How to choose capacitors to start an electric motor. What capacitors are needed to start an electric motor? What microfarad is needed for a 1 kV motor?
But the operating voltage of our household network is 220 V. And in order to connect an industrial three-phase motor to a regular consumer network, phase-shifting elements are used:
- starting capacitor;
- working capacitor.
Connection diagrams for an operating voltage of 380 V
Industrially produced asynchronous three-phase motors can be connected in two main ways:
- star connection";
- delta connection".
Electric motors are structurally made of a movable rotor and a housing into which a stationary stator is inserted (can be assembled directly in the housing or inserted there). The stator consists of 3 equal windings, wound in a special way and located on it.
In a star connection, the ends of all three motor windings are connected together, and three phases are supplied to their beginnings. When connecting windings in a triangle, the end of one is connected to the beginning of the next.
Engine operating principle
When an electric motor is operating, connected to a three-phase 380 V network, voltage is sequentially applied to each of its windings and a current flows through each of them, creating an alternating magnetic field that affects the rotor, movably mounted on bearings, which causes it to rotate. To start with this type of operation, no additional elements are needed.
If one of the three-phase asynchronous electric motors is connected to a single-phase 220 V network, then no torque will arise and the motor will not start.
To run three-phase devices from a single-phase network, many different options have been invented.
One of the simplest and most common among them is the use of phase shift. For this purpose, various phase-shifting capacitors are used for electric motors, through which the third phase contact is connected.
To implement this, you can use a special switch that has two pairs of switchable contacts. When the button is pressed, one pair is fixed until the next press of the Stop button, and the second will be closed only when the Start button is pressed. This prevents motor failure.
Connection diagrams for operating voltage 220 V
Due to the fact that there are two main options for connecting electric motor windings, there will also be two circuits for supplying a household network. Designations:
- “P” – switch that performs the start;
- “P” is a special switch designed to reverse the engine;
- “Sp” and “Cr” are starting and running capacitors, respectively.
When connected to a 220 V network, three-phase electric motors have the opportunity to change the direction of rotation to the opposite. This can be done using the “P” toggle switch.
Attention! The direction of rotation can only be changed when the supply voltage is turned off and the electric motor is completely stopped, so as not to break it.
“Сп” and “Ср” (working and starting capacitors) can be calculated using a special formula: Ср=2800*I/U, where I is the current consumed, U is the rated voltage of the electric motor. After calculating Cp, you can select Sp. The capacity of the starting capacitors should be at least twice as large as that of the Average. For convenience and simplification of choice, the following values can be taken as a basis:
- M = 0.4 kW Av = 40 μF, Sp = 80 μF;
- M = 0.8 kW Av = 80 μF, Sp = 160 μF;
- M = 1.1 kW Av = 100 μF, Sp = 200 μF;
- M = 1.5 kW Av = 150 μF, Sp = 250 μF;
- M = 2.2 kW Av = 230 μF, Sp = 300 μF.
Where M is the rated power of the electric motors used, Cp and Sp are working and starting capacitors.
When using asynchronous electric motors designed for an operating voltage of 380 V in the domestic sphere, by connecting them to a 220 V network, you lose about 50% of the rated power of the motors, but the rotor speed remains unchanged. Keep this in mind when choosing the power needed for the job.
Power losses can be reduced by using a “triangle” connection of the windings; in this case, the efficiency of the electric motor will remain somewhere at the level of 70%, which will be significantly higher than when connecting the windings “star”.
Therefore, if it is technically feasible to change the star connection to a delta connection in the junction box of the electric motor itself, then do it. After all, purchasing an “additional” 20% of power will be a good step and help in your work.
When choosing starting and operating capacitors, keep in mind that their rated voltage must be at least 1.5 times greater than the mains voltage. That is, for a 220 V network, it is advisable to use containers designed for a voltage of 400 - 500 V for startup and stable operation.
Motors with an operating voltage of 220/127 V can only be connected as a star. If you use another connection, you will simply burn it when starting up, and all that remains is to scrap it all.
If you cannot find a capacitor used for start-up and operation, then you can take several of them and connect them in parallel. The total capacity in this case is calculated as follows: Total = C1+C2+....+Sk, where k is the required number.
Sometimes, especially under heavy load, it overheats greatly. In this case, you can try to reduce the degree of heating by changing the capacitance Cp (working capacitor). It is gradually reduced, while checking the engine heating. Conversely, if the operating capacity is insufficient, then the output power produced by the device will be small. In this case, you can try increasing the capacitor capacity.
For a faster and easier start-up of the device, if possible, disconnect the load from it. This applies specifically to those engines that have been converted from a 380 V network to a 220 V network.
