Types of starting capacitors. Capacitors for asynchronous motors. Options for switching circuits - which method to choose
When connecting a 380 V asynchronous three-phase electric motor to a single-phase 220 V network, it is necessary to calculate the capacitance of the phase-shifting capacitor, or rather two capacitors - the working and starting capacitor. Online calculator for calculating capacitor capacity for a three-phase motor at the end of the article.
How to connect an asynchronous motor?
The asynchronous motor is connected according to two schemes: triangle (more efficient for 220 V) and star (more efficient for 380 V).
In the picture at the bottom of the article you will see both of these connection diagrams. Here, I think, it’s not worth describing the connection, because... this has been described a thousand times on the Internet.
Basically, many people have a question about what capacities of the working and starting capacitors are needed.
Start capacitor
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It is worth noting that on small electric motors used for domestic needs, for example, for a 200-400 W electric sharpener, you can not use a starting capacitor, but get by with one working capacitor, I have done this more than once - a working capacitor is quite enough. Another thing is if the electric motor starts with a significant load, then it is better to use a starting capacitor, which is connected in parallel with the working capacitor by pressing and holding the button while the electric motor accelerates, or using a special relay. The starting capacitor capacity is calculated by multiplying the working capacitor capacity by 2-2.5; this calculator uses 2.5.
It is worth remembering that as the asynchronous motor accelerates, it requires less capacitor capacity, i.e. You should not leave the starting capacitor connected for the entire operating time, because A large capacity at high speeds will cause overheating and failure of the electric motor.
How to choose a capacitor for a three-phase motor?
The capacitor used is non-polar, for a voltage of at least 400 V. Either a modern one, specially designed for this (3rd figure), or a Soviet type MBGCh, MBGO, etc. (Fig. 4).
So, to calculate the capacitances of the starting and running capacitors for an asynchronous electric motor, enter the data in the form below, you will find this data on the nameplate of the electric motor, if the data is unknown, then to calculate the capacitor you can use the average data that is inserted into the form by default, but the power of the electric motor is needed must be specified.
Online calculator for calculating capacitor capacity
Calculation of capacitor capacity22:
The function of stabilizers is that they act as capacitive energy fillers for stabilizer filter rectifiers. They can also transmit signals between amplifiers. To start and operate for a long period of time, capacitors are also used in the AC system for asynchronous motors. The operating time of such a system can be varied using the capacitance of the selected capacitor.
The first and only main parameter of the above-mentioned tool is capacity. It depends on the area of the active connection, which is isolated by a dielectric layer. This layer is practically invisible to the human eye; a small number of atomic layers form the width of the film.
An electrolyte is used if it is necessary to restore the oxide film layer. For proper operation of the device, the system must be connected to a network with alternating current of 220 V and have a clearly defined polarity.
That is, a capacitor is created in order to accumulate, store and transmit a certain amount of energy. So why are they needed if you can connect the power source directly to the engine. It's not that simple. If you connect the motor directly to a power source, then at best it will not work, at worst it will burn out.
In order for a three-phase motor to work in a single-phase circuit, you need a device that can shift the phase by 90° on the working (third) terminal. The capacitor also plays the role of a kind of inductor, due to the fact that alternating current passes through it - its surges are leveled out due to the fact that, before operation, in the capacitor, negative and positive charges are evenly accumulated on the plates, and then transferred to the receiving device.
There are 3 main types of capacitors:
- Electrolytic;
- Non-polar;
- Polar.
Description of types of capacitors and calculation of specific capacitance
When choosing the best option, you need to consider several factors. If the connection occurs through a single-phase network with a voltage of 220 V, then a phase-shifting mechanism must be used to start. Moreover, there should be two of them, not only for the capacitor itself, but also for the engine. The formulas used to calculate the specific capacitance of a capacitor depend on the type of connection to the system; there are only two of them: triangle and star.
I 1 – rated motor phase current, A (Amps, most often indicated on the motor packaging);
U network – network voltage (the most standard options are 220 and 380 V). There are also higher voltages, but they require completely different types of connections and more powerful motors.
Sp = Wed + Co
where Cn is the starting capacitance, Cp is the working capacitance, Co is the switched capacitance.
So as not to strain with calculations, smart people derived average, optimal values, knowing the optimal power of electric motors, which is denoted by M. An important rule is that the starting capacity should be greater than the working capacity.
