When operating or manufacturing this or that equipment, it often becomes necessary to connect an asynchronous three-phase motor to a regular 220 V network. This is quite realistic and not even particularly difficult, the main thing is to find a way out of the following possible situations if there is no suitable single-phase motor, and a three-phase one is lying without business, and also if there is three-phase equipment, but in the workshop there is only a single-phase network.
To begin with, it makes sense to recall the diagram for connecting a three-phase motor to a three-phase network.
Connection diagram for a 220 V three-phase electric motor according to the “Star” and “Triangle” circuits
For ease of understanding, the magnetic starter and other switching units are not shown. As can be seen from the diagram, each motor winding is powered by its own phase. In a single-phase network, as its name suggests, there is only one “phase”. But it is also enough to power a three-phase electric motor. Let's take a look at an asynchronous motor connected to 220 V.
How to connect a three-phase electric motor 380 V to 220 V through a capacitor according to the “Star” and “Triangle” circuit: diagram.
Here, one winding of a three-phase electric motor is directly connected to the network, the other two are connected in series, and voltage is supplied to their connection point through the phase-shifting capacitor C1. C2 is the starting button and is turned on by button B1 with self-return only at the moment of starting: as soon as the engine starts, it must be released.
Several questions immediately arise:
- How effective is this scheme?
- How to ensure engine reverse?
- What capacities should capacitors have?
In order to make the motor rotate in the other direction, it is enough to “reverse” the phase arriving at the connection point of windings B and C (Triangle connection) or to winding B (Star circuit). The circuit, which allows you to change the direction of rotation of the rotor by simply clicking the SB2 switch, will look like this.
Reversing a 380V three-phase motor operating in single-phase network
It should be noted here that almost any three-phase motor is reversible, but you need to select the direction of rotation of the motor before starting it. It is impossible to reverse the electric motor while it is running! First you need to de-energize the electric motor, wait for it to stop completely, select the desired direction of rotation with the SB1 toggle switch, and only then apply voltage to the circuit and briefly press button B1.
Capacitances of phase-shifting and starting capacitors
To calculate the capacity of a phase-shifting capacitor, you need to use a simple formula:
- C1 = 2800/(I/U) - for inclusion according to the “Star” circuit;
- C1 = 4800/(I/U) - for switching on according to the “Triangle” scheme.
Here:
- C1 is the capacity of the phase-shifting capacitor, μF;
- I is the rated current of one motor winding, A;
- U is the voltage of a single-phase network, V.
But what to do if the rated current of the windings is unknown? It can be easily calculated by knowing the motor power, which is usually printed on the device nameplate. To calculate we use the formula:
I = P/1.73*U*n*cosф, where:
- I—current consumption, A;
- U—mains voltage, V;
- n - efficiency;
- cosф - power factor.
The symbol * denotes the multiplication sign.
The capacity of the starting capacitor C2 is selected 1.5–2 times greater than the capacity of the phase-shifting one.
When calculating a phase-shifting capacitor, you need to keep in mind that an engine operating at less than full load may overheat at the design capacitor capacity. In this case, its denomination must be reduced.
Efficiency
Unfortunately, a three-phase motor, when powered by one phase, will not be able to develop its rated power. Why? IN normal mode Each of the motor windings develops a power of 33.3%. When the motor is turned on, for example, in a “triangle” mode, only one winding C operates in normal mode, and at the point of connection of windings B and C, with a correctly selected capacitor, the voltage will be 2 times lower than the supply voltage, which means the power of these windings will drop 4 times - i.e. only 8.325% each. Let's do a simple calculation and calculate the total power:
33,3 + 8,325 + 8,325 = 49.95%.
So, even theoretically, a three-phase motor connected to a single-phase network develops only half of its rated power, and in practice this figure is even less.
A way to increase the power developed by the motor
It turns out that it is possible to increase engine power, and significantly. To do this, you don’t even have to complicate the design, but just connect a three-phase motor according to the diagram below.
Asynchronous motor - 220 V connection using an improved circuit
Here windings A and B are already operating in nominal mode, and only winding C delivers a quarter of the power:
33,3 + 33,3 + 8,325 = 74.92%.
Not bad at all, isn't it? The only condition for this connection is that windings A and B must be turned on in antiphase (marked with dots). Reversing such a circuit is done in the usual way - by switching the polarity of the capacitor-winding C circuit.
One final note. In place of the phase-shifting and starting capacitor, only non-polar paper devices can operate, for example, MBGCH, which can withstand a voltage one and a half to two times higher than the supply voltage.
Of all types of electric drives, the most widespread are the ones. They are unpretentious in maintenance, there is no brush-collector unit. If you don't overload them, don't get them wet, and periodically service or change the bearings, then it will last almost forever. But there is one problem - most of the asynchronous motors that you can buy at the nearest flea market are three-phase, as they are intended for industrial use. Despite the trend towards switching to three-phase power supply in our country, the vast majority of houses still have single-phase input. Therefore, let's figure out how to connect a three-phase motor to a single-phase and three-phase network.
What is a star and triangle in an electric motor?
First, let's figure out what the winding connection diagrams are. It is known that a single-speed three-phase asynchronous electric motor has three windings. They are connected in two ways, according to the diagrams:
- star;
- triangle.
