DIY asynchronous electric motor. Can an asynchronous motor work as a generator - how to use it at home? The scope of application is quite wide

A pensioner makes windmills and saves on electricity

Pensioner from Amur region decided inalone to fightincrease in tariffs forelectricity. The desire to do the almost impossible arose afterthe next bills arrived forpublic utilities.

Then the former power engineer drew up his own plan for electrification of the entire site. Now the blades are spinning at the top and the lights at the bottom are lighting up. ABOUT how the wind brought change

Asynchronous electric motor as a generator

Job asynchronous electric motor in generator mode

The article describes how to build a three-phase (single-phase) 220/380 V generator based on an asynchronous electric motor alternating current.

A three-phase asynchronous electric motor, invented at the end of the 19th century by the Russian electrical engineer M.O. Dolivo-Dobrovolsky, has now become predominantly widespread in industry, agriculture, and also in everyday life. Asynchronous electric motors are the simplest and most reliable to operate. Therefore, in all cases where this is permissible under the conditions of the electric drive and there is no need for reactive power compensation, asynchronous AC motors should be used.

There are two main types of asynchronous motors:with squirrel-cage rotor and with phase rotor . An asynchronous squirrel-cage electric motor consists of a stationary part - the stator and a moving part - the rotor, rotating in bearings mounted in two motor shields. The stator and rotor cores are made of separate electrical steel sheets insulated from one another. A winding made of insulated wire. A rod winding is placed into the grooves of the rotor core or molten aluminum is poured. Jumper rings short-circuit the rotor winding at the ends (hence the name short-circuited). Unlike a squirrel-cage rotor, a winding made like a stator winding is placed in the slots of a phase-wound rotor. The ends of the winding are brought to slip rings mounted on the shaft. Brushes slide along the rings, connecting the winding to a starting or control rheostat. Asynchronous electric motors with a wound rotor are more expensive devices, require qualified maintenance, are less reliable, and therefore are used only in those industries where they cannot be done without them. For this reason, they are not very common, and we will not consider them further.

Along the stator winding included in three-phase circuit, a current flows, creating a rotating magnetic field. The magnetic field lines of the rotating stator field cross the rotor winding bars and induce in them electromotive force(EMF). Under the influence of this EMF, current flows in the short-circuited rotor rods. Magnetic fluxes arise around the rods, creating a general magnetic field of the rotor, which, interacting with the rotating magnetic field of the stator, creates a force causing the rotor to rotate in the direction of rotation magnetic field stator. The rotor rotation frequency is slightly less than the rotation frequency of the magnetic field created by the stator winding. This indicator is characterized by slip S and is for most engines in the range from 2 to 10%.

Most commonly used in industrial installationsthree-phase asynchronous electric motors, which are produced in the form of unified series. These include the single 4A series with a rated power range from 0.06 to 400 kW, the machines of which are highly reliable, have good performance and meet world standards.

Autonomous asynchronous generators are three-phase machines that convert the mechanical energy of the primary engine into electrical energy alternating current. Their undoubted advantage over other types of generators is the absence of a commutator-brush mechanism and, as a consequence, greater durability and reliability. If an asynchronous motor disconnected from the network is set into rotation from any prime mover, then in accordance with the reversibility principle electric machines When the synchronous rotation speed is reached, a certain EMF is generated at the terminals of the stator winding under the influence of the residual magnetic field. If you now connect a battery of capacitors C to the terminals of the stator winding, then a leading capacitive current will flow in the stator windings, which in this case is magnetizing. The battery capacity C must exceed a certain critical value C0, depending on the parameters of the autonomous asynchronous generator: only in this case does the generator self-excite and a three-phase symmetrical voltage system is installed on the stator windings. The voltage value ultimately depends on the characteristics of the machine and the capacitance of the capacitors. Thus, an asynchronous squirrel-cage electric motor can be converted into asynchronous generator.

Standard circuit for connecting an asynchronous electric motor as a generator.

You can select the container so that Rated voltage and the power of the asynchronous generator were equal to the voltage and power, respectively, when operating as an electric motor.

Table 1 shows the capacitances of the capacitors for excitation of asynchronous generators (U=380 V, 750...1500 rpm). Here reactive power Q is determined by the formula:

Q = 0.314 U2 C 10-6,

where C is the capacitance of the capacitors, μF.

As can be seen from the above data, the inductive load on the asynchronous generator, which reduces the power factor, causes a sharp increase in the required capacity. To maintain a constant voltage with increasing load, it is necessary to increase the capacitor capacity, that is, connect additional capacitors. This circumstance must be considered as a disadvantage of the asynchronous generator.

