The design of a three-phase electric motor is an electrical machine that requires three-phase alternating current networks for normal operation. The main parts of such a device are the stator and the rotor. The stator is equipped with three windings shifted between each other by 120 degrees. When three-phase voltage appears in the windings, magnetic fluxes are formed at their poles. Due to these flows, the engine rotor begins to rotate.
In industrial production and in everyday life, three-phase asynchronous motors are widely used. They can be single-speed, when the motor windings are connected by star and delta, or multi-speed, with the ability to switch from one circuit to another.
Star and delta connection of windings
All three-phase electric motors have windings connected in a star or delta configuration.
When connecting the windings in a star circuit, their ends are connected at one point in the zero node. Therefore, we get one more additional zero output. The other ends of the windings are connected to the phases of the 380 V network.
The delta connection consists of a series connection of windings. The end of the first winding is connected to the starting end of the second winding, and so on. Ultimately, the end of the third winding will connect to the beginning of the first winding. Three-phase voltage is supplied to each connection node. The triangle connection is distinguished by the absence of a neutral wire.
Both types of compounds have received approximately the same distribution and do not have significant distinctive features.
There is also a combined connection when both options are used. This method is used quite often; its goal is a smooth start of the electric motor, which cannot always be achieved with conventional connections. At the moment of direct start-up, the windings are in the star position. Next, a relay is used that provides switching to the triangle position. Due to this, the starting current decreases. The combined circuit is most often used when starting high-power electric motors. Such motors also require a significantly higher starting current, approximately seven times the rated value.
Electric motors can be connected in other ways when a double or triple star is used. These connections are used for motors with two or more variable speeds.
Starting a three-phase electric motor with star-delta switching
This method is used to reduce the starting current, which can be approximately 5-7 times the rated current of the electric motor. Units with too much power have a starting current at which fuses easily blow, circuit breakers turn off and, in general, the voltage drops significantly. With such a decrease in voltage, the incandescence of the lamps decreases, the torque of other electric motors decreases, and the contactors spontaneously turn off. Therefore, different methods are used to reduce the inrush current.
Common to all methods is the need to reduce the voltage in the stator windings during direct start-up. To reduce the starting current, the stator circuit can be supplemented with a choke, rheostat or automatic transformer during starting.
The most widespread is switching the winding from a star to a triangle position. In the star position, the voltage becomes 1.73 times less than the rated value, therefore the current will be less than at full voltage. During start-up, the motor speed increases, the current decreases and the windings switch to the delta position.
Such switching is allowed in electric motors that have a lightweight starting mode, since the starting torque is reduced by approximately two times. This method is used to switch those engines that can be structurally connected into a triangle. They must have windings capable of operating at .
When to switch from triangle to star
When it is necessary to make a star and delta connection of the electric motor windings, you should remember that it is possible to switch from one type to another. The main option is the star-delta switching circuit. However, if necessary, the reverse option is also possible.
Everyone knows that electric motors that are not fully loaded have a decrease in power factor. Therefore, it is advisable to replace such engines with devices with lower power. However, if replacement is impossible and there is a large power reserve, a delta-star switch is performed. The current in the stator circuit should not exceed the nominal value, otherwise the electric motor will overheat.
Typical cases of star and delta connections of generators, transformers and power receivers are discussed in the articles “Star connection diagram” and “Triangle connection diagram”. Let us now dwell on the most important question about power when connecting in star and delta, since for the operation of each mechanism driven by an electric motor or receiving power from a generator or transformer, it is ultimately important namely power.
In AC networks there are:
apparent power S = E × I or S = U × I;
active power P = E × I×cos φ
or P = U × I×cos φ
;
reactive power Q = E × I× sin φ
or Q = U × I× sin φ
,
Where E– electromotive force (emf); U– voltage at the terminals of the electrical receiver; I– current; φ – phase angle between current and voltage 1.
When determining the power of generators, the formulas include e. d.s., when determining the power of electrical receivers - the voltage at their terminals. When determining the power of electric motors, the efficiency factor is also taken into account, since the power on its shaft is indicated on the electric motor plate.