Such connection methods are typical for any type of three-phase load, and not just for electric motors. Below is how they look in the diagram:
The power wires are connected to the terminal block, which is located in a special box. It is called Brno or Borno. Wires from the windings are routed into it and secured to terminal blocks. The box itself is removed from the motor housing, as are the terminal blocks located in it.
Depending on the design of the engine, there may be 3 wires, or there may be 6 wires. If there are 3 wires, then the windings are already connected according to a star or delta circuit and, if necessary, it will not be possible to quickly reconnect them; to do this, you need to open the case, look for the connection point, disconnect it and make taps.
If there are 6 wires in the brno, which is more common, then depending on the characteristics of the engine and the voltage of the supply network (read about this below), you can connect the windings as you see fit. Below you see the brno and the terminal blocks that are installed in it. For a 3-wire version there will be 3 pins in the terminal block, and for a 6-wire version there will be 6 pins.
The beginnings and ends of the windings are connected to the studs not just “at random” or “as convenient”, but in a strictly defined order, so that with one set of jumpers you can connect both a triangle and a star. That is, the beginning of the first winding is above the end of the third, the beginning of the second is the end of the first, and the beginning of the third is above the end of the second.
Thus, if you install jumpers on the lower contacts of the terminal block in line, you get a star connection of the windings, and by installing three jumpers vertically parallel to each other, you get a delta connection. On “factory equipped” engines, copper bars are used as jumpers, which is convenient to use for connection - no need to bend wires. 
By the way, on the covers of the electric motor, the location of the jumpers is often marked in accordance with these diagrams.
Connection to a three-phase network
Now that we have figured out how the windings are connected, let's figure out how they connect to the network.
Motors with 6 wires allow the windings to be switched for different supply voltages. This is how electric motors with supply voltages became widespread:
- 380/220;
- 660/380;
- 220/127.
Moreover, the higher voltage is for the star connection circuit, and the lower voltage is for the delta connection.
The fact is that a three-phase network does not always have the usual voltage of 380V. For example, on ships there is a network with an isolated neutral (without zero) for 220V, and in old Soviet buildings of the first half of the last century, a 127/220V network is sometimes found now. While a network with a linear voltage of 660V is rare, it is more common in production.
You can read about the differences between phase and line voltage in the corresponding article on our website:.
So, if you need to connect a three-phase electric motor to a 380/220V network, inspect its nameplate and find the supply voltage.
Electric motors on the nameplate that indicate 380/220 can only be connected with a star to our networks. If instead of 380/220 it says 660/380, connect the windings with a triangle. If you are unlucky and have an old 220/127 engine, you need either a step-down transformer or a single-phase one with a three-phase output (3x220). Otherwise, connecting it to three phases 380/220 will not work.
The worst case scenario is when the rated voltage of the motor is three wires with an unknown winding connection diagram. In this case, you need to open the case and look for the point of their connection and, if possible, and they are connected in a triangle pattern, convert them into a star circuit.
We’ve sorted out the connection of the windings, now let’s talk about what types of connections there are for a three-phase electric motor to a 380V network. The diagrams are shown for contactors with coils with a rated voltage of 380V; if you have 220V coils, connect them between phase and zero, that is, the second wire to zero, and not to phase “B”.
Electric motors are almost always connected via (or). You can see the connection diagram without reverse and self-retaining below. It works in such a way that the motor will rotate only when the button on the control panel is pressed. In this case, the button is selected without fixing, i.e. makes or opens contacts while held down, like those used in keyboards, mice, and doorbells.
The principle of operation of this circuit: when you press the “START” button, current begins to flow through the coil of the KM-1 contactor, as a result the contactor armature is attracted and the power contacts of KM-1 are closed, the engine begins to work. When you release the START button, the engine will stop. QF-1 is one that de-energizes both the power circuit and the control circuit.
If you need to press a button and the shaft starts to rotate, instead of the button, install a toggle switch or a button with a locking mechanism, that is, the contacts of which, after pressing, remain closed or open until the next press.
But this is not done often. Much more often, electric motors are started from remote controls with buttons without locking. Therefore, one more element is added to the previous circuit - the block contact of the starter (or contactor), connected in parallel to the “START” button. This circuit can be used to connect electric fans, hoods, machine tools and any other equipment whose mechanisms rotate in only one direction.
The principle of operation of the circuit:
When the QF-1 circuit breaker is switched to the on state, voltage appears on the power contacts of the contactor and the control circuit. The “STOP” button is normally closed, i.e. its contacts open when it is pressed. Through “STOP”, voltage is supplied to the normally open “START” button, the block contact and, ultimately, the coil, so when you press it, the coil control circuit will be de-energized and the contactor will turn off.
In practice, in a push-button post, each button has a normally open and normally closed pair of contacts, the terminals of which are located on different sides of the button (see photo below).
When you press the “START” button, current begins to flow through the coil of the contactor or starter KM-1 (on modern contactors designated as A1 and A2), as a result its armature is attracted and the power contacts of KM-1 are closed. KM-1.1 is a normally open (NO) contactor block contact; when voltage is applied to the coil, it closes simultaneously with the power contacts and bypasses the “START” button.
