Turning on of the 3-phase engine in single-phase network

Among different ways of start of three-phase electric motors in single-phase network most idle time is based on connection of the third winding via the phase-shifting condenser. The net power developed by the engine in this case is 50...60% of its power in three-phase inclusion.

Электрическая принципиальная схема подключения 3-х фазного двигателя

Electric key diagram of connection of the 3 phase engine.

Not all three-phase electric motors , however, well work at connection to single-phase network. It is possible to distinguish from such electric motors, for example, model with the double cage of the short-circuited rotor of the MA series.

In this regard at the choice of three-phase electric motors for work it is necessary to give preference in single-phase network to engines of series A, JSC, AO2, NPA, UAD, etc.

It is necessary for normal operation of the electric motor with condenser start-up that the capacity of the used condenser changed depending on speed. In practice this condition to execute quite difficult therefore use two-level engine management. At launch of the engine connect two condensers, and after dispersal switch-off one condenser and leave only the working condenser.

Calculation of parameters and elements of the electric motor

Рисунок 1. Принципиальная схема включения трехфазного электродвигателя в сеть 220 В: С р - рабочий конденсатор; С п - пусковой конденсатор; П1 - пакетный выключатель

Figure 1. Key diagram of turning on of the three-phase electric motor in network of 220 V: With р - the working condenser; With п - the starting condenser; P1 - the package switch.

If, for example, in the passport of the electric motor tension of its power supply 220/380 B is specified, then the engine is turned on in single-phase network according to the scheme presented on fig. 1.

After turning on of the package P1 switch contacts of P1.1 and P1.2 become isolated, after that it is necessary to press the Dispersal button at once.

After the set of turns the button is released. Reversal of the electric motor is carried out by switching of the phase on its winding by the SA1 toggle-switch.

Capacity of the working condenser of Wednesday in case of connection of windings of the engine in "triangle" is determined by the formula:

Расчет емкости при треугольнике, where

And in case of connection of windings of the engine in "star" is determined by the formula:

Расчет емкости при звезде, where

The current consumed by the electric motor in the above-stated formulas, at the known power of the electric motor, can be calculated from the following expression:

Расчет тока по мощности, where

Рисунок 2. Принципиальная схема соединения электролитических конденсаторов для использования их в качестве пусковых конденсаторов

Figure 2. The key diagram of connection of electrolytic capacitors for their use as starting condensers.

Capacity of the starting condenser of the Joint venture is chosen in 2... 2,5 times are more than the capacity of the working condenser. These condensers have to be expected tension 1,5 times more than mains voltage.

For network of 220 V it is better to use MBGO, MBPG, MBGCh condensers with operating voltage of 500 V and above. On condition of short-term inclusion as starting condensers it is possible to use also electrolytic capacitors like K50-3, EGTs-M, KE-2 with operating voltage not less than 450 V.

For bigger reliability electrolytic capacitors connect consistently, connecting among themselves their minus conclusions, and shunt diodes (fig. 2)

The general capacity of the connected condensers will be:


In practice the size of tanks of working and starting condensers is chosen depending on engine capacity. Value of tanks of working and starting condensers of the three-phase electric motor depending on its power at inclusion in network of 220 Century.

Power of three-phase
engine, kW:

Minimum capacity of the worker
condenser of Wednesday, microfarad:

Minimum capacity of starting
condenser of Wednesday, microfarad:

It should be noted that at the electric motor with condenser start-up in the mode of no-load operation on the winding fed via the condenser the current for 20...30% exceeding rated proceeds. In this regard, if the engine is often used in the underloaded mode or empty, the capacitor capacitance of Wednesday should be reduced. Can happen that during the overload the electric motor stopped, then for its start connect the starting condenser again, having unloaded in general or having lowered it to the minimum.

Capacity of the starting condenser of the Joint venture can be reduced at launch of electric motors idling or with small loading. For inclusion, for example, the AO2 electric motor with a power of 2,2 kW for 1420 about. / mines it is possible to use the working condenser with a capacity of 230 microfarad, and starting - 150 microfarad. In this case the electric motor is surely started at small loading on the shaft.

The portable universal block for launch of three-phase electric motors with power about 0,5 kW from network of 220 V

Рисунок 3. Принципиальная схема переносного универсального блока для пуска трехфазных электродвигателей мощностью около 0,5 кВт от сети 220 В без реверса

Figure 3. The key diagram of the figurative universal block for launch of three-phase electric motors with power about 0,5 kW from network of 220 V without reverse.

For start of electric motors of different series with power about 0,5 kW from single-phase network without reversal it is possible to assemble the portable universal starting block (fig. 3).

By pressing the SB1 button the magnetic KM1 actuator (the SA1 toggle-switch is closed) and the KM contact system 1.1 works, the KM 1.2 connects the M1 electric motor to network of 220 Century.

