Three-phase motor in single-phase network.

Sometimes in life situations when you want to include some kind of industrial equipment in the ordinary domestic mains supply.Immediately there is a problem with the number of wires.The machine, designed for use in enterprises, conclusions, usually three, and sometimes four, and.What to do with them, where they connected?Those who tried to try different options, making sure that the engines just do not want to spin.Is it even possible to connect a single-phase three-phase motor?Yes, you can achieve a rotation.Unfortunately, in this case, the power will inevitably fall by almost half, but in some situations it is - the only way out.

phase voltage networks and their relationship

In order to understand how to connect to a conventional three-phase motor outlet must figure out how to relate voltage industrial network.It is well-known stress values ​​- 220 and 380 volts.Previously, it was still 127, but in the fifties of this option was abandoned in favor of a higher.Where did these "magic numbers"?Why not 100 or 200, or 300?It seems to be easier to round numbers count.

Most of the industrial electrical equipment is designed for connection to three-phase AC power.The voltage of each phase against the neutral wire is 220 volts, just like on your home outlet.Where did come from 380?It is easy enough to consider an isosceles triangle with angles of 60, 30 and 30 degrees, which is a vector diagram of voltages.The length of the longest side is equal to the length of the thigh multiplied by cos 30 °.After simple calculations we can see that 220 x cos 30 ° = 380.

device is a three-phase motor

Not all types of industrial motors can be operated from a single phase.The most common of them - the "workhorses" that make up the majority of electric machines in any enterprise - asynchronous machines with a capacity of 1 - 1.5 kVA.How does a three-phase motor in three-phase network, for which it was intended?

The inventor of this revolutionary device was the Russian scientist Mikhail Dolivo-Dobrovolsky.This outstanding electrical engineer was a supporter of the theory of three-phase mains, which in our time has become dominant.Three-phase induction motor operates on the principle of induction currents of the stator windings in the rotor conductors are closed.As a result of their occurrence on a short-circuited windings in each of which there is a magnetic field that interacts with the field lines of the stator.So it turns out the torque, resulting in a circular motion of the motor axis.

windings are arranged at an angle of 120 °, thus rotating field generated by each phase successively pushing each side of the rotor is magnetized.

triangle or a star?

phase motor in three-phase network can be switched on in two ways - with the participation of the neutral wire or without it.The first method is called a "star", in this case, each of the windings is in phase voltage (phase-to-zero), equal in our case 220 V. Wiring three-phase motor "triangle" involves a series connection of three windings and supply line (380)voltage switching nodes.In the second case, the motor will produce greater than about half the power.

How to turn the motor in the opposite direction?

three-phase motor may imply a change in the rotational direction is reversed, that is the reverse.To do this, you just need to swap the two wires of the three.

For the convenience of schema changes in the motor terminal box provides jumpers made, usually copper.To activate the "star" gently to combine three output winding wires together."Triangle" get a little more complicated, but it will cope with any semi-skilled electrician.

phase-shifting capacity

So, sometimes the question arises, how to connect a three-phase motor in ordinary household outlet.If you just try to connect the two wires to the plug, it will not rotate.In order to get business, you need to simulate the phase shifting the supply voltage at a certain angle (preferably 120 °).To achieve this effect, it is possible, if we apply the phase-shifting element.Theoretically, it could be an inductance, resistance, and even, but more often three-phase motor single phase is included with capacitances (capacitors), referred to in the diagrams Latin letter S.

As regards the application of chokes, it is difficult because of the difficulty in determining theirvalue (if it is not indicated on the device).To measure the L requires a special device or collected for this scheme.Besides the choice of available chokes usually limited.However, any experimental phase-shifting element can pick up, but it is troublesome.

What happens when you turn on the engine?On one of the connection points supplied zero, on the other - a phase, and the third - a kind of tension, moved at an angle relative to the phase.Clearly layman and that operation of the engine will not be complete in terms of mechanical shaft power, but in some cases sufficient rotation of the fact.However, when you run there may be some problems, such as lack of start time, able to move the rotor space.What should I do in this case?

start capacitor

At the time of start-up shaft more efforts are needed to overcome the forces of inertia and friction.To increase torque, install an additional capacitor connected to the circuit only at the start and then turned off.For these purposes, the best option is to use the closing pushbutton position.Wiring three-phase motor with starting capacitor is shown below, it is simple and straightforward.At the time of the supply voltage, press the "Start" button and start capacitor will create additional phase shift.After the engine untwisted to the desired RPM, the button can (and should) let go, and the scheme will only work capacity.

Calculation of containers

So, we found out that in order to enable the three-phase motor in single-phase networks require additional wiring diagram, which, in addition to the start button, includes two capacitors.They need to know the value, otherwise the system will not operate.To begin with we define the value of capacitance needed to get the rotor to pull away.With parallel connection, it is the sum of:

C = a + Cp where:

With Art - starting off after takeoff additional capacity;

Cp - run capacitor, which provides rotation.