After you release the “START” button, the engine will continue to operate, since current is now supplied to the contactor coil through the KM-1.1 block contact.
This is called “self-recovery”.
The main difficulty that beginners have in understanding this basic circuit is that it is not immediately clear that the push-button station is located in one place, and the contactors in another. At the same time, KM-1.1, which is connected parallel to the “START” button, can actually be located tens of meters away.
If you need the electric motor shaft to rotate in both directions, for example, on a winch or other lifting mechanism, as well as on various machines (lathes, etc.) - use a connection diagram for a three-phase motor with reverse.
By the way, this circuit is often called a “reversing starter circuit.”
A reversible connection diagram consists of two non-reversible diagrams with some modifications. KM-1.2 and KM-2.2 are normally closed (NC) block contacts of contactors. They are included in the control circuit of the coil of the opposite contactor, this is the so-called “fool protection”, it is needed to prevent this from happening in the power circuit.
Between the “FORWARD” or “BACK” button (their purpose is the same as in the previous diagram for “START”) and the coil of the first contactor (KM-1), a normally closed (NC) block contact of the second contactor (KM-2) is connected. . Thus, when KM-2 turns on, the normally closed contact opens accordingly and KM-1 will no longer turn on, even if you press “FORWARD”.
Conversely, the NC from KM-2 is installed in the control circuit of KM-1 to prevent their simultaneous activation.
To start the motor in the opposite direction, that is, turn on the second contactor, you need to turn off the existing contactor. To do this, press the “STOP” button, and the control circuit of the two contactors is de-energized, and after that press the start button in the opposite direction of rotation.
This is necessary to prevent a short circuit in the power circuit. Pay attention to the left side of the diagram; the differences in connecting the power contacts KM-1 and KM-2 are in the order of connecting the phases. As you know, to change the direction of rotation of an asynchronous motor (reverse), you need to swap 2 of the 3 phases (any), here the 1st and 3rd phases were swapped.
Otherwise, the operation of the circuit is similar to the previous one.
By the way, Soviet starters and contactors had combined block contacts, i.e. one of them was closed, and the second was open; in most modern contactors you need to install a block contact attachment on top, which has 2-4 pairs of additional contacts just for these purposes.
Connection to a single-phase network
To connect a three-phase 380V electric motor to a single-phase 220V network, a circuit with phase-shifting capacitors (starting and running) is most often used. Without capacitors, the engine may start, but only without a load, and you will have to turn its shaft by hand when starting.
The problem is that for the IM to operate, it requires a rotating magnetic field, which cannot be obtained from a single-phase network without additional elements. But by connecting one of the windings through, you can shift the voltage phase to -90˚ and with the help of +90˚ relative to the phase in the network. We discussed the issue of phase shift in more detail in the article:.
Most often, capacitors are used for phase shifting, rather than chokes. In this way, not a rotating one is obtained, but an elliptical one. As a result, you lose about half of the nominal power. Single-phase IMs work better with this connection, due to the fact that their windings are initially designed and located on the stator for such a connection.
You can see typical motor connection diagrams without reverse for star or delta circuits below.
In the diagram below, it is needed to discharge the capacitors, since after turning off the power, voltage will remain at its terminals and you may get an electric shock.
You can select the capacitor capacity for connecting a three-phase motor to a single-phase network based on the table below. If you observe a difficult and lengthy startup, you often need to increase the starting (and sometimes working) capacity.
If the engine is powerful or starts under load (for example, in a compressor), you also need to connect a starting capacitor.
To simplify switching on, instead of the “ACceleration” button, use “PNVS”. This is a button for starting motors with a starting capacitor. It has three contacts, phase and zero are connected to two of them, and a starting capacitor is connected through the third. There are two keys on the front panel - “START” and “STOP” (as on AP-50 machines).
When you turn on the engine and press the first key all the way, three contacts close, after the engine has spun up and you release “START”, the middle contact opens, and the two outer ones remain closed, and the starting capacitor is removed from the circuit. When you press the “STOP” button, all contacts open. The connection diagram is almost the same.
You can watch the following video for details about what it is and how to properly connect the NVDS:
The connection diagram for a 380V electric motor to a single-phase 220V network with reverse is shown below. Switch SA1 is responsible for reverse.
The windings of a 380/220 motor are connected in a triangle, and for motors 220/127 – in a star, so that the supply voltage (220 volts) corresponds to the rated voltage of the windings. If there are only three outputs, and not six, then you will not be able to change the winding connection diagrams without opening them. There are two options here:
- Rated voltage 3x220V - you're in luck and use the circuits above.
- Rated voltage 3x380V - you are less lucky, since the engine may start poorly or not start at all if you connect it to a 220V network, but it’s worth a try, it might work!
But when connecting a 380V electric motor to 1 phase 220V through capacitors, there is one big problem - power loss. They can reach 40-50%.
The main and effective way to connect without losing power is to use a frequency converter. Single-phase frequency converters output 3 phases with a linear voltage of 220V without zero. In this way you can connect motors up to 5 kW; for higher power, it is simply very rare to find converters that can work with single-phase input. In this case, you will not only receive full engine power, but will also be able to fully regulate its speed and reverse it.
