From the history of the issue. Today, in my work, the question has arisen about participating in a project to introduce our own small-scale generation at the enterprise. Previously, I had experience working with synchronous electric motors, but minimal experience with generators.
Considering the proposals of various manufacturers, in one of them I discovered a method of exciting a synchronous generator using a subexciter based on a generator on permanent magnets(PMG). Let me mention that the generator excitation system is planned to be brushless. I described an example of synchronous electric motors earlier.
And so, from the description of the permanent magnet generator (PMG) as a sub-exciter of the excitation winding of the generator exciter it follows:
1. Air-to-water heat exchanger. 2. Permanent magnet generator. 3. Excitation device. 4. Rectifier. 5. Radial fan. 6. Air channel.
Here the excitation system consists of auxiliary windings or permanent magnet generator, automatic voltage regulator (AVR), CT and VT for current and voltage sensing, built-in exciter and rotary rectifier. As standard, turbogenerators are equipped with a digital AVR, which provides PF (power factor) regulation and various functions monitoring and protection (excitation limitation, overload detection, redundancy capability, etc.). The DC excitation current coming from the AVR is amplified by the rotating excitation device and then rectified by the rotating rectifier. The rotating rectifier consists of diodes and voltage stabilizers.
Schematic representation of a turbogenerator excitation system using PMG:
Solution using a permanent magnet generator (PMG) on the main shaft with generator rotor and brushless exciter:
Actually, on this moment talk about the benefits this method regulating arousal is not possible for me. I think that as time goes by and I gain more information and experience, I will share with you my experience of using PMG.
Generator- a device that converts one type of energy into another.
In this case, we consider the conversion of mechanical rotational energy into electrical energy.
There are two types of such generators. Synchronous and asynchronous.
Synchronous generator. Operating principle
A distinctive feature of a synchronous generator is the tight connection between the frequency f variable EMF induced in the stator winding and rotor speed n, called synchronous speed:
n = f/p
Where p– the number of pole pairs of the stator and rotor windings.
Usually the rotation speed is expressed in rpm, and the EMF frequency in Hertz (1/sec), then for the number of revolutions per minute the formula will take the form:
n = 60·f/p
In Fig. 1.1 presented functional diagram synchronous generator. On stator 1 there is a three-phase winding, which is not fundamentally different from a similar winding of an asynchronous machine. An electromagnet with excitation winding 2 receiving power is located on the rotor DC, as a rule, through sliding contacts made by means of two slip rings located on the rotor and two fixed brushes.
In some cases, permanent magnets can be used in the rotor design of a synchronous generator instead of electromagnets, then the need for contacts on the shaft is eliminated, but the possibilities for stabilizing output voltages are significantly limited.
The drive motor (PD), which is a turbine, internal combustion engine or other source of mechanical energy, drives the generator rotor into rotation at a synchronous speed. In this case, the magnetic field of the rotor electromagnet also rotates at a synchronous speed and induces variable EMF in the three-phase stator winding E A, E B and E C, which, being identical in value and shifted in phase relative to each other by 1/3 of the period (120°), form a symmetrical three-phase EMF system.
When the load is connected to the stator winding terminals C1, C2 and C3, currents appear in the stator winding phases I A, I B, I C, which create a rotating magnetic field. The rotation frequency of this field is equal to the rotation frequency of the generator rotor. Thus, in a synchronous generator, the magnetic field of the stator and the rotor rotate synchronously. Instantaneous value of the EMF of the stator winding in the synchronous generator under consideration
e = 2Blwv = 2πBlwDn
Here: B– magnetic induction in the air gap between the stator core and the rotor poles, T;
l– active length of one slot side of the stator winding, i.e. stator core length, m;
w– number of turns;
v = πDn – linear speed movement of the rotor poles relative to the stator, m/s;
D– internal diameter of the stator core, m.
The EMF formula shows that at a constant rotor speed n the shape of the graph of the variable EMF of the armature (stator) winding is determined exclusively by the law of distribution of magnetic induction B in the gap between the stator and the rotor poles. If the graph of magnetic induction in the gap is a sinusoid B = Bmax sinα, then the EMF of the generator will also be sinusoidal. In synchronous machines, they always strive to obtain an induction distribution in the gap as close to sinusoidal as possible.