If the phase powers S a ( P a, Q a); S b( P b, Q b); S c ( P c , Q c) are identical and respectively equal S f, P f and Q f, then the power of a three-phase system, expressed in terms of phase quantities, is equal to the sum of the powers of the three phases and is:
full S= 3 × S f;
active P= 3 × P f;
reactive Q= 3 × Q f.
Power when connected to a star
When connected to a star, the line currents I and phase currents I f are equal, and between phase
and linear voltages there is a relationship U= √3 × U f where from U f = U / √3.
Comparing these formulas, we see that the powers expressed in terms of linear quantities when connected into a star are equal to:
full S= 3 × S f = 3 × ( U/ √3) × I= √3 × U × I;
active P= √3 × U × I×cos φ
;
reactive Q= √3 × U × I× sin φ
.
Power in delta connection
When connected into a triangle, linear U and phase U f voltages are equal, and there is a relationship between phase and linear currents I= √3 × I f where from I f = I / √3.
Therefore, the powers expressed through linear quantities when connected into a triangle are equal to:
full S= 3 × S f = 3 × U × ( I/ √3) = √3 × U × I;
active P= √3 × U × I×cos φ
;
reactive Q= √3 × U × I× sin φ
.
Important note. The same type of power formulas for star and triangle connections sometimes causes misunderstandings, as it leads insufficiently experienced people to the wrong conclusion that the type of connections is always indifferent. Let us show with one example how erroneous this view is.
The electric motor was connected in a delta and operated from a 380 V network at a current of 10 A with full power
S= 1.73 × 380 × 10 = 6574 VA.
Then the electric motor was reconnected to a star. At the same time, each phase winding had a 1.73 times lower voltage, although the voltage in the network remained the same. The lower voltage caused the current in the windings to decrease by 1.73 times. But this is not enough. When connected into a triangle, the linear current was 1.73 times greater than the phase current, and now the phase and linear currents are equal.
Thus, the line current when reconnected to a star decreased by 1.73 × 1.73 = 3 times.
In other words, although the new power needs to be calculated according to the same formula, but you should substitute it other quantities, namely:
S 1 = 1.73 × 380 × (10 / 3) = 2191 VA.
From this example it follows that when an electric motor is reconnected from a delta to a star and powered from the same electrical network, the power developed by the electric motor decreases by 3 times.
What happens when switching from star to delta and back again in the most common cases?
We stipulate that we are not talking about internal reconnections (which are carried out in the factory or in specialized workshops), but about reconnections on the device panels, if the beginnings and ends of the windings are located on them.
1. When switching star to delta windings of generators or secondary windings of transformers the network voltage decreases by 1.73 times, for example from 380 to 220 V. The power of the generator and transformer remains the same. Why? Because the voltage of each phase winding remains the same and the current in each phase winding is the same, although the current in the line wires increases by 1.73 times.
When switching windings of generators or secondary windings of transformers from delta to star the opposite phenomena occur, that is, the linear voltage in the network increases by 1.73 times, for example from 220 to 380 V, the currents in the phase windings remain the same, the currents in the linear wires decrease by 1.73 times.
This means that both generators and secondary windings of transformers, if they have all six ends connected, are suitable for networks with two voltages that differ by a factor of 1.73.
2. When switching star to triangle lamps(provided they are connected to the same network in which the lamps turned on by the star burn at normal incandescence) the lamps will burn out.
When switching lamps from triangle to star(provided that the lamps, when connected in a triangle, burn at normal incandescence) the lamps will give a dim light. This means that, for example, 127 V lamps in a 127 V network must be connected in a triangle. If they have to be powered from a 220 V network, a star connection with a neutral wire is necessary (for more details, see the article “Star connection diagram”). Only lamps of the same power, evenly distributed between the phases, can be connected into a star without a neutral wire, such as, for example, in theater chandeliers.
