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There are situations in life when you need to start a 3-phase asynchronous electric motor from a household network. The problem is that you only have one phase and zero at your disposal.
What to do in such a situation? Can a three-phase motor be connected to a single-phase network?
If you approach work wisely, everything is real. The main thing is to know the basic schemes and their features.
Design features
Before starting work, understand the design of the IM (asynchronous motor).
The device consists of two elements – a rotor (moving part) and a stator (stationary unit).
The stator has special grooves (recesses) in which the winding is placed, distributed so that the angular distance is 120 degrees.
The windings of the device create one or more pairs of poles, the number of which determines the frequency with which the rotor can rotate, as well as other parameters of the electric motor – efficiency, power and other parameters.
When an asynchronous motor is connected to a three-phase network, current flows through the windings at different time intervals.
A magnetic field is created that interacts with the rotor winding and causes it to rotate.
In other words, an effort appears that turns the rotor at different time intervals.
If you connect the blood pressure to a network with one phase (without performing preparatory work), the current will appear in only one winding.
The torque generated will not be enough to displace the rotor and keep it rotating.
That is why, in most cases, the use of starting and working capacitors is required to ensure the operation of a three-phase motor. But there are other options as well.
How to connect an electric motor from 380 to 220V without capacitor?
As noted above, a capacitor is most often used to start an EM with a squirrel-cage rotor from a network with one phase.
It is he who ensures the start-up of the device at the first moment of time after the supply of a single-phase current. In this case, the capacity of the starting device should be three times the same parameter for the working capacity.
For HELLs with a power of up to 3 kilowatts and used at home, the price of starting capacitors is high and sometimes commensurate with the cost of the motor itself.
Consequently, many are increasingly avoiding containers that are used only at the time of start-up.
The situation is different with working capacitors, the use of which allows you to load the motor at 80-85 percent of its power. In the absence of them, the power indicator can drop to 50 percent.
Nevertheless, capacitor-free starting of a 3-phase motor from a single-phase network is possible thanks to the use of bidirectional keys that are triggered for short periods of time.
The required torque is provided due to the displacement of the phase currents in the IM windings.
Today, two schemes are popular, suitable for motors up to 2,2 kW.
It is interesting that the start-up time of the IM from a single-phase network is not much lower than in the usual mode.
The main elements of the circuit are triacs and symmetrical dinistors. The first ones are controlled by bipolar pulses, and the second – by signals coming from the half-cycle of the supply voltage.
Scheme No. 1.
Suitable for 380 Volt motors up to 1 rpm with delta-connected windings.
An RC circuit acts as a phase shifter. By changing the resistance R2, it is possible to achieve a voltage across the capacitor that is displaced by a certain angle (relative to the voltage of the household network).
The main task is performed by the symmetrical dynistor VS2, which at a certain point in time connects the charged capacitance to the triac and activates this switch.
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Scheme No. 2.
Suitable for electric motors with a speed of up to 3000 rpm and for blood pressure, characterized by increased resistance at the time of start.
For such motors, a higher starting current is required, therefore an open star circuit is more relevant.
Feature – the use of two electronic switches replacing phase-shifting capacitors. During the adjustment process, it is important to ensure the required shift angle in the phase windings.
This is done as follows:
- The voltage is supplied to the electric motor through a manual starter (it must be connected in advance).
- After pressing the button, it is required to select the starting moment using the resistor R
When implementing the considered schemes, it is worth considering a number of features:
- For the experiment, we used radiatorless triacs (types TS-2-25 and TS-2-10), which proved to be excellent. If you use triacs on a plastic case (imported), you cannot do without radiators.
- Symmetrical dynistor type DB3 can be replaced by KP Despite the fact that the KP1125 is made in Russia, it is reliable and has less switching voltage. The main disadvantage is the scarcity of this dinistor.
How to connect through capacitors
First, decide which scheme is assembled on ED. To do this, open the cover-bar, where the blood pressure terminals are led out, and see how many wires come out of the device (most often there are six).
