It happens like this: there is a good small machine, drilling or turning, powered by 380 volts. It could be installed in a home workshop or in the country. But the trouble is, in these rooms there are only ordinary sockets.
In cases where it is necessary to connect a three-phase electric motor, and only a single-phase voltage source is available, you can get out of the situation by powering one of the windings through a phase-shifting element - a starting capacitor. So you can get a replacement for the third phase of the voltage shifted by 120 degrees.
In the ideal case, a large capacitance is required to accelerate the motor, and upon reaching the rated angular velocity, another, smaller one. To achieve this, a scheme is used that makes it possible to turn off excess capacity both manually and automatically, leaving only its working value.
In the event that the windings are connected by a star, the working capacitance of the working capacitor is determined by the formula:
Av=2800 (I/U)
In the case of a triangle connection, the dependence is different:
Av=4800 (I/U)
However, connecting the motor windings in a triangle is undesirable, because in this case the AC voltage should be 380 volts on each of them, and only 220 volts in the household network.
For ease of calculating the capacitance value of a capacitor, you can use the formula according to which
Ср=Р/10, Sp=R/5, where
P - power, Watt;
Сп - capacitance of the starting capacitor, mF;
Сп - capacitance of the starting capacitor, mF.
Thus, the capacity of the starting capacitor should be one and a half to two and a half times the working one.
Standard AC voltage is 220 volts. The question arises of how to determine the magnitude of the current that appears in the above formula.
It's easy. The rated power of the engine is known, it is indicated on a plate attached to its body and serving as a kind of passport for it.
I=P / (1, 73 U cos φ), where
I – current value, Amp;
U - voltage (220 Volts);
φ – phase angle.
By correctly calculating and selecting a starting capacitor for an electric motor, you can connect almost all types of three-phase electric motors to a single-phase network. Some of them will work better, that is, their characteristics will be closer to the passport ones when they are normally turned on (for example, the AOL, UAD, APN series). Series MA, characterized in that their design uses a double cage scheme of a squirrel-cage rotor,will show the worst results.
When choosing a starting capacitor, one should take into account the fact that at the moment of start there will be current values that are many times higher than the nominal value. Thus, it should be remembered that the cross section of the conductors that provide power to the motor must be chosen with a margin.
Now about which starting capacitor can be used to connect a three-phase motor to a single-phase network. Electrolytic capacitances can be used, but the presence of rectifier diodes in the circuit will complicate it and reduce the overall reliability of the entire system. Even Henry Ford rightly argued that the fewer parts, the lower the likelihood of a breakdown.
It is easier and more reliable to install a paper capacitor starting. The voltage indicated on its case must exceed 220 volts.