3. Everything said about lamps also applies to resistances, electric ovens and similar electrical receivers.
4. Capacitors, from which batteries are assembled to increase cos φ , have a rated voltage, which indicates the voltage of the network to which the capacitor is to be connected. If the network voltage is, for example, 380 V, and the rated voltage of the capacitors is 220 V, they should be connected in a star. If the mains voltage and the rated voltage of the capacitors are the same, the capacitors are connected in a triangle.
5. As explained above, when switching electric motor from delta to star its power is reduced approximately threefold. And vice versa, if the electric motor is switched from star to triangle, the power increases sharply, but at the same time the electric motor, if it is not designed to operate at a given voltage and delta connection, will burn.
Starting a squirrel-cage electric motor with star-delta switching
used to reduce the starting current, which is 5–7 times higher than the operating current of the motor. For motors of relatively high power, the starting current is so high that it can cause fuses to blow, turn off the circuit breaker and lead to a significant decrease in voltage. Reducing the voltage reduces the heat of the lamps, reduces the torque of electric motors 2, and can cause the contactors and magnetic starters to turn off. Therefore, they strive to reduce the starting current, which is achieved in several ways. All of them ultimately come down to reducing the voltage in the stator circuit for the start-up period. To do this, a rheostat, inductor, autotransformer are introduced into the stator circuit for the start-up period, or the winding is switched from star to delta. Indeed, before start-up and during the first start-up period, the windings are connected in a star. Therefore, a voltage is supplied to each of them, 1.73 times less than the rated one, and, therefore, the current will be significantly less than when the windings are turned on at full network voltage. During the starting process, the electric motor increases speed and the current decreases. Then the windings are switched into a triangle.
Warnings:
1. Switching from star to delta is only permissible for engines with light starting mode, since when connected to a star, the starting torque is approximately half as much as it would be with a direct start. This means that this method of reducing the starting current is not always suitable, and if it is necessary to reduce the starting current and at the same time achieve a large starting torque, then they take an electric motor with a wound rotor, and a starting rheostat is introduced into the rotor circuit.
2. You can switch from star to delta only those electric motors that are designed to operate when connected in a delta, that is, having windings designed for linear mains voltage.
Switching from delta to star
It is known that underloaded electric motors operate with a very low power factor cos φ . Therefore, it is recommended to replace underloaded electric motors with less powerful ones. If, however, it is impossible to perform a replacement, and the power reserve is large, then an increase in cos is possible φ switching from triangle to star. In this case, it is necessary to measure the current in the stator circuit and make sure that when connected to a star it does not exceed the rated current under load; otherwise the electric motor will overheat.
1 Active power is measured in watts (W), reactive power is measured in reactive volt-amperes (var), apparent power is measured in volt-amperes (VA). Values 1000 times larger are respectively called kilowatt (kW), kilovar (kvar), kilovolt-ampere (kV×A).
2 The torque of an electric motor is proportional to the square of the voltage. Therefore, when the voltage decreases by 20%, the torque decreases not by 20, but by 36% (1² - 0.82² = 0.36).
The most common question for those beginning to study the design of transformers or other electrical devices is “What are star and triangle?” We will try to explain in our article how they differ and how they are structured.
Let's consider the winding connection diagrams using the example of a three-phase transformer. In its structure, it has a magnetic circuit consisting of three rods. Each rod has two windings - primary and secondary. High voltage is applied to the primary, and low voltage is removed from the secondary and goes to the consumer. In the symbol, the connection diagram is indicated by a fraction (for example, Y⁄∆ or Y/D or U/D), the numerator value is the connection of the high voltage (HV) winding, and the denominator value is the low voltage (LV).
