Connecting a three-phase motor to a three-phase network. Three-phase motor - into a single-phase network 3-phase motor 220 volts

Three-phase motor connection diagrams - motors designed to operate from a three-phase network have much higher performance than single-phase 220 volt motors. Therefore, if there are three phases of alternating current in the workroom, then the equipment must be installed taking into account the connection to the three phases. As a result, a three-phase motor connected to the network provides energy savings and stable operation of the device. No need to connect additional elements to start. The only condition for good operation of the device is error-free connection and installation of the circuit, in compliance with the rules.

Three-phase motor connection diagrams

Of the many circuits created by specialists, two methods are practically used for installing an asynchronous motor.

• Star diagram.
• Triangle diagram.

The names of the circuits are given according to the method of connecting the windings to the supply network. To determine on an electric motor which circuit it is connected to, you need to look at the specified data on a metal plate that is installed on the motor housing.

Even on old motor samples, it is possible to determine the method of connecting the stator windings, as well as the mains voltage. This information will be correct if the engine has already been in operation and there are no operational problems. But sometimes you need to make electrical measurements.

Star connection diagrams for a three-phase motor make it possible to start the motor smoothly, but the power is 30% less than the rated value. Therefore, in terms of power, the triangle circuit remains the winner. There is a feature regarding the current load. The current increases sharply during startup, this negatively affects the stator winding. The heat generated increases, which has a detrimental effect on the winding insulation. This leads to insulation failure and damage to the electric motor.

Many European devices supplied to the domestic market are equipped with European electric motors operating with voltages from 400 to 690 V. Such 3-phase motors must be installed in a 380 volt network of domestic voltage only using a triangular stator winding pattern. Otherwise, the motors will immediately fail. Russian motors for three phases are connected in a star. Occasionally, a delta circuit is installed to obtain the maximum power from the engine, used in special types of industrial equipment.

Manufacturers today make it possible to connect three-phase electric motors according to any circuit. If there are three ends in the mounting box, then the factory star circuit has been produced. And if there are six terminals, then the motor can be connected according to any scheme. When mounting in a star, you need to combine the three terminals of the windings into one unit. The remaining three terminals are supplied to phase power with a voltage of 380 volts. In a triangle circuit, the ends of the windings are connected in series in order to each other. Phase power is connected to the node points of the ends of the windings.

Checking the motor connection diagram

Let's imagine the worst case scenario for connecting the windings, when the wire terminals are not marked at the factory, the circuit assembly is carried out in the inside of the motor housing, and one cable is brought out. In this case, it is necessary to disassemble the electric motor, remove the covers, disassemble the internal part, and deal with the wires.

Stator phase determination method

After disconnecting the lead ends of the wires, use a multimeter to measure the resistance. One probe is connected to any wire, the other is brought in turn to all wire terminals until a terminal belonging to the winding of the first wire is found. Do the same for the other terminals. It must be remembered that marking the wires in any way is mandatory.

If there is no multimeter or other device available, then use homemade probes made from a light bulb, wires and batteries.

Winding polarity

To find and determine the polarity of the windings, you need to apply some techniques:

• Connect pulsed direct current.
• Connect an alternating current source.

Both methods operate on the principle of applying voltage to one coil and transforming it along the magnetic circuit of the core.

How to check the polarity of the windings with a battery and a tester

A voltmeter with increased sensitivity is connected to the contacts of one winding, which can respond to a pulse. Voltage is quickly connected to the other coil with one pole. At the moment of connection, the deviation of the voltmeter needle is monitored. If the arrow moves to the positive, then the polarity coincides with the other winding. When the contact opens, the arrow will go to minus. For the 3rd winding the experiment is repeated.

By changing the terminals to another winding when the battery is turned on, it is determined how correctly the markings of the ends of the stator windings are made.

AC test

Any two windings are connected in parallel with their ends to the multimeter. The voltage is turned on to the third winding. They look at what the voltmeter shows: if the polarity of both windings matches, then the voltmeter will show the voltage value, if the polarities are different, then it will show zero.

The polarity of the 3rd phase is determined by switching the voltmeter, changing the position of the transformer to another winding. Next, control measurements are made.

Star diagram

This type of three-phase motor connection circuit is formed by connecting the windings in different circuits, united by a neutral and a common phase point.

Such a circuit is created after the polarity of the stator windings in the electric motor has been checked. A single-phase voltage of 220V is supplied through a machine to the beginning of 2 windings. Capacitors are inserted into the gap into one: working and starting. The neutral power wire is connected to the third end of the star.

The capacitance value of capacitors (working) is determined by the empirical formula:

C = (2800 I) / U

For the starting circuit, the capacity is increased by 3 times. When the motor is operating under load, it is necessary to control the magnitude of the winding currents by measurements and adjust the capacitance of the capacitors according to the average load of the mechanism drive. Otherwise, the device will overheat and an insulation breakdown will occur.