Conclusion on the topic
If you want to use an industrial three-phase electric motor for your needs, then you need to assemble an additional connection diagram for it, taking into account all the conditions necessary for this. And be sure to remember that this is electrical equipment and you must follow all safety standards and regulations when working with it.
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 network. Known to many who are familiar with electrical engineering, the ability of a capacitor to give electric current a “head start” by 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
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:
Many owners quite often find themselves in a situation where they need to connect a device such as a three-phase asynchronous motor to various equipment in the garage or country house, which can be a sanding or drilling machine. This poses a problem, since the source is designed for single-phase voltage. What to do here? In fact, this problem is quite easy to solve by connecting the unit according to the circuits used for capacitors. To realize this idea, you will need a working and starting device, often referred to as a phase shifter.
Selection of capacity
To ensure proper operation of the electric motor, certain parameters must be calculated.
For run capacitor
To select the effective capacity of the device, it is necessary to perform calculations using the formula:
- I1 is the nominal value of the stator current, for measuring which special clamps are used;
- Umains – single-phase network voltage, (V).
After performing the calculations, you will get the capacitance of the working capacitor in microfarads.
It may be difficult for some to calculate this parameter using the above formula. However, in this case, you can use another scheme for calculating capacity, where you do not need to carry out such complex operations. This method allows you to quite simply determine the required parameter based only on the power of the asynchronous motor.
Here it is enough to remember that 100 watts of power of a three-phase unit should correspond to about 7 µF of the working capacitor capacity.
When making calculations, you need to monitor the current that flows to the stator phase winding in the selected mode. It is considered unacceptable if the current is greater than the nominal value.
For starting capacitor
There are situations when the electric motor has to be turned on under conditions of heavy load on the shaft. Then one running capacitor will not be enough, so you will have to add a starting capacitor to it. The peculiarity of its operation is that it will only work during the startup period of the device for no more than 3 seconds, for which the SA key is used. When the rotor reaches the rated speed level, the device turns off.
If, through an oversight, the owner left the starting devices turned on, this will lead to the formation of a significant imbalance in the currents in the phases. In such situations, there is a high probability of engine overheating. When determining the capacitance, it should be assumed that the value of this parameter should be 2.5-3 times greater than the capacitance of the working capacitor. By acting in this way, it is possible to ensure that the starting torque of the engine reaches the nominal value, as a result of which no complications arise during its startup.
To create the required capacitance, capacitors can be connected in parallel or series circuits. It should be borne in mind that the operation of three-phase units with a power of no more than 1 kW is permitted if they are connected to a single-phase network with a working device. Moreover, here you can do without a starting capacitor.
Type
After calculations, you need to determine what type of capacitor can be used for the selected circuit
The best option is to use the same type for both capacitors. Typically, the operation of a three-phase motor is ensured by paper starting capacitors, enclosed in a sealed steel housing such as MPGO, MBGP, KBP or MBGO.
Most of these devices are made in the form of a rectangle. If you look at the case, their characteristics are given there:
- Capacitance (uF);
- Operating voltage (V).
Application of electrolytic devices
When using paper starting capacitors, you need to remember the following negative point: they are quite large in size, while providing a small capacity. For this reason, for efficient operation of a small-power three-phase motor, it is necessary to use a fairly large number of capacitors. If desired, paper ones can be replaced with electrolytic ones. In this case, they must be connected in a slightly different way, where additional elements must be present, represented by diodes and resistors.
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 device, 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.
Voltage
When considering various types of starting rectifiers for a three-phase motor connected to a single-phase network, one should also take into account such a parameter as the operating 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.
To avoid making mistakes 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.
Calculation of the capacity of the working rectifier:
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.
If you take two capacitors and connect them to each other in a parallel circuit, the resulting capacitance will be 20 µF. In this case, the operating voltage will be equal to 160V. To achieve the required level of 320 V, you need to take these two rectifiers and connect them to another pair of capacitors connected in parallel, but using a series circuit. As a result, the total capacitance will be 10 μF. When the battery of working capacitors is ready, connect it to the engine. Then all that remains is to run it in a single-phase network.
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.
Video: connecting a single-phase motor to a single-phase network
The decision to use a starting capacitor should be made based on specific conditions; most often, a working capacitor is sufficient. However, if the engine being used is subjected to increased load, it is recommended to stop operation. In this case, it is necessary to correctly determine the required capacity of the device to ensure efficient operation of the unit.
To ensure reliable operation of the electric motor, starting capacitors are used.
The greatest load on the electric motor occurs at the moment of its start. It is in this situation that the starting capacitor begins to work. We also note that in many situations the start-up is carried out under load. In this case, the load on the windings and other components is very high. What design allows you to reduce the load?
All capacitors, including starting capacitors, have the following features:
- As a dielectric special material is used. In this case, an oxide film is often used, which is applied to one of the electrodes.
