everything is a little simpler. In any sane textbook with the title “ Electrical machines”, at the end of the section devoted to the theory of an asynchronous motor, the issue of operation of an asynchronous motor in single-phase mode is considered, with various schemes winding connections. Formulas for calculating the capacity of working and starting capacitors are also given there. Exact calculation is quite complicated - you need to know the specific parameters of the engine. The simplified calculation method is as follows: Star Srab = 2800 (Inom / Uset); Descent = Trigger 2÷3 (under difficult launch conditions, multiplicity 5); Triangle Serb = 4800 (Inom / Uset); Descent = Trigger 2÷3 (under difficult launch conditions, multiplicity 5); where, Srab is the capacity of the working capacitor, μF; Descent - capacity starting capacitor, µF; Inom – rated phase current of the motor at rated load, A; Uset – voltage of the network to which the motor will be connected, V. Calculation example. Initial data: we have an asynchronous electric motor - 4 kW; winding connection diagram –Δ / Y voltage U – 220 / 380 V; current I – 8 / 13.9 A. For motor currents: 8 A is the phase current (i.e. the current of each of the three windings) of the motor on the delta and the star, and it is also the linear current on the star; 13.9 A is the linear current of the motor on the triangle (we won’t need it in the calculations). Well, and, in fact, the calculation itself: Star Srab = 2800 (Inom / Uset) = 2800 (8 / 220) = 101.8 uF Release = Slab 2÷3 = 101.8 2÷3 = 203.6÷305, 4 µF (under severe starting conditions - 509 µF) Triangle Cut = 4800 (Inom / Uset) = 4800 (8 / 220) = 174.5 µF Release = Cut 2÷3 = 174.5 2÷3 = 349÷523, 5 µF (under severe starting conditions - 872.5 µF) Type of working capacitor - polypropylene (imported SVV-60 or domestic analogue - DPS). The voltage of the condenser is at least 400 V according to alternation (example of marking: AC ~ 450 V), for Soviet paper MBGOs the operating voltage should be at least 500 V, if less, connect in series, but this is a loss of capacity, of course - so many condensers will have to be dialed) . For starting capacitors, it is better, of course, to also use polypropylene or paper, but this will be expensive and cumbersome. To reduce the cost, you can take polar electrolytics (these are those that have “+” and/or “–” on the body), having first made two polar electrolytes, one non-polar, by connecting two capacitors with minuses together (you can also connect them with pluses, but of some capacitors, the minus is connected to the body of these capacitors and if you connect them with pluses, then you will have to isolate these capacitors not only from the surrounding hardware, but also from each other, otherwise short circuit), and leave the remaining two pluses for connection to the motor windings (not we forget that when serial connection two identical capacitors, their total capacitance is halved, and the operating voltage is doubled - for example, by connecting in series (minus to minus) two 400 V 470 μF capacitors, we get one non-polar capacitor with an operating voltage of 800 V and a capacity of 235 μF). The operating voltage of each of the two series-connected electrolytes must be at least 400 V. We collect the required starting capacitance (if necessary) by parallel connecting such dual (i.e., already non-polar) electrolytes - when connecting capacitors in parallel, the operating voltage remains unchanged, and the capacitances are summed up (the same as when connecting batteries in parallel). There is no need to invent this “collective farm” with dual electrolytes - there are ready-made starting non-polar electrolytes - for example, type CD-60. But, in any case, with electrolytes (both non-polar, and even more so with polar) there is one BUT - such capacitors can be turned on in a 220 V network (polar ones are better not turned on at all) only while the engine is starting - electrolytes cannot be used as working capacitors - will explode (polar almost immediately, non-polar a little later). With a working capacitor on the delta, the motor loses 25-30% of its three-phase power, on a star 45-50%. Without a working capacitor, depending on the winding connection diagram, the power loss will be more than 60%. And one more thing about the condensers: there are a lot of videos on YouTube where people select working capacitors based on the sound of the engine at idle (without load) and, frightened by the increased hum of the engine, reduce the capacity of the working capacitors until this hum decreases to more or less acceptable. This is an incorrect selection of a working air conditioner - this reduces the engine power under load. Yes, increased motor hum is not very good, but it is not too dangerous for the windings if the capacity of the working capacitor is not too high. The fact is that ideally, the capacity of the working capacitor should change smoothly, depending on the engine load - the greater the load, the greater the capacity should be. But it is quite difficult to make such a smooth adjustment of the capacity; it is both expensive and cumbersome. Therefore, a capacity is selected that will correspond to a specific motor load - usually the nominal load. When the capacity of the working capacitor corresponds to the calculated load of the engine, the magnetic field of the stator is circular and the hum is minimal. But when the capacity of the working capacitor exceeds the load of the motor, the magnetic field of the stator becomes elliptical, pulsating, uneven, and it is this pulsating magnetic field that causes a hum, due to the uneven rotation of the rotor - the rotor, rotating in one direction, simultaneously jerks back and forth , and with increased currents in the windings, the motor develops less power. Therefore, if the engine hums at medium loads and at idle, then this is not so bad, but if the hum is observed at full load, then this indicates that the capacity of the working condenser is clearly overestimated. In this case, reducing the capacitance will reduce the currents in the motor windings and its heating, level (“round”) the magnetic field of the stator (i.e., reduce the hum) and increase the power developed by the motor. But leave the engine idling long time with a working condenser designed for the full power of the engine, it is still not worth it - in this case, there will be an increased voltage on the working capacitor (up to 350 V), and flow will flow through the winding connected in series with the working capacitor increased current(30% more than nominal - on a triangle, and 15% - on a star). As the load on the motor increases, the voltage on the working conductor and the current in the motor winding connected in series with the working conductor will decrease.
