How to determine the starting capacitor from the working one. How to choose capacitors to start an electric motor

In one of the previous articles, we talked about the selection of working capacitors for operating a 3 ph. (380 Volt) asynchronous electric motor from 1 ph. network (220 Volt). Namely about . Thank you, my readers, for the many reviews and thanks, because if it weren’t for you, I would have abandoned this business a long time ago. In one of the letters sent to me by email there were questions: “Why didn’t you tell me about the starting capacitors?”, “Why doesn’t my engine start, because I did everything as it was written.” But it’s true that there are not always enough “working” capacitors to start an electric motor under load, and the question arises: “What to do?” And here’s what: “We need starting capacitors.” But now we’ll talk about how to choose them correctly.

And so we have: 3 phase motor, based on which we selected a working capacitor capacity of 60 μF. For the starting capacitor, we take a capacity 2 - 2.5 times greater than the capacity of the working capacitor. Thus, we will need a capacitor with a capacity of 120 - 150 μF. In this case, the operating voltage of these capacitors should be 1.5 times more voltage networks. Now many people have a question: “Why not 300 μF or even 1000 μF, because you can’t spoil the porridge with oil?” But in this case, everything should be in moderation, with too much capacity starting capacitors nothing very terrible will happen, but the efficiency of starting the electric motor will be worse. Thus, you should not spend extra money on buying too large a container.

But what kind of capacitors are needed to start an electric motor?

If we need a small capacity of the starting capacitor, then capacitors of the same type that we used for the working capacitors are quite suitable. But what if we need a fairly large capacity? For such a purpose, it is not advisable to use this type of capacitors due to their high cost and size (when assembling a large bank of capacitors, its dimensions will be large). For such purposes, we use special starting (starting) capacitors, which are now on sale in a large assortment. Such capacitors come in different shapes and types, but their names contain a marking (inscription): “Start”, “Starting”, “Motor Start” or something like that, they all serve to start the electric motor. But for better persuasiveness, it is better to ask the seller when purchasing, he will always tell you.

But now you say: “What about the capacitors from old Soviet b/w TVs, the so-called “electrolytes”?”

What can I tell you about this? I don’t use them myself, and I don’t recommend them to you, and even discourage you. This is because their use as starting capacitors is not entirely safe. Because they can swell or, even worse, explode. In addition, this type of capacitors dries out over time and loses its nominal capacity, and we cannot know exactly which one we are using at the moment.

And so we have an electric motor, a working and a starting capacitor. How do we connect all this?

To do this, we need a PVS button.

The PNVS button (push starter with start contact) has three contacts: two outer ones - with locking and one in the middle - without locking. It serves to turn on the starting capacitor, and when you stop pressing the button it returns to its original position (the starting capacitor “Sp” is turned on only when starting the engine, and the working capacitor “Cp” is constantly in operation), the other two extreme contacts remain switched on and are switched off when the Stop button is pressed. The “Start” button must be held until the shaft speed reaches maximum speed, and only then released. Also, do not forget that the capacitor tends to have an electric current charge, and you may get injured electric shock. To prevent this from happening, after finishing work, disconnect the electric motor from the network and turn on the “Start” button for one or two seconds so that the capacitors can discharge. Or place a resistor of about 100 kilo-ohms in parallel with the starting capacitor so that the capacitor is discharged onto it.

Three-phase electric motors asynchronous type are very common today, so many people need to connect them to various equipment when carrying out work in the garage or at their summer cottage.

This process can be problematic because many power supplies are designed for single-phase voltage. This issue can be resolved by using special circuits that imply the presence of a working and a starting one.

How to choose a capacitor

Initially, a working capacitor is purchased, its selection is made taking into account the rated electric current of the starter and voltage indicators in a single-phase network. When using three phase motor, with a power of about 100 W, a working capacitor with a capacity of 7 μF is usually sufficient.

Special clamps are used for measurements; when making calculations, it is important to monitor the electric current entering the stator phase winding: its indicators should not exceed the nominal value.

In some cases, such measures are not enough and a starting capacitor must be added to the circuit; the need for it usually arises when there is excessive load on the shaft at the time of switching on.

Its work and functions will be as follows:

The owner of the equipment must remember the need to disconnect the starting capacitors, otherwise there is a serious risk of overheating of the asynchronous electric motor due to significant current imbalance in the phases.

