In this article, I would like to tell you how to easily and quickly run a three-phase motor into a single-phase network. Many people had old 3-phase motors lying around in their dacha or garage, taken from somewhere, but it was impossible to connect them due to the lack of 380V voltage. A simple “capacitor” can replace a 380V line (for engines only).
And so what we need to know and have to start the engine:
We need to decide how we will connect the stator windings: first way star, with this connection the engine will only produce 50% from its power, second method triangle he produces much better 70% depending on the engine power, so I often choose the second method to achieve the most power.
When connecting the stator windings with a triangle a motor whose power is less than or equal to 1.5 kW and starts without load power, the circuit has the following form:
If the engine has a power of more than 1.5 kW or has load power at start, then starting capacitors must also be added to the circuit.
If you need to start the engine in different directions, using scientific reverse, you need to add a toggle switch as in the picture:
What capacitors and how many are needed to start the engine:
Brands of capacitors MBGO, MBGCh, BGT, MBPG with an operating voltage of at least 300V, you can find this data on the capacitor itself.
But simply taking and connecting any capacitor of these brands will not work; each capacitor has a capacitance measured in microfarads, so we need to calculate what capacity we will need for the working capacitors and separately for the starting capacitors using the following formula:
For workers Ср=4800х(I/U)
For launchers Sp=Срх (2 or 3 times)
It is important to know that we calculate operating capacitors for the rated power of the engine, and when the engine is not fully loaded, it will heat up and you will have to turn off some of the capacitors to reduce the current in the stator windings, but reducing the capacitor will lead to a decrease in the developing power. Also, running the engine at idle for a long time can lead to its combustion. As for capacitors, remember that a capacitor is a capacity that can hold a large voltage that is dangerous to human health, also when starting powerful engines of more than three kW, make sure that the wiring will withstand a long time engine operation and will not fail.
Asynchronous three-phase motors are common in production and everyday life. The peculiarity is that they can be connected to both three-phase and single-phase networks. In the case of single-phase motors, this is impossible: they only operate when powered by 220V. What are the ways to connect a 380 Volt motor? Let's look at how to connect stator windings depending on the number of phases in the power supply using illustrations and a training video.
There are two basic schemes (video and diagrams in the next subsection of the article):
- triangle,
- star.
The advantage of a delta connection is that it operates at maximum power. But when the electric motor is turned on, high starting currents are produced in the windings, which are dangerous for equipment. When connected by a star, the motor starts smoothly, since the currents are low. But it will not be possible to achieve maximum power.
In connection with the above, motors when powered by 380 Volts are connected only by a star. Otherwise, high voltage when switched on by a delta can develop such inrush currents that the unit will fail. But under high load, the output power may not be enough. Then they resort to a trick: they start the engine with a star for safe inclusion, and then switch from this circuit to a delta for gaining high power.
Triangle and star
Before we look at these diagrams, let's agree:
- The stator has 3 windings, each of which has 1 beginning and 1 end. They are brought out in the form of contacts. Therefore, for each winding there are 2 of them. We will designate: winding - O, end - K, beginning - N. In the diagram below there are 6 contacts, numbered from 1 to 6. For the first winding, the beginning is 1, the end is 4. According to the accepted notation, this is HO1 and KO4. For the second winding - NO2 and KO5, for the third - HO3 and KO6.
- There are 3 phases in the 380 Volt electrical network: A, B and C. Let’s leave their symbols the same.
When connecting the windings of an electric motor with a star, first connect all the beginnings: HO1, HO2 and HO3. Then KO4, KO5 and KO6 are respectively supplied with power from A, B and C.
When connecting an asynchronous electric motor with a triangle, each beginning is connected to the end of the winding in series. The choice of the order of winding numbers is arbitrary. It may turn out: NO1-KO5-NO2-KO6-NO3-KO2.
Star and delta connections look like this:
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.
Not every average person understands what electrical circuits are. In apartments they are 99% single-phase, where the current flows to the consumer through one wire and returns through the other (zero). A three-phase network is a system for transmitting electric current that flows through three wires and returns one at a time. Here the return wire is not overloaded due to the phase shift of the current. Electricity is generated by a generator driven by an external drive.
An increase in the load in the circuit leads to an increase in the current passing through the generator windings. As a result, the magnetic field resists rotation of the drive shaft to a greater extent. The number of revolutions begins to decrease and commands an increase in drive power, for example by supplying more fuel to the internal combustion engine. The speed is restored and more electricity is generated.