Now you know how to connect a three-phase motor for 220 and 380 Volts, as well as what is needed for this. We hope the information provided helped you understand the issue!
Materials
There are situations in life when you need to start a 3-phase asynchronous electric motor from a household network. The problem is that you only have one phase and “zero” at your disposal.
What to do in such a situation? Is it possible to connect a three-phase motor to a single-phase network?
If you approach your work wisely, everything is possible. The main thing is to know the basic schemes and their features.
Design features
Before starting work, understand the design of the IM (induction motor).
The device consists of two elements - a rotor (moving part) and a stator (fixed unit).
The stator has special grooves (recesses) into which the winding is placed, distributed in such a way that the angular distance is 120 degrees.
The windings of the device create one or more pairs of poles, the number of which determines the frequency with which the rotor can rotate, as well as other parameters of the electric motor - efficiency, power and other parameters.
When an asynchronous motor is connected to a three-phase network, current flows through the windings at different time intervals.
A magnetic field is created that interacts with the rotor winding and causes it to rotate.
In other words, a force appears that turns the rotor at different time intervals.
If you connect the IM to a network with one phase (without performing preparatory work), the current will appear in only one winding.
The torque generated will not be enough to move the rotor and keep it spinning.
That is why, in most cases, the use of starting and operating capacitors is required to ensure the operation of a three-phase motor. But there are other options.
How to connect an electric motor from 380 to 220V without a capacitor?
As noted above, to start an electric motor with a squirrel-cage rotor from a single-phase network, a capacitor is most often used.
It is this that ensures the device starts at the first moment after the single-phase current is supplied. In this case, the capacity of the starting device should be three times higher than the same parameter for the working capacity.
For motors with a power of up to 3 kilowatts and used at home, the price of starting capacitors is high and sometimes comparable to the cost of the motor itself.
Consequently, many are increasingly avoiding containers used only at the moment of start-up.
The situation is different with working capacitors, the use of which allows you to load the motor at 80-85 percent of its power. If they are absent, the power indicator may drop to 50 percent.
However, capacitorless starting of a 3-phase motor from a single-phase network is possible thanks to the use of bidirectional switches that operate for short periods of time.
The required torque is provided by the displacement of phase currents in the windings of the IM.
Today, two schemes are popular, suitable for motors with power up to 2.2 kW.
It is interesting that the start-up time of the IM from a single-phase network is not much lower than in the usual mode.
The main elements of the circuit are triacs and symmetrical dinistors. The first are controlled by multi-polar pulses, and the second by signals coming from the half-cycle of the supply voltage.
Scheme No. 1.
Suitable for 380 Volt electric motors up to 1,500 rpm with delta windings.
The RC circuit acts as a phase-shifting device. By changing the resistance R2, it is possible to achieve a voltage across the capacitor that is shifted by a certain angle (relative to the household network voltage).
The main task is performed by the symmetrical dinistor VS2, which at a certain point in time connects a charged capacitance to the triac and activates this switch.
Scheme No. 2.
Suitable for electric motors with a rotation speed of up to 3000 rpm and for motors with increased resistance at start-up.
Such motors require more starting current, so an open star circuit is more relevant.
Feature - the use of two electronic keys, replacing phase-shifting capacitors. During the adjustment process, it is important to ensure the required shift angle in the phase windings.
This is done as follows:
- Voltage is supplied to the electric motor through a manual starter (it must be connected in advance).
- After pressing the button, you need to select the starting moment using resistor R
When implementing the considered schemes, it is worth considering a number of features:
- For the experiment, radiatorless triacs (types TS-2-25 and TS-2-10) were used, which showed excellent results. If you use triacs on a plastic case (imported), you cannot do without radiators.
- A symmetrical DB3 type dinistor can be replaced with a KP. Despite the fact that the KP1125 is made in Russia, it is reliable and has a lower switching voltage. The main drawback is the scarcity of this dinistor.
How to connect via capacitors
First, decide which circuit is assembled on the ED. To do this, open the bar cover where the blood pressure terminals are output, and see how many wires come out of the device (most often there are six).
The designations are as follows: C1-C3 are the beginnings of the winding, and C4-C6 are its ends. If the beginnings or ends of the windings are combined with each other, this is a “star”.
The most difficult situation is if six wires simply come out of the housing. In this case, you need to look for the corresponding designations on them (C1-C6).
To implement a scheme for connecting a three-phase electric motor to a single-phase network, two types of capacitors are required - starting and working.
The first ones are used to start the electric motor at the first moment. As soon as the rotor spins to the required number of revolutions, the starting capacitance is excluded from the circuit.
If this does not happen, there may be serious consequences, including engine damage.
Main function work capacitors take over. Here it is worth considering the following points:
- Working capacitors are connected in parallel;
- The rated voltage must be at least 300 Volts;
- The capacity of the working capacitors is selected taking into account 7 µF per 100 W;
- It is desirable that the type of working and starting capacitor be identical. Popular options are MBGP, MPGO, KBP and others.
If you take these rules into account, you can extend the life of the capacitors and the electric motor as a whole.
