Inclusion of a three-phase electric motor in a single-phase network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Many DIY enthusiasts often try to adapt three-phase electric motors for various home-made machines: grinding, drilling, woodworking and others. But the trouble is - not everyone knows how to power such an electric motor from a single-phase network.

Among the various methods of starting three-phase electric motors, the simplest and most effective is by connecting the third winding through a phase-shifting capacitor. The useful power developed by the electric motor is 50-60% of its power in three-phase mode. However, not all three-phase electric motors work well from a single-phase network. These include, for example, electric motors with a double cage squirrel-cage rotor of the MA series. Therefore, preference should be given to three-phase electric motors of the A, DO, AO2, AOL, APN, UAD, etc. series.

In order for a capacitor start motor to work properly, the capacitance of the capacitor must change with the speed. Since this condition is difficult to fulfill in practice, the engine is usually controlled in two stages - first it is turned on with a starting capacitor, and after acceleration it is disconnected, leaving only the working one.

If the electric motor's passport indicates a voltage of 220/380 V, then you can turn on the motor in a single-phase network with a voltage of 220 V according to the diagram shown in Figure 1. When you press the SB1 button, the M1 electric motor begins to accelerate, and when it picks up speed, the button is released - SB1.2 .1.1 opens, and SB1.3 and SBXNUMX remain closed. They are opened to stop the electric motor.

Rice. 1. Electric circuit for connecting a three-phase electric motor to a single-phase network

When connecting the motor windings into a "triangle", the capacitance of the working capacitor is determined by the formula:

where Cp is the capacitance of the capacitor, μF; I - current consumed by the electric motor, A; U - network voltage, V.
If the power of the electric motor is known, the current consumed by it is determined by the formula:

where P is the power of the electric motor (indicated in the passport), W; U - network voltage, V; n - efficiency; cosф - power factor. /The capacity of the starting capacitor is chosen to be 2-2,5 times greater than the working one, and their permissible voltages must be at least 1,5 times the network voltage. For a 220 V network, it is better to use capacitors of the MBGO, MBGP, MBGCh brands with an operating voltage of 500 V and higher. Electrolytic capacitors K50-3, EGC-M, KE-2 with an operating voltage of at least 450 V (subject to short-term switching) can also be used as starting capacitors. For greater reliability, they are connected according to the circuit shown in Figure 2. The total capacity is equal to C/2. Shunt the starting capacitors with a resistor with a resistance of 200-500 kOhm, through which the remaining electrical charge will “drain”.

 
Rice. 2. Connection diagram of electrolytic capacitors

The operation of an electric motor with capacitor starting has some peculiarities. When operating in idle mode, a current flows through the winding fed through the capacitor, 20-40% higher than the rated one. Therefore, if the electric motor will often be used in underloaded mode or idle, the capacitance of the capacitor Cp should be reduced. If overloaded, the electric motor may stop; then to start it, reconnect the starting capacitor (by removing or reducing to a minimum the load on the shaft). In practice, the capacitance values ​​of working and starting capacitors, depending on the power of the electric motor, are determined from the table.

To start the electric motor at idle or with a light load, the capacitance of the capacitor Cn can be reduced. For example, to turn on an AO2 electric motor with a power of 2,2 kW at 1420 rpm, you can use a 230 μF capacitor as a working capacitor, and a starting capacitor - 150 μF. In this case, the electric motor starts confidently with a small load on the shaft. Reversing the electric motor is carried out by switching the phase on its winding with toggle switch SA1 (Fig. 1).

Rice. 3. Electrical circuit of the starting device for a three-phase electric motor with a power of 0,5 kW

Figure 3 shows an electrical diagram of a portable universal unit for starting three-phase electric motors with a power of about 0,5 kW from a single-phase network without reversing. When you press the SB1 button, the magnetic starter KM1 is triggered (toggle switch SA1 is closed) and its contact system KM1.1, KM1.2 connects the electric motor M1 to the 220 V network. At the same time, the third contact group KM1.3 blocks the SB1 button. After complete acceleration of the electric motor, the starting capacitor C1 is turned off using toggle switch SA1. Stop the electric motor by pressing button SB2. The device uses a magnetic starter of the PML type, designed for alternating current with a voltage of 220 V; SB1, SB2 - paired PKE612 buttons, SA1 - toggle switch T2-1; resistors: R1 - wire PE-20, R2 - MLT-2, C1, C2 - MBGCh capacitors for a voltage of 400 V (C2 is made up of two parallel-connected capacitors of 20 μF X 400 V); HL1 - KM-24 lamp (24 V, 100 mA). M1 - electric motor 4A71A4 (AO2-21-4) 0,55 kW, 1420 rpm.

The starting device is mounted in a tin casing measuring 170x140x70 mm (Fig. 4). On the top panel there are the “Start” and “Stop” buttons, a signal lamp and a toggle switch for turning off the starting capacitor. On the front side wall there is a homemade three-pin connector made from three pieces of copper tube and a round electrical plug, in which a third pin is added.

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Rice. 4. Appearance of the starting device: 1 - housing, 2 - carrying handle, 3 - signal lamp, 4 - toggle switch for turning off the starting capacitor, 5 - "Start" and "Stop" buttons, 6 - modified electrical plug, 7 - panel with sockets connector

Using the SA1 toggle switch (Fig. 3) is not entirely convenient. Therefore, it is better if the starting capacitor is turned off automatically using an additional relay K1 (Fig. 5) of the MKU-48 type. When you press the SB1 button, it is triggered and its contact pair K1.1 turns on the magnetic starter KM1, and K1.2 turns on the starting capacitor Sp. In turn, the magnetic starter KM1 is self-locking using its contact system KM1.1, and KM1.2 and KM1.3 connect the electric motor to the network. The SB1 button is kept pressed until the electric motor fully accelerates, and then released - relay K1 is de-energized and turns off the starting capacitor, which is discharged through resistor R2. At the same time, the magnetic starter KM1 remains turned on, providing power to the electric motor in operating mode. Stop the electric motor by pressing the SB2 “Stop” button.

Rice. 5. Electric circuit of the starting device with automatic disconnection of the capacitor Sp.

In conclusion, a few words about improvements that expand the capabilities of the starting device. Capacitors Cp and Sp can be made composite in steps of 10-20 µF and connected with multi-position switches (or two to four toggle switches) depending on the parameters of the electric motors being started. We recommend replacing the HL1 incandescent lamp with a quenching wire-wound resistor with a neon lamp with an additional low-power resistor; instead of paired PKE612 buttons, use two single buttons of any type; fuses can be replaced with automatic fuses for the appropriate cut-off current.

Author: S. Rybas; Publication: cxem.net

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