When solving the power supply issues of a newly constructed building, its owner is faced with numerous tasks that need to be solved by technical and organizational means.
In this case, you should first decide on the required number of phases required to power electrical appliances. Usually people are content with single-phase power supply, and a certain category chooses three-phase, guided by the tasks facing them.
Comparison of the advantages and disadvantages of single-phase and three-phase connection at home
When choosing a circuit, you should take into account its influence on the wiring design and operating conditions created by different systems.
Power consumption
There is hope among individual homeowners that the transition to three-phase power will allow them to increase the permitted power consumption and use electricity more intensively. However, this issue must be resolved in the sales organization, which most likely no longer has extra reserves. Therefore, it is unlikely that it will be possible to significantly increase electricity consumption in this way.
The amount of permitted power that you will be provided with will become the basis for creating. Due to its distribution over two wires in a single-phase circuit, the cross-sectional thickness of the cable cores is always required greater than in a three-phase circuit, where the load is evenly distributed over three symmetrical circuits.
With the same power, lower rated currents will flow in each core of a three-phase circuit. They will require reduced ratings of circuit breakers. Despite this, their dimensions, like other protections and the electric meter, will still be larger due to the use of a triple design. A larger distribution board will be required. Its size can significantly limit the free space inside small rooms.
Three-phase consumers
Asynchronous electric motors of mechanical drives and other electrical appliances designed for operation in a three-phase network are more efficient and work optimally in it. To make them, it is necessary to create voltage converters that will consume additional energy. Moreover, in most cases there is a decrease in the efficiency of such mechanisms and power consumption on the converter.
The use of three-phase consumers is based on an even distribution of the load in each phase, and the connection of powerful single-phase devices can create a phase-by-phase imbalance of currents when some of them begin to flow through the working zero core.
If there is a large current imbalance in the overloaded phase, the voltage decreases: incandescent lamps begin to glow dimly, electronic devices malfunction, and electric motors perform worse. In this situation, owners of three-phase wiring can reconnect part of the load to an unloaded phase, and consumers of a two-wire circuit need to use voltage stabilizers or backup sources.
Operating conditions of electrical wiring insulation
Owners of a three-phase circuit must take into account the effect of a line voltage of 380, and not a phase voltage of 220 volts. Its rating poses a greater danger to humans and the insulation of electrical wiring or devices.
Equipment dimensions
Single-phase electrical wiring and all its components are more compact and require less installation space.
Based on a comparison of these characteristics, we can conclude that a three-phase connection of a private house can often be impractical in modern conditions. It makes sense to use it if there is a need to operate powerful three-phase consumers such as electric boilers or machine equipment for constant operation in certain seasons.
Most household electrical needs can easily be provided by single-phase electrical wiring.
How to make a three-phase connection to a private house
When the issue of three-phase connection of a private house is acute, you will have to:
1. prepare technical documentation;
2. solve technical issues.
What documents need to be prepared
Only the following certificates and passports can ensure the legality of a three-phase connection:
1. technical specifications from the energy supply organization;
2. project for the production of electrical power supply to the building;
3. act of differentiation by balance sheet;
4. protocols for measuring the main electrical parameters of the assembled house connection circuit by the electrical laboratory (installation is permitted after receipt of the first three documents) and an inspection report of electrical equipment;
5. conclusion of an agreement with an energy sales organization, giving the right to receive a power supply order.
Specifications
To obtain them, you must submit an application in advance to the power supply organization, which must reflect the requirements for the subscriber and the electrical installation, indicating:
connection methods;
use of protections;
locations of electrical appliances and switchboards;
restricting access of unauthorized persons;
load characteristics.
Power supply production project
It is developed by a design organization on the basis of current standards and operating rules for electrical installations in order to provide a team of electricians with detailed information on the technology of installing an electrical circuit.
The project includes:
1. explanatory note with the report;
2. executive circuit and installation diagrams;
3. statements;
4. requirements of regulatory documents and regulations.
Act of differentiation by balance sheet
The boundaries of responsibility between the power supply organization and the consumer are determined, the permitted power, the reliability category of the power receiver, the power supply circuit, and some other information are indicated.