Capacity calculations must be made taking into account the rated power of the electric motor. If the motor is underloaded, overheating is inevitable, and then the capacity of the working capacitor will have to be reduced.
If you choose a capacitor with a capacitance less than acceptable, the efficiency of the electric motor will be low.
Remember that even after the circuit is turned off, the voltage remains on the capacitors, so it is worth discharging the device before starting work.
Also note that connecting an electric motor with a power of 3 kW or more to conventional wiring is prohibited, as this can lead to disconnection or burnout of the plugs. In addition, there is a high risk of insulation melting.
To connect ED 380 to 220V using capacitors, proceed as follows:
- Connect the containers to each other (as mentioned above, the connection should be parallel).
- Connect the parts with two wires to the electric motor and a single-phase alternating voltage source.
- Turn on the engine. This is done in order to check the direction of rotation of the device. If the rotor moves in the desired direction, no additional manipulations are needed. Otherwise, the wires connected to the winding should be swapped.
With a capacitor, an additional simplified one is for a star circuit.
With a capacitor, an additional simplified one is for a triangle circuit.
How to connect with reverse
There are situations in life when you need to change the direction of rotation of the motor. This is also possible for three-phase electric motors used in a household network with one phase and zero.
To solve the problem, it is necessary to connect one terminal of the capacitor to a separate winding without the possibility of breaking, and the second - with the possibility of transferring from the “zero” to the “phase” winding.
To implement the circuit, you can use a switch with two positions.
The wires from “zero” and “phase” are soldered to the outer terminals, and the wire from the capacitor is soldered to the central terminal.
How to connect in a star-delta connection (with three wires)
For the most part, domestically produced EDs already have a star circuit assembled. All that is required is to reassemble the triangle.
The main advantage of the star/delta connection is the fact that the motor produces maximum power.
Despite this, such a scheme is rarely used in production due to the complexity of implementation.
To connect the motor and make the circuit operational, three starters are required.
The current is connected to the first (K1), and the stator winding is connected to the other. The remaining ends are connected to starters K3 and K2.
When the K3 starter is connected to the phase, the remaining ends are shortened and the circuit is converted into a “star”.
Please note that simultaneous activation of K2 and K3 is prohibited due to the risk of a short circuit or knocking out of the AV supplying the ED.
To avoid problems, a special interlock is provided, which means turning off one starter when turning on the other.
The operating principle of the circuit is simple:
- When the first starter is connected to the network, the time relay starts and supplies voltage to the third starter.
- The engine starts working in a star configuration and starts working with more power.
- After some time, the relay opens contacts K3 and connects K2. In this case, the electric motor operates in a “triangle” pattern with reduced power. When it is necessary to turn off the power, K1 turns on.
Results
As can be seen from the article, it is possible to connect a three-phase electric motor to a single-phase network without loss of power. At the same time, for home use, the simplest and most affordable option is using a starting capacitor.
5 / 5 ( 1 vote)
1. Connecting a three-phase electric motor - general diagram
When an electrician gets a job at any industrial enterprise, he must understand that he will have to deal with a large number of three-phase electric motors. And any self-respecting electrician (I’m not talking about those who do wiring in an apartment) should clearly know the wiring diagram for a three-phase motor.
I immediately apologize that in this article I often call a contactor a starter, although I have already explained in detail that. What can you do, I'm tired of this name.
The article will discuss connection diagrams for the most common asynchronous electric motor through a magnetic starter. But not only. I will also tell you about the methods and principles of protecting the engine from overheating and overload.
Various electric motor connection diagrams, their pros and cons. From simple to complex. Circuits that can be used in real life are designated: PRACTICAL DIAGRAM. So let's begin.
Connecting a three-phase motor
This means an asynchronous electric motor, winding connection - star or triangle, connection to a 380V network.
For the engine to operate, a working neutral conductor N (Neutral) is not needed, but a protective conductor (PE, Protect Earth) must be connected for safety reasons.
In the most general case, the diagram will look like this, as shown at the beginning of the article. Indeed, why not turn on the engine like a regular light bulb, only the switch will be a “three-key”?
2. Connecting the engine through a switch or circuit breaker
But no one even turns on a light bulb just like that; the lighting network and, in general, any load is always turned on only through circuit breakers.
Diagram of connecting a three-phase motor to the network via a circuit breaker
Therefore, in more detail, the general case will look like this:
3. Connecting the motor via a circuit breaker. PRACTICAL SCHEME
Diagram 3 shows a circuit breaker that protects the motor from overcurrent (“rectangular” bends in the supply lines) and from short circuits (“round” bends). By circuit breaker I mean a regular three-pole circuit breaker with a load thermal characteristic of C or D.
Let me remind you that in order to approximately select (estimate) the required thermal current of the thermal protection setting, you need to multiply the rated power of the three-phase motor (indicated on the nameplate) by 2.
Circuit breaker for turning on the electric motor. The current is 10A, through which you can turn on a 4 kW motor. No more and no less.
Scheme 3 has the right to life (due to poverty or ignorance of local electricians).
It works great, just like it has for many years. And one “fine” day the twist will burn out. Or the engine will burn out.
