Design and purpose of capacitors
This element of the electrical circuit consists of two plates (plates). The plates are positioned relative to each other so that there is a gap between them. When a capacitor is connected to an electric current circuit, charges accumulate on the plates. Due to the physical gap between the plates, the device has low conductivity.
Attention! This gap can be air or filled with a dielectric. The following dielectrics are used: paper, electrolyte, oxide films.
The main feature of such a two-terminal network is the ability to accumulate electric field energy and instantly transfer it to the load (charge and discharge).
Part device
The first prototype of the container was the Leyden jar, created in 1745 in the city of Leiden by the German von Kleist. The jar was lined with copper foil inside and out. This is how the idea of creating covers came about.
Leyden jars connected in parallel
The graphic designation of a two-terminal network on diagrams and drawings is two vertically located lines (like plates) with a gap between them.
Designation on diagrams
Functionality
Operating principle of the electric motor
In DC circuits, the element accumulates charge on the plates for some time and does not allow electrons to pass through the dielectric. This means that at the initial moment a direct current passes through the part until the end of the charge. The same thing happens during discharge.
Important! The element passes a current that changes periodically through itself. This is possible because the two-terminal circuit is cyclically recharged when the polarity of the electricity changes.
Characteristics
The voltage created on the plates of a two-terminal network is equal to the potential difference:
High voltage capacitors
U = ϕ1 – ϕ2.
Knowing the voltage and charge, you can calculate the capacitance (C). This is one of the main characteristics of a two-terminal network:
С = q/U,
Where:
- C – capacitance, F (farad);
- q – charge accumulated by a two-terminal network, C (coulomb);
- U – voltage, V.
Electrical capacity is a physical quantity that is determined by dividing the plate charge by the potential difference between the plates. The unit of measurement for C is farad (F).
For your information. A capacitance equal to 1 F is a large value, so in practice it is measured: in microfarads (μF), picofarads (pF), nanofarads (nF).
Capacitance measurement table
Other parameters of the two-terminal network include:
- energy density;
- Rated voltage;
- polarity.
When the mass of the part body is significantly less than the total mass of the electrolyte and plates, then the highest energy density is achieved.
Nominal voltage is the voltage at which the element can operate for a long time without disruption (deviation) of performance characteristics.
Capacitive two-terminal devices are:
- non-polar;
- polar (electrolytic).
When connected, non-polar parts are not oriented towards the polarity of the power supply's charge terminals. The peculiarity of electrolytic cells is associated with the chemical reaction between the dielectric and the electrolyte. These models have an anode (positive terminal) and a cathode (negative terminal).
Advantages and disadvantages
Advantages of electrolytic capacitors:
- Large capacity;
- Compactness.
Flaws:
- Over time, the electrolyte dries out and capacity is lost;
- Works only at low frequencies;
- Operating limitations and risk of swelling/explosion.
Let's take a closer look at the advantages and disadvantages of electrolytes.
Large capacity
Electrolytic capacitors have a large capacity, and this is their distinctive and most important feature among other capacitors.
Capacitance is indicated in microfarads (μF), since electrolytes with lower values are not produced.
They are usually produced from several microfarads to several f (1,000,000 microfarads).
Compactness
Thanks to chemical sources, high-capacity capacitors are much more compact than if they were made from ceramic or film.
The capacitance of a capacitor can be increased only due to its plates, dielectric and geometry. Therefore, electrolytes lead in terms of capacity/dimensions ratio.
Ionistors
A type of electrolytic capacitors are ionistors. They have a larger capacity (for example, 3000 F), and operate primarily as a backup or self-contained low-voltage power supply for the circuit. And also supports the circuit in sleep mode without another source.
Electrolyte drying out
The main problem with such capacitors is drying out of the electrolyte. Typically, this problem manifests itself due to the fact that the equipment is not used for a long time or the operating conditions are violated (overheating of the case). Because of this, the electrolyte begins to dry out, resulting in a loss of capacity.