The rotation frequency of an asynchronous generator in normal mode must exceed the asynchronous one by a slip value S = 2...10%, and correspond to the synchronous frequency. Not fulfilling this condition will lead to the fact that the frequency of the generated voltage may differ from the industrial frequency of 50 Hz, which will lead to unstable operation of frequency-dependent consumers of electricity: electric pumps, washing machines, devices with transformer input. A decrease in the generated frequency is especially dangerous, since in this case the inductive resistance of the windings of electric motors and transformers decreases, which can cause their increased heating and premature failure. An ordinary asynchronous squirrel-cage electric motor of appropriate power can be used as an asynchronous generator without any modifications. The power of the electric motor-generator is determined by the power of the connected devices. The most energy-intensive of them are:

· household welding transformers;

· electric saws, electric jointers, grain crushers (power 0.3...3 kW);

· electric furnaces of the "Rossiyanka" and "Dream" types with a power of up to 2 kW;

· electric irons (power 850…1000 W).

I would especially like to dwell on the operation of household welding transformers. Their connection to an autonomous source of electricity is most desirable, because when operating from an industrial network, they create a number of inconveniences for other electricity consumers. If household welding transformer designed to work with electrodes with a diameter of 2...3 mm, then it full power is approximately 4...6 kW, the power of the asynchronous generator to power it should be within 5...7 kW. If a household welding transformer allows working with electrodes with a diameter of 4 mm, then in the heaviest mode - “cutting” metal, the total power consumed by it can reach 10...12 kW, respectively, the power of an asynchronous generator should be within 11...13 kW.

As a three-phase capacitor bank, it is good to use so-called reactive power compensators, designed to improve cosφin industrial lighting networks. Their typical designation: KM1-0.22-4.5-3U3 or KM2-0.22-9-3U3, which is deciphered as follows. KM - cosine capacitors impregnated with mineral oil, the first number is the size (1 or 2), then the voltage (0.22 kV), power (4.5 or 9 kvar), then the number 3 or 2 means three-phase or single-phase version, U3 (temperate climate of the third category).

In the case of self-manufacturing of the battery, you should use capacitors such as MBGO, MBGP, MBGT, K-42-4, etc. for an operating voltage of at least 600 V. Electrolytic capacitors cannot be used.

The option discussed above for connecting a three-phase electric motor as a generator can be considered classic, but not the only one. There are other methods that have proven themselves just as well in practice. For example, when a bank of capacitors is connected to one or two windings of an electric motor generator.

Two-phase mode of an asynchronous generator.

Fig.2 Two-phase mode of an asynchronous generator.

This circuit should be used when there is no need to obtain three-phase voltage. This switching option reduces the working capacity of the capacitors, reduces the load on the primary mechanical engine in idle mode, etc. saves "precious" fuel.

As low-power generators that produce an alternating single-phase voltage of 220 V, you can use single-phase asynchronous squirrel-cage electric motors for household use: from washing machines such as "Oka", "Volga", watering pumps "Agidel", "BTsN", etc. Their capacitor battery can connect in parallel to the working winding, or use an existing phase-shifting capacitor connected to starting winding. The capacity of this capacitor may need to be increased slightly. Its value will be determined by the nature of the load connected to the generator: for active loads (electric furnaces, lighting bulbs, electric soldering irons) it is required small capacity, inductive (electric motors, TVs, refrigerators) - more.

Fig. 3 Low-power single-phase generator asynchronous motor.

Now a few words about the primary mechanical engine, which will drive the generator. As you know, any transformation of energy is associated with its inevitable losses. Their value is determined by the efficiency of the device. Therefore, the power of a mechanical motor must exceed the power of an asynchronous generator by 50...100%. For example, with an asynchronous generator power of 5 kW, the power of a mechanical motor should be 7.5...10 kW. Using a transmission mechanism, the speed of the mechanical engine and the generator are matched so that the operating mode of the generator is set at the average speed of the mechanical engine. If necessary, you can briefly increase the generator power by increasing the speed of the mechanical engine.

Each autonomous power plant must contain the required minimum of attachments: an AC voltmeter (with a scale of up to 500 V), a frequency meter (preferably) and three switches. One switch connects the load to the generator, the other two switch the excitation circuit. The presence of switches in the excitation circuit makes it easier to start a mechanical engine, and also allows you to quickly reduce the temperature of the generator windings; after completion of work, the rotor of the unexcited generator is rotated for some time by the mechanical engine. This procedure extends the active life of the generator windings.