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
Picking up best option There are several factors to consider. 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 themselves with calculations, smart people came up with average, optimal values, knowing the optimal power of electric motors, which is designated M. An important rule is that the starting capacity must 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 “triangle” connection diagram, Engine efficiency in this case it will be equal to 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 assembled circuit, the main thing is not to get confused in the sequence of connecting the wires.
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 because not everyone knows how to use the three-phase version of a single-phase motor. 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 on alternating voltage, since the risk of dielectric disappearance increases, which will inevitably lead to overheating and an emergency situation - fire or 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 of 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. Such information will be useful during the purchase and will help you purchase the necessary device, since 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
Typically, various capacitors for asynchronous electric motors are used to operate 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 at 450V 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
When connecting an asynchronous electric motor to a single-phase 220/230 V network, it is necessary to ensure a phase shift on the stator windings in order to simulate a rotating magnetic field (RPF), which causes the motor rotor shaft to rotate when it is connected to the “native” three-phase AC networks. Known to many who are familiar with electrical engineering, the ability of a capacitor to give electric current a “head start” of π/2=90° compared to voltage provides a good service, since this creates the necessary torque that forces the rotor to rotate in already “non-native” networks.
But the capacitor must be selected for these purposes, and it must be done with high precision. That is why readers of our portal are provided with an absolutely free use of a calculator for calculating the capacity of the working and starting capacitor. After the calculator, the necessary explanations will be given on all its points.
Calculator for calculating the capacitance of working and starting capacitors
Good afternoon, dear readers of the blog site
In the “Accessories” section we will consider capacitors for single-phase. For three-phase motors, when connected to the power supply, a rotating magnetic field arises, due to which the motor starts. Unlike three-phase motors, single-phase motors have two windings in the stator: a working winding and a starting winding. The working winding is connected directly to the single-phase power supply, and the starting winding is connected in series with the capacitor. A capacitor is necessary to create a phase shift between the currents of the working and starting windings. The greatest torque in the motor occurs when the phase shift of the winding currents reaches 90°, and their amplitudes create a circular rotating field. A capacitor is an element of an electrical circuit and is designed to use its capacity. It consists of two electrodes or, more correctly, plates, which are separated by a dielectric. Capacitors have the ability to store electrical energy. In the International System of Units SI, a unit of capacitance is taken to be the capacitance of a capacitor whose potential difference increases by one volt when a charge of one coulomb (C) is imparted to it. The capacitance of capacitors is measured in farads (F). The capacity of one farad is very large. In practice, smaller units of microfarad (μF) are used; one μF equals 10 -6 F, picofarads (pF) one pF equals 10 -12 µF. In single-phase asynchronous engines Depending on the power, capacitors with a capacity of several to hundreds of microfarads are used.
Basic electrical parameters and characteristics
The main electrical parameters include: rated capacitance of the capacitor and rated operating voltage. In addition to these parameters, there is also the temperature coefficient of capacitance (TKE), loss tangent (tgd), and electrical insulation resistance.
Capacitor capacity. The ability of a capacitor to accumulate and hold an electric charge is characterized by its capacitance. Capacitance (C) is defined as the ratio of the charge accumulated in the capacitor (q) to the potential difference across its electrodes or the applied voltage (U). The capacitance of capacitors depends on the size and shape of the electrodes, their location relative to each other, as well as the dielectric material that separates the electrodes. The greater the capacitance of the capacitor, the greater the charge accumulated by it. Specific capacitance of the capacitor - expresses the ratio of its capacity to volume. The nominal capacitance of a capacitor is the capacitance that the capacitor has according to regulatory documentation. The actual capacitance of each individual capacitor differs from the nominal one, but it must be within the permissible deviations. The values of the nominal capacity and its permissible deviation in various types fixed capacitors are set as standard.
Rated voltage- this is the voltage value indicated on the capacitor at which it operates under specified conditions for a long time and at the same time maintains its parameters within acceptable limits. The value of the rated voltage depends on the properties of the materials used and the design of the capacitors. During operation, the operating voltage on the capacitor should not exceed the rated voltage. For many types of capacitors, the permissible rated voltage decreases as the temperature increases.
Temperature coefficient of capacity (TKE)– this is a parameter expressing the linear dependence of the capacitance of the capacitor on the ambient temperature. In practice, TKE is defined as the relative change in capacitance with a temperature change of 1°C. If this dependence is nonlinear, then TKE of the capacitor is characterized by a relative change in capacitance during the transition from normal temperature (20 ± 5 ° C) to the permissible operating temperature. For capacitors used in single-phase motors, this parameter is important and should be as small as possible. Indeed, during operation of the engine, its temperature rises, and the capacitor is located directly on the engine in the capacitor box.
