On starting a three-phase motor from a single-phase network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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The article discusses the negative aspects of starting three-phase asynchronous motors with a squirrel-cage rotor from a single-phase AC network by switching its stator windings from a star to a delta, starting schemes are given that eliminate these disadvantages.

In [1], it is proposed to run powerful (2...7 kW) and high-speed (3000 rpm and more) three-phase electric motors (EM) from a single-phase network by switching its stator windings from a star to a triangle. Moreover, the normal working connection of the windings is a triangle connection. That is, the note is about the launch of serial three-phase asynchronous electric motors, designed for a voltage of 220/380 V.

The proposed method of starting, according to the existing classification, refers to the methods of starting at low voltage and, as you know, the main purpose of such a start is to reduce the starting current of the ED, because. the latter at the time of inclusion in the network operates almost in short circuit mode.

Starting an ED at low voltage is used with limited network power. In addition to the specified empirical formula in [2], which determines the possibility of direct start-up of EM, there are also the following restrictions on the power of motors started directly from the network.

When the ED is powered by a transformer that operates on a purely power network, the maximum motor power should be 20% of the transformer power for frequent starts and 30% for rare ones.

In cases of transformer operation for a mixed load (power and lighting), the maximum power of the ED is 4% of the transformer power with frequent starts and 8% with rare ones.

When the ED is powered from a low power power plant - 12% of the power plant power. Thus, EMs that go beyond these limits must be started at reduced voltage, which is suggested by the author of the note [1], by switching the EM windings from a star to a triangle.

However, for this starting method, in addition to [2], the following considerations must be made.

1. Asynchronous EM at industrial frequency cannot have a rotation speed of more than 3000 rpm, as the author points out [1]:

n1=60f1/p=60Ч50=3000 об/мин,

where p is the number of pole pairs of the machine.

Only asynchronous dual-powered motors, which the author of this article intends to talk about in the future, allow doubling the rotation speed of asynchronous motors, namely: to obtain an additional non-standard synchronous rotation speed of 2000 rpm and 6000 rpm at an industrial frequency of 50 Hz.

Therefore, what the author of [1] means when he writes about starting an ED at 3000 rpm or more, it is difficult to say.

2. The practical implementation of the well-known start-up method proposed by him assumes that the ED has six output ends. Since the ED of the most common 4A series with a power of 0,06 ... 0,37 and 0,55 ... 11 kW has three outputs (C1, C2, C3) when connecting the windings with a star or a triangle [3], then "our craftsmen" both experienced and will continue to experience serious difficulties in launching three-phase electric motors of this series of the power range specified by the author [1] (2...7 kW), because for them, it is impossible to use the proposed switching of the stator windings from star to delta.

If we touch on asynchronous EMs of the new unified AI series [4], developed at one time jointly with the countries of Interelectro, which corresponded to the promising level of development of world electrical engineering, then a similar picture is observed here: EMs with a power of 0,55 to 11 kW are made for voltages of 220, 380 and 660 V when phases are coupled into a triangle or a star with three output ends (C1, C2, C3).

Thus, the proposed solution cannot be used here either. Let us now take the general-purpose ED of the older A2 and A02 series, which was developed in 1957-1959. and had nine dimensions, then these EMs up to the fifth dimension inclusive (0,8 ... 13 kW) were also made with three output ends (C1, C2, C3) for voltages of 220, 380, 660 V with a triangle or star winding connection scheme [4].

Thus, this series also does not fit the proposed solution of the author [1].

Therefore, the proposed solution for starting three-phase electric motors with a power of 2 ... 7 kW from a single-phase network by switching its windings from a star to a delta can have a very, very limited application (for electric motors that are made by special order of the consumer with six output ends) or disassembly is necessary ED, which is naturally undesirable.

3. From the technical literature of forty years ago [5], it is known that starting methods by switching windings from a star to a triangle, by changing the number of pole pairs have almost no practical application, but mainly use a reactor or autotransformer start.

As noted in [6], the disadvantage of starting an EM by switching its windings from a star to a triangle compared to a reactor or autotransformer is the fact that during starting switching, the circuits of the EM windings are broken, and this leads to switching overvoltages, which naturally reduces reliability. work of ED and switching devices. In addition, when switching, there are significant shocks in the mechanical part of the EM, especially when the start is carried out under load.

In [7], the reasons for the operation of the EM protection when switching from a star to a delta are explained. The fact is that with such a switch, a current surge often occurs in the power supply circuit of the ED, which can exceed the value of the usual starting current by 2,88 times. This surge of current leads to the operation of the protection in the power supply circuit of the ED. To avoid this, a method for seamless switching from star to delta is proposed. In this case, the inrush current at the moment of switching does not exceed the value of the inrush current during direct start of the EM.

Figure 1 shows a diagram of the continuous switching of the windings of a three-phase asynchronous EM from a star to a triangle. The table shows the switching sequence of switching equipment contacts for this circuit.

As can be seen from the circuit diagram, it is relatively complex, requiring four magnetic starters and three starting resistors.