With a power of 0.4 to 0.8 kW: working capacitance – 40 µF, starting power – 80 µF, From 0.8 to 1.1 kW: 80 µF and 160 µF, respectively. From 1.1 to 1.5 kW: Av – 100 µF, Sp – 200 µF. From 1.5-2.2 kW: Av – 150 µF, Sp 250 µF; At 2.2 kW, the operating power should be at least 230 μF, and the starting power should be 300 μF.
When a motor designed to operate at 380 V is connected to an AC network with a voltage of 220 V, half of the rated power is lost, although this does not affect the rotor rotation speed. When calculating power, this is an important factor; these losses can be reduced with a “delta” connection diagram; the engine efficiency in this case will be 70%.
It is better not to use polar capacitors in a system connected to an alternating current network, in this case the dielectric layer is destroyed and the device heats up and, as a result, a short circuit occurs Connection diagram "Triangle"
The connection itself is relatively easy; the current-carrying wire is connected to and from the motor (or motor) terminals. That is, if we take it more simply, there is a motor; it contains three current-carrying conductors. 1 – zero, 2 – working, 3 – phase.
The power wire is stripped and there are two main wires in a blue and brown winding, the brown one is connected to terminal 1, one of the capacitor wires is also connected to it, the second capacitor wire is connected to the second working terminal, and the blue power wire is connected to the phase.
If the motor power is small, up to one and a half kW, in principle only one capacitor can be used. But when working with loads and high powers, it is mandatory to use two capacitors; they are connected in series, but between them there is a trigger mechanism, popularly called “thermal”, which turns off the capacitor when the required volume is reached.
A quick reminder that the lower wattage starting capacitor will be turned on for a short period of time to increase the starting torque. By the way, it is fashionable to use a mechanical switch, which the user himself will turn on for a given time.
You need to understand that the motor winding itself already has a star connection, but electricians use wires to turn it into a delta. The main thing here is to distribute the wires that go into the junction box.
Connection diagram “Triangle” and “Star” Connection diagram "Star"
But if the engine has 6 outputs - terminals for connection, then you need to unwind it and see which terminals are interconnected. After that, it is reconnected to the same triangle.
To do this, change the jumpers, let's say there are 2 rows of terminals on the engine, 3 each, they are numbered from left to right (123.456), using wires they are connected in series 1 to 4, 2 to 5, 3 to 6, you first need to find the regulatory documents and look on which relay the winding starts and ends.
In this case, the conditional 456 will become: zero, working and phase - respectively. A capacitor is connected to them, as in the previous circuit.
When the capacitors are connected, all that remains is to test the assembled circuit, the main thing is not to get confused in the sequence of connecting the wires.
An asynchronous three-phase motor can be connected without much damage to a conventional single-phase electrical network through capacitors. With their help, the launch and achievement of the desired operating modes with such a power system is ensured. There are working and starting capacitors.
Differences between them
They lie in their purpose, capacity, method of connection, as well as operating conditions. The first difference is that the worker (first) the capacitor serves to shift the phases. As a result, a rotating magnetic field appears between the windings, which is necessary to drive the motor, which is without mechanical load. Such an electric motor is used, for example, in a grinding machine.
Starting (second) provides an increase in the starting torque of the motor, which is under mechanical load, due to which it more easily reaches the desired mode. The resources of one worker may not be enough, which is why the engine rotor simply does not start rotating. The use is justified in conjunction with machine tools, lifting mechanisms, pumps and similar heavy equipment. It can also be used with a more powerful three-phase motor if there is not enough worker to start it reliably.
The capacity of both capacitors will also be different. It is directly proportional to the power of the electric motor and inversely to the network voltage. Depending on the winding connection diagram, a correction factor is introduced. The starting capacity can be twice that of the working one.
Connection methods
In the most common case, the first capacitor is connected to the gap in one of the windings of an asynchronous electric motor, which is also often called “auxiliary”. The other is connected directly to the electrical network, and the third remains unused. This type of circuit is called “star”. There is also a triangle connection. It varies in connection method and complexity.
The second capacitive element, unlike the working one, is connected in parallel to the latter through a button or centrifugal switch. In the first case, control is carried out by a person, and in the second - by the drive itself. Both of these switches briefly close this circuit when the electric motor starts, and after it reaches operating mode, they open it.
