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Connecting a three-phase motor to a single-phase network. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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Electric motors, as you know, are single-phase and three-phase; household electrical network has one phase. The question arises: is it possible to connect a three-phase motor to a single-phase network. Despite the seemingly insoluble contradiction, such a connection can be made, and there are several ways.
The first two ways to connect electric motors (Fig. 98) are based on the use of working (Cp) and starting (Cp) capacitors.
Rice. 98. Scheme of connecting a three-phase electric motor to a single-phase network using capacitors: a - when the electric motor is turned on "in a star"; b - when the electric motor is turned on "in a triangle".
The starting capacitor increases the starting torque, and after starting the engine, it is turned off. But if the engine is started without load, then the capacitor Sp is not included in the circuit.
For a working capacitor included in the circuit, it is necessary to calculate the capacitance. The calculation is made according to the formula: Cp \u1d K (1nom / i), where Cp is the working capacitance of the capacitor for the rated load (in microfarads - μF); 2800nom - rated current (in amperes - A); U - rated voltage in a single-phase network (in volts - V); K is a coefficient that depends on the engine switching circuit. When the electric motor is turned on "in a star" K = 4800, when turned on "in a triangle" K = XNUMX.
For the rated current and voltage, the values \uXNUMXb\uXNUMXbof the specified parameters given in the technical passport of the electric motor are taken. To connect three-phase motors to a single-phase network using capacitors, the following types are used: KBGMN (paper, hermetic, in a metal case, normal), BGT (paper, hermetic, heat-resistant), MBGCH (metal-paper, hermetic, frequency). If it becomes necessary to change the direction of rotation of the electric motor (reversing), then this is easy to do by switching the mains wire from one capacitor terminal to another.
Starting capacitors can have the following technical parameters: the voltage on the capacitor at rated load must be equal to the voltage in the network (and when the engine is underloaded, the voltage on the capacitor must be 1,15 times the voltage in the network); starting capacity should be 2,5-3 working capacity.
As a starting capacitor, a cheap electrolytic capacitor of the EP type is most often used. But when using an electrolytic capacitor, it should be remembered that it has a large discharge current, remaining charged even after the voltage is turned off. Therefore, after each disconnection, the capacitor must be discharged using some kind of resistance, for example, several incandescent lamps connected in series.
The use of capacitors to turn on a three-phase motor in a single-phase network is very effective, since it allows you to get a power that is 65-85% of that indicated in the motor passport. But here difficulties may arise with the selection of the desired capacitance of capacitors. Therefore, switching methods using active resistances have become much more widespread (Fig. 99).
Immediately before connecting the electric motor to a single-phase network, the starting resistance should be turned on; turn off the starting resistance only after the engine reaches a speed close to the nominal. Unfortunately, when using methods for connecting a three-phase motor to a single-phase network using active resistance, it is possible to obtain power from the motor that does not exceed half of its rated power.
When connecting a three-phase motor to a single-phase network using capacitors, it must be taken into account that the rated voltage of capacitors of the KBG-MN and BGT types is given for their operation at direct current. When they work on alternating current, the value of the allowable voltage should not exceed the values \u13b\uXNUMXbspecified in Table. XNUMX.
Table 13. Permissible stress values
If a three-phase motor is connected to a single-phase network according to the scheme (see Fig. 98, b), then the starting torque will be almost half that when switched on according to another scheme (see Fig. 98, a). To change the direction of movement of the rotor (reversal) of an electric motor connected according to the scheme (see Fig. 98, b), it is necessary to swap the conclusions C2 and C5 of the starting winding. Before reversing, the motor must be disconnected from the mains.
The value of starting active resistances is determined according to the table. 14 depending on the power of the electric motor in three-phase mode.
Table 14 Starting resistance values
Author: Korshevr N.G.
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