Capacitor for an electric motor: selection tips and rules for connecting a starting capacitor. Capacitor for an electric motor: selection tips and rules for connecting a starting capacitor Capacitor for a 1 kV station

In the household, sometimes there is a need to run a 3-phase asynchronous electric motor (AM). If you have a 3-phase network, this is not difficult. In the absence of a 3-phase network, the engine can be started from a single-phase network by adding capacitors to the circuit.

Structurally, the IM consists of a stationary part - the stator, and a moving part - the rotor. Windings are placed in slots on the stator. The stator winding is a three-phase winding, the conductors of which are evenly distributed around the circumference of the stator and laid in phases in slots with an angular distance of 120 el. degrees. The ends and beginnings of the windings are led out into the junction box. The windings form pairs of poles. The rated rotor speed of the motor depends on the number of pole pairs. Most general industrial motors have 1-3 pairs of poles, less often 4. IMs with a large number of pole pairs have low efficiency, larger dimensions, and therefore are rarely used. The more pole pairs, the lower the motor rotor speed. General industrial motors are produced with a number of standard rotor speeds: 300, 1000, 1500, 3000 rpm.

The rotor of the IM is a shaft on which there is a short-circuited winding. In low- and medium-power motors, the winding is usually made by pouring molten aluminum alloy into the grooves of the rotor core. Together with the rods, short-circuited rings and end blades are cast, which ventilate the machine. In high-power machines, the winding is made of copper rods, the ends of which are connected to short-circuited rings by welding.

When the IM is turned on in a 3-phase network, current begins to flow through the windings in turn at different times. In one period of time, the current passes along the pole of phase A, in another along the pole of phase B, in the third along the pole of phase C. Passing through the poles of the windings, the current alternately creates a rotating magnetic field that interacts with the rotor winding and causes it to rotate, as if pushing it in different planes at different times.

If you turn on the IM in a 1-phase network, the torque will be created by only one winding. Such a moment will act on the rotor in one plane. This moment is not enough to move and rotate the rotor. To create a phase shift of the pole current relative to the supply phase, phase-shifting capacitors are used in Fig. 1.

Capacitors can be used of any type, except electrolytic. Capacitors such as MBGO, MBG4, K75-12, K78-17 are well suited. Some capacitor data is shown in Table 1.

If it is necessary to gain a certain capacitance, then the capacitors should be connected in parallel.

The main electrical characteristics of the IM are given in the data sheet, Fig. 2.

Fig.2

From the passport it is clear that the motor is three-phase, with a power of 0.25 kW, 1370 rpm, it is possible to change the winding connection diagram. The connection diagram for the windings is “triangle” at a voltage of 220V, “star” at a voltage of 380V, respectively, the current is 2.0/1.16A.

The star connection diagram is shown in Fig. 3. With this connection, a voltage is supplied to the electric motor windings between points AB (linear voltage U l) that is times greater than the voltage between points AO (phase voltage U f).

Fig.3 Star connection diagram.

Thus, the linear voltage is several times greater than the phase voltage: . In this case, the phase current I f is equal to the linear current I l.

Let's look at the triangle connection diagram in Fig. 4:

Fig.4 Delta connection diagram

With such a connection, the linear voltage U L is equal to the phase voltage U f., and the current in the line I l is times greater than the phase current I f:.

Thus, if the IM is designed for a voltage of 220/380 V, then to connect it to a phase voltage of 220 V, a “triangle” connection diagram for the stator windings is used. And for connecting to a linear voltage of 380 V - a star connection.

To start this IM from a single-phase network with a voltage of 220V, we should turn on the windings according to the “delta” circuit, Fig. 5.

Fig.5 Connection diagram of the EM windings according to the “triangle” diagram

The connection diagram of the windings in the output box is shown in Fig. 6

Fig.6 Connection in the ED output box according to the “triangle” diagram

To connect an electric motor according to the “star” circuit, it is necessary to connect two phase windings directly to a single-phase network, and the third through a working capacitor C p to any of the network wires in Fig. 6.

The connection in the terminal box for the star circuit is shown in Fig. 7.

Fig. 7 Connection diagram of the EM windings according to the “star” scheme

The connection diagram of the windings in the output box is shown in Fig. 8

Fig.8 Connection in the ED output box according to the “star” scheme

The capacity of the working capacitor C p for these circuits is calculated by the formula:
,
where I n - rated current, U n - rated operating voltage.

In our case, to switch on the “triangle” circuit, the capacitance of the working capacitor is C p = 25 µF.

The operating voltage of the capacitor should be 1.15 times the rated voltage of the supply network.

To start an IM of small power, a working capacitor is usually sufficient, but with a power of more than 1.5 kW, the engine either does not start or picks up speed very slowly, so it is necessary to also use a starting capacitor C p. The capacity of the starting capacitor should be 2.5-3 times greater than the capacity of the working capacitor capacitor.