Working conditions
They are different for each capacitor. Since the first of them is permanently connected to the motor winding, this circuit forms an elementary oscillatory circuit. Because of this, at certain moments a voltage is formed at its terminals that exceeds the input voltage by two and a half to three times. This circumstance should be taken into account when selecting; you need to focus on parts designed for 500-600 volts.
Starting capacitors for electric motors - 220 V operate under other, less severe conditions, unlike working ones. The voltage applied to this capacitive element exceeds the main voltage by approximately 1.15 times. It is attached to the circuits from time to time, which also has a positive effect on its operating conditions and significantly extends its service life.
The most commonly used domestic paper or oil-filled capacitors brands MBGO or MBGCH. Their advantage is their resistance to high AC voltages. But there is also a drawback - the large size. As an alternative solution, oxide capacitors can be used. They are connected not directly, but through diodes, according to certain circuits.
Conventional electrolytic capacitors used in various devices, and designed for considerable operating voltages, are suitable for asynchronous motors only as starting motors. This is due to the fact that large reactive power passes through them due to the low resistance of the windings. Connecting capacitive elements with violations or deviations from the circuit will lead to damage or boiling of the electrolyte, which can cause harm to the motor and personnel.
So a few tips can be deduced from this, how to distinguish a starting capacitor from a working one:
- The first of them plays a supporting role. It is connected in parallel to the worker while the engine is starting - for a few seconds to make the start easier.
- The second of them is permanently connected, providing the necessary phase shift, as a result of which a three-phase motor can operate from a single-phase network.
If you mix up the capacitors, serious problems will arise. The capacity of the worker should also not be too large, otherwise the motor will heat up, and the increase in power and torque will increase slightly.
Motors, which are called single-phase, usually have two windings on the stator. One of them is called the main or working one, the other is called the auxiliary or starting one. The need to have two spatially shifted windings, powered by currents shifted by 90 degrees to obtain starting torque. 
The motors are called single-phase because they are originally designed to be powered by single-phase alternating current.
The time shift of currents is ensured by including a phase-shifting element - a resistor or electric capacitor.
In motors with a starting resistor (often the starting phase is performed with increased resistance), the magnetic field is elliptical; in engines with a starting electric capacitor, the field is closer to circular. The auxiliary winding is switched off after the engine accelerates and the engine operates as a single-phase single-winding. Its resulting field is sharply elliptical. For this reason, single-phase motors have low energy performance and low overload capacity.
In engines with a permanently switched on capacitor, the capacitance of the latter is selected, as a rule, from the conditions for ensuring a circular field in nominal mode. In this case, the magnetic field at start-up is far from circular and the starting torque is therefore small. To improve the starting properties, a starting electrical capacitor is connected in parallel to the working capacitor at the start.
In electric drives with easy starting conditions, single-phase motors with shielded poles are often used. In such motors, the role of the auxiliary phase is played by short-circuited turns placed on the salient poles of the stator. Since the spatial angle between the axes of the main phase (excitation winding) and the turn is much less than 90°, the field in such a motor is sharply elliptical. Therefore, the starting and operating properties of motors with shaded poles are low.
Single-phase asynchronous motors with a squirrel-cage rotor are used: with increased resistance of the starting phase, with a starting capacitor, with a running capacitor, both, as well as motors with shielded poles.
Basic technical data of single-phase IM for voltage 220 V: k, - starting current multiplicity; kp - multiplicity of starting torque; km - multiple of the maximum torque or overload capacity of the engine.
Basic parameters of electrical capacitors
A capacitor is a concentrator of electric field energy with an electrical capacitance and consists of conductive electrodes separated by a dielectric - plates with leads for connection to an electrical circuit.
The capacitance of a capacitor is the ratio of the amount of charge on the capacitor to the potential difference across its plates, which is imparted to the capacitor:
The unit of capacitance in the international SI system is taken to be a farad (F) - the capacitance of a capacitor whose potential increases by one volt (V) when a charge of one coulomb (C) is imparted to it. This is a very large value, so for practical purposes smaller units of capacitance are used: microfarad (μF), nanofarad (nf) and picofarad (pF):
1 f = 106 µF = 109 nF = 1012 pF.
The capacitance of the capacitor depends on the area of the capacitor plate S, the thickness of the dielectric layer separating them d and the electrical properties of the dielectric, characterized by dielectric constant e:
The nominal capacitance of the capacitor is called the capacitance indicated on its body. Nominal capacity values are standardized.