If you use such a circuit, you need to carefully select the current of the machine so that it is 10-20% greater than the operating current of the motor. And select the characteristic of the thermal release D, so that during a difficult start the machine does not trip.
For example, a 1.5 kW engine. We estimate the maximum operating current - 3A (real operating current may be less, we need to measure it). This means that the three-pole circuit breaker must be set to 3 or 4A, depending on the starting current.
The advantage of this motor connection diagram is the price and ease of execution and maintenance. For example, where there is one engine, and it is turned on manually for the entire shift. The disadvantages of such a scheme with switching on via an automatic machine are:
What's new in the VK group? SamElectric.ru ?
Subscribe and read the article further:
- Inability to regulate the thermal current of the machine. In order to reliably protect the engine, the shutdown current of the circuit breaker must be 10-20% greater than the rated operating current of the engine. The motor current must be periodically measured with clamps and, if necessary, the thermal protection current must be adjusted. But a regular machine does not have the ability to adjust (.
- Inability to remotely and automatically turn on/off the engine.
These shortcomings can be eliminated; the diagrams below will show how.
A manual starter or automatic motor is a more advanced device. It has “Start” and “Stop” buttons, or an “On-Off” knob. Its advantage is that it is specially designed for starting and protecting the engine. The start is still manual, but the operating current can be adjusted within certain limits.
4. Connecting the motor via a manual starter. PRACTICAL SCHEME
Since motors usually have , then motor protection circuit breakers (automatic motors), as a rule, have a thermal protection characteristic of type D. That is it can withstand short-term (starting) overloads of approximately 10 times the nominal value.
Here's what's on the side:
Motor circuit breaker - characteristics on the side wall
Setting current (thermal) – from 17 to 23 A, set manually. Cut-off current (trigger during short circuit) – 297 A.
In principle, a manual starter and an automatic motor are the same device. But the starter shown in the photo can switch the power supply to the engine. And the automatic motor constantly supplies power (three phases) to the contactor, which, in turn, switches the power to the motor. In short, the difference is in the connection diagram.
The advantage of the scheme is that you can adjust the thermal current setting. The downside is the same as in the previous scheme, there is no remote activation.
Motor connection diagram via magnetic starter
This wiring diagram for a three-phase motor should be given the closest attention. It is most common in all industrial equipment produced until about the 2000s. And in new Chinese simple machines it is still used to this day.
An electrician who does not know it is like a surgeon who cannot distinguish an artery from a vein; as a lawyer who does not know Article 1 of the Constitution of the Russian Federation; like a dancer who does not distinguish a waltz from a tectonic.
In this circuit, three phases go to the motor not through the machine, but through the starter. And the starter is turned on/off using the “ Start" And " Stop”, which can be brought to the control panel via 3 wires of any length.
5. Diagram of connecting the motor through a starter with start-stop buttons
Here, the control circuit power comes from phase L1 (wire 1 ) through a normally closed (NC) “Stop” button (wire 2 ).
If you now press the “Start” button, the power circuit of the coil of the KM electromagnetic starter will close (wire 3 ), its contacts will close, and three phases will go to the motor. But in such schemes, in addition to three “power” contacts, the starter has one more additional contact. It is called a “locking” or “self-latching contact”.
When the electromagnetic starter is turned on by pressing the SB1 “Start” button, the self-retaining contact also closes. And if it is closed, then even if the “Start” button is pressed, the power circuit of the starter coil will still remain closed. And the engine will continue to run until the “Stop” button is pressed.
Since the topic of magnetic starters is very extensive, it is included in a separate article. The article has been significantly expanded and supplemented. Everything is covered there - connecting various loads, protection (thermal and short-circuit), reversing circuits, control from different points, etc. The numbering of the schemes has been preserved. I recommend.
Connecting a three-phase motor via electronic devices
All methods of starting the engine described above are called Starting by direct voltage supply. Often, in powerful drives, such a start-up is a difficult test for the equipment - belts burn, bearings and fasteners break, etc.
Therefore, the article would be incomplete if I did not mention current trends. Nowadays, electronic power devices are increasingly used to connect a three-phase motor instead of electromagnetic starters. By this I mean:
- Solid state relays - their power elements are thyristors (triacs), which are controlled by an input signal from a button or from a controller. There are both single-phase and three-phase. .
- Soft (soft) starters (soft starters, soft starters) are advanced solid state machines. You can set the protection current, acceleration/deceleration time, turn on reverse, etc. And on this topic. Practical use soft starters – .
The old specific method of connecting two-speed motors is described in the article. Keywords– Rarity, Retro, USSR.
I’ll end here, thank you for your attention, I couldn’t cover everything, write questions in the comments!
1.1. Selecting a three-phase motor for connection to a single-phase network.
Among the various methods of starting three-phase electric motors in a single-phase network, the simplest is based on connecting the third winding through a phase-shifting capacitor. The useful power developed by the engine in this case is 50...60% of its power in three-phase operation. Not all three-phase electric motors, however, work well when connected to a single-phase network. Among such electric motors we can highlight, for example, those with a double cage squirrel-cage rotor of the MA series. In this regard, when choosing three-phase electric motors for operation in a single-phase network, preference should be given to motors of the A, AO, AO2, APN, UAD, etc. series.
