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

It happens like this: there is a good small lathe, drill 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 sockets.

In those cases where it is necessary to connect a three-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 phase shifted by 120 degrees.

In the ideal case, a large capacitance is required to accelerate the motor, and upon reaching the nominal angular velocity, another, smaller one. To achieve this, a circuit is used that makes it possible to disconnect redundant capacity in both manual and automatic modes, 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 dependence is different:

Cp = 4800 (I / U)

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.

For the sake of simplicity in calculating the capacitance of a capacitor, one can use the formula according to which

Cp = P / 10,

Cn = P / S,

Where

P - power, watts;

Сп - capacitance of the starting capacitor, mF;

Cn is the capacity 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 capacity.

The voltage in the standard AC is 220 volts. The question arises of how to determine the value of the current appearing 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),

Where

I is the current value, Ampere;

U - voltage (220 volts);

Φ is the angle of phase shift.

Correctly having calculated and having picked up a starting condenser for the electric motor, it is possible to connect practically 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 design, will show the worst results.

Choosing the capacitor starting, you should take into account the fact that at the time of the 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 used 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 starting. The voltage indicated on its housing must exceed 220 volts.