So, if the air gap δ
is constant (Fig. 1.2), then magnetic induction B in the air gap is distributed according to a trapezoidal law (graph 1). If the edges of the rotor poles are “beveled” so that the gap at the edges of the pole pieces is equal to δ
max (as shown in Fig. 1.2), then the graph of the distribution of magnetic induction in the gap will approach a sinusoid (graph 2), and, consequently, the graph of the EMF induced in the generator winding will approach a sine wave. EMF frequency of synchronous generator f(Hz) proportional to synchronous rotor speed n(r/s)
Where p– number of pairs of poles.
The generator in question (see Fig. 1.1) has two poles, i.e. p = 1.
To obtain EMF industrial frequency(50 Hz) in such a generator the rotor must be rotated at a frequency n= 50 r/s ( n= 3000 rpm).
Methods for exciting synchronous generators
The most common way to create the main magnetic flux of synchronous generators is electromagnetic excitation, which consists of placing an excitation winding at the rotor poles, when a direct current passes through it, an MMF appears, creating a magnetic field in the generator. Until recently, special DC generators were mainly used to power the excitation winding independent excitation called pathogens IN(Fig. 1.3, a). Field winding ( OB) receives power from another generator (parallel excitation), called a subexciter ( PV). The rotor of the synchronous generator, exciter and subexciter are located on a common shaft and rotate simultaneously. In this case, the current enters the excitation winding of the synchronous generator through slip rings and brushes. To regulate the excitation current, regulating rheostats are used, which are connected to the exciter excitation circuit. r 1 and subexciter r 2. In synchronous generators of medium and high power, the process of regulating the excitation current is automated.
Also used in synchronous generators contactless system electromagnetic excitation, in which the synchronous generator does not have slip rings on the rotor. In this case, an inverted synchronous generator is used as an exciter alternating current IN(Fig. 1.3, b). Three-phase winding 2 exciter, in which an alternating EMF is induced, is located on the rotor and rotates together with the excitation winding of the synchronous generator and their electrical connection carried out through a rotating rectifier 3 directly, without slip rings or brushes. DC power supply to the field winding 1 pathogen B is carried out from the subexciter PV– DC generator. The absence of sliding contacts in the excitation circuit of a synchronous generator makes it possible to increase its operational reliability and increase efficiency.
In synchronous generators, including hydraulic generators, the principle of self-excitation has become widespread (Fig. 1.4, a), when the alternating current energy required for excitation is taken from the stator winding of the synchronous generator and through a step-down transformer and a rectifying semiconductor converter PP converted to DC energy. The principle of self-excitation is based on the fact that the initial excitation of the generator occurs due to the residual magnetism of the machine.
In Fig. 1.4, b shows the block diagram automatic system self-excitation of a synchronous generator ( SG) with a rectifier transformer ( VT) and thyristor converter ( TP), through which AC electricity from the stator circuit SG after conversion to direct current, it is supplied to the excitation winding. The thyristor converter is controlled by means of an automatic excitation regulator ARV, the input of which receives voltage signals at the input SG(via voltage transformer TN) and load current SG(from current transformer TT). The circuit contains a protection block ( BZ), providing protection for the field winding ( OB) from overvoltage and current overload.
The power spent on excitation usually ranges from 0.2 to 5% of the useful power (a lower value applies to high-power generators).
In generators low power The principle of excitation by permanent magnets located on the rotor of the machine is used. This method of excitation makes it possible to rid the generator of the excitation winding. As a result, the design of the generator is significantly simplified, becoming more economical and reliable. However, due to the high cost of materials for the manufacture of permanent magnets with a large supply of magnetic energy and the complexity of their processing, the use of permanent magnet excitation is limited to machines with a power of no more than a few kilowatts.
Synchronous generators form the basis of the electric power industry, since almost all electricity throughout the world is generated through synchronous turbo or hydro generators.
Synchronous generators are also widely used as part of stationary and mobile electrical installations or stations complete with diesel and gasoline engines.
Asynchronous generator. Differences from synchronous
Asynchronous generators are fundamentally different from synchronous generators in the absence of a strict relationship between the rotor speed and the generated EMF. The difference between these frequencies is characterized by the coefficient s- sliding.
s = (n - n r)/n
Here:
n- rotation frequency of the magnetic field (EMF frequency).
n r- rotor speed.