For normal operation of a capacitor-start electric motor, it is necessary that the capacitance of the capacitor used varies depending on the speed. In practice, this condition is quite difficult to fulfill, so two-stage motor control is used. When starting the engine, two capacitors are connected, and after acceleration, one capacitor is disconnected and only the working capacitor is left.
1.2. Calculation of parameters and elements of an electric motor.
If, for example, the electric motor’s data sheet indicates its supply voltage is 220/380, then the motor is connected to a single-phase network according to the diagram shown in Fig. 1
After turning on the batch switch P1, contacts P1.1 and P1.2 close, after which you must immediately press the “Acceleration” button. After gaining speed, the button is released. Reversing the electric motor is carried out by switching the phase on its winding with toggle switch SA1.
The capacity of the working capacitor Cp in the case of connecting the motor windings in a “triangle” is determined by the formula:
And in the case of connecting the motor windings in a “star”, it is determined by the formula:
The current consumed by the electric motor in the above formulas, with a known power of the electric motor, can be calculated from the following expression:
The capacity of the starting capacitor Sp is chosen 2..2.5 times greater than the capacity of the working capacitor. These capacitors must be designed for a voltage of 1.5 times the mains voltage. For a 220 V network, it is better to use capacitors such as MBGO, MBPG, MBGCh with an operating voltage of 500 V and higher. Subject to short-term switching on, electrolytic capacitors of the K50-3, EGC-M, KE-2 types with an operating voltage of at least 450 V can be used as starting capacitors. For greater reliability, electrolytic capacitors are connected in series, connecting their negative terminals together, and are shunted diodes (Fig. 2)
The total capacitance of the connected capacitors will be (C1+C2)/2.
In practice, the capacitance values of the working and starting capacitors are selected depending on the engine power according to the table. 1
Table 1. The value of the capacitances of the working and starting capacitors of a three-phase electric motor depending on its power when connected to a 220 V network.
It should be noted that in an electric motor with capacitor starting in no-load mode, a current flows through the winding fed through the capacitor by 20...30% higher than the rated one. In this regard, if the engine is often used in underloaded mode or idling, then in this case the capacitance of the capacitor C p should be reduced. It may happen that during an overload the electric motor stops, then to start it, the starting capacitor is connected again, removing the load altogether or reducing it to a minimum.
The capacity of the starting capacitor C p can be reduced when starting electric motors at idle or with a light load. To turn on, for example, an AO2 electric motor with a power of 2.2 kW at 1420 rpm, you can use a working capacitor with a capacity of 230 μF, and a starting capacitor - 150 μF. In this case, the electric motor starts confidently with a small load on the shaft.
1.3. Portable universal unit for starting three-phase electric motors with a power of about 0.5 kW from a 220 V network.
To start electric motors of various series, with a power of about 0.5 kW, from a single-phase network without reversing, you can assemble a portable universal starting unit (Fig. 3)
When you press the SB1 button, the magnetic starter KM1 is triggered (toggle switch SA1 is closed) and its contact system KM 1.1, KM 1.2 connects the electric motor M1 to the 220 V network. At the same time, the third contact group KM 1.3 closes the SB1 button. After complete acceleration of the engine, turn off the starting capacitor C1 using toggle switch SA1. The engine is stopped by pressing the SB2 button.
1.3.1. Details.
The device uses an electric motor A471A4 (AO2-21-4) with a power of 0.55 kW at 1420 rpm and a magnetic starter of the PML type, designed for alternating current voltage of 220 V. Buttons SB1 and SB2 are paired type PKE612. Toggle switch T2-1 is used as switch SA1. In the device, the constant resistor R1 is wire-wound, type PE-20, and the resistor R2 is type MLT-2. Capacitors C1 and C2 type MBGCh for a voltage of 400 V. Capacitor C2 is made up of parallel connected capacitors of 20 μF 400 V. Lamp HL1 type KM-24 and 100 mA.
The starting device is mounted in a metal case measuring 170x140x50 mm (Fig. 4)
Rice. 4 Appearance starting device and panel drawing pos.7.
On top panel The housing contains the "Start" and "Stop" buttons - a signal lamp and a toggle switch to turn off the starting capacitor. On the front panel of the device case there is a connector for connecting an electric motor.
To turn off the starting capacitor, you can use an additional relay K1, then there is no need for toggle switch SA1, and the capacitor will turn off automatically (Fig. 5)
When you press the SB1 button, relay K1 is triggered and contact pair K1.1 turns on the magnetic starter KM1, and K1.2 turns on the starting capacitor C. The magnetic starter KM1 is self-locking using its contact pair KM 1.1, and contacts KM 1.2 and KM 1.3 connect the electric motor to the network. The "Start" button is kept pressed until the engine fully accelerates, and then released. Relay K1 is de-energized and turns off the starting capacitor, which is discharged through resistor R2. At the same time, the magnetic starter KM 1 remains switched on and provides power to the electric motor in operating mode. To stop the electric motor, press the "Stop" button. In an improved starting device according to the diagram in Fig. 5, you can use a relay of the MKU-48 type or the like.