It is possible to restore the capacitor's capacity by diluting the dried electrolyte with distilled water (like a battery), but this is not profitable. The best and most reliable way is to replace the old one with a new one, similar in parameters.
Low Frequency Operation
This is more of a feature than a drawback. Large capacitances mean high reactance for high frequencies.
Therefore, such capacitors are used in low-frequency circuits. For example, in power supplies as filters and ripple smoothing.
When a capacitor swells and explodes
Since capacitors of this type are chemical sources, the polarity of the connection must be observed.
Capacitors, like batteries, can swell and explode. Sometimes this happens due to improper activation or overheating.
If you connect the minus of the source to the plus of the capacitor and the plus of the source to the minus of the capacitor, then boiling of the electrolyte will immediately begin. This effect occurs due to a reverse chemical reaction. The capacitor may explode.
In old capacitors of the K-50 type, the body was monolithic, and it exploded loudly and quite destructively.
In modern electrolytes, there is a small cut on the body, which, if the electrolyte boils, allows hot steam to escape.
Sometimes they are simply blown in without breaking the seal, and there are also cases when the capacitor completely loses its seal.
However, the notch on the case has greatly reduced explosions, so capacitors are now more likely to swell rather than explode.
On the housings of modern capacitors, a vertical line indicates the negative contact.
Carefully set and record the previous position, because many manufacturers put their own designations.
For example, among radio amateurs, negative contacts are usually drawn in the form of a square.
On the contrary, printed circuit board manufacturers draw square contact pads for the plus of the capacitor. And then, not everyone does this.
Since there is such confusion among both radio amateurs and manufacturers, always pay attention to this. where the positive contact is indicated. And write down the previous position of the part, otherwise it is fraught with an explosion.
Simple ways to connect an electric motor
The simplest way to turn on motors is to connect to a three-phase network. The electrical windings of the motor are connected in two ways:
- star;
- triangle.
The connection order is indicated on the terminal cover on the reverse side.
Connection diagram
Attention! Connecting the windings in a delta quickly brings the motor to maximum power, but then the starting current increases sevenfold. A soft start, in the absence of a starting rheostat, is difficult.
The star connection of the windings allows the motor to operate steadily and for a long time with a smooth start. The machine can withstand short-term overloads and does not overheat. Its power is slightly lower than with an alternative connection.
The beginnings of the windings can be connected to one point during manufacturing. Only three of their ends are brought out to the terminal block. Therefore, the conclusions are simply connected to the network phases. The direction of rotation is selected by reversing the connection of the terminals to two adjacent phases.
A motor with only three wires out
Specifics of circuits with capacitors
When selecting the types of switching on of electric machines using starting and working two-terminal circuits for a 220 volt network, the following are distinguished:
- inclusion in the “triangle”;
- star connection.
For your information. What are the differences between starting and working two-terminal networks? “Starting” are elements that are used only for starting, and “working” are those that are constantly used in operation.
Connection diagrams for a 380 V line
There is no need to use capacitive elements when connecting a 3-phase motor to a 380 volt network.
Connecting the motor to a three-phase network
Schemes for connecting to a single-phase network
When installing a single-phase motor in a single-phase line, it is started using an additional winding. Such a motor has three outputs:
- from the working coil;
- from additional;
- common output for both windings.
When there is no marking, the coils are “ringed” by a tester to determine the correct connection.
Circuit for starting a single-phase motor
Assembly type "Triangle"
To connect an asynchronous three-phase machine to a single-phase line, it is possible to use a delta connection. The starting capacitance is turned on according to the diagram.
Turning on the motor via a delta connection
Assembly type "Star"
The principle of assembling the starting circuit of a 3-phase motor, the windings of which are connected by a star, is similar. When it is possible to independently make such a connection of the windings, it is carried out on the terminal block.
Star connection
Voltage
It would be a mistake to use a rectifier whose voltage is an order of magnitude higher than required. In addition to the high costs of purchasing it, you will have to allocate more space for it due to its large dimensions.
At the same time, you should not consider models in which the voltage has a lower value than the network voltage. Devices with such characteristics will not be able to effectively perform their functions and will soon fail.