If using a generator it is intended to power equipment that is normally connected to an alternating current network (for example, lighting in a residential building, household electrical appliances), then it is necessary to provide a two-phase switch that will turn off the power supply during generator operation. this equipment from the industrial network. It is necessary to disconnect both wires: “phase” and “zero”.

In conclusion, some general advice.

1. The alternator is a hazardous device. Use 380 V only when absolutely necessary; in all other cases, use 220 V.

2. According to safety requirements, the electric generator must be equipped with grounding.

3. Pay attention to the thermal mode of the generator. He "does not like" idling. The thermal load can be reduced by more carefully selecting the capacitance of the exciting capacitors.

4. Don't go wrong with power electric current produced by the generator. If one phase is used when operating a three-phase generator, its power will be 1/3 total power generator, if two phases are 2/3 of the total generator power.

5. The frequency of the alternating current produced by the generator can be indirectly controlled by the output voltage, which in the “no-load” mode should be 4...6% higher than the industrial value of 220/380 V.

This task requires a number of manipulations, which must be accompanied by a clear understanding of the principles and modes of operation of such equipment.

What it is and how it works

An asynchronous type electric motor is a machine in which electrical energy is transformed into mechanical and thermal energy. Such a transition becomes possible due to the phenomenon of electromagnetic induction that occurs between the stator and rotor windings. A feature of asynchronous motors is the fact that the rotation speed of these two key elements is different.

The design features of a typical electric motor can be seen in the illustration. Both the stator and the rotor are coaxial round section objects are made by collecting a sufficient number of plates from special steel. The stator plates have grooves on the inside of the ring and, when combined, form longitudinal grooves into which the winding is wound. copper wire. For the rotor, its role is played by aluminum rods; they are also inserted into the grooves of the core, but are closed on both sides by locking plates.

When voltage is applied to the stator windings, an electromagnetic field appears on them and begins to rotate. Due to the fact that the rotor rotation speed is obviously lower, an EMF is induced between the windings and the central shaft begins to move. Non-synchronism of frequencies is associated not only with theoretical foundations process, but also with the actual friction of the shaft support bearings, it will slow it down somewhat relative to the stator field.

What is an electric generator?

The generator is an electric machine that converts mechanical and thermal energy to electric. From this point of view, it is a device directly opposite in principle of operation and mode of operation to an asynchronous motor. Moreover, the most common type of electric generators are induction.

As we remember from the theory described above, this becomes possible only when there is a difference in the revolutions of the magnetic fields of the stator and rotor. One logical conclusion follows from this (taking into account also the principle of reversibility, mentioned at the beginning of the article) - it is theoretically possible to make a generator from an asynchronous machine, in addition, this is a problem that can be solved independently by rewinding.

Engine operation in generator mode

Any asynchronous electric generator is used as a kind of transformer, where mechanical energy from the rotation of the motor shaft, is converted into alternating current. This becomes possible when its speed becomes higher than synchronous (about 1500 rpm). Classic scheme for converting and connecting an engine in electric generator mode with output three-phase current You can easily assemble it yourself:

To save on electricity bills, our readers recommend the Electricity Saving Box. Monthly payments will be 30-50% less than they were before using the saver. It removes the reactive component from the network, resulting in a reduction in load and, as a consequence, current consumption. Electrical appliances consume less electricity and costs are reduced.

To achieve such a starting speed, it is necessary to apply a fairly large torque (for example, by connecting an internal combustion engine in a gas generator or an impeller in a windmill). As soon as the rotation speed reaches the synchronous value, the capacitor bank begins to operate, creating capacitive current. Due to this, self-excitation of the stator windings occurs and electric current is generated (generation mode).

A necessary condition stable operation of such an electric generator with an industrial network frequency of 50 Hz, is the compliance of its frequency characteristics:

1. Its rotation speed must exceed the asynchronous speed (the frequency of operation of the motor itself) by a slip percentage (from 2 to 10%);
2. The generator rotation speed must match the synchronous speed.

How to assemble an asynchronous generator yourself?

Having acquired knowledge, ingenuity and the ability to work with information, you can assemble/remake a working generator from an engine with your own hands. To do this, you need to perform the exact steps in the following sequence:

1. The real (asynchronous) rotation speed of the engine, which is planned to be used as an electric generator, is calculated. To determine the speed of a unit connected to the network, you can use a tachograph;
2. The synchronous frequency of the motor is determined, which will also be asynchronous for the generator. Here the amount of slip is taken into account (2-10%). Let's say the measurements showed a rotation speed of 1450 rpm. The required operating frequency of the electric generator will be:

n GEN = (1.02…1.1)n DV = (1.02…1.1)·1450 = 1479…1595 rpm;

1. Selection of a capacitor of the required capacity (standard comparative data tables are used).

You can put an end to this, but if tension is required single-phase network 220V, then the operating mode of such a device will require the introduction of a step-down transformer into the previously described circuit.

