How to choose a capacitor to start an electric motor? How to choose capacitors for an electric motor How many microfarads are needed per 1 kilowatt
Each object is initially supplied with three-phase current. The main reason is the use in power plants of generators with three-phase windings, out of phase with each other by 120 degrees and generating three sinusoidal voltages. However, with further distribution of current, only one phase is supplied to the consumer, to which all existing electrical equipment is connected.
Sometimes there is a need to use non-standard devices, so you have to solve the problem of how to choose a capacitor for a three-phase motor. As a rule, it is necessary to calculate the capacity of a given element that ensures stable operation of the unit.
The principle of connecting a three-phase device to one phase
In all apartments and most private houses, all internal energy supply is carried out through single-phase networks. Under these conditions, it is sometimes necessary to perform . This operation is quite possible from a physical point of view, since individual phases differ from each other only by a time shift. Such a shift can be easily organized by including any reactive elements in the circuit - capacitive or inductive. It is they who perform the function of phase-shifting devices when working and starting elements are used.
It should be taken into account that the stator winding itself has inductance. In this regard, it is quite enough to connect a capacitor with a certain capacitance outside the engine. At the same time, the stator windings are connected in such a way that the first of them shifts the phase of the other winding in one direction, and in the third winding the capacitor performs the same procedure, only in the other direction. As a result, the required phases are formed in the amount of three, extracted from a single-phase supply wire.
Thus, the three-phase motor acts as a load for only one phase of the connected power supply. As a result, an imbalance is formed in the energy consumed, which negatively affects the overall operation of the network. Therefore, this mode is recommended to be used for a short time for low-power electric motors. Connecting the windings to a single-phase network can be done.
Schemes for connecting a three-phase motor to a single-phase network
When a three-phase electric motor is planned to be connected to a single-phase network, it is recommended to give preference to a delta connection. An information plate attached to the housing warns about this. In some cases there is a “Y” designation here, which means a star connection. It is recommended to reconnect the windings in a delta configuration to avoid large power losses.
The electric motor is connected to one of the phases of a single-phase network, and the other two phases are created artificially. For this, a working capacitor (Cp) and a starting capacitor (Sp) are used. At the very beginning of starting the engine, a high level of starting current is required, which cannot be provided by the running capacitor alone. A starting or starting capacitor, connected in parallel with the working capacitor, comes to the rescue. With low engine power, their performance is equal. Specially produced starting capacitors are marked “Starting”.
These devices operate only during start-up periods in order to accelerate the engine to the required power. It is subsequently turned off using a push-button or double switch.
Types of starting capacitors
Small electric motors, the power of which does not exceed 200-400 watts, can operate without a starting device. For them, one working capacitor is quite enough. However, if there are significant loads at the start, additional starting capacitors are necessarily used. It is connected in parallel with the working capacitor and during the acceleration period is kept in the on position using a special button or relay.
To calculate the capacitance of the starting element, it is necessary to multiply the capacitance of the working capacitor by a factor equal to 2 or 2.5. During acceleration, the engine requires less and less capacity. In this regard, you should not keep the starting capacitor constantly on. High capacity at high speeds will lead to overheating and failure of the unit.
The standard capacitor design consists of two plates located opposite each other and separated by a dielectric layer. When choosing a particular element, it is necessary to take into account its parameters and technical characteristics.
All capacitors are presented in three main types:
- Polar. Cannot operate electric motors connected to alternating current. A collapsing dielectric layer can lead to heating of the unit and subsequent short circuit.
- Non-polar. Received the greatest distribution. They can operate in any connection options due to the identical interaction of the plates with the dielectric and the current source.
- Electrolytic. In this case, the electrodes are a thin oxide film. They can reach a maximum possible capacity of up to 100 thousand uF, ideal for low-frequency motors.
Selecting a capacitor for a three-phase motor
Capacitors intended for a three-phase motor must have a fairly high capacity - from tens to hundreds of microfarads. Electrolytic capacitors are not suitable for these purposes because they require a unipolar connection. That is, specifically for these devices it will be necessary to create a rectifier with diodes and resistances.
