, and b) the system speed regulation of the primary motor in the generator was used to control the frequency isolated.
But the network that has connected the alternator power is infinite, which indicates that the frequency and voltage are constant and are imposed by the network.

Effect of varying the intensity of excitation
To couple this machine to the network will have to produce an emf E 0 equal in magnitude and phase voltage V of the network. E 0 and V, are identical and therefore no current will flow through the armature of the alternator. Although the generator has been connected to the network, not supply (or receive) any power: it is said to be working in floating mode. If now the exciting current increases, increasing the induced emf E 0, which being above the line voltage will cause a circulating current induced by I = (E 0-V) / jX s = E x / jX s, the current lags the voltage difference Ex an angle of 90 degrees.

Effects of changes of mechanical torque (speed regulator)
The active power supplied by a synchronous machine connected to a network of infinite power comes from the mechanical power supplied by the turbine, which in turn depends upon the entry of water (or steam in the case of heating) to the same , which is governed by the position of speed regulator. If considered as a new baseline floating mode and opens the water inlet to the turbine, the rotor speed which will cause the voltage produced is below the line voltage at an angle. The electrical power transferred by the generator to the grid will be: P = 3E0Vsenä/Xs, which is a function of power angle ä, which indicates that if the excitation is constant, ie the emf remains fixed E0, as active power increases, the gap grows to between V and E0. In short, we can say that the change in speed regulator of the turbine causes a change in the active power that gives the machine, which physically is reflected as a change in the angle at which the emf is the voltage V E0
For a given excitation, the active power will be maximum for a = pi / 2, which corresponds to the limit of static overload capacity or static stability limit of the alternator. A further increase in the entrance of the prime mover (turbine) causes the decrease active power and excess power is converted into acceleration torque which causes an increase in the generator speed, getting out of sync.
If it receives active power of the network and a positive imaginary part, which means that inductive reactive power delivered to the network or otherwise, which receives power capacitive network, said that the machine is overexcited.

Synchronous motor. Features and applications
The synchronous machine can move from functioning as a generator to work as an engine by disconnecting the primary motor starter, then a couple helpful exercise in transforming electrical energy axis of the net absorbed into mechanical energy of rotation. The engine speed is expressed by the relationship: n = 60f / p which is the network synchronism.
The synchronous motor has the major drawback of the pair retains a unique sense only when the machine is already synchronized, ie when the rotor rotates at the same speed as the armature field. If the rotor is at rest or rotates at different speed than that of the synchronism, the average torque that develops when connected to the network is zero.
In synchronous motors that can start in a vacuum, the implementation is done by means of an auxiliary motor, usually asynchronous with the same number of poles as the main engine, so that you get a nearly synchronous rotation speed and connection network is performed using synchronization equipment as was done in an alternator coupled to the network. Can also be used for this purpose dc motors because of their advantage of speed control, or induction motors with a pair of poles less than the synchronous motor.
Another more practical procedure for the implementation of these engines is their starter as asincron. For this purpose it is necessary to place a squirrel cage winding on the poles of the machine. To make asynchronous boot the field winding must be closed on an ohmic resistance whose magnitude is 10-15 times higher than their own. This process is called self-clocking the engine. After the operation of synchronous motor starting, it can control its flow and excitation for the machine to work on under-excitation regime or overdrive in order to regulate their pdfs, and which make this machine can fulfill the dual mission to drag a load mechanics and compensate the reactive current of the network.
Generally, the squirrel cage and placed in these engines used here to produce an asynchronous boot is placed on generators and also called the damper winding because it reduces the fluctuations that occur in the transitional processes of the machines Synchronous: coupling to the network, abrupt changes in electrical load or mechanics. The effect of damper windings is zero in steady state, since turning the machine synchronous speed not induce currents in them.
The synchronous motor can be used to move mechanical loads. In its power not less than 1 hp cc used for excitation and its operation is based on variation of the rotor reluctance (reluctance motors). They are also used hysteresis synchronous motors are used to boost el.ectricos clocks and other time measuring devices.
For great powers, one of the biggest advantages of this engine versus asynchronous is the possibility of regulating the fdp

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