2. The use of electrolytic capacitors in electric motor starting circuits.
When connecting three-phase asynchronous electric motors to a single-phase network, as a rule, ordinary paper capacitors are used. Practice has shown that instead of bulky paper capacitors, you can use oxide (electrolytic) capacitors, which are smaller in size and more affordable to purchase. An equivalent replacement diagram for conventional paper is shown in Fig. 6
Positive half wave alternating current passes through the chain VD1, C2, and the negative VD2, C2. Based on this, it is possible to use oxide capacitors with a permissible voltage that is half that of conventional capacitors of the same capacity. For example, if in a circuit for a single-phase network with a voltage of 220 V a paper capacitor with a voltage of 400 V is used, then when replacing it, according to the above diagram, you can use an electrolytic capacitor with a voltage of 200 V. In the above diagram, the capacitances of both capacitors are the same and are selected in the same way as the method for selecting paper capacitors capacitors for the starting device.
2.1. Connecting a three-phase motor to a single-phase network using electrolytic capacitors.
The diagram for connecting a three-phase motor to a single-phase network using electrolytic capacitors is shown in Fig. 7.
In the above diagram, SA1 is the engine rotation direction switch, SB1 is the engine acceleration button, electrolytic capacitors C1 and C3 are used to start the engine, C2 and C4 are used during operation.
Selection of electrolytic capacitors in the circuit shown in Fig. 7 is best done using current clamps. Currents are measured at points A, B, C and equality of currents at these points is achieved by stepwise selection of capacitor capacitances. Measurements are carried out with the engine loaded in the mode in which it is expected to operate. Diodes VD1 and VD2 for a 220 V network are selected with a maximum permissible reverse voltage of at least 300 V. The maximum forward current of the diode depends on the engine power. For electric motors with a power of up to 1 kW, diodes D245, D245A, D246, D246A, D247 with a direct current of 10 A are suitable. With a higher motor power from 1 kW to 2 kW, you need to take more powerful diodes with the corresponding forward current, or put several less powerful diodes in parallel , installing them on radiators.
Please note that if the diode is overloaded, breakdown may occur and alternating current will flow through the electrolytic capacitor, which can lead to its heating and explosion.
3. Connection of powerful three-phase motors to a single-phase network.
The capacitor circuit for connecting three-phase motors to a single-phase network makes it possible to obtain no more than 60% of the rated power from the motor, while the power limit of the electrified device is limited to 1.2 kW. This is clearly not enough to operate an electric planer or electric saw, which should have a power of 1.5...2 kW. The problem in this case can be solved by using a higher power electric motor, for example, with a power of 3...4 kW. Motors of this type are designed for a voltage of 380 V, their windings are star-connected and the terminal box contains only 3 terminals. Connecting such a motor to a 220 V network leads to a reduction in the rated power of the motor by 3 times and by 40% when operating in a single-phase network. This reduction in power makes the engine unsuitable for operation, but can be used to spin the rotor idle or with minimal load. Practice shows that most electric motors confidently accelerate to rated speed, and in this case, starting currents do not exceed 20 A.
3.1. Refinement of a three-phase motor.
The easiest way to convert a powerful three-phase motor into operating mode is to convert it to a single-phase operating mode, while receiving 50% of the rated power. Switching the motor to single-phase mode requires slight modification. Open the terminal box and determine which side of the motor housing cover the winding terminals fit on. Unscrew the bolts securing the cover and remove it from the engine housing. Find the place where the three windings are connected to a common point and solder an additional conductor with a cross-section corresponding to the cross-section of the winding wire to the common point. The twist with a soldered conductor is insulated with electrical tape or a polyvinyl chloride tube, and the additional terminal is pulled into the terminal box. After this, the housing cover is replaced.
The electric motor switching circuit in this case will have the form shown in Fig. 8.
During engine acceleration, a star connection of the windings is used with the connection of a phase-shifting capacitor Sp. In operating mode, only one winding remains connected to the network, and the rotation of the rotor is supported by a pulsating magnetic field. After switching the windings, the capacitor Cn is discharged through the resistor Rр. The operation of the presented circuit was tested with an AIR-100S2Y3 type engine (4 kW, 2800 rpm) installed on a homemade woodworking machine and showed its effectiveness.
3.1.1. Details.
In the switching circuit of electric motor windings, a packet switch with an operating current of at least 16 A should be used as a switching device SA1, for example, a switch of type PP2-25/N3 (two-pole with neutral, for a current of 25 A). Switch SA2 can be of any type, but with a current of at least 16 A. If motor reversal is not required, then this switch SA2 can be excluded from the circuit.
A disadvantage of the proposed scheme for connecting a powerful three-phase electric motor to a single-phase network can be considered the sensitivity of the motor to overloads. If the load on the shaft reaches half the engine power, then the shaft rotation speed may decrease until it stops completely. In this case, the load is removed from the motor shaft. The switch is first moved to the “Acceleration” position, and then to the “Work” position and further work continues.
In order to improve the starting characteristics of motors, in addition to the starting and running capacitor, you can also use inductance, which improves the uniformity of phase loading. All this is written in the article Devices for starting a three-phase electric motor with low power losses
When writing the article, some of the materials from the book by V.M. Pestrikov were used. "Home electrician and more..."
All the best, write to © 2005