The main criterion for choosing a starting capacitor is its capacity, it should be at least 2-3 times higher than the same parameter of the working capacitor. If the calculation was made correctly, then at the moment of starting the engine reaches its nominal values ​​and no problems are observed.

When making a choice, you also need to pay attention to the following points:

1. You can use paper or electrolytic capacitors. The first option is the most common, although it has a significant drawback, which is the combination of large dimensions and low capacity, which creates the need to use a large number of devices with high engine power. Because of this, many people turn to electrolytic devices, which require the addition of resistors and diodes to the circuit. This practice is considered undesirable, since there is always a risk that the diodes will not cope with their task, which can lead to negative and dangerous consequences, including overheating of equipment and explosions of the starting capacitor. If you are unable or unwilling to use paper models, you can turn to more modern version: launch of models equipped with an improved metallized coating. Most of them are designed to operate with voltages ranging from 400 to 450 V.
2. The operating voltage indicator is another important criterion for choosing three-phase motor rectifiers. Many people mistakenly purchase devices with very high performance when there is no need for such a resource; this leads to an increase in financial expenses for the purchase and the allocation of a large amount of space for the installation of large equipment. At the same time, it is important to ensure that the voltage indicator is no less than in the electrical network, otherwise the selected model will not be able to function properly and will very quickly fail. To implement optimal choice it is necessary to make the following calculation: multiply the actual voltage present in the network by a factor of 1.15. Thanks to this, you will get an indicator of the required voltage, but it should not be less than 300V.

In most cases, paper models equipped with a protective case made of steel are well suited for the purposes described. They actually always have a rectangular shape; the main operating parameters are usually indicated on the body.

Connecting the starting capacitor to the electric motor

When implementing such schemes in practice and connecting starting devices, it will be necessary to do the following:

1. Initially check the starting capacitor using to make sure it works.
2. Select the most suitable connection scheme I, here the owner of the equipment is given complete freedom. The winding and capacitor terminals of most motors are located in.
3. In some situations, it becomes necessary to modify the existing scheme, in this case it is necessary to independently recalculate the main indicators according to the schemes already considered.

Models

Many models of such devices differ not in capacity, but in the type of design. Below are examples of some accessories that are suitable for connecting electric motors:

It is a polypropylene device that is equipped with a metallized coating. This is the most modern and optimal option, its cost is about 300 rubles.

HTC film type have the same capacity as SVV-60, but they usually cost no more than 200 rubles.

E92 is a Russian-made analogue with an identical capacity, while such a device is a budget option that can be purchased at a price of 100-150 rubles.

1. Initially, you need to make sure that it is advisable to include a starting device in the circuit, since in some situations you can do without it.
2. In the absence of confidence in own strength when implementing the selected scheme connection, it is better to seek help from professionals.
3. Depending on the circumstances and characteristics of the situation, you can implement both serial and parallel connection circuit.

It is good if you can connect the motor to the required type of voltage. What if this is not possible? This becomes a headache since not everyone knows how to use the three-phase version of the motor based on single-phase networks. This problem appears in various cases, it may be necessary to use a motor to sand or drilling machine- capacitors will help. But they come in many types, and not everyone can understand them.

To give you an idea of ​​their functionality, we’ll next look at how to choose a capacitor for an electric motor. First of all, we recommend deciding on the correct capacity of this auxiliary device and how to accurately calculate it.

What is a capacitor?

Its device is simple and reliable - inside two parallel plates, in the space between them, a dielectric is installed, which is necessary for protection against polarization in the form of a charge created by the conductors. But different types of capacitors for electric motors are different, so it’s easy to make a mistake at the time of purchase.

Let's look at them separately:

Polar versions are not suitable for connection based AC voltage, since the risk of dielectric disappearance increases, which will inevitably lead to overheating and an emergency situation - a fire or a short circuit.

Non-polar versions are distinguished by high-quality interaction with any voltage, which is due to the universal plating option - it is successfully combined with increased current power and various types dielectrics.

Electrolytic, often called oxide, is considered best for low frequency motors as their maximum capacitance can reach 100,000 IF. This is possible due to the thin type of oxide film included in the design as an electrode.

Now check out the photo of capacitors for an electric motor - this will help you distinguish them by appearance. This information will be useful during the purchase and will help you purchase required device because they are all similar. But the help of the seller may also be useful - it’s worth using his knowledge if you don’t have enough of your own.