Each rod has both a primary winding and a secondary winding (three primary and three secondary windings). Each winding has a beginning and an end. The windings can be connected to each other using a star or delta method. For clarity, let us denote the above schematically (Fig. 1)
When connected by a star, the ends of the windings are connected together, and three phases go from the beginning to the consumer. From the terminal connections of the ends of the windings, the neutral wire N (also known as neutral) is removed. The result is a four-wire, three-phase system, which is often found along overhead power lines (Fig. 2)
The advantages of this connection scheme are that we can get 2 types of voltage: phase (phase + neutral) and linear. In such a connection, the linear voltage is √3 times greater than the phase voltage. Knowing that the phase voltage gives us 220V, then multiplying it by √3 = 1.73, we get approximately 380V - linear voltage. But as for the electric current, in this case the phase current is equal to the linear one, because Both linear and phase currents leave the winding in the same way, and it has no other path. It is also worth noting that only in the star connection there is a neutral wire, which is a load “equalizer” so that the voltage does not change or jump.
Let us now consider the connection of the windings with a triangle. If we connect the end of phase A to the beginning of phase B, connect the end of phase B to the beginning of phase C, and connect the end of phase C to the beginning of phase A, we will get a triangle winding connection diagram. Those. in this circuit the windings are connected in series. (Fig. 3)
Basically, this connection scheme is used for symmetrical loads, where the load does not change between phases. In such a connection, the phase voltage is equal to the linear voltage, but the electric current, on the contrary, is different in such a circuit. The linear current is √3 times greater than the phase current. The delta connection of the winding provides ampere-turn balance for zero current
sequences. In simple terms, the delta connection circuit provides balanced voltage.
Let's summarize. For a basic definition of power transformer winding connection diagrams, it is necessary to understand that the difference between these connections is that in a star, all three windings are connected together by one end of each winding at one (neutral) point, and in a triangle, the windings are connected in series. The star connection allows us to create two types of voltage: linear (380V) and phase (220V), and in a triangle only 380V.
The choice of winding connection diagram depends on a number of reasons:
- Transformer power circuits
- Transformer power
- Voltage level
- Load asymmetry
- Economic considerations
For example, for networks with a voltage of 35 kV and more, it is more profitable to connect the transformer winding with a star circuit, grounding the zero point. In this case, it turns out that the voltage of the transformer terminals and transmission line wires relative to the ground will always be √3 times less than linear, which will lead to a reduction in the cost of insulation.
In practice, the following groups of compounds are most often encountered: Y/Y, D/Y, Y/D.
The group of winding connections Y/Y (star/star) is most often used in low-power transformers that supply symmetrical three-phase electrical appliances/receivers. It is also sometimes used in high-power circuits when grounding of the neutral point is required.
The winding connection group D/Y (delta/star) is used mainly in high-power step-down transformers. Most often, transformers with such a connection operate as part of power supply systems for low-voltage current distribution networks. Typically, the star's neutral point is grounded to accommodate both line-to-line and phase-to-phase voltages.
The Y/D (star/delta) winding connection group is mainly used in the main transformers of large power stations and non-distribution substations.
Three-phase asynchronous motors, which are often used due to their widespread use, consist of a stationary stator and a moving rotor. Winding conductors are laid in the stator slots with an angular distance of 120 electrical degrees, the beginnings and ends of which (C1, C2, C3, C4, C5 and C6) are brought out into the junction box. The windings can be connected according to a “star” (the ends of the windings are connected to each other, the supply voltage is supplied to their beginnings) or a “triangle” (the ends of one winding are connected to the beginning of another).
In the distribution box, the contacts are usually shifted - opposite C1 is not C4, but C6, opposite C2 - C4.
When a three-phase motor is connected to a three-phase network, a current begins to flow through its windings at different times in turn, creating a rotating magnetic field that interacts with the rotor, causing it to rotate. When the motor is turned on in a single-phase network, no torque is created that can move the rotor.
Among the different ways to connect three-phase electric motors to a single-phase network, the simplest is to connect the third contact through a phase-shifting capacitor.
The rotation speed of a three-phase motor operating from a single-phase network remains almost the same as when it is connected to a three-phase network. Unfortunately, this cannot be said about power, the losses of which reach significant values. The exact values of power loss depend on the connection diagram, engine operating conditions, and the capacitance value of the phase-shifting capacitor. Approximately, a three-phase motor in a single-phase network loses about 30-50% of its power.
