/ / How to calculate a starting capacitor for connecting a three-phase motor to a single-phase network

How to calculate a starting capacitor for connecting a three-phase motor to a single-phase network

It happens: There is a nice small lathe, drilling or lathe, powered by 380 volts. It could be installed in a home workshop or at a dacha. Yes, that's the trouble, in these rooms there are only ordinary outlets.

start capacitor

In those cases where you need to connectthree-phase electric motor, and only a single-phase voltage source is available, it is possible to leave the position by feeding one of the windings through the phase-shifting element-the starting capacitor. So you can get a replacement for the third, shifted by 120 degrees of the voltage phase.

Ideally, to accelerate the motora large value of capacity, and upon reaching the nominal angular velocity - another, smaller. To achieve this, a circuit is used that allows you to turn off excess capacity in both manual and automatic mode, 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:

Cp = 2800 (I / U)

In the case of a triangle connection, the relationship is different:

Cp = 4800 (I / U)

starting capacitor for electric motor

However, connecting the windings of the motor with a triangle is undesirable, because in this case the AC voltage should be 380 volts on each of them, and in the household network - only 220.

To simplify the calculation of the capacitance of a capacitor, one can use the formula according to which

Cp = P / 10,

Cn = P / S,


P - power, Watt;

Сп - capacitance of the starting capacitor, mF;

Cn is the capacity of the starting capacitor, mF.

capacitance of the starting capacitor

Thus, the capacity of the starting capacitor should be one and a half to two and a half times the working capacity.

The voltage in the standard AC is 220 volts. The question arises of how to determine the value of the current that appears in the above formula.

It is not difficult. The rated power of the engine is known, it is indicated on the plate fixed on its body and serves as a kind of passport.

I = P / (1.73 U cos φ),


I is the current value, Ampere;

U - voltage (220 volts);

φ is the angle of phase shift.

Correctly having calculated and having picked up startingcapacitor for the electric motor, it is possible to 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 when their regular inclusion (for example, the series AOL, UAD, APN). The MA series, which is characterized by the use of a double squirrel cage rotor circuit in their design, will show the worst results.

Choosing a capacitor starting, you should considerthe fact that at the moment of start there will be current values ​​many times exceeding the nominal value. Thus, it should be remembered that the cross section of the conductors supplying the motor must be selected with a margin.

Now, about which capacitor can be startedUse to connect a three-phase motor to a single-phase network. Electrolytic tanks can be used, but the presence of rectifier diodes in the circuit will complicate it and reduce the overall reliability of the entire system. Henry Ford rightly argued that the fewer details, the lower the probability of breakage.

It is easier and more reliable to install a paper capacitor with a starting capacitor. The voltage indicated on its housing must exceed 220 volts.

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