Protocols of electrical measurements
They are carried out by the electrical measuring laboratory after complete completion of installation work. In case of receiving positive measurement results reflected in the protocols, an equipment inspection report is provided with a conclusion giving the right to contact the electricity sales organization.
Agreement with energy sales
After its conclusion, based on documents from the electrical laboratory, you can contact the electricity supply organization to include the installed electrical installation in work according to a special order.
Technical issues of three-phase connection of a private house
The principle of supplying electrical energy to a detached residential building is carried out according to the following principle: voltage is supplied from the transformer substation via the power line through four wires, including three phases (L1, L2, L3) and one common neutral conductor PEN. Such a system is carried out according to the method that is still most widespread in our country.
The power line can most often be overhead or, less commonly, cable. Faults can occur on both structures, which can be resolved more quickly with overhead power lines.
Features of PEN conductor separation
The energy industry is gradually beginning to modernize old power transmission lines and transfer them to the new TN-C-S standard, and those under construction are immediately built according to TN-S standards. In it, the fourth conductor PEN from the supply substation is supplied not with one, but with two branched conductors: PE and N. As a result, these circuits already use five conductors for conductors.
The three-phase connection of a private house is based on the fact that all these conductors are connected to the input device of the building, and from it the electricity is supplied to the electric meter and then to the distribution board for internal distribution to the premises and consumers of the building.
Almost all household appliances operate on a phase voltage of 220 volts, which is present between the working zero N and one of the potential conductors L1, L2 or L3. And between the linear wires a voltage of 380 volts is formed.
Inside the input device using the TN-C-S standard, the working zero N and the protective PE are separated from the PEN conductor, which is connected here to the main grounding bus. It is connected to the repeated grounding circuit of the building.
All connections of conductors on the GZSh are made with a bolted connection with washers and nuts, firmly tightening the threaded connection. This achieves a minimum value of the transient electrical resistance at the junction of the contacts. Each cable is connected to a separate mounting hole for convenient disconnection of the circuit in order to carry out various measurements.
The main material for GZSh is copper, and in some cases it is possible to use steel alloys. The use of aluminum for the main protective busbar is prohibited. You cannot install lugs made of aluminum alloys on the wires connected to it.
From the input device, the working and protective zeros come in isolated chains, which are prohibited from being combined at any other point in the electrical wiring diagram.
According to the old rules in force in the TN-C grounding circuit, the PEN conductor was not split, and the phase voltage was taken directly between it and one of the linear potentials.
The final distance of the line between its support before entering the house is laid through the air or underground. It's called a branch. It is on the balance sheet of the electricity supply organization, and not the owner of the residential building. Therefore, all work to connect a house on this site must be carried out with the knowledge and decision of the owner of the power line. Accordingly, by law they will require approval and payment.
For an underground cable line, the branch is mounted in a metal cabinet, which is placed close to the route, and for overhead power lines - directly on the support. In both cases, it is important to ensure the safety of their operation, block access to unauthorized people and provide reliable protection against damage by vandals.
Selecting the location for splitting the PEN conductor
It can be done:
1. on the nearest support;
2. or on the input panel located on the wall or inside the house.
In the first case, the responsibility for safe operation lies with the electricity supply organization, and in the second, with the owner of the building. Access for residents of the house to work at the end of the PEN conductor located on the support is prohibited by the rules.
It should be taken into account that the wires on the overhead line can break for various reasons and malfunctions may occur on them. During an accident on the supply power line with a break in the PEN conductor, its current will flow through the wire connected to the additional ground loop. Its material and cross-section must reliably withstand such increased power. Therefore, they are chosen no thinner than the main core of the power line.
When splitting is performed directly on the support, a line called re-grounding is laid to it and the circuit. It is convenient to make it from a metal strip buried 0.3÷1 m into the ground.
Since it creates a path for lightning to flow into the ground during a thunderstorm, it must be diverted away from paths and places where people may be accommodated. It is rational to lay it under the fence of the building and in similar hard-to-reach places, and make all connections by welding.