If a capacitor is needed to operate a three-phase electric motor

It is necessary to correctly calculate the capacitance of the electric motor capacitor, which can be done using a complex formula or using a simplified method. To do this, the power of the electric motor is specified; for every 100 watts, about 7-8 μF of the capacitor capacity will be required.

But during calculations it is necessary to take into account the level of voltage impact on the winding part of the stator. It must not exceed the nominal level.

If the engine can start only based on maximum load, you will have to add a starting capacitor. It is distinguished by its short duration of operation, since it is used for approximately 3 seconds before the rotor speed reaches its peak.

It must be taken into account that it will require a power increased by 1.5 times, and a capacity increased by approximately 2.5 - 3 times, than that of the network version of the capacitor.

If a capacitor is needed to operate a single-phase electric motor

Usually different capacitors for asynchronous electric motors are used to work with a voltage of 220 V, taking into account installation in a single-phase network.

But the process of using them is a little more complicated because three-phase electric motors work using a constructive connection, and for single-phase versions it will be necessary to provide offset torque at the rotor. This is achieved by using an increased amount of winding to start, and the phase is shifted by the forces of the capacitor.

What is the difficulty in choosing such a capacitor?

In principle, there is no greater difference, but different capacitors for asynchronous electric motors will require a different calculation of the permissible voltage. About 100 watts will be required for each microfarad of device capacity. And they differ in the available operating modes of electric motors:

• A starting capacitor and a layer of additional winding are used (only for the starting process), then the calculation of the capacitance of the capacitor is 70 μF for 1 kW of electric motor power;
• A working version of a capacitor with a capacity of 25 - 35 µF is used based on an additional winding with a constant connection during the entire duration of operation of the device;
• A working version of the capacitor is used based on parallel connection of the starting version.

But in any case, it is necessary to monitor the level of heating of engine elements during its operation. If overheating is noticed then action must be taken.

In the case of a working version of the capacitor, we recommend reducing its capacity. We recommend using capacitors that operate on a power basis of 450 V or more as they are considered the best option.

To avoid unpleasant moments, before connecting to the electric motor, we recommend that you verify the functionality of the capacitor using a multimeter. In the process of creating the necessary connection with the electric motor, the user can create a fully operational circuit.

Almost always, the terminals of the windings and capacitors are located in the terminal part of the motor housing. Due to this, you can create virtually any modernization.

Important: The starting version of the capacitor must have an operating voltage of at least 400 V, which is associated with the appearance of a surge of increased power up to 300 - 600 V that occurs during the process of starting or shutting down the engine.

So, what is the difference between a single-phase asynchronous version of an electric motor? Let's look at this in detail:

• It is often used for household appliances;
• To start it, an additional winding is used and an element for phase shifting is required - a capacitor;
• Connects based on multiple circuits using a capacitor;
• To improve the starting torque, a starting version of the capacitor is used, and the performance is increased by using a running version of the capacitor.

Now you have the necessary information and know how to connect a capacitor to an induction motor for maximum efficiency. You also have acquired knowledge about capacitors and how to use them.

Photo of capacitors for an electric motor

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 motor, the stator provides spatial movement 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 increment in torque 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 The electric motor uses a run capacitor. 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 ability parallel connection among themselves. 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 starting virtual 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 passport value is divided by 220 (the value effective voltage electrical network). 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 with 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 the engine starts, 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 amount of 220 V AC voltage used for technical characteristics engines, 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 varieties 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 it 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, subject to the right conditions operation, 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 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.

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.

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 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 conditions For starting, single-phase IMs 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 are used asynchronous motors with a squirrel-cage rotor: with increased resistance of the starting phase, with a starting capacitor, with a running capacitor, with both, as well as motors with shielded poles.

Basic technical data of single-phase IM for voltage 220 V: k, - multiplicity starting current; 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 an energy concentrator with electrical capacitance. electric field and consists of conductive electrodes separated by a dielectric - plates with leads for connection to the 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 its documentation at which it can operate in given conditions during the 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 with increasing temperature (usually 70...85 °C) permissible voltage decreases. Rated voltages capacitors are installed 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 capacity, use the letter K (capacitor of constant capacity) and numbers that determine the type of dielectric.