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” 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
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.
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.
In addition, there must be one more element. This is the starting capacitor. It is designed to start the engine itself and should only work at the moment of starting for about 2-3 seconds. If it is left on for a long time, the motor windings will quickly overheat and it will fail.
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 comply with all safety standards and regulations when working with it.
added a comment on YouTube:
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, with different winding connection diagrams, is considered. 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 of the 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 Descent = 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 previously 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 two identical capacitors are connected in series, 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 (it is better not to turn on polar ones 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 this pulsating magnetic field 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 scary, 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 leaving the engine idling for a long time with a working condenser designed for the full power of the engine is still not worth it - in this case, there will be an increased voltage on the working capacitor (up to 350 V), and along the winding connected in series with the working capacitor, an increased current will flow (30% more than the rated current on the delta, and 15% more on the 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.
And most asynchronous motors are designed for 380 V and three phases. And when making homemade drilling machines, concrete mixers, emery machines and others, it becomes necessary to use a powerful drive. A motor from an angle grinder, for example, cannot be used - it has a lot of revolutions and little power, so you have to use mechanical gearboxes, which complicate the design.
Design features of asynchronous three-phase motors
Asynchronous AC machines are a godsend for any owner. It’s just that connecting them to a household network turns out to be problematic. But you can still find a suitable option, the use of which will result in minimal power losses.
Before you need to understand its design. It consists of the following elements:
- The rotor is made according to the “squirrel cage” type.
- Stator with three identical windings.
- Terminal box.
There must be a metal nameplate on the engine - all the parameters are written on it, even the year of manufacture. The wires from the stator go into the terminal box. Using three jumpers, all wires are connected to each other. Now let's look at what motor connection diagrams exist.
Star connection
Each winding has a beginning and an end. Before you connect a 380 to 220 motor, you need to find out where the ends of the windings are. To make a star connection, it is enough to install jumpers so that all ends are closed. Three phases must be connected to the beginning of the windings. When starting the engine, it is advisable to use this particular circuit, since high currents are not induced during operation.
But it is unlikely that it will be possible to achieve high power, so hybrid circuits are used in practice. The motor is started with the windings turned on in a star configuration, and when it reaches a stable mode, it switches to a delta configuration.
Connection diagram for delta windings
The disadvantage of using such a circuit in a three-phase network is that large currents are induced in the windings and wires. This leads to damage to electrical equipment. But when working on a 220 V household network, no such problems are observed. And if you are thinking about how to connect a 380 to 220 V asynchronous motor, then the answer is obvious - only by using a delta circuit. In order to make a connection according to this scheme, you need to connect the beginning of each winding to the end of the previous one. Power must be connected to the vertices of the resulting triangle.
Connecting the motor using a frequency converter
This method is at the same time the simplest, most progressive and expensive. Although, if you need the functionality of an electric drive, you won't regret any money. The cost of the simplest frequency converter is about 6,000 rubles. But with its help it will not be difficult to connect a 380 V motor to 220 V. But you need to choose the right model. Firstly, you need to pay attention to which network the device is allowed to connect to. Secondly, pay attention to how many outputs it has.
For normal operation in domestic conditions, you need the frequency converter to be connected to a single-phase network. And the output should have three phases. It is recommended to carefully study the operating instructions so as not to make a mistake with the connection, otherwise the powerful transistors installed in the device may burn out.
Using capacitors
When using a motor with a power of up to 1500 W, you can install only one capacitor - a working one. To calculate its power, use the formula:
Serb=(2780*I)/U=66*P.
I - operating current, U - voltage, P - engine power.
To simplify the calculation, you can do it differently - for every 100 W of power, 7 μF of capacitance is needed. Therefore, for a 750W motor you need 52-55uF (you need to experiment a bit to get the right phase shift).
In the event that a capacitor of the required capacity is not available, you need to connect in parallel those that are available, using the following formula:
Comm=C1+C2+C3+...+Cn.
A starting capacitor is required when using motors whose power exceeds 1.5 kW. The starting capacitor works only in the first seconds of switching on to give a “push” to the rotor. It is turned on via a button parallel to the working one. In other words, it causes a stronger phase shift. This is the only way to connect a 380 to 220 motor through capacitors.
The essence of using a working capacitor is to obtain the third phase. The first two are zero and phase, which is already in the network. There should be no problems connecting the motor; the most important thing is to hide the capacitors away, preferably in a sealed, strong case. If the element fails, it may explode and harm others. The capacitor voltage must be at least 400 V.
Connection without capacitors
But you can connect a 380 to 220 motor without capacitors; you don’t even have to buy a frequency converter for this. All you have to do is rummage around in the garage and find a few main components:
- Two transistors type KT315G. The cost on the radio market is about 50 kopecks. per piece, sometimes even less.
- Two thyristors type KU202N.
- Semiconductor diodes D231 and KD105B.
You will also need capacitors, resistors (fixed and one variable), and a zener diode. The entire structure is enclosed in a housing that can protect against electric shock. The elements used in the design must operate at voltages up to 300 V and currents up to 10 A.
It is possible to carry out both mounted and printed mounting. In the second case, you will need foil material and the ability to work with it. Please note that domestic thyristors of the KU202N type get very hot, especially if the drive power is over 0.75 kW. Therefore, install the elements on aluminum radiators; if necessary, use additional airflow.
Now you know how to independently connect a 380 motor to a 220 motor (into a household network). There is nothing complicated about this, there are many options, so you can choose the most suitable one for a specific purpose. But it’s better to spend money once and purchase it; it increases the number of drive functions many times over.