When splitting is carried out in the water shield of a building, emergency currents will flow through the branch line with connected wires, which only conductors with the cross-section of the phase conductors of the power line can withstand.
Electricity input distribution device
It differs from a simple input device in that its design includes elements that distribute electricity among consumer groups inside the building. It is mounted at the electrical cable input in an extension or some separate room.
The ASU is installed inside a metal cabinet, where all three phases, a PEN conductor and a re-grounding circuit bus are connected in the building connection diagram using the TN-C-S system.
For TN-S, five wires are inserted into the input distribution cabinet - three phases and two zeros: working and protective, as shown in the picture below.
Inside the input switchgear cabinet, the phase conductors are connected to the terminals of the input circuit breaker or power fuses, and the PEN conductor is connected to its busbar. Through it, it is split into PE and N with the formation of the main grounding bus and its connection to the repeated grounding loop.
Voltage increase limiters operate on a pulse principle, protect the circuit diagram of phases and working zero from the effects of possible penetration of extraneous external discharges, divert them through the PE conductor and the main protective bus with a ground loop to the ground potential.
When high-voltage pulse discharges of high power occur in the supply line and pass through a serial chain of a circuit breaker and an SPD, it is quite possible that the power contacts of the machine will fail due to burning and even welding.
Therefore, the protection of this chain with powerful fuses, carried out by simply burning out the fuse link, remains relevant and is widely used in practice.
A three-phase electric meter takes into account the power consumed. After this, the connected loads are distributed among consumption groups through properly selected circuit breakers and residual current devices. There may also be an additional RCD at the input, which performs fire-fighting functions for all electrical wiring of the building.
After each group of RCDs, consumers can be further divided by degree of protection with individual circuit breakers or dispensed with, as shown in different sections in the diagram.
Cables going to groups of end consumers are connected to the output terminals of the shield and protection.
Branch design features
Most often, the three-phase connection of a private house to the supply power line is carried out by an overhead line, on which a short circuit or break may occur. To prevent them, you should pay attention to:
the overall mechanical strength of the created structure;
quality of insulation of the outer layer;
material of current-carrying conductors.
Modern self-supporting aluminum cables are lightweight and have good conductive properties. They are well suited for installing an air branch. With three-phase power supply to consumers, a SIP core cross-section of 16 mm2 will be sufficient for long-term production of 42 kW, and 25 mm2 - 53 kW.
When a branch is made using an underground cable, pay attention to:
the configuration of the route being laid, its inaccessibility to damage by unauthorized people and machinery when working in the ground;
protection of the ends coming out of the ground with metal pipes to a height no less than average human height. The best option is to completely place the cable in the pipe up to the entry into the control unit and the distribution cabinet.
For underground installation, use only a single piece of cable with strong armor tape or protect it with pipes or metal boxes. In this case, copper conductors are preferable to aluminum ones.
The technical aspects of three-phase connection of a private house in most cases require greater costs and effort than with a single-phase circuit.
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, a model 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.
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 V, 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:
- U - network voltage, V.
And in the case of connecting the motor windings in a “star”, it is determined by the formula:
- Ср - capacity of the working capacitor, in μF;
- I is the current consumed by the electric motor, in A;
- U - network voltage, V.
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:
- P - engine power, in W, indicated in its passport;
- h - efficiency;
- cos j - power factor;
- U - network voltage, V.
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 such as K50-3, EGC-M, KE-2 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 leads together, and shunted with diodes (Fig. 2)
The total capacitance of the connected capacitors will be:
In practice, the capacitance values of the working and starting capacitors are selected depending on the engine power. 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.
Three-phase power
engine, kW:
- 0,4;
- 0,6;
- 0,8;
- 1,1;
- 1,5;
- 2,2.
Minimum worker capacity
capacitor Cp, µF:
- 100;
- 150;
- 230.
Minimum starting capacity
capacitor Cp, µF:
- 120;
- 160;
- 200;
- 250;
- 300.
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, which is 20...30% higher than the rated one. In this regard, if the engine is often used in underloaded mode or idling, 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.
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 a 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.