It is best to connect the motor to operation through the PNVS switch, as shown in the figure.

It already contains a pair of closure contacts, which together supply voltage to 2 circuits by means of the “Start” button. When the button is released, the circuit breaks. This contact is used to start the circuit. A complete power shutdown is done by clicking on “Stop”.

Triangle diagram

The diagram for connecting a three-phase motor with a delta is a repetition of the previous version in startup, but differs in the method of connecting the stator windings.

The currents passing in them are greater than the values ​​of the star circuit. The operating capacitances of capacitors require increased rated capacitances. They are calculated using the formula:

C = (4800 I) / U

The correct choice of capacitances is also calculated by the ratio of currents in the stator coils by measuring with a load.

Motor with magnetic starter

A three-phase electric motor operates through a similar circuit with a circuit breaker. This circuit additionally has an on and off block, with Start and Stop buttons.

One phase, normally closed, connected to the motor, is connected to the Start button. When it is pressed, the contacts close and current flows to the electric motor. It must be taken into account that when the Start button is released, the terminals will open and the power will turn off. To prevent this situation from happening, the magnetic starter is additionally equipped with auxiliary contacts, which are called self-retaining. They block the chain and prevent it from breaking when the Start button is released. You can turn off the power using the Stop button.

As a result, a 3-phase electric motor can be connected to a three-phase voltage network using completely different methods, which are selected according to the model and type of device, and operating conditions.

Connecting a motor from a machine

A general version of this connection diagram looks like in the figure:

Shown here is a circuit breaker that turns off the power supply to the electric motor in case of excessive current load and short circuit. The circuit breaker is a simple 3-pole circuit breaker with a thermal automatic load characteristic.

For an approximate calculation and assessment of the required thermal protection current, it is necessary to double the rated power of a motor designed to operate from three phases. The rated power is indicated on a metal plate on the motor housing.

Such connection diagrams for a three-phase motor may well work if there are no other connection options. The duration of the work cannot be predicted. This is the same if you twist an aluminum wire with a copper one. You never know how long it will take for the twist to burn out.

When using a connection diagram for a three-phase motor, you need to carefully select the current for the machine, which should be 20% greater than the operating current of the motor. Select the thermal protection properties with a reserve so that the blocking does not work during startup.

If, for example, the motor is 1.5 kilowatts, the maximum current is 3 amperes, then the machine needs at least 4 amperes. The advantage of this motor connection scheme is low cost, simple design and maintenance.

If the electric motor is in one number and works a full shift, then there are the following disadvantages:

• It is impossible to adjust the thermal current of the circuit breaker. To protect the electric motor, the protective shutdown current of the machine is set to 20% greater than the operating current of the motor rating. The electric motor current must be measured with clamps after a certain time, and the thermal protection current must be adjusted. But a simple circuit breaker does not have the ability to adjust the current.
• You cannot turn the electric motor off and on remotely.

Perhaps the most common and simplest way to connect a three-phase electric motor to a single-phase network in the absence of a supply voltage of ~ 380 V is the method using a phase-shifting capacitor, through which the third winding of the electric motor is powered. Before connecting a three-phase electric motor to a single-phase network, make sure that its windings are connected in a delta (see figure below, option 2), since this connection will give minimal power losses to a 3-phase motor when it is connected to the network ~ 220 V.

The power developed by a three-phase electric motor connected to a single-phase network with such a winding connection diagram can be up to 75% of its rated power. In this case, the engine rotation speed is practically no different from its frequency when operating in three-phase mode.

The figure shows the terminal blocks of electric motors and the corresponding winding connection diagrams. However, the design of the electric motor terminal box may differ from that shown below - instead of terminal blocks, the box may contain two separated bundles of wires (three in each).

These bundles of wires represent the "beginnings" and "ends" of the motor windings. They need to be “ringed” in order to separate the windings from each other and connect them according to the “triangle” pattern we need - in series, when the end of one winding is connected to the beginning of another, etc. (C1-C6, C2-C4, C3-C5).

When a three-phase electric motor is connected to a single-phase network, a starting capacitor Cp is added to the delta circuit, which is used for a short time (only for starting) and a working capacitor Cp.

As a SB button to start the electric. For a low-power engine (up to 1.5 kW), you can use the usual “START” button, used in the control circuits of magnetic starters.

For engines of higher power, it is worth replacing it with a more powerful switching device - for example, an automatic machine. The only inconvenience in this case will be the need to manually turn off the capacitor Sp automatically after the electric motor picks up speed.

Thus, the circuit implements the possibility of two-stage control of the electric motor, reducing the total capacitance of the capacitors when the engine “accelerates”.