Not all three-phase electric motors are capable of working well in single-phase networks, but most of them cope with this task quite satisfactorily - except for the loss of power. Basically, for operation in single-phase networks, asynchronous motors with a squirrel-cage rotor (A, AO2, AOL, APN, etc.) are used.
Asynchronous three-phase motors are designed for two rated mains voltages - 220/127, 380/220, etc. The most common electric motors with an operating voltage of the windings are 380/220V (380V for star, 220 for delta). Higher voltage for star, lower for delta. In the passport and on the motor plate, among other parameters, the operating voltage is indicated winding voltage, their connection diagram and the possibility of changing it.
Designation on the plate A indicates that the motor windings can be connected either as a “triangle” (at 220V) or a “star” (at 380V). When connecting a three-phase motor to a single-phase network, it is advisable to use a delta circuit, since in this case the motor will lose less power than when connected to a star.
Tablet B informs that the motor windings are connected in a star configuration, and the distribution box does not provide the ability to switch them to delta (there are only three terminals). In this case, you can either accept a large loss of power by connecting the motor in a star configuration, or, by penetrating the electric motor winding, try to bring out the missing ends in order to connect the windings in a delta configuration.
If the operating voltage of the engine is 220/127V, then the engine can only be connected to a single-phase 220V network using a star circuit. If you connect 220V in a delta circuit, the engine will burn out.
Beginnings and ends of windings (various options)
Perhaps the main difficulty in connecting a three-phase motor to a single-phase network is to understand the wires going into the junction box or, in the absence of one, simply leading out of the motor.The simplest case is when the windings in an existing 380/220V motor are already connected in a delta circuit. In this case, you just need to connect the current supply wires and the working and starting capacitors to the motor terminals according to the connection diagram.
If the windings in the motor are connected by a “star”, and it is possible to change it to a “triangle”, then this case also cannot be classified as complex. You just need to change the connection diagram of the windings to a “triangle”, using jumpers for this.
Determination of the beginnings and ends of windings. The situation is more complicated if 6 wires are brought out into the junction box without indicating their belonging to a specific winding and marking the beginnings and ends. In this case, it comes down to solving two problems (But before doing this, you need to try to find some documentation for the electric motor on the Internet. It may describe what wires of different colors belong to.):
- identifying pairs of wires belonging to one winding;
- finding the beginning and end of the windings.
The first task is solved by “ringing” all the wires with a tester (measuring resistance). If you don’t have a device, you can solve the problem using a flashlight light bulb and batteries, connecting the existing wires in a circuit in series with the light bulb. If the latter lights up, it means that the two ends being tested belong to the same winding. In this way, three pairs of wires (A, B and C in the figure below) belonging to three windings are determined.
The second task (determining the beginning and end of the windings) is somewhat more complicated and requires a battery and a pointer voltmeter. Digital is not suitable due to inertia. The procedure for determining the ends and beginnings of the windings is shown in diagrams 1 and 2.
To the ends of one winding (for example, A) a battery is connected to the ends of the other (for example, B) - pointer voltmeter. Now, if you break the contact of the wires A with a battery, the voltmeter needle will swing in one direction or another. Then you need to connect a voltmeter to the winding WITH and do the same operation with breaking the battery contacts. If necessary, change the polarity of the winding WITH(switching ends C1 and C2) you need to ensure that the voltmeter needle swings in the same direction, as in the case of the winding IN. The winding is checked in the same way. A- with a battery connected to the winding C or B.
As a result of all manipulations, the following should happen: when the battery contacts break from any of the windings, an electric potential of the same polarity should appear on the other 2 (the device needle swings in one direction). Now all that remains is to mark the terminals of one bundle as the beginning (A1, B1, C1), and the terminals of the other as the ends (A2, B2, C2) and connect them according to the required circuit - “triangle” or “star” (if the motor voltage is 220/127V ).
Retrieving missing ends. Perhaps the most difficult case is when the engine has a star connection of the windings, and there is no way to switch it to a delta (only three wires are brought into the distribution box - the beginning of the windings C1, C2, C3) (see figure below) . In this case, to connect the motor according to the "triangle" diagram, it is necessary to bring the missing ends of the windings C4, C5, C6 into the box.