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):
- 1- body;
- 2 - carrying handle;
- 3 - signal lamp;
- 4 - toggle switch to turn off the starting capacitor;
- 5 - “Start” and “Stop” buttons;
- 6 - modified electric plug;
- 7- panel with connector sockets.
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 there is a connector for.
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. KM1 is self-blocking 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.
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.
The replacement diagram for a conventional paper capacitor 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 circuit, you can use an electrolytic capacitor with a voltage of 200 V. In the above circuit, the capacitances of both capacitors are the same and are selected in the same way as the method for selecting paper capacitors for starting device.
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 engine 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 the fact that if the diode is overloaded, its breakdown may occur and alternating current will flow through the electrolytic capacitor, which can lead to its heating and explosion.
Connecting 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 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.
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.
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, after which further work is continued.
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.
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 reach 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.
In the household, sometimes there is a need to run a 3-phase asynchronous electric motor (AM). If you have a 3-phase network, this is not difficult. In the absence of a 3-phase network, the engine can be started from a single-phase network by adding capacitors to the circuit.
Structurally, the IM consists of a stationary part - the stator, and a moving part - the rotor. Windings are placed in slots on the stator. The stator winding is a three-phase winding, the conductors of which are evenly distributed around the circumference of the stator and laid in phases in slots with an angular distance of 120 el. degrees. The ends and beginnings of the windings are led out into the junction box. The windings form pairs of poles. The rated rotor speed of the motor depends on the number of pole pairs. Most general industrial motors have 1-3 pairs of poles, less often 4. IMs with a large number of pole pairs have low efficiency, larger dimensions, and therefore are rarely used. The more pole pairs, the lower the motor rotor speed. General industrial motors are produced with a number of standard rotor speeds: 300, 1000, 1500, 3000 rpm.
The rotor of the IM is a shaft on which there is a short-circuited winding. In low- and medium-power motors, the winding is usually made by pouring molten aluminum alloy into the grooves of the rotor core. Together with the rods, short-circuited rings and end blades are cast, which ventilate the machine. In high-power machines, the winding is made of copper rods, the ends of which are connected to short-circuited rings by welding.
When the IM is turned on in a 3-phase network, current begins to flow through the windings in turn at different times. In one period of time, the current passes along the pole of phase A, in another along the pole of phase B, in the third along the pole of phase C. Passing through the poles of the windings, the current alternately creates a rotating magnetic field that interacts with the rotor winding and causes it to rotate, as if pushing it in different planes at different times.
If you turn on the IM in a 1-phase network, the torque will be created by only one winding. Such a moment will act on the rotor in one plane. This moment is not enough to move and rotate the rotor. To create a phase shift of the pole current relative to the supply phase, phase-shifting capacitors are used in Fig. 1.
Capacitors can be used of any type, except electrolytic. Capacitors such as MBGO, MBG4, K75-12, K78-17 are well suited. Some capacitor data is shown in Table 1.
If it is necessary to gain a certain capacitance, then the capacitors should be connected in parallel.
The main electrical characteristics of the IM are given in the data sheet, Fig. 2.
Fig.2
From the passport it is clear that the motor is three-phase, with a power of 0.25 kW, 1370 rpm, it is possible to change the winding connection diagram. The connection diagram for the windings is “triangle” at a voltage of 220V, “star” at a voltage of 380V, respectively, the current is 2.0/1.16A.
The star connection diagram is shown in Fig. 3. With this connection, a voltage is supplied to the electric motor windings between points AB (linear voltage U l) that is times greater than the voltage between points AO (phase voltage U f).
Fig.3 Star connection diagram.
Thus, the linear voltage is several times greater than the phase voltage: . In this case, the phase current I f is equal to the linear current I l.
Let's look at the triangle connection diagram in Fig. 4:
Fig.4 Delta connection diagram
With such a connection, the linear voltage U L is equal to the phase voltage U f., and the current in the line I l is times greater than the phase current I f:.
Thus, if the IM is designed for a voltage of 220/380 V, then to connect it to a phase voltage of 220 V, a “triangle” connection diagram for the stator windings is used. And for connecting to a linear voltage of 380 V - a star connection.