Along with it the third contact unit of KM 1.3 closes the SB1 button.

After full dispersal of the engine by the SA1 toggle-switch switch-off the starting C1 condenser.

The stop of the engine is carried out by pressing the SB2 button.


In the device the 0,55 kW A471A4 (AO2-21-4) electric motor for 1420 is used about. / the min. and magnetic PML actuator expected alternating current by voltage of 220 V. The SB1 and SB2 buttons - coupled the PKE612 type. As the SA1 switch the T2-1 toggle-switch is used. In the device the constant R1 resistor - wire, the PE-20 type, and the R2 resistor of the MLT-2 type. C1 and C2 condensers of the MBGCh type on tension of 400 Century. The C2 condenser is made from in parallel the connected condensers on 20 microfarad 400 V. Lampa by KM-24 HL1 and 100 mA.

Рисунок 4. Схема пускового устройства в металлическом корпусе размером 170х140х50 мм

Figure 4. The scheme of the starting arrangement in the metal body of 170х140х50 mm in size.

The starting arrangement is mounted in the metal body of 170х140х50 mm in size (fig. 4):

On the upper panel of the body the Start-up and Stop buttons - the signal lamp and the toggle-switch for shutdown of the starting condenser are located. On the front panel of the body of the device there is the connector for connection of the electric motor.

For shutdown of the starting condenser it is possible to use the additional K1 relay, then need for the SA1 toggle-switch disappears, and the condenser will be automatically switched-off (fig. 5).

Рисунок 5. Принципиальная схема пускового устройства с автоматическим отключением пускового конденсатора

Figure 5. The key diagram of the starting arrangement with automatic shutdown of the starting condenser.

By pressing the SB1 button works the K1 relay and in contact couple of K1.1 turns on the magnetic KM1 actuator, and K1.2 - the starting condenser of the Joint venture. The magnetic KM1 actuator samoblokirutsya by means of the contact couple of KM 1.1, and contacts of KM 1.2 and KM 1.3 connect the electric motor to network.

The Start-up button is held pressed before full dispersal of the engine, and later released. The K1 relay is deenergized and switches-off the starting condenser which is discharged via the R2 resistor. In the same time the KM magnetic actuator 1 remains included and provides the power supply of the electric motor in the operating duty.

For the stop of the electric motor it is necessary to press the Feet button. In the advanced starting arrangement according to the scheme of fig. 5 it is possible to use the MKU-48 relay or to it similar.

Use of electrolytic capacitors in schemes of start of electric motors

At turning on of three-phase asynchronous electric motors in single-phase network, as a rule, use normal paper capacitors. Practice showed that instead of bulky paper capacitors it is possible to use oxidic (electrolytic) condensers which have smaller dimensions and are more available in respect of purchase.

Рисунок 6. Принципиальная схема замены бумажного конденсатора (а) электролитическим (б, в)

Figure 6. Key diagram of replacement of the paper capacitor (a) electrolytic (c).

The scheme of replacement обычног the paper capacitor is given in fig. 6.

The alternating-current positive half wave passes through the chain of VD1, S2, and negative VD2, S2. Proceeding from it it is possible to use oxidic condensers with allowable voltage twice smaller, than for normal condensers of the same capacity.

For example, if in the scheme for single-phase network voltage of 220 V uses the paper capacitor on voltage of 400 V, then at its replacement according to the above-stated scheme the provided scheme of capacity of both condensers can use the electrolytic capacitor on 200 V.V tension are identical and are chosen similar to the technique of the choice of paper capacitors for the starting arrangement.

Turning on of the three-phase engine in single-phase network with use of electrolytic capacitors

The scheme of turning on of the three-phase engine in single-phase network with use of electrolytic capacitors is provided on fig. 7.

In the provided scheme SA1 - the switch of the rotation sense of the engine, SB1 - the button of dispersal of the engine, electrolytic capacitors of C1 and C3 are used for launch of the engine, C2 and C4 - during the operating time.

It is better to make selection of electrolytic capacitors in the scheme of fig. 7 by means of clamp-on ammeters. Measure currents in points And, In, With and try to obtain equality of currents in these points by step selection of capacitor capacities. Measurements carry out at the loaded engine in that mode in which its operation is supposed.

Рисунок 7. Принципиальная схема включения трехфазного двигателя в однофазную сеть при помощи электролитических конденсаторов

Figure 7. The key diagram of turning on of the three-phase engine in single-phase network by means of electrolytic capacitors.

VD1 and VD2 diodes for network of 220 V are chosen with the reverse most allowable voltage not less than 300 V. The peak forward current of the diode depends on engine capacity. D245, D245A, D246, D246A, D247 diodes with the forward current 10 A will be suitable for electric motors with power up to 1 kW.