To do this, gain access to the motor winding by removing the cover and possibly removing the rotor. The place of adhesion is found and released from insulation. The ends are separated and flexible stranded insulated wires are soldered to them. All connections are reliably insulated, the wires are secured with a strong thread to the winding and the ends are brought out to the terminal board of the electric motor. They determine whether the ends belong to the beginnings of the windings and connect them according to the “triangle” pattern, connecting the beginnings of some windings to the ends of others (C1 to C6, C2 to C4, C3 to C5). The job of bringing out missing ends requires some skill. The motor windings may contain not one, but several solders, which are not so easy to understand. Therefore, if you do not have the proper qualifications, you may have no choice but to connect a three-phase motor in a star configuration, accepting a significant loss of power.
Schemes for connecting a three-phase motor to a single-phase network
Delta connection. In the case of a household network, from the point of view of obtaining greater output power, the most appropriate is a single-phase connection of three-phase motors in a delta circuit. Moreover, their power can reach 70% of the nominal. Two contacts in the distribution box are connected directly to the wires of a single-phase network (220V), and the third is connected through a working capacitor Cp to any of the first two contacts or network wires.
Start-up support. A three-phase motor without a load can also be started from a working capacitor (more details below), but if the electric motor has some kind of load, it either will not start or will pick up speed very slowly. Then, for a quick start, an additional starting capacitor Sp is required (calculation of the capacitor capacity is described below). The starting capacitors are turned on only while the engine is starting (2-3 seconds, until the speed reaches approximately 70% of the nominal), then the starting capacitor must be disconnected and discharged.
Connecting a three-phase electric motor to a single-phase network using a delta circuit with a starting capacitor Sp
It is convenient to start a three-phase motor using a special switch, one pair of contacts of which closes when the button is pressed. When it is released, some contacts open, while others remain on - until the "stop" button is pressed.
Reverse. The direction of rotation of the motor depends on which contact ("phase") the third phase winding is connected to.
The direction of rotation can be controlled by connecting the latter, through a capacitor, to a two-position toggle switch connected by its two contacts to the first and second windings. Depending on the position of the toggle switch, the engine will rotate in one direction or the other.
The figure below shows a circuit with a starting and running capacitor and a reverse button, which allows for convenient control of a three-phase motor.
Star connection. A similar diagram for connecting a three-phase motor to a network with a voltage of 220V is used for electric motors whose windings are designed for a voltage of 220/127V.
The required capacity of working capacitors for operating a three-phase motor in a single-phase network depends on the connection diagram of the motor windings and other parameters. For a star connection, the capacitance is calculated using the formula:
For a triangle connection:
Where Cp is the capacitance of the working capacitor in microfarads, I is the current in A, U is the network voltage in V. The current is calculated by the formula:
I = P/(1.73 U n cosph)
Where P is the electric motor power kW; n - engine efficiency; cosф - power factor, 1.73 - coefficient characterizing the relationship between linear and phase currents. The efficiency and power factor are indicated in the data sheet and on the engine plate. Typically their value is in the range of 0.8-0.9.
In practice, the capacitance value of the working capacitor when connected in a triangle can be calculated using the simplified formula C = 70 Pn, where Pn is the rated power of the electric motor in kW. According to this formula, for every 100 W of electric motor power, about 7 μF of working capacitor capacity is required.
The correct selection of capacitor capacity is checked by the results of engine operation. If its value is greater than required under given operating conditions, the engine will overheat. If the capacitance is less than required, the motor output will be too low. It makes sense to select a capacitor for a three-phase motor, starting with a small capacitance and gradually increasing its value to the optimal one. If possible, it is better to select the capacitance by measuring the current in the wires connected to the network and to the working capacitor, for example, with a current clamp. The current value should be as close as possible. Measurements should be made in the mode in which the engine will operate.