If the engine power is small (up to 1 kW), then it will be possible to start it without a starting capacitor, leaving only the running capacitor Cp in the circuit.

• C slave = 2800. I / U, µF - for motors connected to a single-phase network with star-connected windings.

This is the most accurate method, however, it requires measuring the current in the motor circuit. Knowing the rated power of the engine, it is better to use the following formula to determine the capacity of the working capacitor:

C slave = 66·Р nom, μF, where Р nom is the rated power of the engine.

Simplifying the formula, we can say that for a three-phase electric motor to operate in a single-phase network, the capacitor capacity for every 0.1 kW of its power should be about 7 μF.

So, for a 1.1 kW motor, the capacitance of the capacitor should be 77 μF. Such a capacity can be obtained by several capacitors connected to each other in parallel (the total capacity in this case will be equal to the total), using the following types: MBGCh, BGT, KGB with an operating voltage exceeding the network voltage by 1.5 times.

By calculating the capacitance of the working capacitor, you can determine the capacitance of the starting capacitor - it should exceed the capacitance of the working capacitor by 2-3 times. Start-up capacitors should be of the same types as the working ones; in extreme cases and under the condition of a very short-term start-up, you can use electrolytic ones - types K50-3, KE-2, EGC-M, designed for a voltage of at least 450 V.

How to connect a three-phase motor to a single-phase network.

connecting a 380 to 220 volt motor

correct selection of capacitors for the electric motor

1.1. Selecting a three-phase motor for connection to a single-phase network.

Among the various methods of starting three-phase electric motors in a single-phase network, the simplest is based on connecting the third winding through a phase-shifting capacitor. The useful power developed by the engine in this case is 50...60% of its power in three-phase operation. Not all three-phase electric motors, however, work well when connected to a single-phase network. Among such electric motors we can highlight, for example, those with a double cage squirrel-cage rotor of the MA series. In this regard, when choosing three-phase electric motors for operation in a single-phase network, preference should be given to motors of the A, AO, AO2, APN, UAD, etc. series.

For normal operation of a capacitor-start electric motor, it is necessary that the capacitance of the capacitor used varies depending on the speed. In practice, this condition is quite difficult to fulfill, so two-stage motor control is used. When starting the engine, two capacitors are connected, and after acceleration, one capacitor is disconnected and only the working capacitor is left.

1.2. Calculation of parameters and elements of an electric motor.

If, for example, the electric motor’s data sheet indicates its supply voltage is 220/380, then the motor is connected to a single-phase network according to the diagram shown in Fig. 1

After turning on the batch switch P1, contacts P1.1 and P1.2 close, after which you must immediately press the “Acceleration” button. After gaining speed, the button is released. Reversing the electric motor is carried out by switching the phase on its winding with toggle switch SA1.

The capacity of the working capacitor Cp in the case of connecting the motor windings in a “triangle” is determined by the formula:

And in the case of connecting the motor windings in a “star”, it is determined by the formula:

The current consumed by the electric motor in the above formulas, with a known power of the electric motor, can be calculated from the following expression:

The capacity of the starting capacitor Sp is chosen 2..2.5 times greater than the capacity of the working capacitor. These capacitors must be designed for a voltage of 1.5 times the mains voltage. For a 220 V network, it is better to use capacitors such as MBGO, MBPG, MBGCh with an operating voltage of 500 V and higher. Subject to short-term switching on, electrolytic capacitors of the K50-3, EGC-M, KE-2 types with an operating voltage of at least 450 V can be used as starting capacitors. For greater reliability, electrolytic capacitors are connected in series, connecting their negative terminals together, and are shunted diodes (Fig. 2)

The total capacitance of the connected capacitors will be (C1+C2)/2.

In practice, the capacitance values ​​of the working and starting capacitors are selected depending on the engine power according to the table. 1

Table 1. The value of the capacitances of the working and starting capacitors of a three-phase electric motor depending on its power when connected to a 220 V network.

It should be noted that in an electric motor with capacitor starting in no-load mode, a current flows through the winding fed through the capacitor by 20...30% higher than the rated one. In this regard, if the engine is often used in underloaded mode or idling, then in this case the capacitance of the capacitor C p should be reduced. It may happen that during an overload the electric motor stops, then to start it, the starting capacitor is connected again, removing the load altogether or reducing it to a minimum.

The capacity of the starting capacitor C p can be reduced when starting electric motors at idle or with a light load. To turn on, for example, an AO2 electric motor with a power of 2.2 kW at 1420 rpm, you can use a working capacitor with a capacity of 230 μF, and a starting capacitor - 150 μF. In this case, the electric motor starts confidently with a small load on the shaft.