The designations are as follows: C1-C3 – the beginning of the winding, and C4-C6 – its ends. If the beginnings or ends of the windings are joined together, this is a “star”.
The situation is most difficult if six wires simply come out of the case. In this case, you need to look for the corresponding designations on them (C1-C6).
To implement the scheme for connecting a three-phase EM to a single-phase network, two types of capacitors are required – starting and working.
The first are used to start the electric motor at the first moment. As soon as the rotor spins up to the required number of revolutions, the starting capacity is excluded from the circuit.
Failure to do so could result in serious consequences such as damage to the motor.
The main function is taken over by working capacitors. Here it is worth considering the following points:
- Working capacitors are connected in parallel;
- The rated voltage must be at least 300 volts;
- The capacity of the working capacities is selected taking into account 7 μF per 100 W;
- It is desirable that the type of working and starting capacitor be identical. Popular options are MBGP, MPGO, KBP and others.
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If you take these rules into account, you can extend the operation of the capacitors and the electric motor as a whole.
The calculation of the capacity should be carried out taking into account the nominal power of the EM. If the motor is underloaded, overheating is inevitable, and then the capacity of the working capacitor will have to be reduced.
If you choose a capacitor with a capacity less than the permissible, then the efficiency of the electric motor will be low.
Remember that even after the circuit is turned off, the voltage remains on the capacitors, so it is worth discharging the device before starting work.
Also note that connecting an electric motor with a power of 3 kW or more to ordinary wiring is prohibited, because this can lead to disconnection of machines or burnout of plugs. In addition, there is a high risk of melting the insulation.
To connect ED 380 to 220V using capacitors, proceed as follows:
- Connect the containers together (as mentioned above, the connection must be parallel).
- Connect the parts with two wires to the EM and an alternating single-phase voltage source.
- Start the engine. This is done in order to check the direction of rotation of the device. If the rotor moves in the desired direction, no further manipulation is required. Otherwise, the wires connected to the winding should be reversed.
With a capacitor, an additional simplified one – for a star circuit.
With a capacitor, an additional simplified one – for a triangle circuit.
How to connect with reverse
There are situations in life when it is required to change the direction of rotation of the motor. This is also possible for three-phase EMs used in a household network with one phase and zero.
To solve the problem, it is required to connect one output of the capacitor to a separate winding without the possibility of rupture, and the second – with the possibility of transferring from “zero” to “phase” winding.
To implement the circuit, you can use a switch with two positions.
Wires from “zero” and “phase” are soldered to the extreme terminals, and a wire from the capacitor is connected to the central one.
How to connect in a star-delta pattern (with three wires)
For the most part, a star diagram has already been assembled in the ED of domestic production. All that is required is to rebuild the triangle.
The main advantage of the star / delta connection is the fact that the motor delivers maximum power.
Despite this, such a scheme is rarely used in production due to the complexity of the implementation.
Three starters are required to connect the motor and make the circuit work.
The current is connected to the first (K1), and the stator winding to the other. The remaining ends are connected to the K3 and K2 starters.
Further, the winding of the last starter (K2) is combined with the remaining phases to create a “delta” circuit.
When the K3 starter is connected to the phase, the remaining ends are shortened, and the circuit is converted into a “star”.
Please note that the simultaneous inclusion of K2 and K3 is prohibited due to the risk of a short circuit or knocking out the AV supplying the EM.
To avoid problems, a special interlock is provided, which means that one starter is disconnected when the other is turned on.
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The principle of the circuit is simple:
- When the first starter is connected to the network, the time relay starts and supplies voltage to the third starter.
- The engine starts in a star pattern and starts to run at higher power.
- After some time, the relay opens contacts K3 and connects K2. In this case, the electric motor operates according to the “triangle” scheme with a reduced power. When you need to turn off the power, K1 turns on.
Results
As you can see from the article, it is possible to connect a three-phase electric motor to a single-phase network without losing power. At the same time, for home conditions, the simplest and most affordable option is using a starting capacitor.