When determining the starting capacity, we proceed, first of all, from the requirements for creating the necessary starting torque. Do not confuse the starting capacitance with the capacitance of the starting capacitor. In the above diagrams, the starting capacitance is equal to the sum of the capacitances of the working (Cp) and starting (Sp) capacitors.
If, due to operating conditions, the electric motor starts without load, then the starting capacitance is usually taken to be equal to the working capacitance, that is, a starting capacitor is not needed. In this case, the switching circuit is simplified and cheaper. To simplify this and, most importantly, reduce the cost of the circuit, it is possible to organize the possibility of disconnecting the load, for example, by making it possible to quickly and conveniently change the position of the engine to loosen the belt drive, or by making a pressure roller for the belt drive, for example, like the belt clutch of walk-behind tractors.
Starting under load requires the presence of additional capacity (Cn) connected while the engine is starting. An increase in the switchable capacitance leads to an increase in the starting torque, and at a certain value, the torque reaches its maximum value. A further increase in capacitance leads to the opposite result: the starting torque begins to decrease.
Based on the condition of starting the engine under a load close to the rated load, the starting capacitance should be 2-3 times greater than the working capacitance, that is, if the capacity of the working capacitor is 80 µF, then the capacitance of the starting capacitor should be 80-160 µF, which will give the starting capacitance (the sum capacity of the working and starting capacitors) 160-240 µF. But if the engine has a small load when starting, the capacity of the starting capacitor may be less or, as stated above, it may not exist at all.
Starting capacitors operate for a short time (only a few seconds during the entire switching period). This allows you to use when starting the engine the cheapest launchers electrolytic capacitors specifically designed for this purpose (http://www.platan.ru/cgi-bin/qweryv.pl/0w10609.html).
Note that for a motor connected to a single-phase network through a capacitor, operating without load, the winding fed through the capacitor carries a current 20-30% higher than the rated one. Therefore, if the engine is used in an underloaded mode, the capacity of the working capacitor should be reduced. But then, if the engine was started without a starting capacitor, the latter may be required.
It is better to use not one large capacitor, but several smaller ones, partly due to the possibility of selecting the optimal capacitance by connecting additional ones or disconnecting unnecessary ones; the latter can be used as starting ones. The required number of microfarads is obtained by connecting several capacitors in parallel, based on the fact that the total capacitance in a parallel connection is calculated using the formula: C total = C 1 + C 1 + ... + C n.
Metallized paper or film capacitors are usually used as workers (MBGO, MBG4, K75-12, K78-17 MBGP, KGB, MBGCh, BGT, SVV-60). The permissible voltage must be at least 1.5 times the mains voltage.
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Today, asynchronous electric motors are popular due to their reliability, excellent performance and relatively low cost. Motors of this type are designed to withstand strong mechanical loads. In order for the unit to start up successfully, it must be connected correctly. For this purpose, star and delta connections are used, as well as a combination of them.
Types of connections
The design of the electric motor is quite simple and consists of two main elements - a stationary stator and an internal rotating rotor. Each of these parts has its own windings that conduct current. The stator one is placed in special grooves with a mandatory distance of 120 degrees.
The principle of operation of the engine is simple - after turning on the starter and applying voltage to the stator, a magnetic field appears, causing the rotor to rotate. Both ends of the windings are brought out into the distribution box and are arranged in two rows. Their conclusions are marked with the letter “C” and receive a numerical designation ranging from 1 to 6.
To connect them, you can use one of three methods:
- "Star";
- "Triangle";
- "Star-triangle".
However, a combined circuit cannot be used if it is necessary to reduce the starting current, but at the same time a large torque is required. In this case, you should use an electric motor with a wound rotor equipped with a rheostat.
If we talk about the advantages of combining two connection methods, then we can note two:
- Thanks to the smooth start, the service life is increased.
- You can create two power levels for the unit.
Today, the most widely used electric motors are those designed to operate in 220 and 380 volt networks. The choice of connection diagram depends on this. Thus, it is recommended to use “delta” at a voltage of 220 V, and “star” at 380 V.