Types of engine-based generators

Buying a standard ready-made electric generator is by no means a cheap pleasure and is unlikely to be affordable for the practical majority of our fellow citizens. An excellent alternative would be homemade generator, it can be assembled with sufficient knowledge of electrical engineering and plumbing. The assembled device can be successfully used as:

1. Self-powered electric generator. The user can obtain with his own hands a device for generating electricity with a long period of action due to self-recharge;
2. Wind generator. A windmill, which rotates under the influence of the wind, is used as a propulsion device necessary to start the engine;
3. Generator with neodymium magnets;
4. Three-phase gas generator;
5. Single-phase low-power generator on motors of electrical appliances, etc.

Converting a standard motor into a working generating device with your own hands is an exciting activity and obviously saves your budget. In this way, you can convert a regular windmill by connecting it to an engine for autonomous energy generation.

In an effort to obtain autonomous sources of electricity, specialists have found a way to convert a three-phase asynchronous AC electric motor into a generator with their own hands. This method has a number of advantages and some disadvantages.

Appearance of an asynchronous electric motor

The section shows the main elements:

1. cast iron body with radiator fins for efficient cooling;
2. a squirrel-cage rotor housing with magnetic field shift lines relative to its axis;
3. switching contact group in a box (borno), for switching stator windings in star or delta circuits and connecting power supply wires;
4. tight tourniquets copper wires stator windings;
5. steel rotor shaft with a groove for fixing the pulley with a wedge key.

A detailed disassembly of the asynchronous electric motor, indicating all the parts, is shown in the figure below.

Detailed disassembly of an asynchronous motor

Advantages of generators converted from asynchronous motors:

1. ease of circuit assembly, no need to disassemble the electric motor, no rewinding of the windings;
2. the ability to rotate the electric current generator with a wind or hydraulic turbine;
3. generator from an asynchronous motor is widely used in motor-generator systems to convert a single-phase 220V AC network into three-phase network with voltage 380V.
4. possibility of using a generator, in field conditions spinning it from internal combustion engines.

As a disadvantage, one can note the difficulty of calculating the capacitance of capacitors connected to the windings; in fact, this is done experimentally.

Therefore it is difficult to achieve maximum power such a generator, there are difficulties with power supply to electrical installations that have great importance starting current, on circular saws with three-phase motors alternating current, concrete mixers and other electrical installations.

Generator operating principle

The operation of such a generator is based on the principle of reversibility: “any electrical installation that converts electrical energy into mechanical energy can perform the reverse process.” The principle of operation of generators is used; rotation of the rotor causes an EMF and the appearance of an electric current in the stator windings.

Based on this theory, it is obvious that an asynchronous electric motor can be converted into an electric generator. In order to consciously carry out reconstruction, it is necessary to understand how the generation process occurs and what is required for this. All motors driven by alternating current are considered asynchronous. The stator field moves slightly ahead of the rotor magnetic field, pulling it along with it in the direction of rotation.

To obtain the reverse process, generation, the rotor field must advance the movement of the stator magnetic field, ideally rotating in the opposite direction. This is achieved by connecting a large capacitor to the power supply network; to increase the capacity, groups of capacitors are used. The capacitor unit is charged by accumulating magnetic energy (an element of the reactive component of alternating current). The charge of the capacitor is in phase opposite to the current source of the electric motor, so the rotation of the rotor begins to slow down, the stator winding generates current.

Conversion

How to practically convert an asynchronous electric motor into a generator with your own hands?

To connect the capacitors you need to unscrew top cover boron (box), where the contact group is located, switching the contacts of the stator windings and the power wires of the asynchronous motor are connected.

Open boron with contact group

The stator windings can be connected in a “Star” or “Triangle” configuration.

Connection circuits "Star" and "Triangle"

The nameplate or product data sheet shows possible connection diagrams and motor parameters for various connections. Indicated:

• maximum currents;
• supply voltage;
• power consumption;
• number of revolutions per minute;
• Efficiency and other parameters.

Engine parameters indicated on the nameplate

IN three phase generator from a do-it-yourself asynchronous electric motor, the capacitors are connected in a similar “Triangle” or “Star” circuit.

The connection option with a “Star” ensures the starting process of generating current at lower speeds than when connecting the circuit in a “Triangle”. In this case, the voltage at the generator output will be slightly lower. Delta connection provides a slight increase in output voltage, but requires higher rpm when starting the generator. In a single-phase asynchronous electric motor, one phase-shifting capacitor is connected.

