Often there is a need in the household or during repair work to connect a three-phase electric motor to a 220 Volt network. These devices operate on a voltage of 380 V. But, as you know, in most homes the mains supply has only 220V. How to connect a three-phase electric motor to a 220V network? We learn about this from our article.
How to connect a three-phase motor to a single-phase network
Consider the sewing machine example. Problems in the factory with the connection, of course, will not arise. But to work in a single-phase network, you need to slightly correct the electric motor. For example, change the connection diagram of the windings from a star shape to a triangle. Of course, you need to adhere to polarity. Thanks to this alteration, it will be possible to connect a three-phase electric motor to a 220V network.
The motor power of the sewing machine is 0.4 kW. If it is possible to purchase starting metal paper capacitors MBTT, MBGO or MBGO with 50 or 100 ΞΌF capacitance and operating voltage from 450 to 600, then there will be no problems with starting. However, they can cost too much. Therefore, it is better to look for alternative βcheapβ solutions to the problem.
This may be a short-term connection of an additional electrolytic capacitor. It should work only two to three seconds, no more. After all, his work is necessary only to start the electric motor. Then the latter will operate in two-phase mode and lose up to half the power. Its stock, however, can be envisaged. By the way, the same power loss will be observed when working with a phase-shifting capacitor.
Lack of method and solution to the problem
Many people know that in an AC network, an electrolytic capacitor heats up very quickly. The electrolyte in it boils and explodes. Practice has shown that this can happen in a period of ten to fifteen seconds. But if you turn on this capacitor only for one and a half seconds, using a little resistance, then the device will not be damaged, since it simply will not have time for heating.
In washing machines, the PNVS button is used for short duration. It is three-pin. Two of them have a fixation, and one dispenses with it. Due to the last contact, the capacitor turns on and ceases to function after the termination of pressing.
The voltage at the electrolytic capacitors must be at least 450V. Therefore, the capacitance can be assembled from several capacitors placed in a protective box. Such a connection scheme has proved its viability in practice. True, the experiments were carried out only with electric motors, the power of which was less than one kW. For more powerful motors, most likely, you will need to include a small resistor with a capacitor with current limitation and the necessary dissipating power.
Second way
Consider how a three-phase electric motor in a single-phase network is connected as a synchronous squirrel-cage rotor .
In practice, even with the best choice of the capacity of the phase-shifting capacitor, the torque will not be higher than thirty-five percent of the nominal. This is due to the fact that the current flowing along one winding is shifted in phase relative to other windings. Therefore, another component is created in the stator magnetic field, in addition to the one that rotates the rotor in the necessary direction.
The formed component rotates in the opposite direction and brakes the rotor, reducing torque on the shaft and wasting energy, heating ordinary and magnetic wires of the motor. But if you turn off the winding, then the torque will increase to forty-one percent. And if you change the direction of the current in it and reconnect it, it will increase even more and can reach up to fifty-eight percent.
How else to improve the process
Such process optimization is possible not only due to a change in the direction of rotation of the component. It also results in the compensation of the fields of other windings, which coincide in direction and do not participate in rotor rotation. Starting of the engine will also improve with the use of two phase-shifting capacitors.
Their capacities should be the same. Such indicators are calculated by a special formula. They are checked by measuring the voltage across the windings and should show approximately the same results.
Equal stresses can be counter-paralleled by a dashed line.
How to connect a three-phase motor to a 220 Volt network
Hams often have to use the motors in question. Therefore, how to connect a three-phase electric motor to a 220V network, they absolutely need to know. It is already known that for this it is not necessary to have a three-phase network. It is better to connect the third winding through a phase-shifting capacitor.
For normal engine operation, the capacitance of the capacitor is changed, taking into account the number of revolutions. In practice, this condition is very difficult to fulfill. They leave the situation in a two-stage way: the engine is turned on with the starting capacity and at the same time they leave the working one. In manual mode, it switches to working.
The capacitor is used only paper type, and its operating voltage should be one and a half times more than the mains voltage. The reversal circuit of an engine with capacitor starting is quite simple. When the switch is activated, the motor changes the direction of rotation. But you need to know the features of the operation of such engines. If the device is idling through the winding, the current will flow from twenty to forty percent more than the nominal. Therefore, when operating under load, the working capacity must be reduced. If the motor reboots, it will turn off, and for a new start it will be necessary to turn on the start capacitor again.
You can connect an electric motor to a 220V network, even three-phase. However, some of them may work poorly. An example is a double-cage squirrel cage rotor MA. But if the switching circuit is performed correctly, and the necessary parameters of the capacitors are correctly selected, the workflow will be excellent. For example, asynchronous motors A, AO2, APN, AO, AOL and UAD are successful options.
Cons of three connection methods
The disadvantages of the above paths are the following:
- half of the rated power is lost;
- when powered by a single-phase network, not all models of electric motors are started;
- operating and starting capacities should be used;
- at idle, the current flows more than twenty to forty percent of the nominal;
- For the automated process of disconnecting the start capacitor and replacing paper elements with electrolytic, additional turns are used.
Fourth way
These shortcomings can be eliminated using the following method. How to connect a three-phase electric motor to a 220V network?
In a three-phase voltage, each curve is shifted by a third compared to the other.
Since the network frequency is fifty hertz, the period will be equal to twenty microseconds. Then his third will be 6.666 ... microseconds. Take a sinusoidal voltage single-phase at 220V and 50 Hz. If you pass it through a delay circuit for a third of the period, you get a shifted voltage, which will be equal in amplitude and frequency to the original. If you let it through the same delay circuit, you get shifted voltage by another third of the period.
Not sure how to connect a three-phase motor to a single-phase network? The scheme should be studied by you as much as possible. And it looks like this.
The mechanism includes a power supply unit and a pulse generator of positive polarity on the transformer. The power supply consists of a second transformer winding, a rectifier bridge and a stabilizer. The generator is assembled in the third winding of the transformer, a resistor and a rectifier on diodes. The zener diode protects the inputs of the part from accidental increase above the permissible voltage, that is, more than twelve volts. In the part is a shaper of rectangular pulses. At the output, rectangular pulses of fifty hertz of positive polarity are supplied.
When transforming a three-phase current, three single-phase or special three-phase transformers with a core in the form of rods can be used. Separate elements must be connected according to the "star-star" scheme.
Conclusion
Thus, the solution to the question of how to connect a three-phase electric motor to a 220V network is possible in several ways. Some of them are more difficult to implement, but the process will go better. Other methods are simpler, but also not without drawbacks.