More details on the calculation of slip and frequency can be found in the article: asynchronous generators. Frequency.
In normal mode, the electromagnetic field of an asynchronous generator under load exerts a braking torque on the rotation of the rotor, therefore, the frequency of change of the magnetic field is less, so the slip will be negative. Generators operating in the area of positive slips include asynchronous tachogenerators and frequency converters.
Asynchronous generators, depending on the specific conditions of use, are made with a squirrel-cage, phase or hollow rotor. Sources of formation required energy rotor excitation can be static capacitors or valve converters with artificial switching of valves.
Asynchronous generators can be classified according to the method of excitation, the nature of the output frequency (variable, constant), method of voltage stabilization, slip operating areas, design and number of phases.
The last two signs characterize design features generators.
The nature of the output frequency and methods of voltage stabilization are largely determined by the method of generating magnetic flux.
Classification according to the method of excitation is the main one.
You can consider generators with self-excitation and independently excited.
Self-excitation in asynchronous generators can be organized:
a) using capacitors connected to the stator or rotor circuit or simultaneously to the primary and secondary circuits;
b) through valve converters with natural and artificial switching of valves.
Independent excitation can be carried out from external source alternating voltage.
Based on the nature of the frequency, self-exciting generators are divided into two groups. The first of them includes sources of almost constant (or constant) frequency, the second variable (adjustable) frequency. The latter are used to power asynchronous motors with a smooth change in rotation speed.
It is planned to consider in more detail the operating principle and design features of asynchronous generators in separate publications.
Asynchronous generators do not require complex components in their design to organize direct current excitation or the use of expensive materials with a large supply of magnetic energy, therefore they are widely used by users of mobile electrical installations due to their simplicity and unpretentiousness in maintenance. Used to power devices that do not require a strict connection to the current frequency.
The technical advantage of asynchronous generators is their resistance to overloads and short circuits.
Some information on mobile generator sets can be found on the page:
Diesel generators.
Asynchronous generator. Characteristics .
Asynchronous generator. Stabilization.
Comments and suggestions are accepted and welcome!
The purpose of this work is to elucidate the energy characteristics of over-unity synchronous generators with permanent magnets, and, in particular, the influence of the load current creating a demagnetizing field (armature reaction) on the load characteristics of such generators. Two disk synchronous generators were tested different power and designs. The first generator is a small synchronous disk generator with one magnetic disk 6 inches in diameter, with six pairs of poles, and a winding disk with twelve windings. This generator is shown on the test bench (Photo No. 1), and its full tests are described in my article entitled: Experimental Studies energy efficiency receiving electrical energy from the magnetic field of permanent magnets." The second generator is a large disk generator with two magnetic disks 14 inches in diameter, with five pairs of poles, and a winding disk with ten windings. This generator has not yet been comprehensively tested, and is shown in photo No. 3, as an independent electrical machine, next to the test bench of a small generator. The rotation of this generator was carried out by a DC motor mounted on its body.
The alternating output voltages of the generators were rectified, smoothed with large capacitors, and the currents and voltages in both generators were measured at direct current with digital multimeters of the DT9205A type. For the small generator, measurements were made at a standard alternating current frequency of 60 Hz, which for the small generator corresponded to 600 rpm . For the small generator, measurements were also made at a multiple of 120 Hz, which corresponded to 1200 rpm. The load in both generators was purely active. In a small generator with one magnetic disk, the magnetic circuit was open, and the air gap between the rotor and stator was about 1 mm. In a large generator, with two magnetic disks, the magnetic circuit was closed, and the windings were placed in an air gap of 12 mm.