1.3. Portable universal unit for starting three-phase electric motors with a power of about 0.5 kW from a 220 V network.

To start electric motors of various series, with a power of about 0.5 kW, from a single-phase network without reversing, you can assemble a portable universal starting unit (Fig. 3)

When you press the SB1 button, the magnetic starter KM1 is triggered (toggle switch SA1 is closed) and its contact system KM 1.1, KM 1.2 connects the electric motor M1 to the 220 V network. At the same time, the third contact group KM 1.3 closes the SB1 button. After complete acceleration of the engine, turn off the starting capacitor C1 using toggle switch SA1. The engine is stopped by pressing the SB2 button.

1.3.1. Details.

The device uses an electric motor A471A4 (AO2-21-4) with a power of 0.55 kW at 1420 rpm and a magnetic starter of the PML type, designed for alternating current voltage of 220 V. Buttons SB1 and SB2 are paired type PKE612. Toggle switch T2-1 is used as switch SA1. In the device, the constant resistor R1 is wire-wound, type PE-20, and the resistor R2 is type MLT-2. Capacitors C1 and C2 type MBGCh for a voltage of 400 V. Capacitor C2 is made up of parallel connected capacitors of 20 μF 400 V. Lamp HL1 type KM-24 and 100 mA.

The starting device is mounted in a metal case measuring 170x140x50 mm (Fig. 4)

Rice. 4 Appearance of the starting device and drawing of the panel, pos. 7.

On the top panel of the case there are “Start” and “Stop” buttons - a signal lamp and a toggle switch to turn off the starting capacitor. On the front panel of the device case there is a connector for connecting an electric motor.

To turn off the starting capacitor, you can use an additional relay K1, then there is no need for toggle switch SA1, and the capacitor will turn off automatically (Fig. 5)

When you press the SB1 button, relay K1 is triggered and contact pair K1.1 turns on the magnetic starter KM1, and K1.2 turns on the starting capacitor C. The magnetic starter KM1 is self-locking using its contact pair KM 1.1, and contacts KM 1.2 and KM 1.3 connect the electric motor to the network. The "Start" button is kept pressed until the engine fully accelerates, and then released. Relay K1 is de-energized and turns off the starting capacitor, which is discharged through resistor R2. At the same time, the magnetic starter KM 1 remains switched on and provides power to the electric motor in operating mode. To stop the electric motor, press the "Stop" button. In an improved starting device according to the diagram in Fig. 5, you can use a relay of the MKU-48 type or the like.

2. The use of electrolytic capacitors in electric motor starting circuits.

When connecting three-phase asynchronous electric motors to a single-phase network, as a rule, ordinary paper capacitors are used. Practice has shown that instead of bulky paper capacitors, you can use oxide (electrolytic) capacitors, which are smaller in size and more affordable to purchase. An equivalent replacement diagram for conventional paper is shown in Fig. 6

The positive half-wave of alternating current passes through the chain VD1, C2, and the negative half-wave VD2, C2. Based on this, it is possible to use oxide capacitors with a permissible voltage that is half that of conventional capacitors of the same capacity. For example, if in a circuit for a single-phase network with a voltage of 220 V a paper capacitor with a voltage of 400 V is used, then when replacing it, according to the above diagram, you can use an electrolytic capacitor with a voltage of 200 V. In the above diagram, the capacitances of both capacitors are the same and are selected in the same way as the method for selecting paper capacitors capacitors for the starting device.

2.1. Connecting a three-phase motor to a single-phase network using electrolytic capacitors.

The diagram for connecting a three-phase motor to a single-phase network using electrolytic capacitors is shown in Fig. 7.

In the above diagram, SA1 is the engine rotation direction switch, SB1 is the engine acceleration button, electrolytic capacitors C1 and C3 are used to start the engine, C2 and C4 are used during operation.

Selection of electrolytic capacitors in the circuit shown in Fig. 7 is best done using current clamps. Currents are measured at points A, B, C and equality of currents at these points is achieved by stepwise selection of capacitor capacitances. Measurements are carried out with the engine loaded in the mode in which it is expected to operate. Diodes VD1 and VD2 for a 220 V network are selected with a maximum permissible reverse voltage of at least 300 V. The maximum forward current of the diode depends on the engine power. For electric motors with a power of up to 1 kW, diodes D245, D245A, D246, D246A, D247 with a direct current of 10 A are suitable. With a higher motor power from 1 kW to 2 kW, you need to take more powerful diodes with the corresponding forward current, or put several less powerful diodes in parallel , installing them on radiators.

Please note that if the diode is overloaded, breakdown may occur and alternating current will flow through the electrolytic capacitor, which can lead to its heating and explosion.