A three-phase system consists of 3 circuits with an EMF of the same frequency and a phase shift of 120°.
Features of connecting power to a private home
Many people believe that a three-phase network in the house increases power consumption. In fact, the limit is set by the electricity supply organization and is determined by the following factors:
- supplier capabilities;
- number of consumers;
- condition of the line and equipment.
To prevent voltage surges and phase imbalance, they should be loaded evenly. The calculation of a three-phase system is approximate, since it is impossible to accurately determine which devices will be connected at a given moment. The presence of pulsed devices currently leads to increased energy consumption during their startup.
The electrical distribution panel for a three-phase connection is larger in size than for a single-phase supply. Options are possible with the installation of a small input panel, and the rest made of plastic for each phase and for outbuildings.
Connection to the main line is carried out using underground and overhead lines. Preference is given to the latter due to the small amount of work, low connection cost and ease of repair.
Nowadays it is convenient to make an air connection using a self-supporting insulated wire (SIP). The minimum cross-section of the aluminum core is 16 mm 2, which is sufficient for a private home.
The SIP is attached to the supports and the wall of the house using anchor brackets with clamps. The connection to the main overhead line and the input cable to the electrical panel of the house is made with branch piercing clamps. The cable is taken with non-combustible insulation (VVGng) and passed through a metal pipe inserted into the wall.
Air connection of three-phase power supply at home
At a distance from the nearest support, it is more necessary to install another pole. This is necessary to reduce loads that lead to sagging or broken wires.
The height of the connection point is 2.75 m and above.
Electrical distribution cabinet
Connection to a three-phase network is made according to the project, where inside the house consumers are divided into groups:
- lighting;
- sockets;
- separate powerful devices.
Some loads can be disconnected for repairs while others are running.
The power of consumers is calculated for each group, where the wire of the required cross-section is selected: 1.5 mm 2 - for lighting, 2.5 mm 2 - for sockets and up to 4 mm 2 - for powerful devices.
The wiring is protected from short circuits and overloads by circuit breakers.
Electric meter
For any connection scheme, a metering device is required. A 3-phase meter can be connected directly to the network (direct connection) or through a voltage transformer (semi-indirect), where the meter readings are multiplied by a coefficient.
It is important to follow the connection order, where odd numbers are power and even numbers are load. The color of the wires is indicated in the description, and the diagram is located on the back cover of the device. The input and corresponding output of a 3-phase meter are indicated by the same color. The most common connection order is when the phases come first and the last wire is zero.
A 3-phase direct connection meter for a home is usually designed for a power of up to 60 kW.
Before choosing a multi-tariff model, you should coordinate the issue with the energy supply company. Modern devices with tarifficators make it possible to calculate electricity charges depending on the time of day, register and record power values over time.
The temperature readings of the devices are selected as widely as possible. On average they range from -20 to +50 °C. The service life of the devices reaches 40 years with a calibration interval of 5-10 years.
The meter is connected after the input three- or four-pole circuit breaker.
Three-phase load
Consumers include electric boilers, asynchronous electric motors and other electrical appliances. The advantage of using them is the uniform distribution of the load in each phase. If a three-phase network contains unevenly connected single-phase powerful loads, this can lead to phase imbalance. At the same time, electronic devices begin to malfunction, and lighting lamps glow dimly.
Connection diagram of a three-phase motor to a three-phase network
The operation of three-phase electric motors is characterized by high performance and efficiency. No additional starting devices are required here. For normal operation, it is important to connect the device correctly and follow all recommendations.
The connection diagram of a three-phase motor to a three-phase network creates a rotating magnetic field with three windings connected in a star or delta.
Each method has its own advantages and disadvantages. The star circuit allows the engine to start smoothly, but its power is reduced by up to 30%. This loss is absent in the delta circuit, but the current load is significantly greater at start-up.
The motors have a connection box where the winding terminals are located. If there are three of them, then the circuit is connected only by a star. With six terminals, the motor can be connected in any way.
Power consumption
It is important for the home owner to know how much energy is consumed. This is easy to calculate for all electrical appliances. Adding up all the powers and dividing the result by 1000, we get the total consumption, for example 10 kW. For household electrical appliances, one phase is sufficient. However, current consumption increases significantly in a private home where there is powerful equipment. One device can have 4-5 kW.
It is important to plan the power consumption of a three-phase network at the design stage in order to ensure symmetry in voltages and currents.