Gradually, the electrolyte in such capacitors dries out, which leads to loss of capacity. In addition, during operation, these elements sometimes explode. If you still decide to use electrolytic devices, you must take these features into account.
Classic examples are the elements presented in the figure. The working capacitor is shown on the left, and the starting capacitor on the right.
The selection of a capacitor for a three-phase motor is carried out experimentally. The capacity of the working device is selected at the rate of 7 μF per 100 W of power. Therefore, 600 W will correspond to 42 µF. The starting capacitor is at least 2 times the operating capacitance. Thus 2 x 45 = 90 uF would be the most appropriate figure.
The choice is made gradually, based on the operation of the engine, since its real power directly depends on the capacity of the capacitors used. In addition, this can be done using a special table. If there is insufficient capacity, the engine will lose its power, and if there is excess capacity, overheating will occur from excessive current. If the capacitor is selected correctly, the engine will operate normally, without jerking or extraneous noise. We select the device more accurately through calculations performed using special formulas.
Capacity calculation
The capacitance of the capacitor for the electric motor is calculated based on the winding connection diagram - star or triangle.
In both cases, the general calculation formula is applied: C slave = k x I f /U network, to which all parameters have the following designations:
- k - is a special coefficient. Its value is 2800 for the star circuit and 4800 for the delta circuit.
- If - rated stator current indicated on the information plate. If it is impossible to read, measurements are taken using special measuring clamps.
- Umains is the supply voltage of 220 volts.
By substituting all the necessary values, you can easily calculate what capacity the working capacitor will have (μF). During calculations, it is necessary to take into account the current supplied to the stator phase winding. It should not exceed the rated value, just as the load on a motor with a capacitor should not exceed 60-80% of the rated power indicated on the information plate.
How to connect starting and running capacitors
The figure shows the simplest diagram of connecting the starting and working elements. The first of them is installed at the top, and the second at the bottom. At the same time, an on and off button is connected to the engine. The most important thing is to carefully understand the wires so as not to mix up the ends.
This scheme allows you to perform a preliminary check with an inaccurate estimate. It is also used after the final selection of the most optimal value.
This selection is carried out experimentally using several capacitors of different capacities. When connected in parallel, their total power will increase. At this time, you need to monitor the operation of the engine. If the operation is stable and smooth, in this case you can buy a capacitor with a capacitance equal to the sum of the capacitances of the test elements.
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
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.
If there is a need to connect an asynchronous three-phase electric motor to a household network, you may encounter a problem - it seems completely impossible to do this. But if you know the basics of electrical engineering, you can connect a capacitor to start an electric motor in a single-phase network. But there are also capacitorless connection options; they are also worth considering when designing an installation with an electric motor.
Simple ways to connect an electric motor
The easiest way is to connect the motor using a frequency converter. There are models of these devices that convert single-phase voltage to three-phase. The advantage of this method is obvious - there is no power loss in the electric motor. But the cost of such a frequency converter is quite high - the cheapest copy will cost 5-7 thousand rubles.
There is another method that is used less frequently - the use of a three-phase asynchronous winding to convert voltage. In this case, the entire structure will be much larger and more massive. Therefore, it will be easier to calculate which capacitors are needed to start the electric motor and install them by connecting them according to the diagram. The main thing is not to lose power, since the operation of the mechanism will be much worse.
Features of the circuit with capacitors
The windings of all three-phase electric motors can be connected according to two schemes:
- “Star” - in this case, the ends of all windings are connected at one point. And the beginnings of the windings are connected to the supply network.
- “Triangle” - the beginning of the winding is connected to the end of the adjacent one. The result is that the connection points of the two windings are connected to the power supply.
The choice of circuit depends on what voltage the motor is supplied with. Typically, when connected to a 380 V AC network, the windings are connected in a “star”, and when operating under a voltage of 220 V - in a “delta”.
In the picture above:
a) star connection diagram;
b) triangle connection diagram.