At the bigger engine capacity from 1 kW to 2 kW it is necessary to take more powerful diodes with the corresponding forward current or to put several less powerful diodes in parallel, having established them on radiators.

It is necessary to pay attention that at the overload of the diode there can be its breakdown and through the electrolytic capacitor alternating current will begin to flow that can lead to its heating and explosion.

Turning on of powerful three-phase engines in single-phase network

The condenser scheme of turning on of three-phase engines in single-phase network allows to receive no more than 60% of rated power while the limit of power of the electrified device is limited to 1,2 kW from the engine. It is obviously not enough for work of the electroplane or the power saw which have to have the power of 1,5...2 kW. The problem in this case can be solved by use of the electric motor of bigger power, for example 3...4 kW. Engines of this kind are expected voltage of 380 V, their windings are connected by "star", and the terminal box contains only 3 conclusions.

Turning on of such engine in network of 220 V leads to decrease in rated power of the engine by 3 times and for 40% during the work in single-phase network. Such power deceleration does the engine unsuitable for work, but can be used for promotion of the rotor empty or with the minimum load. Practice shows that the most part of electric motors surely accelerates to rated turns, and in this case starting currents do not exceed 20 A.

Completion of the three-phase engine

It is most just possible to make the transfer of the powerful three-phase engine in the operating duty if to change it on the single-phase operating mode, receiving at the same time 50% of rated power. Switching of the engine in the single-phase mode demands its small completion.

Рисунок 8. Принципиальная схема коммутации обмоток трехфазного электродвигателя для включения в однофазную сеть

Figure 8. The key diagram of switching of windings of the three-phase electric motor for inclusion in single-phase network.

Open the terminal box and define from what party of the cover of the body of the engine conclusions of windings approach. Turn off bolts of fastening of the cover and take out it from the engine body. Find the junction of three windings in the generic point and podpaivat to the generic point the additional conductor with the section corresponding to winding wire section. The twist with the soldered conductor is isolated the insulating tape or the polyvinylchloride tube, and the additional conclusion is stretched in the terminal box. After that the cover of the body is established into place.

The scheme of switching of the electric motor will have in this case the appearance shown in fig. 8.

During dispersal of the engine connection of windings by "star" with connection of the phase-shifting condenser of the Joint venture is used. In the operating duty in network remains included only one winding, and rotation of the rotor is supported by the pulsing magnetic field. After switching of windings the condenser of the Joint venture is discharged via the Rp resistor. Work of the provided scheme was tested with the AIR-100S2Y3 engine (4 kW, 2800 about. / mines), established on the self-made woodworking machine, also showed the efficiency.


In the scheme of switching of windings of the electric motor as the SA1 switch it is necessary to use the ganged switch on the working current not less than 16 And, for example the PP2-25/N3 switch (bipolar with the neutral, on current 25 A). The SA2 switch can be any type, but on current not less than 16 And. If the reverse of the engine is not required, then this SA2 switch can be excluded from the scheme.

The lack of the offered scheme of turning on of the powerful three-phase electric motor in single-phase network can be considered sensitivity of the engine to overloads. If loading on the shaft reaches the half of engine capacity, then there can be the reduction in the rate of rotation of the shaft up to its full stop. In this case it is unloaded the engine shaft. The switch is turned at first to position "Dispersal", and then to the provision "Work" then continue further work.

To improve control characteristics of engines, except the starting and working condenser it is possible to use also inductance that improves uniformity of loading of phases.

  1. Stanislav

    tell me please according to fig. 8 how to calculate Rp face value

    To answer
  2. Anatoly

    The formula 1/Sobshch = 1/C1+1/C2 is right:
    1/Sobshch = C2+C1/C1*C2
    Собщ * (C2+C1) = C1*C2
    Собщ = C1*C2/C2+C1

    To answer
  3. Kostya

    People do not suffer just take 2 identical engines, transfer them to the triangle, i.e. 220vi having included 1 in network 220 you will take away with 3 phases on 220V for other engine are more useless, приченм the 2nd will be will disperse itself and its power will be lost much less than with condensers, the scheme is tested on the circular saw …

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  4. Sergey

    The formula of calculation of capacity of consistently connected condensers in the explanation to fig. 2 is given incorrectly. Instead of (C1+C2)/2 should write (C1*C2) / (C1+C2).

    To answer
    • admin

      Thanks for the found mistake.

      To answer
  5. Mikhail

    There is one more scheme of turning on of 3 phase engines, power remains big than via the condenser. The truth it will suit not all, but when the engine does not pull, for example any electric tool з x phase, it is applicable. The electric motor the power twice more powerful than that which should be started is necessary, we start it via the condenser (it is possible the string if on couple of starts, the second is connected parallel to the first and everything works, there is phases distortion but it in the admission. It not mine, saw in practice.

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