When describing the physical processes in both generators, the axiom is that permanent magnets have a constant magnetic field, and it can neither be reduced nor increased. This is important to take into account when analyzing the nature of the external characteristics of these generators. Therefore, we will consider only the changing demagnetizing field of the load windings of generators as a variable. The external characteristic of a small generator, at a frequency of 60 Hz, is shown in Fig. 1, which also shows the output power curve of the generator Pgen, and the KPI curve. Character of the curve external characteristics generator can be explained based on the following considerations - if the magnitude of the magnetic field on the surface of the magnet poles is constant, then as it moves away from this surface it decreases, and, being outside the body of the magnet, it can change. At low load currents, the field of the load windings of the generator interacts with the weakened, scattered part of the magnet field and greatly reduces it. As a result of their common field decreases greatly, and the output voltage drops sharply along a parabola, since the power of the demagnetizing current is proportional to its square. This is confirmed by the picture of the magnetic field of the magnet and winding obtained using iron filings. In photo No. 1, only the picture of the magnet itself is visible, and it is clearly visible that power lines the fields are concentrated at the poles, in the form of clumps of sawdust. Closer to the center of the magnet, where the field is generally zero, the field weakens greatly, so that it cannot even move the sawdust. It is this weakened field that nullifies the reaction of the armature of the winding, at a low current of 0.1A, as can be seen in photo No. 2. With a further increase in the load current, the stronger magnetic fields located closer to their poles decrease, but the winding cannot reduce the ever-increasing field of the magnet further, and the curve of the external characteristic of the generator gradually straightens and turns into a direct dependence of the output voltage of the generator on the load current . Moreover, on this linear part of the load characteristic, the voltages under load decrease less than on the nonlinear part, and the external characteristic becomes stiffer. It approaches the characteristic of a conventional synchronous generator, but with a lower initial voltage. In industrial synchronous generators, up to 30% voltage drop under rated load is allowed. Let's see what percentage of voltage drop a small generator has at 600 and 1200 rpm. At 600 rpm, its idle voltage was 26 Volts, and under a load current of 4 Amps, it dropped to 9 Volts, that is, decreased by 96.4% - this is a very high voltage drop, more than three times the norm. At 1200 rpm, the idle voltage was already 53.5 Volts, and under the same load current of 4 Amps, it dropped to 28 Volts, that is, it had already decreased by 47.2% - this is already closer to the permissible 30%. However, let us consider numerical changes in the rigidity of the external characteristic of this generator over a wide range of loads. The rigidity of the load characteristic of the generator is determined by the rate at which the output voltage drops under load, so let’s calculate it starting from the no-load voltage of the generator. A sharp and nonlinear decrease in this voltage is observed up to approximately a current of one Ampere, and is most pronounced up to a current of 0.5 Ampere. So, with a load current of 0.1 Ampere, the voltage is 23 Volts and drops, compared to the no-load voltage of 25 Volts, by 2 Volts, that is, the rate of voltage drop is 20 V/A. With a load current of 1.0 Ampere, the voltage is already 18 Volts, and drops by 7 Volts, compared to the no-load voltage, that is, the rate of voltage drop is already 7 V/A, that is, it has decreased by 2.8 times. This increase in the rigidity of the external characteristic continues with a further increase in the generator load. So, with a load current of 1. 7 Amperes, the voltage drops from 18 Volts to 15.5 Volts, that is, the rate of voltage drop is already 3.57 V/A, and with a load current of 4 Amps, the voltage drops from 15.5 Volts to 9 Volts, that is, the rate of voltage drop decreases to 2.8 V/A. This process is accompanied by a constant increase in the output power of the generator (Fig. 1), while simultaneously increasing the rigidity of its external characteristics. An increase in output power at these 600 rpm also ensures a fairly high generator KPI of 3.8 units. Similar processes occur at double synchronous speed of the generator (Fig. 2), also a strong quadrature decrease in the output voltage at low load currents, with a further increase in the rigidity of its external characteristics with increasing load, the differences are only in the numerical values. Let's take only two extreme cases of generator load - minimum and maximum currents. So, with a minimum load current of 0.08 A, the voltage is 49.4 V, and drops by 4.1 V compared to a voltage of 53.5 V. That is, the rate of voltage drop is 51.25 V/A, or more than twice that speed at 600 rpm. With a maximum load current of 3.83 A, the voltage is already 28.4 V, and drops, compared to 42 V at a current of 1.0 A, by 13.6 V. That is, the rate of voltage drop was 4.8 V/ Ah, and 1.7 times this speed at 600 rpm. From this we can conclude that an increase in the rotation speed of the generator significantly reduces the rigidity of its external characteristic in its initial section, but does not significantly reduce it in the linear section of its load characteristic. It is characteristic that in this case, with a full generator load of 4 Amps, the percentage voltage drop is less than at 600 rpm. This is explained by the fact that the output power of the generator is proportional to the square of the generated voltage, that is, the rotor speed, and the power of the demagnetizing current is proportional to the square of the load current. Therefore, at the rated, full load of the generator, the demagnetizing power, in relation to the output, is less, and the percentage voltage drop is reduced. Home positive feature more high speed rotation of a small generator is a significant increase in its KPI. At 1200 rpm, the generator EPI increased from 3.8 units at 600 rpm to 5.08 units.