Since a single-phase network clearly lacks one supply wire, it needs to be made artificially. For this purpose, capacitors are used that shift the phase by 120 degrees. These are working capacitors; they are not enough when starting electric motors with a power of over 1500 W. To start powerful engines, you will need to additionally include another container, which will facilitate work during the start.
Working capacitor capacity
In order to find out what capacitors are needed to start an electric motor when operating on a 220 V network, you need to use the following formulas:
- When connected in a star configuration C (slave) = (2800 * I1) / U (network).
- When connected in a "triangle" C (slave) = (4800 * I1) / U (network).
Current I1 can be measured independently using clamps. But you can also use this formula: I1 = P / (1.73 U (network) cosφ η).
The value of power P, supply voltage, power factor cosφ, efficiency η can be found on the tag, which is riveted on the motor housing.
A simplified version of calculating a working capacitor
If all these formulas seem a little complicated to you, you can use their simplified version: C (slave) = 66 * P (motor).
And if we simplify the calculation as much as possible, then for every 100 W of electric motor power a capacitance of about 7 μF is required. In other words, if you have a 0.75 kW motor, then you will need a run capacitor with a capacity of at least 52.5 uF. After selection, be sure to measure the current when the motor is running - its value should not exceed the permissible values.
Start capacitor
In the event that the motor is subject to heavy loads or its power exceeds 1500 W, a phase shift alone cannot be done. You will need to know what other capacitors are needed to start an electric motor of 2.2 kW and higher. The starter is connected in parallel with the worker, but only it is excluded from the circuit when the idle speed is reached.
Be sure to turn off the starting capacitors - otherwise phase imbalance and overheating of the electric motor occurs. The starting capacitor should be 2.5-3 times larger in capacity than the working capacitor. If you consider that a capacitance of 80 μF is required for normal operation of the motor, then to start you need to connect another block of capacitors of 240 μF. You can hardly find capacitors with such a capacitance on sale, so you need to make a connection:
- When the capacitances are added in parallel, the operating voltage remains the same as indicated on the element.
- In a series connection, the voltages are added, and the total capacitance will be equal to C (total) = (C1*C2*..*CX)/(C1+C2+..+CX).
It is advisable to install starting capacitors on electric motors whose power is over 1 kW. It is better to reduce the power rating a little to increase the degree of reliability.
What type of capacitors to use
Now you know how to select capacitors to start an electric motor when operating on a 220 V AC network. After calculating the capacitance, you can begin to select a specific type of element. It is recommended to use elements of the same type as working and starting elements. Paper capacitors perform well; their designations are as follows: MBGP, MPGO, MBGO, KBP. You can also use foreign elements that are installed in computer power supplies.
The operating voltage and capacitance must be indicated on the body of any capacitor. One drawback of paper cells is that they are large in size, so to operate a powerful engine you will need a rather large battery of cells. It is much better to use foreign capacitors, since they are smaller in size and have a larger capacity.
Using Electrolytic Capacitors
You can even use electrolytic capacitors, but they have a peculiarity - they must operate on direct current. Therefore, to install them in the structure, you will need to use semiconductor diodes. It is undesirable to use electrolytic capacitors without them - they tend to explode.
But even if you install diodes and resistors, this cannot guarantee complete safety. If the semiconductor breaks through, then alternating current will flow to the capacitors, resulting in an explosion. The modern element base allows the use of high-quality products, for example, polypropylene capacitors for operation on alternating current with the designation SVV.
For example, the designation of elements SVV60 indicates that the capacitor is designed in a cylindrical housing. But SVV61 has a rectangular body. These elements operate under a voltage of 400... 450 V. Therefore, they can be used without problems in the design of any device that requires connecting an asynchronous three-phase electric motor to a household network.
Operating voltage
One important parameter of capacitors must be taken into account - operating voltage. If you use capacitors to start an electric motor with a very large voltage reserve, this will lead to an increase in the dimensions of the structure. But if you use elements designed to operate with a lower voltage (for example, 160 V), this will lead to rapid failure. In order for capacitors to function normally, their operating voltage must be approximately 1.15 times greater than the network voltage.