Connection diagram of capacitors on a generator in a “Triangle”

Capacitors of the KBG-MN model or other brands of at least 400 V non-polar are used; bipolar electrolytic models are not suitable in this case.

What does a poleless capacitor of the KBG-MN brand look like?

Calculation of capacitor capacity for the motor used

In synchronous generators, the generation process is excited on the armature windings from the current source. 90% of asynchronous motors have squirrel-cage rotors, without winding; excitation is created by a residual static charge in the rotor. It is enough to create an EMF at the initial stage of rotation, which induces current and recharges the capacitors through the stator windings. Further recharging already comes from the generated current; the generation process will be continuous as long as the rotor rotates.

It is recommended to install the automatic load connection to the generator, sockets and capacitors in a separate closed panel. Lay the connecting wires from the boron generator to the switchboard in a separate insulated cable.

Even when the generator is not working, you must avoid touching the capacitor terminals of the socket contacts. The charge accumulated by the capacitor remains long time and may give you an electric shock. Ground the housings of all units, motor, generator, control panel.

Installation of a motor-generator system

When installing a generator with a motor with your own hands, you must take into account that the specified number of rated revolutions of the asynchronous electric motor used at idle is greater.

Scheme of a motor-generator on a belt drive

On an engine of 900 rpm at idle speed there will be 1230 rpm, in order to obtain sufficient power at the output of a generator converted from this engine, you must have a number of revolutions 10% higher than idle speed:

1230 + 10% = 1353 rpm.

Belt drive is calculated using the formula:

Vg = Vm x Dm\Dg

Vg – required generator rotation speed 1353 rpm;

Vm – motor rotation speed 1200 rpm;

Dm – pulley diameter on the motor is 15 cm;

Dg – diameter of the pulley on the generator.

Having a 1200 rpm motor where the pulley is Ø 15 cm, all that remains is to calculate Dg - the diameter of the pulley on the generator.

Dg = Vm x Dm/ Vg = 1200 rpm x 15cm/1353 rpm = 13.3 cm.

Generator with neodymium magnets

How to make a generator from an asynchronous electric motor?

This homemade generator eliminates the use of capacitor units. The source of the magnetic field, which induces EMF and creates current in the stator winding, is built on permanent neodymium magnets. In order to do this yourself, you must sequentially perform the following steps:

• Remove the front and rear covers of the asynchronous motor.
• Remove the rotor from the stator.

What does the rotor of an asynchronous motor look like?

• The rotor is ground, the top layer 2 mm larger than the thickness of the magnets is removed. IN living conditions It is not always possible to bore the rotor with your own hands, in the absence of turning equipment and skills. You need to contact specialists in turning workshops.
• On a sheet plain paper a template for placement is being prepared round magnets, Ø 10-20mm, thickness up to 10 mm, with an attractive force of 5-9 kg, per sq/cm, size depends on the size of the rotor. The template is glued to the surface of the rotor, the magnets are placed in strips at an angle of 15 - 20 degrees relative to the rotor axis, 8 pieces per strip. The figure below shows that on some rotors there are dark-light stripes of displacement of the magnetic field lines relative to its axis.

Installing magnets on the rotor

• The rotor on magnets is calculated so that there are four groups of strips, in a group of 5 strips, the distance between the groups is 2Ø of the magnet. The gaps in the group are 0.5-1Ø of the magnet, this arrangement reduces the force of sticking of the rotor to the stator; it must be rotated with the efforts of two fingers;
• The magnetic rotor, made according to the calculated template, is poured epoxy resin. After it dries a little, the cylindrical part of the rotor is covered with a layer of fiberglass and again impregnated with epoxy resin. This will prevent the magnets from flying out when the rotor rotates. Upper layer on magnets should not exceed the original diameter of the rotor, which was before the groove. Otherwise, the rotor will not fall into place or will rub against the stator winding when rotating.
• After drying, the rotor can be put back in place and the lids closed;
• To test an electric generator, it is necessary to turn the rotor with an electric drill, measuring the voltage at the output. The number of revolutions when the desired voltage is reached is measured by a tachometer.
• Knowing required amount generator speed, the belt drive is calculated according to the method described above.

An interesting application option is when an electric generator based on an asynchronous electric motor is used in a self-feeding electric motor-generator circuit. When part of the power generated by the generator goes to the electric motor, which spins it. The rest of the energy is spent on payload. By implementing the principle of self-feeding, it is practically possible to for a long time provide the house with autonomous power supply.

Video. G generator from an asynchronous motor.