The large generator has a conceptually different design, based on the application of Kirchhoff's second law in magnetic circuits. This law states that if in a magnetic circuit there are two or several sources of MMF (in the form of permanent magnets), then in the magnetic circuit these MMF are algebraically summed up. Therefore, if we take two identical magnets and connect one of their unlike poles with a magnetic circuit, then a double MMF appears in the air gap of the other two unlike poles. This principle is used in the design of a large generator. The windings are the same flat in shape as in the small generator, and are placed in this resulting air gap with double MMF. Tests showed how this affected the external characteristics of the generator. Tests of this generator were carried out at a standard frequency of 50Hz, which, just like in a small generator, corresponds to 600 rpm. An attempt was made to compare the external characteristics of these generators at the same no-load voltages. To do this, the rotation speed of the large generator was reduced to 108 rpm, and its output voltage was reduced to 50 volts, a voltage close to the no-load voltage of the small generator at a rotation speed of 1200 rpm. The external characteristic of a large generator obtained in this way is shown in the same figure No. 2, which also shows the external characteristic of a small generator. A comparison of these characteristics shows that with such a very low output voltage for a large generator, its external characteristic turns out to be very soft, even in comparison with the not so harsh external characteristic of a small generator. Since both subunit generators are capable of self-rotation, it was necessary to find out what was required for this in their energy characteristics. Therefore, an experimental study was carried out on the power consumed by the drive electric motor, without consuming free energy from a large generator, that is, measuring the no-load losses of the generator. These studies were carried out for two different reduction gear ratios between the motor shaft and the generator shaft, with the aim of their effect on the idle power consumption of the generator. All these measurements were carried out in the range from 100 to 1000 rpm. The supply voltage of the drive electric motor and its current consumption were measured, and the idle power of the generator was calculated with gear ratios of 3.33 and 4.0. Figure 3 shows graphs of changes in these values. The supply voltage of the drive electric motor increased linearly with increasing speed at both gear ratios, and the consumed current had a slight nonlinearity caused by the quadratic dependence of the electrical component of power on the current. The mechanical component of power consumption, as is known, linearly depends on the rotation speed. It has been observed that increasing the gear ratio reduces the current consumption throughout the entire speed range, and especially at high speeds. And this naturally affects the power consumption - this power decreases in proportion to the increase in the gear ratio, and in this case by about 20%. The external characteristics of the large generator were taken only with a gear ratio of four, but at two speeds - 600 (frequency 50 Hz) and 720 (frequency 60 Hz). These load characteristics are shown in Fig. 4. These characteristics, unlike the characteristics of a small generator, are linear in nature, with a very small voltage drop under load. So, at 600 rpm, the no-load voltage of 188 V under a load current of 0.63 A dropped by 1.0 V. At 720 rpm, the no-load voltage of 226 V under a load current of 0.76 A also dropped by 1.0 B. With a further increase in the generator load, this pattern remained, and we can assume that the rate of voltage drop is approximately 1 V per Ampere. If we calculate the percentage voltage drop, then for 600 revolutions it was 0.5%, and for 720 revolutions 0.4%. This voltage drop is caused only by the voltage drop across the active resistance of the generator winding circuit - the winding itself, the rectifier and the connecting wires, and it is approximately 1.5 Ohms. The demagnetizing effect of the generator winding under load did not manifest itself, or manifested itself very weakly at high load currents. This is explained by the fact that the doubled magnetic field in such a narrow air gap, where the generator winding is located, cannot overcome the armature reaction, and non-voltage is generated in this doubled magnetic field of the magnets. Home distinctive feature The external characteristics of a large generator is that even at low load currents they are linear, there are no sharp voltage drops, as in a small generator, and this is explained by the fact that the existing armature reaction cannot manifest itself, cannot overcome the field of permanent magnets. Therefore, the following recommendations can be made for developers of permanent magnet CE generators:
1. Do not use open magnetic circuits in them under any circumstances, this leads to strong dissipation and underutilization of the magnetic field.
