We turn on the three-phase motor. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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We turn on the three-phase motor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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 ways to start three-phase electric motors, the simplest and most effective is to connect the third winding through a phase-shifting capacitor. Useful power, developed at the same time by an electric motor, is 50-60% of its power in a 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 of a squirrel-cage rotor of the MA series. Therefore, preference should be given to three-phase electric motors of the A, AO, 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 voltage of 220/380 V is indicated in the passport of the electric motor, then you can turn on the motor in a single-phase network with a voltage of 220 V according to the scheme shown in Figure 1. When you press the 5V1 button, the M1 electric motor starts to accelerate, and when it picks up speed, the button is released - SВ1.2 .1.1 opens, while SB1.3 and SBXNUMX remain closed. They are opened to stop the motor.

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

Wed = 4800*I/U,

where Cp is the capacitance of the capacitor, uF; I - current consumed by the electric motor, A; and - network voltage, V.

If the power of the electric motor is known, the current consumed by it is determined by the formula:

The capacity of the starting capacitor is chosen 2-2,5 times more than the working one, and their allowable voltages must be at least 1,5 times the mains voltage. For a 220 V network, it is better to use capacitors of the MBGO, MBGP, MBGCH brand 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 can also be used as starting ones (subject to short-term switching). For greater reliability, they are connected according to the circuit shown in Figure 2. The total capacitance is equal to c / 2. Shunt the starting capacitors with a resistor with a resistance of 2C0-500 kOhm, through which the remaining electric charge will "drain".

The operation of an electric motor with a capacitor start has some features. When operating in idle mode, a current flows through the winding fed through the capacitor, which is 20-40% higher than the nominal one. Therefore, if the electric motor is often used in underloaded mode or idle, the capacitance of the capacitor Cp should be reduced.

In case of overload, the electric motor may stop, then to start it, reconnect the starting capacitor (by removing or reducing the load on the shaft to a minimum).

In practice, the values of the capacities of the working and starting capacitors, depending on the power of the electric motor, are determined from the table.

To start the motor at idle or with a small load, the capacitance of the capacitor Sp 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 microfarad capacitor as a working capacitor, a 150 microfarad starting capacitor. At the same time, 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 the SA1 toggle switch (Fig. 1).

We turn on the three-phase motor
Rice. 1. Electric circuit for connecting a three-phase electric motor to a single-phase network
Rice. 2. Connection diagram of electrolytic capacitors
Rice. 3. The electrical circuit of the starting device for a three-phase electric motor with a power of 0,5 kW.

Figure 3 shows the 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 the SB1 button is pressed, the KM1 magnetic starter is activated (5A1 toggle switch is closed) and with its contact system KM1.1, KM1.2 connects the M1 electric motor to the 220 V network. At the same time, the third contact group KM1.3 blocks the SB1 button. After full acceleration of the electric motor, the starting capacitor C1 is turned off by the toggle switch SA1. Stop the electric motor by pressing the button SB2.

The device uses a magnetic starter type PML, designed for alternating current with a voltage of 220 V; SB1, SB2 - paired buttons PKE612, SA1 - toggle switch T2-1; resistors: R1 - wire PE-20; R2 - MLT-2; HL1 - lamp KM-2 (400 V, 2 mA). M20 - electric motor 400A1A24 (A24-100-1) for 4 kW, 71 rpm.

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

We turn on the three-phase motor
Rice. 4. Appearance of the starting device: 1 - body, 2 - carrying handle. 3 - signal lamp, 4 - toggle switch to turn off the starting capacitor, 5 - "Start" and "Stop" buttons, 6 - modified electrical plug, 7 - panel with connector sockets.

Using the SA1 toggle switch (Fig. 3) is not very convenient. Therefore, it is better if the starting capacitor is automatically switched off using an additional relay Kt (Fig. 5) of the MKU-48 type.

We turn on the three-phase motor
Rice. 5. Electric circuit of the starting device with automatic disconnection of the capacitor Sp.

When you press the button S81, it works and with its contact pair K1.1 turns on the magnetic starter KM1, and K1.2 - the starting capacitor Sp. In turn, the KM1 magnetic starter is self-blocking using its KM1.1 contact system, and KM1.2 and KM1.3 connect the electric motor to the network. The 5B1 button is kept pressed until the electric motor is fully accelerated, 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 KM1 magnetic starter remains on, providing power to the electric motor in operating mode. Stop the electric motor by pressing the button SB2 "Stop".

In conclusion, a few words about improvements that expand the capabilities of the launcher. Capacitors Cp and Cp can be made composite with steps of 10-20 microfarads and connected with multi-position switches (or two to four toggle switches), depending on the parameters of the starting motors. We recommend replacing the HL1 incandescent lamp with a quenching wire resistor with a neon one 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 cutoff current.

Author: S.Rybas

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