A four-wire wire with three phases and a neutral enters the house. The voltage of the electrical network is Between the phases and the neutral wire, electrical appliances are connected to In addition, there may be a three-phase load.
The power calculation of a three-phase network is carried out in parts. First, it is advisable to calculate purely three-phase loads, for example a 15 kW electric boiler and a 3 kW asynchronous electric motor. The total power will be P = 15 + 3 = 18 kW. In this case, current I = Px1000/(√3xUxcosϕ) flows in the phase wire. For household electrical networks cosϕ = 0.95. Substituting numerical values into the formula, we obtain the current value I = 28.79 A.
Now you need to define single-phase loads. Let them be P A = 1.9 kW, P B = 1.8 kW, P C = 2.2 kW for the phases. The mixed load is determined by summation and is 23.9 kW. The maximum current will be I = 10.53 A (phase C). Adding it to the current from the three-phase load, we get I C = 39.32 A. The currents in the remaining phases will be I B = 37.4 kW, I A = 37.88 A.
When calculating the power of a three-phase network, it is convenient to use power tables taking into account the type of connection.
Using them it is convenient to select circuit breakers and determine wiring cross-sections.
Conclusion
With proper design and maintenance, a three-phase network is ideal for a private home. It allows you to evenly distribute the load across phases and connect additional power from electrical consumers, if the wiring cross-section allows.
The magnetic starter has power contacts designed for switching circuits under load and block contacts which are used in control circuits.
Contacts are divided into normally open- contacts that are in their normal position, i.e. before applying voltage to the coil of the magnetic starter or before mechanical impact on them, are in an open state and normally closed- which in their normal position are in a closed state.
The new magnetic starters have three power contacts and one normally open block contact. If it is necessary to have a larger number of block contacts (for example, during assembly), an attachment with additional block contacts (contact block) is additionally installed on the magnetic starter on top, which, as a rule, has four additional block contacts (for example, two normally closed and two normally open).
Buttons for controlling an electric motor are included in push-button stations; push-button stations can be one-button, two-button, three-button, etc.
Each button of the push-button post has two contacts - one of them is normally open, and the second is normally closed, i.e. Each of the buttons can be used both as a “Start” button and as a “Stop” button.
Electric motor direct connection diagram
This diagram is the simplest diagram for connecting an electric motor; it does not have a control circuit, and the electric motor is turned on and off by an automatic switch.
The main advantages of this scheme are its low cost and ease of assembly, but the disadvantages of this scheme include the fact that circuit breakers are not intended for frequent switching of circuits; this, in combination with inrush currents, leads to a significant reduction in the service life of the machine; in addition, this scheme does not include Possibility of additional motor protection.
Connection diagram for an electric motor via a magnetic starter
This scheme is also often called simple motor starting circuit, in it, unlike the previous one, in addition to the power circuit, a control circuit also appears.
When you press the SB-2 button (the “START” button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 when the button is released SB-2 its contact opens again, but the coil of the magnetic starter is not de-energized, because its power will now be provided through the KM-1.1 block contact (i.e. the KM-1.1 block contact bypasses the SB-2 button). Pressing the SB-1 button (the “STOP” button) leads to a break in the control circuit, de-energizing the magnetic starter coil, which leads to the opening of the magnetic starter contacts and, as a result, to stopping the electric motor.
Reversible motor connection diagram (How to change the direction of rotation of an electric motor?)
To change the direction of rotation of a three-phase electric motor, you need to swap any two phases supplying it:
If it is necessary to frequently change the direction of rotation of the electric motor, the following is used:
This circuit uses two magnetic starters (KM-1, KM-2) and a three-button post; the magnetic switches used in this circuit, in addition to a normally open block contact, must also have a normally closed contact.
When you press the SB-2 button (START 1 button), voltage is applied to the coil of the magnetic starter KM-1, while the starter closes its power contacts KM-1 starting the electric motor, and also closes its block contact KM-1.1 which bypasses the button SB-2 and opens its block contact KM-1.2 which protects the electric motor from turning on in the opposite direction (when the SB-3 button is pressed) until it stops first, because An attempt to start the electric motor in the opposite direction without first disconnecting the KM-1 starter will result in a short circuit. To start the electric motor in the opposite direction, you need to press the “STOP” button (SB-1), and then the “START 2” button (SB-3), which will power the coil of the KM-2 magnetic starter and start the electric motor in the opposite direction.
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