4. The author of the note [1] proposes a single-phase switching on of the ED when its windings are connected with a "bare" triangle in the operating mode and nothing more. As is known, the use of the overall power in this case will be 50 ... 60% and the useful power of the electric motor will be approximately 1 ... 3,5 kW for the power range of 1 ... it decreases significantly, while the magnetic field of the ED becomes elliptical. An elliptical field is characterized by the inconsistency of the instantaneous rotation speed of the spatial vector of the resulting magnetomotive force and, accordingly, the magnetic field of the EM, which can cause vibrations, especially at low moments of inertia of the rotor, which is typical for high-speed EMs, for which, in fact, it is proposed by the author [ 2] use the method of starting by switching windings (7 rpm or more). The elliptical field assumes the presence in the ED of a direct (rotating) moment and a reverse (braking) moment. The presence of reverse torque leads to a deterioration in performance in single-phase mode, namely: the motor has significantly worse values ​​of efficiency and power factor.

In order to improve the energy performance of EMs when operating in a single-phase mode, to better use the size of the power, it is necessary to operate them with a working capacitor, for example, as shown in [2]. In this case, the envelope power utilization reaches 80...100%, and the value of the power factor approaches unity. This means that the electric motor practically does not consume reactive energy from the network, as a result, the operation mode of the power line is facilitated, and its throughput increases.

Figure 2 shows an autotransformer circuit for starting a three-phase EM in a single-phase mode. The circuit contains a conventional laboratory autotransformer (LATR), for example, a nine-amp one, which allows you to smoothly start an ED with a power of up to about 2 ... 3 kW. If there are six leads of the stator winding of the EM, two of them - A and B - are turned on in the opposite direction. By interchanging the ends of the winding C, you can change the direction of rotation of the EM.

Before turning on the EM in the network, the LATR engine is set to its lowest position, then the package switch A1 is turned on and the voltage on the ED is gradually increased by moving the engine upwards, setting the rated voltage on the engine, even if it is lowered in the network. The circuit also allows, within certain limits, to regulate the rotational speed of the ED by changing the voltage at its terminals.

The overall power utilization for this circuit is 80...94%, the power factor is close to unity, the starting torque is about three times higher compared to other circuits.

If there are only three leads of the stator winding C1, C2, C3 of the motor, the latter is connected to the output terminals 1 and 3 of the LATR by the leads C1 and C2 (see Fig. 2).

The autotransformer can also be connected in series to the EM circuit, as shown in Fig. 3, for the case, for example, when the EM has only three output terminals C1, C2, C3. In this case, it turns into an adjustable choke (inductive reactance). Before starting, the LATR mobile contact is set to the extreme right position, i.e. its entire winding is connected in series with the ED. As the latter accelerates, the LATR winding is gradually taken out of operation by moving the movable contact to the extreme left position, as shown in Fig. 3 by a dotted line. This ends the start of the ED.

Naturally, for the circuit shown in Fig. 3, instead of LATR, you can use a laboratory sliding wire resistor (rheostat), for example, of the RSP type for 7 Ohms and a current of 7 ... 10 A, which is quite sufficient and much cheaper for an ED with a power of up to 2 ...XNUMX kW in single-phase mode. At the same time, its movable contact (slider) must be connected to one of the extreme conclusions for reliability. When starting an ED with a rheostat, it should be borne in mind that the rheostat must be taken out of operation smoothly and completely, without delaying its slider in intermediate positions, which is necessary to prevent its overheating and possible failure.

Instead of an adjustable wire resistor, an unregulated one can also be used, and at the end of the start-up of the ED, it must be shunted with a package switch SA2.

It is also possible to start the ED at low voltage using simple booster devices [8]. Figure 4 shows a diagram of such a connection by using two booster transformers, which are conventional step-down transformers of the OSO-0,25 type with a power of 250 W, a voltage of 220/36 V and a current of the secondary (through) winding of 6,1 A (called in everyday life "boilers"). It is possible to use one (or two) transformer type OSM-O,4 with a power of 400 W, which has two secondary windings, which makes it possible to use them as a feedthrough when connected in series.

The corresponding windings of each of the transformers VT1 and VT2 are connected in opposite directions. Moreover, their secondary windings are connected in series and in accordance, and the primary windings are connected in parallel and in accordance. As a result, a reduced voltage of about 150 V is applied to the ED and the inrush current, respectively, will be reduced. In order to exclude switching overvoltages during switching, the primary windings are shunted with a 1 W resistor R50.

Before starting the EM, the contacts of the SA2 switch are closed and the contacts of the SA3 switch are opened. The engine is turned on with a batch switch SA1. After the acceleration of the latter, the contacts SA2 are opened, and SA3 are closed, thereby connecting the ED directly to the network without breaking its power supply circuit. In this case, the primary windings of the transformers are disconnected from the network, and the secondary windings are shunted by the contacts of the SAZ switch and do not participate in the work. It is desirable to synchronize the operation of the SA2 and SAZ switches: when SA2 is turned on, SA3 should open and, conversely, when SA2 is turned off, SAZ should close.