3. Connection of powerful three-phase motors to a single-phase network.

The capacitor circuit for connecting three-phase motors to a single-phase network makes it possible to obtain no more than 60% of the rated power from the motor, while the power limit of the electrified device is limited to 1.2 kW. This is clearly not enough to operate an electric planer or electric saw, which should have a power of 1.5...2 kW. The problem in this case can be solved by using a higher power electric motor, for example, with a power of 3...4 kW. Motors of this type are designed for a voltage of 380 V, their windings are star-connected and the terminal box contains only 3 terminals. Connecting such a motor to a 220 V network leads to a reduction in the rated power of the motor by 3 times and by 40% when operating in a single-phase network. This reduction in power makes the engine unsuitable for operation, but can be used to spin the rotor idle or with minimal load. Practice shows that most electric motors confidently accelerate to rated speed, and in this case, starting currents do not exceed 20 A.

3.1. Refinement of a three-phase motor.

The easiest way to convert a powerful three-phase motor into operating mode is to convert it to a single-phase operating mode, while receiving 50% of the rated power. Switching the motor to single-phase mode requires slight modification. Open the terminal box and determine which side of the motor housing cover the winding terminals fit on. Unscrew the bolts securing the cover and remove it from the engine housing. Find the place where the three windings are connected to a common point and solder an additional conductor with a cross-section corresponding to the cross-section of the winding wire to the common point. The twist with a soldered conductor is insulated with electrical tape or a polyvinyl chloride tube, and the additional terminal is pulled into the terminal box. After this, the housing cover is replaced.

The electric motor switching circuit in this case will have the form shown in Fig. 8.

During engine acceleration, a star connection of the windings is used with the connection of a phase-shifting capacitor Sp. In operating mode, only one winding remains connected to the network, and the rotation of the rotor is supported by a pulsating magnetic field. After switching the windings, the capacitor Cn is discharged through the resistor Rр. The operation of the presented circuit was tested with an AIR-100S2Y3 type engine (4 kW, 2800 rpm) installed on a homemade woodworking machine and showed its effectiveness.

3.1.1. Details.

In the switching circuit of electric motor windings, a packet switch with an operating current of at least 16 A should be used as a switching device SA1, for example, a switch of type PP2-25/N3 (two-pole with neutral, for a current of 25 A). Switch SA2 can be of any type, but with a current of at least 16 A. If motor reversal is not required, then this switch SA2 can be excluded from the circuit.

A disadvantage of the proposed scheme for connecting a powerful three-phase electric motor to a single-phase network can be considered the sensitivity of the motor to overloads. If the load on the shaft reaches half the engine power, then the shaft rotation speed may decrease until it stops completely. In this case, the load is removed from the motor shaft. The switch is first moved to the “Acceleration” position, and then to the “Work” position and further work continues.

In order to improve the starting characteristics of motors, in addition to the starting and running capacitor, you can also use inductance, which improves the uniformity of phase loading. All this is written in the article Devices for starting a three-phase electric motor with low power losses

When writing the article, some of the materials from the book by V.M. Pestrikov were used. "Home electrician and more..."

All the best, write to © 2005

It consists of two main parts - the stator and the rotor. The stator is the stationary part, the rotor is the rotating part. The rotor is placed inside the stator. There is a small distance between the rotor and stator, called an air gap, usually 0.5-2 mm.

Asynchronous motor stator

Asynchronous motor rotor

Stator consists of a body and a core with a winding. The stator core is assembled from thin sheet technical steel, usually 0.5 mm thick, coated with insulating varnish. The laminated core design contributes to a significant reduction in eddy currents arising during the process of magnetization reversal of the core by a rotating magnetic field. The stator windings are located in the slots of the core.

Housing and stator core of an asynchronous electric motor

Design of a laminated core of an asynchronous motor

Rotor consists of a core with a short-circuited winding and a shaft. The rotor core also has a laminated design. In this case, the rotor sheets are not varnished, since the current has a low frequency and the oxide film is sufficient to limit eddy currents.

Principle of operation. Rotating magnetic field

The principle of three-phase operation is based on the ability of a three-phase winding, when connected to a three-phase current network, to create a rotating magnetic field.

Launch

Stop

Rotating magnetic field of an asynchronous electric motor

The rotation frequency of this field, or the synchronous rotation frequency, is directly proportional to the frequency of the alternating current f 1 and inversely proportional to the number of pole pairs p of the three-phase winding.

,

• where n 1 is the rotation frequency of the stator magnetic field, rpm,
• f 1 – alternating current frequency, Hz,
• p – number of pole pairs

Rotating magnetic field concept

To understand the rotating magnetic field phenomenon better, consider a simplified three-phase winding with three turns. Current flowing through a conductor creates a magnetic field around it. The figure below shows the field created by three-phase alternating current at a specific point in time

Launch

Stop

Magnetic field of a straight conductor with direct current

Magnetic field created by the winding

The components of alternating current will change over time, causing the magnetic field they create to change. In this case, the resulting magnetic field of the three-phase winding will take different orientations, while maintaining the same amplitude.

