Many owners quite often find themselves in a situation where it is necessary to connect in the garage or in the country such a device as three-phase asynchronous motor to various equipment, in the quality of which emery or drilling can act machine. This raises a problem, since the source is designed for single-phase voltage. What to do here? In fact, this problem can be solved quite easily by connecting the unit according to the circuits used for capacitor ones. To realize this idea, you will need a working and starting device, often referred to as phase shifters.
1 Capacity selection
- 1.1 For working capacitor
- 1.2 For starting capacitor
- 2 Type of
- 3 Electrolytic Application
- 4.1 Video: Connecting a single-phase motor to a single-phase network
To ensure the correct operation of the electric motor, it is necessary to calculate certain parameters.
For working capacitor
To find the effective capacity of the device, it is necessary to perform calculations using the formula:
- I1 is the nominal indicator of the stator current, for the measurement of which special mites are used;
- U networks - network voltage with a single phase, (V).
After performing the calculations, we obtain the capacitance of the working capacitor in μF.
It may be difficult for someone to calculate this parameter using the above formula. However, in this case, you can use another capacity calculation scheme, where you do not need to carry out such complex operations. This method allows you to simply determine the required parameter based only on the power of the asynchronous motor.
Here it is enough to remember that 100 watts of power of a three-phase unit should correspond to about 7 μF of the capacity of the working capacitor.
When calculating you need to monitor the current that flows to the stator phase winding in the selected mode. Invalid is considered if the current has a greater value than the nominal value.
For starting capacitor
There are situations when the electric motor has to be turned on under conditions of heavy load on the shaft. Then one working capacitor will not be enough, so you will need to add a starting capacitor to it. A feature of his work is that it will work only during the launch of the device for no more than 3 seconds, which is what the SA key is used. When the rotor reaches the rated speed, the device shuts down.
If due to an oversight, the owner left the starting devices turned on, this will lead to the formation of a significant bias on the currents in the phases. In such situations, the probability of overheating of the engine. When determining the capacity, it should be assumed that the value of this parameter should be 2.5–3 times greater than the capacity of the working capacitor. By acting in this way, it is possible to ensure that the engine starting torque reaches the nominal rate, as a result of which there are no complications during its launch.
To create the required capacitance capacitors can be connected in parallel and in series. It should be borne in mind the operation of three-phase units with a capacity of not more than 1 kW is allowed if they are connected to a single-phase network in the presence of a working device. And here you can do without a starting capacitor.
After calculations, you need to determine which type of capacitor can be used for the selected circuit.
The best option when using the same type for both capacitors. Typically, the operation of a three-phase motor is provided by paper starting capacitors dressed in a steel hermetic housing of type MPGO, MBGP, KBP or MBGO.
Most of these devices are made in the form of a rectangle. If you look at the case, then there are given their characteristics:
- Capacity (μF);
- Operating voltage (V).
Using paper starting capacitors, you need to remember the following negative point: they are quite large, while providing a small capacity. For this reason, for the effective operation of a three-phase motor of small power, it is necessary to use a sufficiently large number of capacitors. If desired, paper can be replaced and electrolytic. In this case, they need to be connected in a slightly different way, where additional elements must be present, represented by diodes and resistors.
However, experts do not recommend the use of electrolytic starting capacitors. This is due to the fact that they have a serious deficiency, which is manifested in the following: if the diode does not cope with alternating current will begin to be sold to the device, and this is already fraught with its heating and subsequent by the blast.
Another reason is that today, on the market, one can find improved metallized polypropylene starting models of alternating current type UHV.
Most often they are designed to work with a voltage of 400-450 V. Just they should be preferred, given that they have repeatedly shown themselves to be good.
Considering the different types of starting rectifiers of a three-phase motor connected to a single-phase network, one should take into account such a parameter as operating voltage.
An error would be the use of a rectifier, the voltage of which exceeds the order required. In addition to the high cost of its acquisition will have to allocate more space for it because of its large size.
At the same time, it is not worthwhile to consider models in which the voltage has a smaller indicator than the network voltage. Devices with such characteristics will not be able to effectively perform their functions and will soon fail.
To reduce to a mistake when choosing the operating voltage, the following calculation scheme should be followed: the final parameter should correspond to the product of the actual mains voltage and the coefficient 1.15, while the calculated value must be at least 300 V.
In that case, if paper rectifiers are selected for operation in an alternating voltage network, their operating voltage should be divided by 1.5-2. Therefore, the operating voltage for a paper capacitor, for which the manufacturer has indicated a voltage of 180 V, in operating conditions in an AC network will be 90-120 V.
In order to understand how the idea of connecting a three-phase electric motor to a single-phase network is realized in practice, let's perform an experiment using an AOL 22-4 unit with a power of 400 (W). The main task that must be solved is to start the engine from a single-phase network with a voltage of 220 V.
The motor used has the following characteristics:
- yesterday's power rating is 400 kW;
- 220V AC mains voltage;
- The current, all characteristics of which were determined using electrical clamp mites in a three-phase mode of operation - 1.9A;
- Wiring connection of the star.
Bearing in mind that the electric motor used has a small power, when connecting it to a single-phase network, you can buy only a working capacitor.
Calculation of the capacity of the working rectifier:
Using the above formulas, we take for the average value of the capacity of the working rectifier indicator 25 microfarads. Here a somewhat large capacitance of 10 μF was chosen. So we will try to find out how this change affects the launch of the device.
Now we need to buy rectifiers, as the last will be used capacitors like MBGO. Next, based on the prepared rectifiers, the required capacity is assembled.
In the course of operation, it should be remembered that each such rectifier has a capacity of 10 μF.
If you take two capacitors and connect them to each other in a parallel circuit, then the total capacity will be 20 μF. In this case, the operating voltage will be equal to 160V. To achieve the required level of 320 V, you need to take these two rectifiers and connect them to the same pair of capacitors connected in parallel, but already using a series circuit. As a result, the total capacity will be 10 microfarads. When the battery is working capacitors will be ready, connect it to the engine. Further it will be necessary only to start it in a single-phase network.
In the process of the experiment with connecting the motor to the single-phase network, the work required less time and effort. Using a similar unit with a selected battery of rectifiers, it should be noted that its effective capacity will be at the level of up to 70-80% of the rated power, while the rotor speed will correspond to the nominal indicator.
Important: if the engine used is designed for a network of 380/220 V, then when connecting to the network, use the "triangle" scheme.
Pay attention to the content of the tag: it happens that there is an image of a star with a voltage of 380 V. In this case, the correct operation of the engine in the network can be ensured by the following conditions. First you have to “gut” a common star, and then connect 6 ends to the terminal block. Search for a common point should be in the frontal part of the engine.
Video: Connecting a single-phase motor to a single-phase network
The decision on the use of the starting capacitor should be made on the basis of specific conditions, most often it is sufficiently working. However, if the engine being used is subjected to increased load, it is recommended to stop operation In this case, it is necessary to correctly determine the required capacity of the device in order to ensure efficient operation of the unit.