IEC (Publication No. 63) has established seven preferred rows for rated capacity values: E3; E6; E12; E24; E48; E96; E192. The numbers after the letter E indicate the number of nominal values in each decimal interval (deca¬de), which correspond to the numbers 1.0; 1.5; 2.2; 3.3; 4.7; 6.8 or numbers obtained by multiplying or dividing by 10″, where n is a positive or negative integer. In the symbol, the nominal capacitance is expressed in microfarads (μF) or picofarads (pF).
A coding system is used to designate nominal capacities. It consists of three or four characters, including two or three numbers and a letter. The letter of the code from the Russian or Latin alphabets denotes the multiplier that makes up the capacitance value and determines the position of the decimal point. The letters P(p), N(p), M(m), I(1), Ф(Р) denote the factors 10~12, 10~9, 10~6, 10-3 and 1, respectively, for the values of capacitance, height ¬wife in farads.
For example, a capacitance of 2.2 pF is designated 2P2 (2p2); 1500 pF - 1H5 (1p5); 0.1 µF - M1 (m1); 10 µF - YuM (Yum); 1 farad - 1F0 (1F0).
The actual value of the capacitance may differ from the nominal value by the permissible deviation in percentage. Permissible deviations vary depending on the type and accuracy of the capacitor within a very wide range from ±0.1 to +80%.
The rated voltage is the voltage indicated on the capacitor or in the documentation for it, at which it can operate under specified conditions during its service life while maintaining parameters within acceptable limits. The rated voltage depends on the design of the capacitor and the properties of the materials used. During operation, the voltage on the capacitor should not exceed the rated voltage. For many types of capacitors, as the temperature increases (usually 70...85 °C), the permissible voltage decreases. The rated voltages of capacitors are set in accordance with the series (GOST 9665-77): 1; 1.6; 2.5; 3.2; 4; 6.3; 10; 16; 20; 25; 32; 40; 50; 63; 80; 100; 125; 160; 200; 250; 315; 350; 400; 450; 500; 630; 800; 1000; 1600; 2000; 2500; 3000; 4000; 5000; 6300; 8000; 10000 V.
The temperature coefficient of capacitance (TKE) determines the relative change in capacitance (in ppm) from temperature when it changes by 1 °C.
The loss tangent (tg8) characterizes the loss of electrical energy in the capacitor. The values of the loss tangent for polystyrene and fluoroplastic capacitors are in the range (15...15)10~4, polycarbonate (15...25)10~4, oxide 5...35%, polyethylene terephthalate 0.01...0.012. The reciprocal of the loss tangent is called the quality factor of the capacitor.
Insulation resistance and leakage current. These parameters characterize the quality of the dielectric and are used in calculations of high-resistance, time-setting and low-current circuits. The highest insulation resistance is for fluoroplastic, polystyrene and polypropylene capacitors, slightly lower for high-frequency ceramic, polycarbonate and lavsan capacitors.
To mark capacitors of constant capacitance, use the letter K (capacitor of constant capacitance) and numbers that determine the type of dielectric.
A conventional synchronous and asynchronous motor is powered from an alternating voltage network. There are also “unusual” engines, for example, powered from the vehicle’s on-board network or from special generators. The principle of their operation is the same, but the frequency of the supply voltage, as a rule, is noticeably higher than 50 Hz.
In an AC electric motor, the stator provides spatial movement of the magnetic field. Without this, the rotor will not be able to start rotating on its own.
The role of capacitors in an electric drive
If the supply voltage is single-phase, using a capacitor you can obtain a magnetic field movement in the stator. To do this, it needs an additional winding. It is connected via a capacitor. The size of its capacity is directly proportional to the starting torque. If you measure its value (ordinate axis) according to the increase in capacitance (abscissa axis), you will get a curve. From a certain value of the capacitance, the torque increment will become smaller and smaller.
The capacitance value, starting from which the increment in torque noticeably decreases, will be optimal for starting this motor. But for an overclocked engine and its long-term operation, the starting capacitor is always too large in capacity. To maintain stable operation of the electric motor, a run capacitor is used. Its capacity is less than that of the starter. You can also choose the right operating capacitor experimentally.