Loss tangent (tgd). The loss of accumulated energy in a capacitor is due to losses in the dielectric and its plates. When alternating current flows through a capacitor, the current and voltage vectors are shifted relative to each other by an angle (d). This angle (d) is called the dielectric loss angle. If there are no losses, then d=0. The loss tangent is the ratio of active power (Pa) to reactive power (Pр) at a sinusoidal voltage of a certain frequency.
Electrical insulation resistance – electrical resistance DC, is defined as the ratio of the voltage (U) applied to the capacitor to the leakage current (I ut ), or conductivity. The quality of the dielectric used characterizes the insulation resistance. For a capacitor with a large capacitance, the insulation resistance is inversely proportional to its plate area, or its capacitance.
Capacitors are very affected by moisture. Asynchronous electric motors used in pumping equipment pump water, and there is a high probability of moisture getting onto the motor and into the condenser box. Exposure to moisture leads to a decrease in insulation resistance (the probability of breakdown increases), an increase in the loss tangent, and corrosion of the metal elements of the capacitor.
In addition, during engine operation, the capacitors are affected by various types mechanical loads: vibration, shock, acceleration, etc. As a result, broken leads, cracks and a decrease in electrical strength may appear.
Working and starting capacitors
Capacitors with an oxide dielectric (previously called electrolytic) are used as working and starting capacitors. Working and starting capacitors capacitors for asynchronous motors are connected to the AC mains and must be non-polar. They have a relatively large 450 volt operating voltage for oxide capacitors, which is twice the industrial voltage. In practice, capacitors with a capacity of the order of tens and hundreds of microfarads are used. As we said above, the run capacitor is used to produce a rotating magnetic field. The starting capacitance is used to produce the magnetic field necessary to increase the starting torque of the electric motor. The starting capacitor is connected in parallel to the working capacitor through a centrifugal switch. When there is a starting capacitance, the rotating magnetic field of an asynchronous motor at the moment of starting approaches circular, and the magnetic flux increases. This increases starting torque and improves engine performance. When the asynchronous motor reaches a speed sufficient to turn off the centrifugal switch, the starting capacitance is switched off and the engine remains in operation only with a working capacitor. The connection diagram for the working and starting capacitors is shown in (Fig. 1).
Circuit with working and starting capacitors
The table shows isolated characteristics workers and launchers capacitors for asynchronous motors.
|
WORKER |
LAUNCHER |
|
| Purpose | For asynchronous electric motors | |
| Connection diagram | In series with the starting winding of the electric motor | Parallel to the run capacitor |
| As | Phase shifting element | Phase shifting element |
| For what | To obtain a circular rotating magnetic field necessary for the operation of the electric motor | To obtain the magnetic field necessary to increase the starting torque of the electric motor |
| On time | During operation of the electric motor | At the moment of starting the electric motor |
Operation, maintenance and repair
When operating pumping equipment with a single-phase asynchronous motor, special attention should be paid to the supply voltage electrical network. In the case of reduced network voltage, as is known, the starting torque and rotor speed are reduced due to increased slip. At low voltage, the load on the run capacitor also increases and the engine starting time increases. In case of significantIf the supply voltage drops by more than 15%, there is a high probability that the asynchronous motor will not start. Very often, at low voltage, the working capacitor fails due to increased currents and overheating. It melts and electrolyte flows out of it. For repairs, it is necessary to purchase and install a new capacitor of appropriate capacity. It often happens that the required capacitor is not at hand. In this case, you can select the required capacity from two or even three and four capacitors by connecting them in parallel. Here you should pay attention to the operating voltage; it should not be lower than the voltage on the factory capacitor. The total capacitance of the capacitor(s) should differ from the nominal value by no more than 5%. If you install a larger capacity, the engine will start and run, but will start to heat up. If you measure the rated current of the motor using clamps, the current will be overestimated. Since the total electrical resistance of the circuit in the motor windings consists of the active resistance of the circuit and the reactance of the motor windings and capacitance, then with increasing capacitance the total resistance increases. The phase shift of the currents in the windings due to an increase in the impedance of the electrical circuit of the windings after starting the engine will greatly decrease, the magnetic field will turn from sinusoidal to elliptical, and the performance characteristics of the asynchronous motor will deteriorate greatly, efficiency will decrease and heat losses will increase.
Sometimes it happens that the starting winding of a single-phase motor fails along with the capacitor. In such a situation, the cost of repairs increases sharply, because it is necessary not only to replace the capacitor, but also to rewind the stator. As you know, stator rewinding is one of the most expensive operations when repairing an engine. It is very rare, but there is also a situation when at low voltage only the starting winding fails, while the capacitor remains operational. To repair the engine, you need to rewind the stator. All these situations with the engine occur at low voltage of a single-phase supply network. To solve this problem, ideally a voltage stabilizer is needed.
Thank you for your attention