Magnetic field created by three-phase current at different times Current flowing in the turns of the electric motor (shift 60°)

Launch

Stop

The effect of a rotating magnetic field on a closed loop

Now let's place a closed conductor inside a rotating magnetic field. A changing magnetic field will give rise to an electromotive force (EMF) in the conductor. In turn, the EMF will cause a current in the conductor. Thus, in a magnetic field there will be a closed conductor with a current, on which a force will act accordingly, as a result of which the circuit will begin to rotate.

The influence of a rotating magnetic field on a closed conductor carrying current

Squirrel-cage rotor of an asynchronous motor

This principle also works. Instead of a current-carrying frame, inside the asynchronous motor there is a squirrel-cage rotor whose design resembles a squirrel wheel. A squirrel-cage rotor consists of rods short-circuited at the ends with rings.

Squirrel cage rotor most widely used in induction motors (shown without shaft and core)

Three-phase alternating current, passing through the stator windings, creates a rotating magnetic field. Thus, also as described earlier, a current will be induced in the rotor bars, causing the rotor to start rotating. In the figure below you can notice the difference between the induced currents in the rods. This occurs due to the fact that the magnitude of the change in the magnetic field differs in different pairs of rods, due to their different locations relative to the field. The change in current in the rods will change with time.

Launch

Stop

Rotating magnetic field penetrating a squirrel-cage rotor

You may also notice that the rotor arms are tilted relative to the axis of rotation. This is done in order to reduce the higher harmonics of the EMF and get rid of torque ripple. If the rods were directed along the axis of rotation, then a pulsating magnetic field would arise in them due to the fact that the magnetic resistance of the winding is much higher than the magnetic resistance of the stator teeth.

Slip of an asynchronous motor. Rotor speed

A distinctive feature of an asynchronous motor is that the rotor speed n 2 is less than the synchronous speed of the stator magnetic field n 1 .

This is explained by the fact that the EMF in the rods of the rotor winding is induced only when the rotation speeds n 2 are unequal

,

• where s is the slip of an asynchronous electric motor,
• n 1 – rotation frequency of the stator magnetic field, rpm,
• n 2 – rotor speed, rpm,

Let us consider the case when the rotor rotation frequency coincides with the rotation frequency of the stator magnetic field. In this case, the relative magnetic field of the rotor will be constant, thus no EMF, and therefore no current, will be created in the rotor rods. This means that the force acting on the rotor will be zero. This will slow down the rotor. After which an alternating magnetic field will again act on the rotor rods, thus the induced current and force will increase. In reality, the rotor will never reach the rotation speed of the stator's magnetic field. The rotor will rotate at a certain speed which is slightly less than the synchronous speed.

The slip of an asynchronous motor can vary in the range from 0 to 1, i.e. 0-100%. If s~0, then this corresponds to the idle mode, when the engine rotor experiences practically no counteracting torque; if s=1 - short circuit mode, in which the motor rotor is stationary (n 2 = 0). Slip depends on the mechanical load on the motor shaft and increases with its growth.

The slip corresponding to the rated load of the motor is called rated slip. For low and medium power asynchronous motors, the rated slip varies from 8% to 2%.

Energy conversion

Field-oriented control of an asynchronous electric motor using a rotor position sensor

Field-oriented control allows you to smoothly and accurately control the movement parameters (speed and torque), but its implementation requires information about the direction and vector of the engine rotor flux linkage.

According to the method of obtaining information about the position of the flux linkage of the electric motor rotor, the following are distinguished:
• field-oriented sensor control;
• field-oriented control without a sensor: the position of the rotor flux linkage is calculated mathematically based on the information available in the frequency converter (supply voltage, stator voltages and currents, resistance and inductance of the stator and rotor windings, number of motor pole pairs).

Field-oriented control of an asynchronous electric motor without a rotor position sensor

To increase efficiency and reduce brush wear, some ADFRs contain a special device (short-circuit mechanism), which, after starting, raises the brushes and closes the rings.

With rheostatic starting, favorable starting characteristics are achieved, since high torque values ​​are achieved at low starting current values. Currently, ADDFs are being replaced by a combination of a squirrel-cage induction motor and a frequency converter.

There are situations in life when you need to connect some industrial equipment to a regular home power supply network. A problem immediately arises with the number of wires. Machines intended for use in enterprises usually have three, but sometimes four, terminals. What to do with them, where to connect them? Those who tried to try various options were convinced that the motors simply did not want to spin. Is it even possible to connect a single-phase three-phase motor? Yes, you can achieve rotation. Unfortunately, in this case, the power drop is inevitable by almost half, but in some situations this is the only way out.