2. The dispersion field is easily overcome by the armature reaction, which leads to a sharp softening of the external characteristics of the generator, and the inability to remove the design power from the generator.
3. You can double the power of the generator, while simultaneously increasing the rigidity of the external characteristic, by using two magnets in its magnetic circuit and creating a field with double the MMF.
4. In this field with double MMF, coils with ferromagnetic cores cannot be placed, because this leads to a magnetic connection of two magnets, and the disappearance of the effect of doubling MMF.
5. In the electric drive of the generator, use the following gear ratio gearbox, which will most effectively allow you to reduce losses at the generator input at idle.
6. I recommend the disk design of the generator, this is the most simple design, available to make at home.
7. Disc design allows the use of a housing and shaft with bearings from a conventional electric motor.
And finally, I wish you perseverance and patience in creating
a real working generator.
Excitation of a synchronous machine and its magnetic fields. Excitation of a synchronous generator.
The excitation winding of a synchronous generator (SG) is located on the rotor and receives direct current power from an external source. It creates the main magnetic field of the machine, which rotates with the rotor and closes along the entire magnetic circuit. During rotation, this field crosses the conductors of the stator winding and induces EMF E10 in them.
To power the excitation winding of powerful S.G. special generators are used - exciters. If they are installed separately, then power is supplied to the field winding through slip rings and a brush device. For powerful turbogenerators, exciters (synchronous generators of “reversed type”) are hung on the generator shaft and then the excitation winding receives power through semiconductor rectifiers mounted on the shaft.
The power spent on excitation is approximately 0.2 - 5% of the nominal power of the S.G., with a smaller value for large S.G.
Medium-power generators often use a self-excitation system - from the stator winding network through transformers, semiconductor rectifiers and rings. In very small S.G. Sometimes permanent magnets are used, but this does not allow the magnitude of the magnetic flux to be adjusted.
The excitation winding can be concentrated (for salient-pole synchronous generators) or distributed (for non-salient-pole synchronous generators).
Magnetic circuit S.G.
Magnetic system S.G. is a branched magnetic circuit with 2 parallel branches. In this case, the magnetic flux created by the excitation winding is closed along the following sections of the magnetic circuit: air gap “?” - twice; stator tooth zone hZ1 – twice; stator back L1; toothed layer of the rotor “hZ2” - twice; rotor back – “LOB”. In salient-pole generators, the rotor has rotor poles “hm” - twice (instead of the tooth layer) and a cross LOB (instead of the rotor back).
Figure 1 shows that the parallel branches of the magnetic circuit are symmetrical. It can also be seen that the main part of the magnetic flux F is closed throughout the magnetic circuit and is coupled to both the rotor winding and the stator winding. A smaller part of the magnetic flux Fsigma (sorry, there is no symbol) closes only around the field winding, and then along the air gap without engaging with the stator winding. This is the magnetic leakage flux of the rotor.
Figure 1. Magnetic circuits S.G.
salient-pole (a) and non-salient-pole (b) type.
In this case, the total magnetic flux Фm is equal to:
where SIGMAm is the magnetic flux dissipation coefficient.
The MMF of the excitation winding per pair of poles in no-load mode can be determined as the sum of the MMF components required to overcome the magnetic resistance in the corresponding sections of the circuit.
The area of the air gap in which the magnetic penetration µ0 = const is constant has the greatest magnetic resistance. In the presented formula, wB is the number of series-connected turns of the field winding per pair of poles, and IBO is the field current in no-load mode.
As the magnetic flux increases, the steel of the magnetic circuit has the property of saturation, therefore the magnetic characteristic of the synchronous generator is nonlinear. This characteristic as the dependence of the magnetic flux on the excitation current Ф = f(IВ) or Ф = f(ФВ) can be constructed by calculation or determined experimentally. It looks like shown in Figure 2.
Figure 2. Magnetic characteristic of S.G.