In order not to make a mistake when choosing the operating voltage, you should adhere to the following calculation scheme: the final parameter must correspond to the product of the actual network voltage and a coefficient of 1.15, and the calculated value must be at least 300 V.
If paper rectifiers are selected for operation in an alternating voltage network, then their operating voltage must be divided by 1.5-2. Therefore, the operating voltage for a paper capacitor, for which the manufacturer specified a voltage of 180 V, under operating conditions in an AC network will be 90-120 V.
In order to understand how the idea of connecting a three-phase electric motor to a single-phase network is implemented in practice, let’s perform an experiment using an AOL 22-4 unit with a power of 400 (W). The main task that must be solved is starting the engine from a single-phase network with a voltage of 220 V.
The electric motor used has the following characteristics:
Keeping in mind that the electric motor used has little power, when connecting it to a single-phase network, you can only buy a working capacitor.
Using the above formulas, we take the average value of the capacitance of the working rectifier to be 25 μF. Here a slightly larger capacitance was chosen, equal to 10 μF. So we will try to find out how such a change affects the launch of the device.
Now we need to buy rectifiers, the latter will be MBGO type capacitors. Next, based on the prepared rectifiers, the required capacity is assembled.
During operation, it should be remembered that each such rectifier has a capacity of 10 μF.
During the experiment with connecting the motor to a single-phase network, the work required less time and effort. When using a similar unit with a selected battery of rectifiers, it should be taken into account that its useful power will be at a level of up to 70-80% of the rated power, while the rotor speed will correspond to the rated value.
Important: if the motor used is designed for a 380/220 V network, then when connecting to the network you should use a “triangle” circuit.
Pay attention to the contents of the tag: it happens that there is an image of a star with a voltage of 380 V. In this case, correct operation of the motor in the network can be ensured by fulfilling the following conditions. First you will have to “gut” the common star, and then connect 6 ends to the terminal block. You should look for a common point in the frontal part of the engine.
Capacity size: working and starting
The specific capacity of these elements can be calculated using an online calculator on the Internet. The calculation is done independently using formulas.
For the trigger element
There are two known formulas for determining the capacity of a starting two-terminal network:
- for the “star” circuit – Cп = 2800*I/U;
- for the “triangle” circuit – Cп = 4800*I/U.
The rated current is calculated using the expression:
I = P/(1.73*U*η*cosϕ.
Here:
- P – motor power;
- U – network voltage;
- η – efficiency;
- cosϕ – power factor.
For work item
You can select a working capacitor based on the following calculation:
Cp = 1/2 Cp.
A running and stable engine requires the use of a working tank to rotate under load.
Capacity calculation methods
To calculate which capacitors are best used to start an electric motor, the following formula is used:
- k is the coefficient, it differs depending on the type of connection, 4800 is a triangle and 2800 is a star;
- If – starter current (indicated on the engine);
- Uc is the network voltage, in this case 220 volts.
The output is a capacitance measured in µF (one millionth of a Farad). It can be calculated in another way, using power as the main parameter.
Every 100 W of motor power corresponds to 7 µF. It should not be forgotten that the starter winding must receive a current no higher than the rated current.
Expert opinion
It-Technology, Electrical power and electronics specialist
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Review of models The decision to use a starting capacitor should be made based on specific conditions; most often, a working capacitor is sufficient. Ask, I'm in touch!
Start capacitor
If you choose one of the metal-paper types as a starting element, then you can choose one such as mbgch.
This is a sealed and high voltage trigger element. It is produced with a constant capacitance value of up to 10 mF and designed for a voltage of 250-1000 V. Such a two-terminal network is used in networks of any kind of current.
Two-terminal network MBGCH
Which type to use
The requirements for capacitors for starting electric motors are simple:
- the capacity is sufficient to start the engine;
- the rated voltage is selected 10-15% higher than the connected one;
- The two-terminal network must work with the applied type of current.
There are small nuances for electric machines that differ in operating principles.