- Large capacity with small overall dimensions - a feature of polar storage devices.
- Non-polar They are more expensive and larger, but they can be used without regard to polarity in the circuit.
This design is a combination of 2 conductors that are separated by a dielectric. The use of modern materials can significantly increase the capacity indicator and reduce its overall dimensions, as well as increase its reliability. Many with impressive performance indicators have dimensions of no more than 50 millimeters.
Purpose and benefits
Capacitors of the type in question are used in the connection system. 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:
- Starting capacity makes it possible to bring the state of the electric field closer to circular.
- Held significant increase in magnetic flux.
- Rising starting torque, engine performance is significantly improved.
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 AC mains can serve as a power source when using this type of capacitor. Almost all used versions are non-polar; they have a comparatively higher operating voltage for oxide capacitors.
The advantages of a network that has a similar element are as follows:
- Easier engine starting.
- Life time the engine is much larger.
The starting capacitor operates for several seconds when the engine starts.
Connection diagrams
wiring diagram for an electric motor with a starting capacitor The circuit that has a starting capacitor in the network has become more widespread.
This scheme has certain nuances:
- Start winding and capacitor turn on when the engine starts.
- Additional winding works for a short time.
- 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.
- There is a fork in front of him, which goes to the switch. In addition to the switch, another element can be used that starts the engine.
- After the switch a starting capacitor is installed. It operates for a few seconds until the rotor picks up speed.
- Both capacitors go to the engine.
You can make a connection in a similar way.
It is worth noting that the working capacitor is present in the circuit almost constantly. Therefore, it is worth remembering that they must be connected in parallel.
Selecting a starting capacitor for an electric motor
A modern approach to this issue involves the use of special calculators on the Internet that perform quick and accurate calculations.
To carry out the calculation, you should know and enter the following indicators:
- Motor winding connection type: triangle or star. The capacitance also depends on the type of connection.
- Engine power is one of the determining factors. This indicator is measured in Watts.
- Mains voltage taken into account in calculations. As a rule, it can be 220 or 380 Volts.
- Power factor– a constant value, which is often 0.9. However, it is possible to change this indicator during calculation.
- Electric motor efficiency also affects the calculations performed. This information, as well as others, can be found by studying the information printed by the manufacturer. If it is not there, you should enter the engine model on the Internet to search for information about what the efficiency is. You can also enter an approximate value, which is typical for such models. It is worth remembering that efficiency may vary depending on the condition of the electric motor.
Such information is entered into the appropriate fields and an automatic calculation is carried out. At the same time, we obtain the capacity of the working condensate, and the starting condensate should have an indicator 2.5 times greater.
You can carry out such a calculation yourself.
To do this, you can use the following formulas:
- For the star winding connection type, Capacitance is determined using the following formula: Cр=2800*I/U. In the case of a triangle connection of the windings, the formula Cр=4800*I/U is used. As you can see from the information above, the type of connection is the determining factor.
- The above formulas determine the need to calculate the amount of current that passes through the system. For this, the formula is used: I=P/1.73Uηcosφ. For the calculation you will need engine performance indicators.
- After calculating the current you can find the capacitance indicator of the working capacitor.
- Launcher, as previously noted, should be 2 or 3 times higher in capacity than the worker.
When choosing, you should also consider the following nuances:
- Interval operating temperature.
- Possible deviation from the design capacity.
- Insulation resistance.
- Loss tangent.
Usually, the above parameters are not paid much attention. However, they can be taken into account to create an ideal electric motor power system.
Overall dimensions can also be a determining factor. In this case, the following dependence can be distinguished:
- Capacity increase leads to an increase in the diametrical size and exit distance.
- Most common maximum diameter 50 millimeters with a capacitance of 400 μF. At the same time, the height is 100 millimeters.
In addition, it is worth considering that on the market you can find models from foreign and domestic manufacturers. As a rule, foreign ones are more expensive, but also more reliable. Russian versions are also often used when creating an electric motor connection network.
Model overview
capacitor CBB-60 There are several popular models that can be found on sale.
It is worth noting that these models differ not in capacity, but in type of design:
- Metallized polypropylene options execution of the SVV-60 brand. The cost of this version is about 300 rubles.
- Film grades NTS are somewhat cheaper. With the same capacity, the cost is about 200 rubles.
- E92– products from domestic manufacturers. Their cost is small - about 120-150 rubles for the same capacity.
There are other models, often differing in the type of dielectric used and the type of insulating material.
- Often, the electric motor can operate without including a starting capacitor in the circuit.
- Include this element in the circuit Only recommended if starting under load.
- Also, greater engine power also requires the presence of similar elements in the circuit.
- Special attention It is worth paying attention to the connection procedure, since violating the integrity of the structure will lead to its malfunction.