The connection diagram of the electric motor windings connected in a delta pattern using starting capacitors C p is shown in Fig. 9.

Fig. 9 Connection diagram of the EM windings according to the “triangle” diagram using starting condensates

The connection diagram of the star motor windings using starting capacitors is shown in Fig. 10.

Fig. 10 Connection diagram of the EM windings according to the “star” circuit using starting capacitors.

Starting capacitors C p are connected in parallel to the working capacitors using the KN button for a time of 2-3 s. In this case, the rotation speed of the electric motor rotor should reach 0.7…0.8 of the rated rotation speed.

To start the IM using starting capacitors, it is convenient to use the button Fig. 11.

Fig.11

Structurally, the button is a three-pole switch, one pair of contacts of which closes when the button is pressed. When released, the contacts open, and the remaining pair of contacts remains on until the stop button is pressed. The middle pair of contacts performs the function of a KN button (Fig. 9, Fig. 10), through which starting capacitors are connected, the other two pairs act as a switch.

It may turn out that in the connection box of the electric motor the ends of the phase windings are made inside the motor. Then the IM can only be connected according to the diagrams in Fig. 7, Fig. 10, depending on power.

There is also a diagram for connecting the stator windings of a three-phase electric motor - partial star Fig. 12. Making a connection according to this diagram is possible if the beginnings and ends of the stator phase windings are brought out into the junction box.

Fig.12

It is advisable to connect an electric motor according to this scheme when it is necessary to create a starting torque exceeding the nominal one. This need arises in drives of mechanisms with difficult starting conditions, when starting mechanisms under load. It should be noted that the resulting current in the supply wires exceeds the rated current by 70-75%. This must be taken into account when choosing the wire cross-section for connecting the electric motor.

Capacitance of the working capacitor C p for the circuit in Fig. 12 is calculated by the formula:
.

The capacitance of starting capacitors should be 2.5-3 times greater than the capacitance C r. The operating voltage of the capacitors in both circuits should be 2.2 times the rated voltage.

Typically, the terminals of the stator windings of electric motors are marked with metal or cardboard tags indicating the beginnings and ends of the windings. If for some reason there are no tags, proceed as follows. First, the belonging of the wires to the individual phases of the stator winding is determined. To do this, take any of the 6 external terminals of the electric motor and connect it to any power source, and connect the second terminal of the source to the control light and, with the second wire from the lamp, alternately touch the remaining 5 terminals of the stator winding until the light comes on. When the light comes on, it means that the 2 terminals belong to the same phase. Conventionally, let's mark the beginning of the first wire C1 with tags, and its end - C4. Similarly, we will find the beginning and end of the second winding and designate them C2 and C5, and the beginning and end of the third - C3 and C6.

The next and main stage will be to determine the beginning and end of the stator windings. To do this, we will use the selection method, which is used for electric motors with a power of up to 5 kW. Let's connect all the beginnings of the phase windings of the electric motors according to the previously connected tags to one point (using a star circuit) and connect the electric motor to a single-phase network using capacitors.

If the engine immediately picks up the rated speed without a strong hum, this means that all the beginnings or all ends of the winding have hit the common point. If, when turned on, the engine hums strongly and the rotor cannot reach the rated speed, then terminals C1 and C4 in the first winding should be swapped. If this does not help, the ends of the first winding must be returned to their original position and now the terminals C2 and C5 are swapped. Do the same; for the third pair if the engine continues to hum.

When determining the beginnings and ends of windings, strictly adhere to safety regulations. In particular, when touching the stator winding clamps, hold the wires only by the insulated part. This must also be done because the electric motor has a common steel magnetic core and a large voltage may appear at the terminals of other windings.

To change the direction of rotation of the rotor of an IM connected to a single-phase network according to the “triangle” circuit (see Fig. 5), it is enough to connect the third phase winding of the stator (W) through a capacitor to the terminal of the second phase winding of the stator (V).

To change the direction of rotation of an IM connected to a single-phase network according to the “star” circuit (see Fig. 7), you need to connect the third phase winding of the stator (W) through a capacitor to the terminal of the second winding (V).

When checking the technical condition of electric motors, you can often notice with disappointment that after prolonged operation, extraneous noise and vibration appear, and the rotor is difficult to turn manually. The reason for this may be the poor condition of the bearings: the treadmills are covered with rust, deep scratches and dents, individual balls and the cage are damaged. In all cases, it is necessary to inspect the electric motor and eliminate any existing faults. In case of minor damage, it is enough to wash the bearings with gasoline and lubricate them.

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.

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 according to the “star” circuit, and when it reaches the established mode, it switches to the “triangle” one.

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.

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 the 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 the same type of elements as working and starting ones. 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.

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.