For a wide range of electricity consumers, buy powerful diesel power plants like TEKSAN TJ 303 DW5C with an output power of 303 kVA or 242 kW does not make sense. Low-power gasoline generators expensive, best option make your own wind generators or a self-powered motor-generator device.

Using this information, you can assemble a generator with your own hands, using permanent magnets or capacitors. This type of equipment is very useful for country houses, in the field, as an emergency power source when there is no voltage in industrial networks. Fully equipped house with air conditioning, electric stoves and heating boilers, they cannot handle a powerful circular saw motor. Temporarily provide electricity Appliances basic necessities may be lighting, refrigerator, TV and others that do not require large powers.

To ensure uninterrupted power supply to the home, alternating current generators driven by diesel or carburetor internal combustion engines are used. But from the electrical engineering course we know that any electric motor is reversible: it is also capable of generating electricity. Is it possible to make a generator from an asynchronous motor with your own hands if you already have one and an internal combustion engine? After all, then you won’t need to buy an expensive power plant, but you can make do with improvised means.

Construction of an asynchronous electric motor

An asynchronous electric motor includes two main parts: a stationary stator and a rotor rotating inside it. The rotor rotates on bearings mounted in removable end parts. The rotor and stator contain electrical windings, the turns of which are laid in grooves.

The stator winding is connected to an alternating current network, single-phase or three-phase. Metal part The stator where it is placed is called a magnetic circuit. It is made of individual thin coated plates that insulate them from each other. This eliminates the occurrence of eddy currents, which make the operation of the electric motor impossible due to excessive losses due to heating of the magnetic circuit.

The terminals from the windings of all three phases are located in a special box on the motor housing. It is called a barno, in which the terminals of the windings are connected to each other. Depending on the supply voltage and technical data of the motor, the terminals are combined either into a star or into a triangle.

The rotor winding of any asynchronous electric motor is similar to a “squirrel cage”, that’s what it’s called. It is made in the form of a series of conductive aluminum rods distributed along the outer surface of the rotor. The ends of the rods are closed, which is why such a rotor is called squirrel-cage.
The winding, like the stator winding, is located inside a magnetic core, also made up of insulated metal plates.

Operating principle of an asynchronous electric motor

When the supply voltage is connected to the stator, current flows through the turns of the winding. It creates a magnetic field inside. Since the current is alternating, the field changes in accordance with the shape of the supply voltage. The arrangement of the windings in space is made in such a way that the field inside it turns out to be rotating.
In the rotor winding, the rotating field induces an emf. And since the turns of the winding are short-circuited, a current appears in them. It interacts with the stator field, this leads to rotation of the electric motor shaft.

The electric motor is called asynchronous because the stator field and rotor rotate with at different speeds. This speed difference is called slip (S).


Where:
n – magnetic field frequency;
nr – rotor rotation frequency.
To adjust the shaft speed in within wide limits, asynchronous electric motors are made with a wound rotor. On such a rotor, windings displaced in space are wound, the same as on the stator. The ends from them are brought out onto rings, and resistors are connected to them using a brush apparatus. The greater the resistance connected to wound rotor, the lower the speed of its rotation will be.

Asynchronous generator

What happens if the rotor of an asynchronous electric motor is rotated? Will it be able to generate electricity, and how to make a generator from an asynchronous motor?
It turns out that this is possible. In order for voltage to appear on the stator winding, it is initially necessary to create a rotating magnetic field. It appears due to the residual magnetization of the rotor of an electric machine. Subsequently, when load current appears, the strength of the rotor magnetic field reaches the required value and stabilizes.
To facilitate the process of the appearance of voltage at the output, a bank of capacitors is used, connected to the stator of the asynchronous generator at the time of startup (capacitor excitation).

But the parameter characteristic of an asynchronous electric motor remains unchanged: the amount of slip. Because of this, the frequency of the output voltage of the asynchronous generator will be lower than the shaft rotation speed.
By the way, the shaft of an asynchronous generator must be rotated at such a speed that the rated rotation speed of the stator field of the electric motor is achieved. To do this, you need to find out the shaft rotation speed from the plate located on the housing. By rounding its value to the nearest whole number, the rotation speed for the rotor of the electric motor being converted into a generator is obtained.

For example, for an electric motor, the plate of which is shown in the photo, the shaft rotation speed is 950 rpm. This means that the shaft rotation speed should be 1000 rpm.

Why is an asynchronous generator worse than a synchronous one?