Moreover, one feature must be taken into account - if you use paper capacitors, then when working in alternating current circuits their voltage must be reduced by 2 times. In other words, if the housing indicates that the element is designed for a voltage of 300 V, then this characteristic is relevant for direct current. Such an element can be used in an alternating current circuit with a voltage of no more than 150 V. Therefore, it is better to assemble batteries from paper capacitors, the total voltage of which is about 600 V.
Connecting an electric motor: a practical example
Let's say you have an asynchronous electric motor designed to be connected to a three-phase AC network. Power - 0.4 kW, motor type - AOL 22-4. Main characteristics for connection:
- Power - 0.4 kW.
- Supply voltage - 220 V.
- The current when operating from a three-phase network is 1.9 A.
- The motor windings are connected using a star circuit.
Now it remains to calculate the capacitors to start the electric motor. The motor power is relatively small, therefore, to use it in a household network, you only need to select a working capacitor; there is no need for a starting capacitor. Using the formula, calculate the capacitance of the capacitor: C (slave) = 66*P (motor) = 66*0.4 = 26.4 µF.
You can use more complex formulas; the capacity value will differ slightly from this. But if there is no capacitor suitable for the capacitance, you need to connect several elements. When connected in parallel, the containers are folded.
note
Now you know which capacitors are best to use to start an electric motor. But the power will drop by about 20-30%. If a simple mechanism is set in motion, it will not be felt. The rotor speed will remain approximately the same as indicated in the passport. Please note that if the motor is designed to operate from a 220 and 380 V network, then it is connected to a household network only if the windings are connected in a triangle. Carefully study the tag; if it only has the designation of a “star” circuit, then in order to work in a single-phase network you will have to make changes to the design of the electric motor.
Instructions
As a rule, to connect a three-phase electric motor, three wires and a supply voltage of 380 are used. There are only two wires in a 220 volt network, so in order for the engine to work, voltage must also be applied to the third wire. For this purpose, a capacitor is used, which is called a working capacitor.
The capacitor capacity depends on the engine power and is calculated by the formula:
C=66*P, where C is the capacitance of the capacitor, μF, P is the power of the electric motor, kW.
That is, for every 100 W of engine power it is necessary to select about 7 µF of capacitance. Thus, a 500-watt motor requires a capacitor with a capacity of 35 µF.
The required capacity can be assembled from several capacitors of smaller capacity by connecting them in parallel. Then the total capacity is calculated using the formula:
Ctotal = C1+C2+C3+…..+Cn
It is important to remember that the operating voltage of the capacitor should be 1.5 times the power supply to the electric motor. Therefore, with a supply voltage of 220 volts, the capacitor should be 400 volts. Capacitors can be used of the following types: KBG, MBGCh, BGT.
To connect the motor, two connection schemes are used - “triangle” and “star”.
If in a three-phase network the motor was connected according to a delta circuit, then we connect it to a single-phase network according to the same circuit with the addition of a capacitor.
The star connection of the motor is carried out according to the following diagram.
To operate electric motors with a power of up to 1.5 kW, the capacity of the working capacitor is sufficient. If you connect a higher power engine, then such an engine will accelerate very slowly. Therefore it is necessary to use a starting capacitor. It is connected in parallel with the run capacitor and is used only during engine acceleration. Then the capacitor is turned off. The capacitor capacity to start the engine must be 2-3 times greater than the operating capacity.
After starting the engine, determine the direction of rotation. Typically you want the motor to rotate clockwise. If the rotation occurs in the desired direction, you do not need to do anything. To change direction, it is necessary to remount the engine. Disconnect any two wires, swap them and reconnect. The direction of rotation will change to the opposite.
When performing electrical installation work, follow safety regulations and use personal protective equipment against electric shock.
Three-phase electric does not contain brushes that can wear out and require periodic replacement. It is less efficient than collector, but much more efficient than asynchronous single-phase. Its disadvantage is its significant dimensions.
Instructions
Find the nameplate on the three-phase electric motor. It shows two voltages, for example: 220/380 V. The engine can be powered by any of these voltages, it is only important to connect its windings correctly: for the lower of the indicated voltages - with a triangle, for the higher - with a star.