You can also start the EM smoothly at low mains voltage using an electronic voltage regulator, for example, as shown in Fig. 5. As a key element in the circuit, a VT1 transistor of the P416, GTZ11I, KTZ61 type is used, which operates in an avalanche mode. Resistors R1, R3, R5-R7 type MLT.

Capacitors C1-C3 types BM, MBM, K73-11 for 400 V are selected during adjustment within 0,1 ... 1,0 μF. Resistor R2 is trimmer, it is tuned to obtain the minimum power in the load at the highest value of R4. Diodes VD1VD4 type D226B or any suitable diode assembly, for example, type KTs405I.

The triac VS1 is selected according to the power of the class being launched by the ED, not lower than the fourth, for example, TS 106-10-4, TS112-10-4 and the like.

At the end of the process of starting the ED, the triac VS1 can be taken out of operation by shunting it with a switch.

I propose a scheme (Fig. 6) for smooth switching of EM windings from a star to a triangle using a three-phase adjustable autotransformer of the RNT type with an open zero, which can be used to start both three-phase and single-phase EMs. This circuit, like all of the above, eliminates the disadvantages of contact switching circuits due to the absence of breaks in the EM stator winding circuit.

It works in the following way. Before starting a three-phase EM, the moving contacts of the RNT autotransformer are set to the lowest position.

In this case, as can be seen from Fig. 6, the EM windings will be connected by a star. Then, with the help of a switching device, voltage is applied to the terminals A, B, C of the engine, which starts at a 1,73 times lower mains voltage. After acceleration of the EM, the moving contacts of the RNT autotransformer are smoothly transferred to the uppermost position, which leads to a smooth transition from connecting the EM windings with a star to connecting them with a triangle and, accordingly, to an increase in the voltage on the windings by 1,73 times, i.e. up to the rated operating voltage at which the EM operates.

In a similar way, the ED is started from a single-phase network by connecting it to it with terminals B and C, and terminal A is connected to terminal B using a working capacitor. At the end of the start-up, the autotransformer winding can be turned off with a three-pole package switch.

Instead of a three-phase RNT autotransformer, three single-phase LATR types can be used, provided that all three moving contacts of each of them move synchronously. The start of all EM according to the above diagrams is carried out in idle mode or with a fan load on the shaft, in the presence of a starting capacitor in the motor circuit, which is not shown in the diagrams.

Conclusions

1. The method proposed by the author [1] for starting three-phase asynchronous motors with a squirrel-cage rotor from a single-phase AC network by switching its stator windings from a star to a triangle in the specified power range (2 ... 7 kW), with rare exceptions, is practically not feasible, since how engines of these capacities are produced with three output ends - C1, C2, C3.

2. Switching the motor stator windings from a star to a delta during start-up by a contact switching device has the following negative aspects, which significantly limit its application in practice:

2.1. The presence of switching surges during switching due to a break in the circuits of the motor stator windings during start-up, which reduces the reliability of the motor and switching equipment.

2.2. It is possible that the motor protection trips during switching due to the large starting current, which can exceed the normal starting current by 2,88 times.

2.3. The presence of mechanical shocks on the motor shaft during switching, which reduces the reliability of the electric drive.

3. In the operating mode, vibrations may occur, especially at low moments of inertia of the rotor, which is typical for high-speed motors (due to the presence of an elliptical magnetic field, which is due to the absence of a working capacitor in the motor circuit).

4. The engine has the worst performance and low energy performance in the operating mode.

5. To eliminate the noted shortcomings, the engine in the operating mode should be operated in the presence of a working capacitor, and starting from low-power networks should be carried out by a smooth or step change in voltage (current) in its circuit without breaking the circuits of the stator windings.

References:

1. Bearded Yu. On the inclusion of a three-phase motor in a single-phase network, facilitating the start//Electrician. - 2002. No. 4. - P.13.
2. Kolomoytsev K.V. Once again about the inclusion of a three-phase motor in a single-phase network // Electrician. - 2001. - No. 12. C.12.
3. Stokolov V.E., Usyshkin G.S. and others. Electrical equipment of forging and pressing machines: Ref. - 2nd ed. - M.: Mashinostroenie, 1981. - 304 p.
4. Handbook of electrical machines: in 2 tons / C 74 Under the general. ed. I.P. Kopylov and V.K. Klyukov. T.1. - M: Energoatomizdat, 1988. - 456 p.
5. Benerman V.I., Loetsky N.N. Design of power electrical equipment for industrial enterprises. - M.-L.: Gosenergoizdat, 1960. - P.83.
6. Voldek A.K. Electrical Machines: Textbook. for stud. universities. Ed. 2nd. - L .: Energy, 1974. - P. 570.
7. RJ. Electrical engineering and power engineering (consolidated volume). 1974, No. 7K p. 9, abstract 7K58.
8. Kolomoytsev K.V. Simple booster devices//Electrician. - 2003. No. 1. - p.6.

Author: A.G. Zyzyuk

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