How good will a homemade generator from an asynchronous motor be? How will it differ from a synchronous generator?
To answer these questions, let us briefly recall the operating principle of a synchronous generator. Through slip rings, direct current is supplied to the rotor winding, the magnitude of which is adjustable. The rotating field of the rotor creates an EMF in the stator winding. To obtain the required generation voltage automatic system excitation adjustment will change the current in the rotor. Since the voltage at the generator output is monitored automatically, as a result of a continuous regulation process, the voltage always remains unchanged and does not depend on the load current.
To start and operate synchronous generators, independent power sources (batteries) are used. Therefore, the start of its operation does not depend either on the appearance of load current at the output or on achieving the required rotation speed. Only the frequency of the output voltage depends on the rotation speed.
But even when the excitation current is received from the generator voltage, everything said above remains true.
A synchronous generator has one more feature: it is capable of generating not only active, but also reactive power. This is very important when powering electric motors, transformers and other units that consume it. The lack of reactive power in the network leads to an increase in heating losses of conductors and windings of electrical machines, and a decrease in the voltage level among consumers relative to the generated value.
To excite an asynchronous generator, the residual magnetization of its rotor is used, which in itself is a random quantity. It is not possible to regulate the parameters that affect the value of its output voltage during operation.

In addition, an asynchronous generator does not generate, but consumes reactive power. It is necessary for him to create excitation current in the rotor. Let's remember about capacitor excitation: by connecting a bank of capacitors at startup, the reactive power required by the generator to start working is created.
As a result, the voltage at the output of the asynchronous generator is not stable and varies depending on the nature of the load. When connected to it large number consumers of reactive power, the stator winding may overheat, which will affect the service life of its insulation.
Therefore, the use of an asynchronous generator is limited. It can operate in conditions close to “greenhouse”: no overloads, inrush load currents, or powerful consumers of the reagent. And at the same time, electrical receivers connected to it should not be critical to changes in the magnitude and frequency of the supply voltage.
Ideal place for the use of an asynchronous generator are systems alternative energy powered by water or wind energy. In these devices, the generator does not directly supply the consumer, but charges battery. From her already, through the converter direct current in variable, the load is powered.
Therefore, if you need to assemble a windmill or a small hydroelectric power station, the best way out is an asynchronous generator. Its main and only advantage works here - simplicity of design. The absence of rings on the rotor and brush apparatus means that during operation it does not need to be constantly maintained: clean the rings, change the brushes, remove graphite dust from them. After all, in order to make a wind generator from an asynchronous motor with your own hands, the generator shaft must be directly connected to the windmill blades. This means that the structure will be located on high altitude. It's a hassle to remove it from there.

Magnetic generator

Why does a magnetic field need to be created using an electric current? After all, there are powerful sources of it - neodymium magnets.
To convert an asynchronous motor into a generator, you will need cylindrical neodymium magnets, which will be installed in place of the standard conductors of the rotor winding. First you need to calculate the required number of magnets. To do this, remove the rotor from the engine being converted into a generator. It clearly shows the places where the winding of the “squirrel wheel” is laid. The dimensions (diameter) of the magnets are selected so that when installed strictly in the center of the conductors of the short-circuited winding, they do not come into contact with the magnets next row. There should be a gap between the rows no less than the diameter of the magnet used.
Having decided on the diameter, calculate how many magnets will fit along the length of the winding conductor from one edge of the rotor to the other. A gap of at least one to two millimeters is left between them. By multiplying the number of magnets in a row by the number of rows (conductors of the rotor winding), the required number is obtained. The height of the magnets should not be very large.
To install magnets on the rotor of an asynchronous electric motor, it will need to be modified: remove it lathe layer of metal to a depth corresponding to the height of the magnet. In this case, the rotor must be carefully centered in the machine so as not to upset its balancing. Otherwise, it will have a displacement of the center of mass, which will lead to beating in operation.

Then they begin to install magnets on the surface of the rotor. Glue is used for fixation. Any magnet has two poles, conventionally called north and south. Within one row, the poles located away from the rotor must be the same. To avoid mistakes in installation, the magnets are first linked together into a garland. They will adhere in a strictly defined way, since they are attracted to each other only by opposite poles. Now all that remains is to mark the poles of the same name with a marker.
In each subsequent row, the pole located outside changes. That is, if you laid out a row of magnets with the pole marked with a marker located outward from the rotor, then the next one is laid out with magnets turned the other way around. And so on.
After gluing the magnets, they need to be fixed with epoxy resin. To do this, around the resulting structure made of cardboard or thick paper make a template into which the resin will be poured. The paper is wrapped around the rotor and covered with tape or tape. One of the end parts is covered with plasticine or also sealed. Then the rotor is installed vertically and epoxy resin is poured into the cavity between the paper and the metal. After it hardens, the devices are removed.
Now we clamp the rotor back into the lathe, center it, and sand the surface filled with epoxy. This is not necessary for aesthetic reasons, but to minimize the impact of possible imbalance resulting from additional parts installed on the rotor.
Sanding is done first with coarse sandpaper. It is attached to wooden block, which is then moved evenly along the rotating surface. You can then use finer grit sandpaper.