To start this IM from a single-phase network with a voltage of 220V, we should turn on the windings according to the “delta” circuit, Fig. 5.
Fig.5 Connection diagram of the EM windings according to the “triangle” diagram
The connection diagram of the windings in the output box is shown in Fig. 6
Fig.6 Connection in the ED output box according to the “triangle” diagram
To connect an electric motor according to the “star” circuit, it is necessary to connect two phase windings directly to a single-phase network, and the third through a working capacitor C p to any of the network wires in Fig. 6.
The connection in the terminal box for the star circuit is shown in Fig. 7.
Fig. 7 Connection diagram of the EM windings according to the “star” scheme
The connection diagram of the windings in the output box is shown in Fig. 8
Fig.8 Connection in the ED output box according to the “star” scheme
The capacity of the working capacitor C p for these circuits is calculated by the formula:
,
where I n - rated current, U n - rated operating voltage.
In our case, to switch on the “triangle” circuit, the capacitance of the working capacitor is C p = 25 µF.
The operating voltage of the capacitor should be 1.15 times the rated voltage of the supply network.
To start an IM of small power, a working capacitor is usually sufficient, but with a power of more than 1.5 kW, the engine either does not start or picks up speed very slowly, so it is necessary to also use a starting capacitor C p. The capacity of the starting capacitor should be 2.5-3 times greater than the capacity of the working capacitor capacitor.
The connection diagram of the electric motor windings connected in a delta pattern using starting capacitors C p is shown in Fig. 9.
Fig. 9 Connection diagram of the EM windings according to the “triangle” diagram using starting condensates
The connection diagram of the star motor windings using starting capacitors is shown in Fig. 10.
Fig. 10 Connection diagram of the EM windings according to the “star” circuit using starting capacitors.
Starting capacitors C p are connected in parallel to the working capacitors using the KN button for a time of 2-3 s. In this case, the rotation speed of the electric motor rotor should reach 0.7…0.8 of the rated rotation speed.
To start the IM using starting capacitors, it is convenient to use the button Fig. 11.
Fig.11
Structurally, the button is a three-pole switch, one pair of contacts of which closes when the button is pressed. When released, the contacts open, and the remaining pair of contacts remains on until the stop button is pressed. The middle pair of contacts performs the function of a KN button (Fig. 9, Fig. 10), through which starting capacitors are connected, the other two pairs act as a switch.
It may turn out that in the connection box of the electric motor the ends of the phase windings are made inside the motor. Then the IM can only be connected according to the diagrams in Fig. 7, Fig. 10, depending on power.
There is also a diagram for connecting the stator windings of a three-phase electric motor - partial star Fig. 12. Making a connection according to this diagram is possible if the beginnings and ends of the stator phase windings are brought out into the junction box.
Fig.12
It is advisable to connect an electric motor according to this scheme when it is necessary to create a starting torque exceeding the nominal one. This need arises in drives of mechanisms with difficult starting conditions, when starting mechanisms under load. It should be noted that the resulting current in the supply wires exceeds the rated current by 70-75%. This must be taken into account when choosing the wire cross-section for connecting the electric motor.
Capacitance of the working capacitor C p for the circuit in Fig. 12 is calculated by the formula:
.
The capacitance of starting capacitors should be 2.5-3 times greater than the capacitance C r. The operating voltage of the capacitors in both circuits should be 2.2 times the rated voltage.
Typically, the terminals of the stator windings of electric motors are marked with metal or cardboard tags indicating the beginnings and ends of the windings. If for some reason there are no tags, proceed as follows. First, the belonging of the wires to the individual phases of the stator winding is determined. To do this, take any of the 6 external terminals of the electric motor and connect it to any power source, and connect the second terminal of the source to the control light and, with the second wire from the lamp, alternately touch the remaining 5 terminals of the stator winding until the light comes on. When the light comes on, it means that the 2 terminals belong to the same phase. Conventionally, let's mark the beginning of the first wire C1 with tags, and its end - C4. Similarly, we will find the beginning and end of the second winding and designate them C2 and C5, and the beginning and end of the third - C3 and C6.