For operation with a three-phase electric motor
In this case, the part carries out a phase shift in the winding of an asynchronous machine, and its capacitance must be high. Creation of a starting torque and further operation under load require a more precise selection of this element characteristic.
Switching on with a single-phase electric motor
Starting capacitors are used here to connect an additional winding. It is designed to start the motor and can be turned on either permanently, through a two-terminal network, or briefly without it.
Part selection features
The selected starting capacitors correspond to the applied voltage. The size of their capacity should not allow the engine to overheat during operation and should be easy to start when turned on. There are no particular difficulties with the selection of elements.
Connection diagrams
The circuit that has a starting capacitor in the network has become more widespread.
This scheme has certain nuances:
- The starting winding and capacitor are turned on when the engine starts.
- The additional winding operates for a short time.
- The thermal relay is included in the circuit to protect the additional winding from overheating.
If it is necessary to provide high torque during startup, a starting capacitor is included in the circuit, which is connected together with the working capacitor. It is worth noting that quite often its capacity is determined empirically to achieve the highest starting torque. Moreover, according to the measurements taken, the value of its capacity should be 2-3 times greater.
The main points of creating an electric motor power circuit include the following:
- From the current source , 1 branch goes to the working capacitor. It works all the time, which is why it got its name.
- In front of it there is a branch that goes to the switch. In addition to the switch, another element can be used that starts the engine.
- after the switch . It operates for a few seconds until the rotor picks up speed.
- Both capacitors go to the motor.
In a similar way, you can connect a single-phase electric motor.
Using Electrolytic Capacitors
To start operation of a three-phase motor from 220V, the starting capacitor must have a large capacity. To move the shaft of an engine with a power of 3 kilowatts, 2100 uF of capacitance is needed. To select such a value of C, you will need a whole battery of non-polar components. Electrolytic two-terminal networks (electrolytes) have a larger capacity with smaller dimensions. But their inclusion in the alternating current circuit for a long time is unacceptable.
Carefully. When the container is connected for a long time, the electrolyte boils and the element explodes.
Wiring diagram for an electrolytic cell to start the engine
Application of electrolytic devices
However, experts do not recommend using electrolytic starting capacitors. This is due to the presence of a serious drawback in them, which manifests itself in the following: if the diode does not cope with its task, alternating current will begin to be supplied to the capacitor, and this is fraught with its heating and subsequent explosion.
Another reason is that today on the market you can find improved polypropylene AC starting models of the SVV type with a metallized coating.
Most often they are designed to operate with a voltage of 400-450 V
. They should be given preference, given that they have repeatedly shown themselves to be good.
What kind of lighting do you prefer?
Built-in Chandelier
Connecting an electric motor with your own hands
How to choose a capacitor for a single-phase motor is already clear. The selection of capacitors for a three-phase motor is considered. How to practically mount a circuit to start an engine, what is necessary for this?
The circuit consists of the following components:
- engine (up to 3 kW);
- capacitors: starting and running, which differ in capacity;
- 220 V PVS start button.
Why do you need a start button? For short-term connection of an electrolytic two-terminal network and starting the motor rotation. The circuit is assembled according to the diagram in the picture below. All connections are made using bolted clamps. Bare sections of wires are subject to mandatory insulation.
Practical connection diagram
The use of starting and operating capacitors makes it possible to start motors in any circuit. The capacity of the two-terminal circuits should be sufficient to start rotation and stable operation under load. It is preferable to use new parts.
Purpose and benefits
Capacitors of the type in question are used in the connection system of an asynchronous motor. In this case, it works only at the time of start-up, until the operating speed is reached.
The presence of such an element in the system determines the following:
- The starting capacitance makes it possible to bring the state of the electric field closer to circular.
- is being carried out .
- increases and engine performance improves significantly.
Without the presence of this element in the system, the service life of the engine is significantly reduced. This is due to the fact that a complex start-up leads to certain difficulties.
The advantages of a network that has a similar element are as follows:
- Easier engine starting.
- service life is significantly longer.
The starting capacitor operates for several seconds when the engine starts.
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