Now the finished rotor can be inserted back into the stator and the resulting structure can be tested. It can be successfully used by those who want to make, for example, a wind generator from an asynchronous motor. There is only one drawback: the cost of neodymium magnets is very high. Therefore, before you start remaking the rotor and spending money on spare parts, you should calculate which option is more economically profitable: making a generator from an asynchronous motor or purchasing a ready-made one.

It was decided to convert an asynchronous motor as a generator for a windmill. This modification is very simple and affordable, so homemade structures In wind turbines you can often see generators made from asynchronous motors.

The modification consists of cutting the rotor under the magnets, then the magnets are usually glued to the rotor according to a template and filled with epoxy resin so that they do not fly off. They also usually rewind the stator with a thicker wire to reduce too much voltage and increase the current. But I didn’t want to rewind this motor and it was decided to leave everything as is, just convert the rotor to magnets. A three-phase asynchronous motor with a power of 1.32 kW was found as a donor. Below is a photo of this electric motor.

asynchronous motor conversion into a generator The rotor of the electric motor was machined on a lathe to the thickness of the magnets. This rotor does not use a metal sleeve, which is usually machined and placed on the rotor under the magnets. The sleeve is needed to enhance magnetic induction, through it the magnets close their fields by feeding each other from underneath and the magnetic field does not dissipate, but goes all the way to the stator. This design uses enough strong magnets 7.6*6mm in size in the amount of 160 pieces, which will provide good EMF even without a sleeve.

First, before gluing the magnets, the rotor was marked into four poles, and the magnets were placed at a bevel. The motor was four-pole and since the stator did not rewound, there should also be four magnetic poles on the rotor. Each magnetic pole alternates, one pole is conventionally “north”, the second pole is “south”. The magnetic poles are made at intervals, so the magnets are grouped closer together at the poles. After being placed on the rotor, the magnets were wrapped with tape for fixation and filled with epoxy resin.

After assembly, the rotor felt sticking, and when the shaft rotated, sticking was felt. It was decided to remake the rotor. The magnets were knocked together with epoxy and placed again, but now they are more or less evenly placed throughout the rotor, below is a photo of the rotor with magnets before being filled with epoxy. After filling, the sticking decreased somewhat and it was noticed that the voltage dropped slightly when the generator rotated at the same speed and the current increased slightly.

After assembling the finished generator, it was decided to twist it with a drill and connect something to it as a load. A 220 volt 60 watt light bulb was connected, at 800-1000 rpm it burned at full intensity. Also, to test what the generator was capable of, a 1 kW lamp was connected; it burned at full intensity and the drill was not strong enough to turn the generator.

Idle on maximum speed drill 2800 rpm, the generator voltage was more than 400 volts. At approximately 800 rpm the voltage is 160 volts. We also tried connecting a 500-watt boiler, after a minute of twisting the water in the glass became hot. These are the tests that the generator, which was made from an asynchronous motor, passed.

Afterwards, a stand with a rotating axis was welded for the generator to mount the generator and tail. The design is made according to a scheme where the wind head is moved away from the wind by folding the tail, so the generator is offset from the center of the axis, and the pin behind is the pin on which the tail is placed.

Here is a photo of the finished wind generator. The wind generator was installed on a nine-meter mast. When the wind was strong, the generator produced an idle voltage of up to 80 volts. They tried connecting a two-kilowatt tenn to it, but after a while the tenn became warm, which means the wind generator still has some power.

Then a controller for the wind generator was assembled and the battery was connected through it for charging. The charging current was quite good, the battery quickly began to make noise, as if it were being charged from a charger.

The data on the electric motor wiring diagram said 220/380 volts 6.2/3.6 A. This means the generator resistance is 35.4 Ohm delta/105.5 Ohm star. If he charged a 12-volt battery according to the scheme of connecting the generator phases in a triangle, which is most likely, then 80-12/35.4 = 1.9A. It turns out that with a wind of 8-9 m/s, the charging current was approximately 1.9 A, which is only 23 watt/hour, not much, but maybe I was wrong somewhere.

Such big losses due to the high resistance of the generator, so the stator is usually rewound with a thicker wire to reduce the resistance of the generator, which affects the current strength, and the higher the resistance of the generator winding, the lower the current strength and the higher the voltage.