How to determine the optimal capacity size
This will require several capacitors connected in parallel. Along the connections, an ammeter measures the current consumed by the electric motor. It will decrease as the total capacity increases. But from a certain value its current will begin to increase. The minimum value of the current corresponds to the optimal value of the capacitance of the working capacitor. For normal operation of the engine, two capacitors are used with the possibility of parallel connection with each other. The connection diagram containing the starting and running capacitor is shown below.
When starting, they are connected, forming the best capacity for accelerating the engine. Why use a separate starting capacitor of the same capacity if the installation turns out to be unreasonably cumbersome. Therefore, it is advantageous to use a container made up of two parts. Although it also includes a run capacitor, it becomes part of the virtual start capacitor at startup. And those that can be switched off are called starting capacitors.
Calculation of working capacity
Experimental determination of the capacitance of capacitors is the most accurate. However, these experiments take a lot of time and are quite labor-intensive. Therefore, in practice, estimation methods are mainly used. They will require the engine power value and coefficients. They correspond to the “star” (12.73) and “triangle” (24) schemes. The power value is necessary to calculate the current strength. To do this, its nameplate value is divided by 220 (the value of the current mains voltage). Power is taken in watts.
- The resulting number is multiplied by the corresponding coefficient and gives the value of microfarads.
Selection of starting capacity
But the mentioned method determines the capacity of the working capacitor. If the engine is used in an electric drive, it may not start with it. An additional starting capacitor will be required. In order not to bother yourself when making a selection, you can start with a container of the same size. If the engine still does not start due to the load on the drive side, it is necessary to add in parallel.
After each connected instance, you need to apply voltage to the engine to check startup. After starting the engine, the last of the connected capacitors will complete the formation of the capacitance required for the engine in starting mode. If for any reason, after being connected to the electrical network, the capacitor is disconnected from it, it must be discharged.
To do this, use a resistor with a value of several kilo-ohms. First, before connecting, its leads must be bent so that their ends are at the same distance as the terminals. The resistor is taken by one of the terminals with pliers with insulated handles. By pressing the resistor leads to the terminals for a few seconds, the capacitor is discharged. After this, it is advisable to check with a multimeter-voltmeter how many volts there are on it. It is desirable that the voltage either resets to zero or remains less than 36 V.
Metal paper and film capacitors
The 220 V AC voltage value used for motor specifications corresponds to the current value. But with it, the amplitude voltage value will be 310 V. It is to this level that the electric motor capacitor will be charged. Therefore, the rated voltage of the starting and operating capacitor is selected with a margin and is at least 350 volts. The most reliable types of them are metal-paper and metal-film capacitors.
But their sizes are large, and the capacity of one capacitor is not enough for most industrial engines. For example, for a 1 kW engine, only the working capacitance is equal to 109.1 μF. Consequently, the starting capacity will be more than 2 times larger. To select a capacitor of the required capacity, for example, for a 3 kW engine if there is already a selected instance for a power of 1 kilowatt, you can take it as a basis. In this case, one capacitor is replaced by three connected in parallel.
For the operation of the engine, it makes no difference which capacitors - one or three - are used when turned on. But it’s better to choose three. This option is economical, despite the larger number of connections. Overvoltage will only damage one of the three. And replacing it will cost less. One large capacitor, when replaced, will have a significantly higher price.
If you need an optimal size specimen, it is selected in the table according to the data provided.
Electrolytic capacitors
The metal film capacitors under consideration are stable, reliable and durable under the correct operating conditions, among which the most important parameter is voltage. But in the electrical network, as a result of switching consumers, as well as for other reasons, overvoltages are possible. If a breakdown of the insulation of the plates occurs, they become unsuitable for further work. But this does not happen often, and the main problem with using these models is the dimensions.
A more compact alternative may be electrolytic capacitors (so-called electrolytes). They have significant differences in their smaller size and structure. Therefore, they can replace several units of metal and paper with 1 electrolyte. But the properties of their structure limit their service life. Although there is a positive side - self-healing after a breakdown. Long-term operation of electrolytes on alternating current is impossible. It will heat up and eventually destroy, at least the safety valve. And even the body.
To prevent such incidents, diodes must be connected. Connecting the starting capacitor with diodes is done as shown in the image below. But this does not mean that any of the electrolyte models with a voltage of 350 V or more can be used. The level of pulsations and their frequency are strictly regulated. If these parameters are exceeded, heating begins. The capacitor may fail. To start and operate engines, special electrolytes with diodes inside are made. Only such models should be used for engines.