Connection of three-phase consumers to a single-phase network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power supply

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In amateur radio literature [1...5], the question of connecting a three-phase consumer to a single-phase network has been repeatedly raised. The authors of the articles point out the disadvantages of the described methods:

- loss of 50% of the nominal power;

- not all brands of electric motors start well when powered from a single-phase network;

- the need to use two containers (starting and working);

- stepwise adjustment of the capacity rating in different operating modes;

- the need to change the capacitance rating when the load on the shaft changes;

- at idle, the current flowing through the motor winding is 40% more than the nominal one;

- extra "bells and whistles" to automate the shutdown of the starting capacitor and when replacing paper capacitors with electrolytic ones.

I propose another option for connecting three-phase consumers to a single-phase network.

If you look at the three-phase voltage graph, you can see that each curve is shifted relative to the other by 1/3 of the period (Fig. 1).

Ris.1

The mains frequency is 50 Hz, therefore, the period T is 20 ms. It follows that 1/3 of the period is 6,666 ... ms. Let Ua in Fig. 1 be a single-phase sinusoidal voltage of 220 V, 50 Hz. Having passed Ua through the delay circuit for 6,666 ... ms, we obtain a voltage Uv shifted by 1/3 of the period, equal in amplitude and frequency to Ua. Having “passed” the voltage Uv through a similar delay circuit, we get the voltage Uс shifted by 1/3 of the period relative to the voltage Uv.

A schematic diagram of such a device is shown in Fig.2.

Fig.2 (click to enlarge)

The device consists of a power supply and a positive polarity pulse generator on the T1 transformer. The power supply includes the winding II of the transformer T1, the rectifier bridge VD1 ... VD4 and the stabilizer DA1. The pulse generator is assembled on the winding III of the transformer T1, the resistor R1 and the rectifier on the diodes VD5, VD6. The zener diode VD7 protects the inputs of the DD1.1 element from accidental overvoltage of more than 12 V. A rectangular pulse shaper is assembled on the DD1.1 element. You can also use the comparator, described in detail in [6]. At the output of the shaper DD1.1 there are rectangular pulses with a frequency of 50 Hz of positive polarity.

Suppose that these are voltage pulses Ua (Fig. 1). Pulses from the output "A" of the element DD1.1 are fed to the input of the delay circuit assembled on the elements DD2.1, DD2.2. R2, C3. At the output of the element DD2.2, pulses appear that are delayed by 1/3 of the period relative to pulses "A", i.e. "B" impulses. Pulses "B" are fed to the input of the second delay circuit on the elements DD2.3, DD2.4, R3, C4, at the output of which (element DD2.4) there are pulses corresponding to the voltage Uc in Fig. 1, shifted by 1/3 of the period relative to "Uv". Pulses "A", "B", "C", shifted relative to each other by 6,666 ... ms, arrive at the key stages VT1, VS 1; VT2, VS2 and VT3, VS3 respectively. From the outputs of the keys (triacs VS1 ... VS3), a pulsed voltage with a frequency of 50 Hz is supplied to the windings of transformers T2 ... T4. From the output windings of the transformers, we obtain sinusoidal voltages shifted by 1/3 of the period or by 120 ° one relative to the other, i.e. three phase voltage.

Details and circuit adjustment. The shaper of rectangular pulses can be made according to any of the known schemes. Instead of diodes VD1 ... VD4, you can use the KTs405 bridge. Triacs VS1 ... VS3 are replaced by thyristors KU202, because DC voltage is applied to their inputs. The time constant t of RC circuits R2, C3 and R3, C4 is calculated by the formula T=1,4RC. Taking the capacitance of the capacitors C3, C4 equal to 0,01 μF, we find the resistance of the resistors R2, R3, which is 476,186 k. In this case, the time constant t is 6,666604 ms, which is almost equal to a shift of 1/3 period. For a more accurate adjustment of m RC circuits, resistors R2, R3 consist of series-connected constant and trimmer resistors with a total value of about 510 k. The trimmer resistor adjusts m RC circuits, controlling the phase shift at the outputs of transformers T2 ... T4 with a phase meter, so that the shift phase was as close as possible to 120°.

When transforming a three-phase current, either three single-phase or special three-phase transformers with a core in the form of three shorted rods are used. The connection diagram of the individual transformers (Fig. 3) corresponds to the star/star connection. Such a connection is shown in Fig. 3 [7].

Ris.3

Transformer T 1 (Fig. 2) - factory. Winding voltages: II - up to 30 V (Umax.in DA1); III-12 V. T2 ... T4 - increasing. The input "+U2" is supplied with a nominal voltage, for which the windings T2 ... T4 are designed, i.e. 12 V at Uii=12 V, 24 V at Uii=24 V, etc. Transformers T2 ... T4 - ready for the corresponding currents and voltages or home-made.

Literature

1. Three-phase electric motor in a single-phase network. - Radio amateur, 1992, No. 12, C.20.
2. Smirnov K. On the operation of a three-phase electric motor in a single-phase network. - Radio amateur, 1993, No. 6, S. 27.
3. Kukharenko A. Three-phase motor in a single-phase network. - Radio amateur, 1996, N2, p.28; 1996, No. 3, p.27.
4. Novik A. Polar capacitor in the alternating current circuit. Radio amateur, 1996, No. 9. C.17.
5. Egovkin V. Choice of diodes for starting oxide capacitors. - Radio amateur, 1997, No. 7, S.12.
6. Ilyin A.G. AC voltage stabilizer. - Radio amateur, 1997, No. 8, C.25.
7. Vorobyov A.V. Electrical engineering and electrical equipment of construction processes. - M.: Publishing house "ASV", 1995.
8. Ginkin G.G. Handbook of radio engineering. - GEI, 1948.

Author: A. Ilyin, St. Petersburg; Publication: N. Bolshakov, rf.atnn.ru

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