Yet we need the value of the rated current I n (it is listed on a label attached to the engine at the factory).This parameter can also be determined by means of simple formulas:

I N = P / (3 x U), where:

U - voltage at a "star" - 220, and if the "triangle", the 380 V;

P - power three-phase motor, it is sometimes in the case of loss tablets is determined by eye.

So, depending on the desired operating power calculated by the formulas:

Cp = Cp = 2800 I n / U - for the "stars";

C p = 4800 I n / U - to the "triangle";

start capacitor must be greater than the working of 2-3.Unit - microfarad.

There are just too simple method of calculating the capacity: C = P / 10, but this formula gives the order numbers rather than its value.However, tinker have anyway.

Why a fit

method of calculation given above are approximate.Firstly, the rated value shown on the body of electric capacity, may differ significantly from the actual.Secondly, paper capacitors (generally speaking, a thing not cheap) is often used second-hand, and they are all sorts of other objects, subject to aging, which leads to an even greater deviation from the specified.Third, the current that will be consumed by the engine depends on the mechanical load on the shaft, and therefore it can only evaluate experimentally.How to do it?

you need a little patience.The result may be quite voluminous set of capacitors connected in parallel and in series.The main thing - after all works properly fix to not fall off the ends soldered by vibrations coming from the engine.And then do not be amiss to once again analyze the results and possibly to simplify the design.

Drafting battery capacities

If the disposal of the master no special electrolytic ticks, allowing to measure current without breaking the chains, it is necessary to connect an ammeter in series with each wire, which is a three-phase motor.In the single-phase network will flow total value, and the selection of capacitors should strive for more uniform loading coils.It should be remembered that in series connection reduces the overall capacity of the law:

1 / C = 1 / C1 + 1 / C2 ... and so on, and in parallel - on the contrary, develops.

is also necessary not to forget about such important parameters as the voltage on the capacitor is calculated.It should not be less than the nominal value of the network, and it is better to reserve.

discharge resistor

scheme three-phase motor connected between one phase and neutral, sometimes supplemented with resistance.It is to start on the capacitor is not accumulated charge remaining after the machine has turned off.This energy can cause electric shock, not dangerous, but extremely unpleasant.In order to protect themselves, should be parallel with the start-up capacity to connect a resistor (for electricians is called "shunt").The value of its resistance is greater - from half to Meg Meg, and it is small in size, so pretty and poluvattnoy power.However, if you are not afraid to be "pinch" that without this part it is possible and necessary.

use electrolytes

As already noted, film or paper capacitances expensive to purchase, and they are not as easy as we would like.It is possible to produce a single-phase connection of three-phase motors using inexpensive and available electrolytic capacitors.At the same time they are very very cheap, too, will not be as to withstand 300 volts DC.For safety, they should be shunted semiconductor diode (D 245 and D 248, for example), but it is useful to remember that the penetration of these devices AC voltage falls to the electrolyte, and at first he will overheat and then explode, loud and impressive.Therefore, unless absolutely necessary it is better to use the same type of paper capacitors, the voltage is constant even though the variables.Some masters quite allow the use of electrolytes in the starting circuit.By short-term exposure to an alternating voltage, they may not have time to explode.It is better not to experiment.

If no capacitors

Where ordinary citizens who do not have access to the best-selling electric and electronic parts, buy them?At flea markets and "flea markets".There they lie, carefully vypayat someone else (usually elderly) hands of the old washing machines, televisions and other published articles and building of civil and industrial engineering.Ask for these products a lot of Soviet-made: the sellers know that the item is needed, then buy it, and if not - and do not take the gift.It happens that just the basic necessities (in this case the capacitor) is just not.And what do you do?No problem!Converge and resistors just need powerful, preferably ceramic and vitrified.Of course, the ideal resistance (active) phase does not shift, but nothing in this world is not perfect, and in this case it is good.Each physical body has a self-inductance, electric capacity and resistivity, be it a tiny speck of dust or a huge mountain.The inclusion of the three-phase motor in the socket, it is possible, if the above scheme to replace the capacitor impedance whose value is calculated according to the formula:

R = (0,86 x U) / kI, where:

kI - the amount of current for three-phase connections, and;

U - our loyal 220 volts.

What engines fit?

Before you buy a lot of money motor that prudent owner is going to use as a drive for the grinding wheel, circular saw, drill or any other useful household devices will not prevent to think about its applicability for this purpose.Not everyone is a three-phase motor in single-phase networks in general be able to work.For example, a series of MA (his squirrel-cage double cage) should be deleted so as not to have to haul home a considerable and useless weight.Generally, it's best to experiment first, or invite an experienced person, electrician, for example, and to consult with them before buying.It will approach the three-phase asynchronous motor series UAD, APN, AO2, AO and, of course, A. These codes indicated on the nameplate.