Usually S.G. designed so that at the nominal value of the magnetic flux F, the magnetic circuit is saturated. In this case, the “ab” section of the magnetic characteristic corresponds to the MMF when overcoming air gap 2Fsigma, and the “vs” section – to overcome the magnetic resistance of the magnetic core steel. Then the attitude 
can be called the saturation coefficient of the magnetic circuit as a whole.
Idle speed of synchronous generator
If the stator winding circuit is open, then in S.G. There is only one magnetic field - created by the MMF of the field winding.
The sinusoidal distribution of the magnetic field induction necessary to obtain the sinusoidal EMF of the stator winding is provided by:
- in salient pole S.G. the shape of the rotor pole pieces (under the middle of the pole the gap is smaller than under its edges) and the bevel of the stator slots.
- in non-salient pole S.G. – by the distribution of the field winding along the rotor slots under the middle of the pole, the gap is smaller than under its edges and the bevel of the stator slots.
In multi-pole machines, stator windings with a fractional number of slots per pole and phase are used.
Figure 3. Ensuring the sinusoidality of the magnetic
excitation fields
Since the EMF of the stator winding E10 is proportional to the magnetic flux ФО, and the current in the excitation winding IVO is proportional to the MMF of the excitation winding FVO, it is easy to construct the dependence: E0 = f(IВО) identical to the magnetic characteristic: Ф = f(FВО). This dependence is called the idle speed characteristic (H.H.H.) S.G. It allows you to determine the parameters of the S.G. and build its vector diagrams.
Usually H.H.H. are constructed in relative units e0 and iBO, i.e. the current value of the quantities is referred to their nominal values
In this case, H.H.H. called normal characteristic. The interesting thing is that normal X.H.H. for almost all S.G. are the same. In real conditions, H.H.H. starts not from the origin of coordinates, but from a certain point on the ordinate axis, which corresponds to the residual EMF e RES., due to the residual magnetic flux magnetic core steel.
Figure 4. Idle characteristics in relative units
Schematic diagrams excitement S.G. with excitation a) and self-excitation b) are shown in Figure 4.
Figure 5. Schematic diagrams of excitation S.G.
Magnetic field S.G. under load.
To load S.G. or increase its load, it is necessary to reduce electrical resistance between the phase terminals of the stator winding. Then currents will flow through the closed circuits of the phase windings under the influence of the EMF of the stator winding. If we assume that this load is symmetrical, then the phase currents create the MMF of the three-phase winding, which has an amplitude
and rotates along the stator with a rotation speed n1 equal to the rotor speed. This means that the MMF of the stator winding F3F and the MMF of the excitation winding FB, stationary relative to the rotor, rotate at the same speeds, i.e. synchronously. In other words, they are motionless relative to each other and can interact.
At the same time, depending on the nature of the load, these MMFs can be differently oriented relative to each other, which changes the nature of their interaction and, consequently, the operating properties of the generator.
Let us note once again that the effect of the MMF of the stator winding F3Ф = Fa on the MMF of the rotor winding FВ is called the “armature reaction”.
In non-salient-pole generators, the air gap between the rotor and stator is uniform, therefore the induction B1 created by the MMF of the stator winding is distributed in space like the MMF F3Ф = Fa sinusoidally, regardless of the position of the rotor and the field winding.
In salient-pole generators, the air gap is uneven due to both the shape of the pole pieces and the interpole space filled with copper field windings and insulating materials. Therefore, the magnetic resistance of the air gap under the pole pieces is significantly less than in the region of the interpolar space. Rotor pole axis S.G. they call it the longitudinal axis d - d, and the axis of the interpolar space is called the transverse axis S.G. q - q.
This means that the induction of the stator magnetic field and the graph of its distribution in space depend on the position of the MMF wave F3F of the stator winding relative to the rotor.
Let us assume that the amplitude of the MMF of the stator winding F3Ф = Fa coincides with the longitudinal axis of the machine d - d, and the spatial distribution of this MMF is sinusoidal. Let us also assume that the excitation current is zero Ivo = 0.
For clarity, let us depict in the figure a linear scan of this MMF, from which it can be seen that the induction of the stator magnetic field in the area of the pole piece is quite large, and in the area of the interpolar space it sharply decreases to almost zero due to the high air resistance.
Figure 6. Linear scan of the MMF of the stator winding along the longitudinal axis.
Such an uneven distribution of induction with amplitude B1dmax can be replaced by a sinusoidal distribution, but with a smaller amplitude B1d1max.