The next and main stage will be to determine the beginning and end of the stator windings. To do this, we will use the selection method, which is used for electric motors with a power of up to 5 kW. Let's connect all the beginnings of the phase windings of the electric motors according to the previously connected tags to one point (using a star circuit) and connect the electric motor to a single-phase network using capacitors.
If the engine immediately picks up the rated speed without a strong hum, this means that all the beginnings or all ends of the winding have hit the common point. If, when turned on, the engine hums strongly and the rotor cannot reach the rated speed, then terminals C1 and C4 in the first winding should be swapped. If this does not help, the ends of the first winding must be returned to their original position and now the terminals C2 and C5 are swapped. Do the same; for the third pair if the engine continues to hum.
When determining the beginnings and ends of windings, strictly adhere to safety regulations. In particular, when touching the stator winding clamps, hold the wires only by the insulated part. This must also be done because the electric motor has a common steel magnetic core and a large voltage may appear at the terminals of other windings.
To change the direction of rotation of the rotor of an IM connected to a single-phase network according to the “triangle” circuit (see Fig. 5), it is enough to connect the third phase winding of the stator (W) through a capacitor to the terminal of the second phase winding of the stator (V).
To change the direction of rotation of an IM connected to a single-phase network according to the “star” circuit (see Fig. 7), you need to connect the third phase winding of the stator (W) through a capacitor to the terminal of the second winding (V).
When checking the technical condition of electric motors, you can often notice with disappointment that after prolonged operation, extraneous noise and vibration appear, and the rotor is difficult to turn manually. The reason for this may be the poor condition of the bearings: the treadmills are covered with rust, deep scratches and dents, individual balls and the cage are damaged. In all cases, it is necessary to inspect the electric motor and eliminate any existing faults. In case of minor damage, it is enough to wash the bearings with gasoline and lubricate them.
Among the different methods of starting three-phase electric motors in a single-phase network, the more common one is based on connecting the third winding through a phase-shifting capacitor. The required 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 section of a 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-started 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 they use two-stage engine control. When starting the motor, two capacitors are connected, and after acceleration, one capacitor is disconnected and only the working capacitor is left.
1.2. Calculation of characteristics and parts of an electric motor.
If, for example, the 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
Scheme for connecting a three-phase electric motor to a 220 V network
C r – working capacitor;
C p – starting capacitor;
P1 – packet switch
After turning on the batch switch P1, contacts P1.1 and P1.2 close, after which you need to 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 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:
, Where
U - network voltage, V
And in the case of connecting the motor windings in a “star”, it is determined by the formula:
, Where
Ср – capacity of the working capacitor in μF;
I – current consumed by the electric motor in A;
U - network voltage, V
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:
, Where
P – motor power in W, indicated in its passport;
h – efficiency;
cos j – power factor;
U - network voltage, V
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 more than 450 V can be used as starting capacitors. For greater reliability, electrolytic capacitors are connected alternately, connecting their negative leads together, and are shunted with diodes (Fig. 2)
Connection diagram of electrolytic capacitors for use as starting capacitors.
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 motor 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 depends on its power when connected to a 220 V network.
It must be emphasized that in an electric motor with capacitor starting in idle mode, a current flows through the winding fed through the capacitor by 20...30% exceeding the rated one. In this regard, if the engine is often used in an underloaded mode or idling, then in this case the capacitance of the capacitor Cp should be reduced. It may happen that during an overload the electric motor slowed down, then to start it, the starting capacitor is connected again, removing the load completely or reducing it to a minimum.
The capacity of the starting capacitor Cn 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 different 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)
Scheme of a portable universal unit for starting three-phase electric motors with a power of about 0.5 kW from a 220 V network without reverse.
