Three-phase motor in a single-phase network. Scheme, description

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Three-phase motor in a single-phase network. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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Sometimes at home it becomes necessary to connect a three-phase AC motor to a single-phase network.

I also had such a need when connecting an industrial sewing machine. In a garment factory, such machines operate in a workshop with a three-phase network, and there are no problems.

The first thing that had to be done was to change the connection diagram of the motor windings from "star" to "triangle", observing the polarity of the winding connection (beginning - end) (Fig. 1). This switching allows you to turn on the electric motor in a single-phase 220 V network.

The power of the electric motor of the sewing machine according to the plate is 0,4 kW. To purchase working, and even more so starting, metal-paper capacitors of the MBGO, MBGP, MBGCH types with a capacity of 50 and 100 microfarads, respectively, for an operating voltage of 450 ... Use instead of paper-metal polar (electrolytic) capacitors and powerful rectifier diodes D600, D242. did not give a positive result. The electric motor stubbornly did not start, apparently due to the finite resistance of the diodes in the forward direction.

Therefore, the idea, absurd at first sight, of starting an electric motor with the help of a short-term connection of a conventional electrolytic capacitor to an alternating current network came to mind (Fig. 2). After starting (acceleration) of the electric motor, the electrolytic capacitor is disconnected, and the electric motor operates in a two-phase mode, losing up to 50% of its power. But if you foresee a power reserve in advance, or it is known for sure that such a reserve exists (as in my case), then you can put up with this drawback. By the way, when the electric motor is running with a working phase-shifting capacitor, the electric motor also loses up to 50% of its power.

Now about the most important. The electrolytic capacitor, being connected directly to the alternating current network, quickly heats up, the electrolyte boils, and it explodes - many people know this. As the experiment showed, it takes about 10 ... 15 s. It is known that the resistance of a capacitor in an alternating current circuit of industrial frequency is determined by the formula.

where C is the capacitance of the capacitor in microfarads.

The magnitude of the current in the circuit with a capacitor

But if the electrolytic capacitor is turned on through a small resistance (in my case, this is the complex resistance of the phase of the motor winding Z = r + jx), and also for a short time, during the acceleration of the motor (somewhere 1 ... 1,5 s), then the electrolytic capacitor is not damaged, as it does not have time to warm up.

The short-term activation can be provided by the PNVS-10UHL2 button,

used in domestic washing machines. The button has three contacts: two - with fixation (SB 1.1, SB1.3) and one - without fixation (SB 1.2). It turns on the capacitor, and when you stop pressing the button, it returns to its original off position.

The formulas for calculating the starting capacitor have been repeatedly printed, but nevertheless I want to repeat them for the circuit for connecting the stator winding of an electric motor into a "triangle".

where U is the mains voltage; In - rated current consumed by the electric motor.

where P is the power of the electric motor, kW; U - network voltage. AT; n is the efficiency of the electric motor (usually 0,8 ... 0,9); cosph - power factor (usually 0,85).

Electrolytic capacitors must be at a voltage of at least 450 V. It is advisable to collect capacity from several capacitors (thermal conditions improve). Capacitors are placed in a protective box.

Four years of experience in the operation of the electric motor showed the viability of the specified scheme for starting it. This scheme was repeated by some of my friends, however, the experiments were carried out with electric motors up to 1 kW. For electric motors over 1 kW, it seems to me that it is necessary to connect a small current-limiting resistor in series with the capacitor with an appropriate power dissipation during the start-up.

Literature

Smirnov K.O. The operation of a three-phase electric motor in a single-phase network. - Radio amateur, 1993, No. 6, S. 27.
Kukharenko A. Three-phase electric motor in a single-phase network. - Radio amateur, 1996, No. 2, S. 28.

Author: V.Bashkatov, Ukraine, Donetsk region, Gorlovka; Publication: N. Bolshakov, rf.atnn.ru

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