If the maximum value of the stator MMF F3Ф = Fa coincides with the transverse axis of the machine, then the magnetic field pattern will be different, as can be seen from the linear scan of the machine MMF.
Figure 7. Linear scan of the MMF of the stator winding along the transverse axis.
Here, too, the amount of induction in the area of the pole tips is greater than in the area of the interpolar space. And it is quite obvious that the amplitude of the main harmonic of the stator field induction B1d1 along the longitudinal axis is greater than the amplitude of the field induction B1q1 along the transverse axis. The degree of reduction in induction B1d1 and B1q1, which is caused by the unevenness of the air gap, is taken into account using the coefficients:
They depend on many factors and, in particular, on the sigma/tau ratio (sorry there is no symbol) ( relative value air gap), from the ratio
(pole overlap coefficient), where VP is the width of the pole piece, and other factors.
Synchronous generators
with excitation from permanent magnets
(developed in 2012)
The proposed generator, according to its operating principle, is a synchronous generator with excitation from permanent magnets. NeFeB magnets creating a magnetic field with an induction of 1.35 Tl, located around the circumference of the rotor with alternating poles.
Electricity is excited in the generator windings. d.s., the amplitude and frequency of which are determined by the rotation speed of the generator rotor.
The design of the generator does not contain a collector with openable contacts. The generator also does not have excitation windings that consume additional current.
Advantages of the generator of the proposed design:
1. Has everything positive features synchronous generators with excitation from permanent magnets:
1) absence of current collecting brushes,
2) absence of excitation current.
2. Most similar generators currently produced with the same power have mass and dimensional parameters of 1.5 - 3 times more.
3. Rated rotation speed of the generator shaft – 1600 about./min. It corresponds to the rotation speed of low-speed diesel drives. Therefore, when converting individual power plants from gasoline engines to diesel engines using our generator, the consumer will receive significant fuel savings and, as a result, the cost per kilowatt-hour will decrease.
4. The generator has a small starting torque (less than 2 N×m), i.e., to start, only 200 drive power is sufficient W, and starting the generator is possible from the diesel engine itself at startup, even without a clutch. Similar market engines have an acceleration period to create a power reserve when starting the generator, since when starting, the gasoline engine operates in a power deficit mode.
5. At a reliability level of 90%, the generator resource is 92 thousand hours (10.5 years of non-stop operation). The operating cycle of the drive engine between major overhauls, declared by manufacturers (as well as market analogues of the generator) is 25 - 40 thousand hours. That is, our generator’s operating reliability exceeds the reliability of serial engines and generators by 2-3 times.
6. Simplicity of manufacturing and assembly of the generator - the assembly site can be a metalworking workshop for piece and small-scale production.
7. Simple adaptation of the generator to the AC output voltage:
1) 36 IN, frequency 50 – 400 Hz
2) 115 IN, frequency 50 – 400 Hz(airfield power plants);
3) 220 IN, frequency 50 – 400 Hz;
4) 380 IN, frequency 50 – 400 Hz.
The basic design of the generator allows the product to be configured to different frequencies and voltages without changing the design.
8. High fire safety. The proposed generator cannot become a source of fire even with short circuit in the load circuit or in the windings, which is included in the design of the system. This is very important when using a generator for an on-board power plant in a confined space on a watercraft, aircraft, as well as private wooden housing construction, etc.
9. Low level noise.
10. High maintainability.
Generator parameters with power 0.5 kW
Generator parameters 2.5 power kW
RESULTS:
The proposed generator can be manufactured for use in electric generating sets with a shaft speed of 1500-1600 rpm. - in diesel, gasoline and steam generator power plants for individual use or in local energy systems. Paired with a multiplier, an electromechanical energy converter can also be used to generate electricity in low-speed generator systems, such as wind power plants, wave power plants, etc. of any power. That is, the scope of application of the electro-mechanical converter makes the proposed complex (multiplier-generator) universal. The weight, size and other electrical and technical parameters given in the text give the proposed design obvious competitive advantages on the market compared to analogues.
The manufacturing principles underlying the design are highly manufacturable, do not fundamentally require precision machine tools and are oriented towards mass serial production. As a result, the design will have a low cost of serial production.