Voltages and their ratio

In order to understand how to connect a three-phase motor to a regular outlet, you need to understand how the voltages in the industrial network relate. The voltage values ​​are well known - 220 and 380 Volts. Previously, there was still 127 V, but in the fifties this parameter was abandoned in favor of a higher one. Where did these “magic numbers” come from? Why not 100, or 200, or 300? It seems that round numbers are easier to count.

Most industrial electrical equipment is designed to be connected to a three-phase network. The voltage of each phase in relation to the neutral wire is 220 Volts, just like in a home socket. Where does 380 V come from? It is very simple, just consider an isosceles triangle with angles of 60, 30 and 30 degrees, which is a vector stress diagram. The length of the longest side will be equal to the length of the thigh multiplied by cos 30°. After some simple calculations, you can make sure that 220 x cos 30° = 380.

Three-phase motor device

Not all types of industrial motors can operate from a single phase. The most common of them are the “workhorses” that make up the majority of electrical machines in any enterprise - asynchronous machines with a power of 1 - 1.5 kVA. How does such a three-phase motor work in the three-phase network for which it is intended?

The inventor of this revolutionary device was the Russian scientist Mikhail Osipovich Dolivo-Dobrovolsky. This outstanding electrical engineer was a proponent of the theory of a three-phase power supply network, which has become dominant in our time. three-phase operates on the principle of induction of currents from the stator windings to closed rotor conductors. As a result of their flow through the short-circuited windings, a magnetic field arises in each of them, interacting with the stator power lines. This produces a torque that leads to circular motion of the motor axis.

The windings are angled 120° so that the rotating field generated by each phase pushes each magnetized side of the rotor in succession.

Triangle or star?

A three-phase motor in a three-phase network can be switched on in two ways - with or without a neutral wire. The first method is called “star”, in this case each of the windings is under (between phase and zero), equal in our conditions to 220 V. The connection diagram of a three-phase motor with a “triangle” involves connecting three windings in series and applying linear (380 V) voltage to switching nodes. In the second case, the engine will produce about one and a half times more power.

How to turn the motor in reverse?

Control of a three-phase motor may require changing the direction of rotation to the opposite, that is, reverse. To achieve this, you just need to swap two of the three wires.

To make it easier to change the circuit, jumpers are provided in the motor terminal box, usually made of copper. For star switching, gently connect the three output wires of the windings together. The “triangle” turns out to be a little more complicated, but any average qualified electrician can handle it.

Phase shifting tanks

So, sometimes the question arises about how to connect a three-phase motor to a regular home outlet. If you just try to connect two wires to the plug, it will not rotate. In order for things to work, you need to simulate the phase by shifting the supplied voltage by some angle (preferably 120°). This effect can be achieved by using a phase-shifting element. Theoretically, this could be inductance or even resistance, but most often a three-phase motor in a single-phase network is switched on using electrical circuits designated by the Latin letter C on the diagrams.

As for the use of chokes, it is difficult due to the difficulty of determining their value (if it is not indicated on the device body). To measure the value of L, a special device or a circuit assembled for this purpose is required. In addition, the choice of available chokes is usually limited. However, any phase-shifting element can be selected experimentally, but this is a troublesome task.

What happens when you turn on the engine? Zero is applied to one of the connection points, phase is applied to the other, and a certain voltage is applied to the third, shifted by a certain angle relative to the phase. It is clear to a non-specialist that the operation of the engine will not be complete in terms of mechanical power on the shaft, but in some cases the very fact of rotation is sufficient. However, already at startup, some problems may arise, for example, the lack of an initial torque capable of moving the rotor from its place. What to do in this case?

Start capacitor

At the moment of starting, the shaft requires additional efforts to overcome the forces of inertia and static friction. To increase the torque, you should install an additional capacitor, connected to the circuit only at the moment of start, and then turned off. For these purposes, the best option is to use a locking button without fixing the position. The connection diagram for a three-phase motor with a starting capacitor is shown below, it is simple and understandable. At the moment the voltage is applied, press the “Start” button, and it will create an additional phase shift. After the engine spins up to the required speed, the button can (and even should) be released, and only the working capacity will remain in the circuit.

Calculation of container sizes

So, we found out that in order to turn on a three-phase motor in a single-phase network, an additional connection circuit is required, which, in addition to the start button, includes two capacitors. You need to know their value, otherwise the system will not work. First, let's determine the amount of electrical capacitance required to make the rotor move. When connected in parallel, it is the sum:

C = C st + Wed, where:

C st - starting additional capacity that can be switched off after takeoff;

C p is a working capacitor that provides rotation.

We also need the value of the rated current I n (it is indicated on the plate attached to the engine at the manufacturer). This parameter can also be determined using a simple formula:

I n = P / (3 x U), where:

U - voltage, when connected as a “star” - 220 V, and if connected as a “triangle”, then 380 V;

P is the power of a three-phase motor; sometimes, if the plate is lost, it is determined by eye.