When you press the SB1 button, the magnetic starter KM1 is triggered (switch SA1 is closed) and its own contact system KM 1.1, KM 1.2 connects the electric motor M1 to the 220 V network. Immediately with this, the 3rd contact group KM 1.3 closes the SB1 button. After the motor has fully accelerated, switch SA1 turns off the starting capacitor C1. The motor 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. Switch T2-1 is used as toggle 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 an iron case measuring 170x140x50 mm (Fig. 4) 
1 – body
2 – carrying handle
3 – signal lamp
4 – start capacitor disconnect switch
5 – “Start” and “Stop” buttons
6 – modified electric plug
7 – panel with connector sockets
On the top panel of the case there are “Start” and “Stop” buttons - a warning light and a 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 be turned off automatically (Fig. 5)
Circuit diagram of a starting device with automatic shutdown of the starting capacitor.
When you press the SB1 button, relay K1 is activated and the contact pair K1.1 turns on the magnetic starter KM1, and K1.2 turns on the starting capacitor Sp. The magnetic starter KM1 is self-blocking using its own contact pair KM 1.1, and contacts KM 1.2 and KM 1.3 connect the electric motor to the network. Keep the “Start” button pressed until the engine fully accelerates, and then release it. Relay K1 is de-energized and turns off the starting capacitor, which is discharged through resistor R2. At this time, the magnetic starter KM 1 remains turned 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 something similar.
2. Introduction of electrolytic capacitors in electric motor starting circuits.
When connecting three-phase asynchronous electric motors to a single-phase network, simple paper capacitors are usually used. But practice has shown that instead of massive paper capacitors, you can use oxide (electrolytic) capacitors, which have the smallest dimensions and are more affordable to purchase. An equivalent replacement diagram for a conventional paper capacitor is shown in Fig. 6
Scheme of replacing a paper capacitor (a) with an electrolytic one (b, c).
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 similar and are selected in the same way as the selection method paper 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 with the introduction of electrolytic capacitors is shown in Fig. 7.
Scheme for connecting a three-phase motor to a single-phase network using electrolytic capacitors.
In the above diagram, SA1 is the toggle switch for the direction of rotation of the motor, SB1 is the motor acceleration button, electrolytic capacitors C1 and C3 are used to start the motor, C2 and C4 are used during operation.
Selection of electrolytic capacitors in the circuit shown in Fig. 7 is best created using current clamps. The currents are determined at points A, B, C and equality of currents at these points is achieved by the method of stepwise selection of capacitor capacitances. Measurements are carried out with the engine loaded in the mode in which its operation is intended. Diodes VD1 and VD2 for a 220 V network are selected with a very permissible reverse voltage of more than 300 V. The maximum forward current of the diode depends on the power of the motor. 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. For higher motor power from 1 kW to 2 kW, you need to take large diodes with a suitable direct current, or put several smaller diodes in parallel, installing them on radiators.
Should be paid ATTENTION the fact that if the diode is overloaded, its 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 allows you to receive less 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 obviously not enough to operate an electric planer or electric saw, which must have a power of 1.5...2 kW. The problem in this case can be solved by introducing 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 unusable for operation, but can be used to spin the rotor idle or with a low load. Practice indicates that most electric motors confidently accelerate to rated speed, and in this case the starting currents do not exceed 20 A.
3.1. Refinement of a three-phase motor.
It is easier to convert a powerful three-phase motor into operating mode by converting it to a single-phase operating mode, while receiving 50% of the rated power. Switching the motor to single-phase mode requires 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 motor housing. Find the place where the 3 windings are connected to a common point and solder an additional conductor with a cross-section appropriate 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 which the housing cover is installed in place.
The electric motor switching circuit in this case will have the form shown in Fig. 8.
Switching diagram of the windings of a three-phase electric motor for inclusion in a single-phase network.
During acceleration of the motor, 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 toggle switch for an operating current of more than 16 A should be used as a switching device SA1, for example, a toggle switch of type PP2-25/N3 (two-pole with neutral, for a current of 25 A). The SA2 toggle switch can be of any type, but for a current of more than 16 A. If motor reverse is not required, then this SA2 toggle switch 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 motor power, then the shaft rotation speed may decrease until it stops completely. In this case, the load is removed from the motor shaft. The toggle switch is first moved to the “Acceleration” position, and later to the “Work” position and subsequent work is continued.
