The electric motor is a converter of single-phase voltage to three-phase. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Three-phase electric motors in everyday life and amateur practice drive a variety of mechanisms - a circular saw, an electric planer, a fan, a drilling machine, a pump. To power such motors from a single-phase network, various capacitive or inductive-capacitive phase-shifting circuits are used. It would be nice to have one such circuit for all motors, but this is not possible due to the need to change the parameters of its elements depending on the power and connection scheme of the motor windings. There is another way out - to get a three-phase voltage from a single-phase one using an electric motor that acts as a generator.

It is known that any electric machine is reversible: the generator can serve as an engine, and vice versa. The rotor of a conventional asynchronous electric motor, after an accidental disconnection of one of the windings, continues to rotate, and there is an EMF between the terminals of the disconnected winding. This phenomenon prompted the idea to use a three-phase asynchronous electric motor to convert a single-phase voltage into a three-phase one.

Under the influence of the stator magnetic field, currents flow in the short-circuited winding of the rotor of the asynchronous motor, turning the rotor into an electromagnet with pronounced poles, inducing a sinusoidal voltage in the stator windings, including those not connected to the network. The phase shift between sinusoids in different windings depends only on the location of the latter on the stator and in a three-phase motor is exactly 120 degrees.

The main condition for the transformation of an asynchronous electric motor into a phase number converter is a rotating rotor. Therefore, it should first be untwisted, for example, using a conventional phase-shifting capacitor, the capacitance of which is calculated by the formula C \u1d K-2800f / Uc, where K \u4800d 42 if the motor windings are connected by a star, or 4 if - by a triangle; Iph - rated phase current of the electric motor, A; Uc - voltage of a single-phase network, V. You can use capacitors MBGO, MBGP, MBGT, K600-42 for an operating voltage of at least 19 V or MBGCH, K250-XNUMX for a voltage of at least XNUMX V. The capacitor is needed only to start the engine-generator, then its chain is broken, and the rotor continues to rotate. Therefore, the capacitance of the phase-shifting capacitor does not affect the quality of the generated three-phase voltage. A three-phase load can be connected to the stator windings. If it is not there, the energy of the supply network is spent only on overcoming friction in the rotor bearings (not counting the usual losses in copper and iron), so the efficiency of the converter is quite high.

Several different electric motors were tested as phase number converters. Those of them, the windings of which are connected by a star with an output from a common point (neutral), were connected according to the circuit shown in Fig. 1.

In the case of connecting the windings with a star without a neutral or a triangle, the schemes shown, respectively, in Fig. 2 and 3.

In all cases, the engine was started by pressing the button SB1 and holding it for 1 ... 5 s until the rotor speed reaches the nominal one. Then the SA1 switch was closed, and the button was released. The test results are shown in the table. The indices in the designations of voltages correspond to the numbers of contacts of the socket X2 (see Fig. 1 - 3), between which they were measured.

The speed of rotation of the rotor of the motor-generator depends little on the voltage of the single-phase supply network. The generated voltages are proportional to the mains voltage, but noticeably less than it, which is due to energy losses for magnetization and the creation of a torque that compensates for mechanical losses in the bearings.

The reduced rated speed of the AOL-22-4 motor indicates its four-pole design (other motors are two-pole). Nevertheless, it successfully works as a converter.

Various three-phase two- and four-pole electric motors with windings connected both in a star and in a triangle were connected to the AOL2 motor as a load:

• AOL-011-2 with a power of 80 W (grinding stone drive);
• UAD-32F with a power of 120 W (fan drive);
• A08 with a power of 1,5 kW (woodworking machine drive).

Under load, the phase and linear voltages changed by 2...5%, the phase shift between them - by 5...6 degrees.

Literature

1. Biryukov S. Three phases - without power loss. - Radio, 2000, No. 7, p. 37 - 39.
2. Belopolsky I. I. Power sources of radio devices. - M.: Energy, 1971.
3. Karvovsky G.A., Okorokov S.P. Handbook of asynchronous motors and ballasts. - M.: Energy, 1969.

Author: V.Kleymenov

Let's try, having a single-phase alternating voltage, to get two missing phases. Let's take a conventional three-phase asynchronous electric motor with a squirrel-cage rotor, which, like the generator, has a rotor and three stator windings shifted in space at an angle of 120 degrees. We apply single-phase voltage to one of the windings. The motor rotor will not be able to start rotating on its own. He needs some way to give the initial impetus. Further, it will rotate due to the interaction with the magnetic field of one stator winding. The magnetic flux of the rotating rotor will induce the induction EMF in the other two stator windings, i.e. the missing phases will be restored.

The rotor can be made to rotate in any way, even the old "grandfather" one, using a rope wound around the shaft. The author used for this a widely used device with a starting capacitor. By the way, its capacitance does not have to be large, since the rotor of an asynchronous converter is set in motion without a mechanical load on the shaft.

One of the disadvantages of such a converter is unequal phase voltages (see the table in the previous article - ed.), which leads to a decrease in the efficiency of the converter itself and the motor-load. If you supplement the device with an autotransformer of the appropriate power, turning it on, as shown in the figure, you can achieve an approximate equality of phase voltages by switching the taps. The stator of a defective electric motor with a power of 17 kW was used as the magnetic circuit of the autotransformer. Winding - 400 turns of enameled wire with a cross section of 4 ... 6 mm2 with taps after every 40 turns.

Finally, some practical advice. It is better to use "low-speed" motors (1000 min-1 and less) as electric motor-converters. They are very easy to start, the ratio of starting current to operating current is much less than that of motors with a speed of 3000 min-1, and therefore, the load on the network is “softer”. The power of the motor used as a converter must be greater than that of the electric drive connected to it. For example, if the converter is a 4 kW motor, the load power should not exceed 3 kW. Always start the converter first, and then connect the three-phase current consumers to it. Turn off the unit in reverse order.

A 4 kW converter manufactured by the author has been used in his personal household for several years. A sawmill, a grain mill, a grinder work from it.

Author: S. Gurov

See other articles Section Electric motors.

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