So, the dependencies of the required operating power are calculated using the formulas:

C p = Wed = 2800 I n / U - for the “star”;

C p = 4800 I n / U - for a “triangle”;

The starting capacitor should be 2-3 times larger than the working capacitor. The unit of measurement is microfarads.

There is also a very simple way to calculate capacity: C = P /10, but this formula gives the order of the number rather than its value. However, in any case you will have to tinker.

Why adjustment is needed

The calculation method given above is approximate. Firstly, the nominal value indicated on the body of the electrical capacitance may differ significantly from the actual one. Secondly, paper capacitors (generally speaking, an expensive thing) are often second-hand, and they, like any other items, are subject to aging, which leads to an even greater deviation from the specified parameter. Thirdly, the current that will be consumed by the motor depends on the magnitude of the mechanical load on the shaft, and therefore it can only be assessed experimentally. How to do it?

This requires a little patience. The result can be a rather voluminous set of capacitors. The main thing is to secure everything well after finishing the work so that the soldered ends do not fall off due to vibrations emanating from the motor. And then it would be a good idea to analyze the result again and, perhaps, simplify the design.

Composing a battery of containers

If the master does not have at his disposal special electrolytic clamps that allow you to measure the current without opening the circuits, then you should connect an ammeter in series to each wire that enters the three-phase motor. In a single-phase network, the total value will flow, and by selecting capacitors one should strive for the most uniform loading of the windings. It should be remembered that when connected in series, the total capacitance decreases according to the law:

It is also necessary not to forget about such an important parameter as the voltage for which the capacitor is designed. It must be no less than the nominal value of the network, or better yet, with a margin.

Discharge resistor

The circuit of a three-phase motor connected between one phase and a neutral wire is sometimes supplemented with resistance. It serves to prevent the charge remaining on the starting capacitor from accumulating after the machine has already been turned off. This energy can cause an electric shock, which is not dangerous, but extremely unpleasant. In order to protect yourself, you should connect a resistor in parallel with the starting capacitance (electricians call this “bypassing”). The value of its resistance is large - from half a megohm to a megohm, and it is small in size, so half a watt of power is enough. However, if the user is not afraid of being “pinched,” then this detail can be completely dispensed with.

Using Electrolytes

As already noted, film or paper electrical containers are expensive, and purchasing them is not as easy as we would like. It is possible to make a single-phase connection to a three-phase motor using inexpensive and readily available electrolytic capacitors. At the same time, they won’t be very cheap either, since they must withstand 300 Volts of DC. For safety, they should be bypassed with semiconductor diodes (D 245 or D 248, for example), but it would be useful to remember that when these devices break through, alternating voltage will hit the electrolyte, and it will first heat up very much, and then explode, loudly and effectively. Therefore, unless absolutely necessary, it is still better to use paper-type capacitors that operate under either constant or alternating voltage. Some craftsmen completely allow the use of electrolytes in starting circuits. Due to short-term exposure to alternating voltage, they may not have time to explode. It's better not to experiment.

If there are no capacitors

Where do ordinary citizens who do not have access to in-demand electrical and electronic parts purchase them? At flea markets and flea markets. There they lie, carefully soldered by someone’s (usually elderly) hands from old washing machines, televisions and other household and industrial equipment that are out of use and out of use. They ask a lot for these Soviet-made products: sellers know that if a part is needed, they will buy it, and if not, they will not take it for nothing. It happens that just the most necessary thing (in this case, a capacitor) is just not there. So what should we do? No problem! Resistors will also do, you just need powerful ones, preferably ceramic and vitrified ones. Of course, ideal resistance (active) does not shift the phase, but nothing is ideal in this world, and in our case this is good. Every physical body has its own inductance, electrical power and resistivity, whether it is a tiny speck of dust or a huge mountain. Connecting a three-phase motor to a power outlet becomes possible if in the above diagrams you replace the capacitor with a resistance, the value of which is calculated by the formula:

R = (0.86 x U) / kI, where:

kI - current value for three-phase connection, A;

U - our trusty 220 Volts.

What engines are suitable?

Before purchasing a motor for a lot of money, which a zealous owner intends to use as a drive for a grinding wheel, circular saw, drilling machine or any other useful household device, it would not hurt to think about its applicability for these purposes. Not every three-phase motor in a single-phase network will be able to operate at all. For example, the MA series (it has a squirrel-cage rotor with a double cage) should be excluded so that you do not have to carry considerable and useless weight home. In general, it is best to experiment first or invite an experienced person, an electrician, for example, and consult with him before purchasing. A three-phase asynchronous motor of the UAD, APN, AO2, AO and, of course, A series is quite suitable. These indices are indicated on the nameplates.