Some electrical appliances, such as electric motors, are powered by a three-phase network. To turn them on, all three phases must be connected at the same time. Sometimes it is necessary to change the direction of rotation of the rotor, or it is necessary to switch a load with a high current. In all these cases, they are used. In order for the device to work well, it is necessary to correctly connect a magnetic starter, or a contactor.
Content
- Using a magnetic starter
- Connection according to the usual scheme
- Reverse application
- Protection methods
- Voltage safety
Using a magnetic starter
Before connecting the starter, you need to understand its device. By itself, the electromagnetic starter (MP) is a relay, but it is capable of switching much higher current. This ability is due to the large contacts, as well as the speed of response. For this, the device has more powerful electromagnets.
An electric magnet is a coil that contains enough turns of insulated wire to allow a current of 24 to 660 volts to flow through it. The coil is located on the core, which increases the magnetic flux. This power is needed to overcome the spring force and increase the contact closure speed.
The spring is placed to quickly open the contacts. The faster the opening occurs, the smaller the arc will be. An electric arc is harmful because a very high temperature is created in it, and this has a detrimental effect on the contacts themselves. More powerful devices - contactors - are also equipped with an arc-extinguishing chamber, which allows you to break a circuit with an even higher current (on powerful contactors up to 1000 A, for MP - from 6.3 A to 250 A).
Although the control coil of the starter is AC powered, any kind of current can be passed through the contacts. Unlike contactors and relays, the MP has two groups of contacts:
- power;
- blocking.
With the help of power contacts, the load is connected, and the blocking contacts are used to protect against incorrect or dangerous connection. Depending on the design, there can be three or four pairs of power contacts. Moreover, each pair has movable and fixed contacts. The latter are connected through metal plates to the terminals located on the body. Wires are connected to them. Blocking contacts can be:
- normally closed;
- normally open.
The control coil is fed through both. Another set of contacts can be added if necessary. All of them are used for control or indication, a small current passes through them, so there are no special requirements for them.
Connection according to the usual scheme
The housing has holes for mounting. Recently, DIN-rail enclosures began to appear. This is a profile used in electrical engineering. It can take one of the following forms:
- Ω-type;
- G-shaped;
- C-type.
Such a MP can be installed in shields. The mounting method is very convenient, it allows you to quickly remove and install the device, saving the installer from long monotonous work.
After installation, proceed to the connection. The connection diagram of the magnetic starter can be of two types:
- ordinary;
- reversible.
A typical wiring scheme uses one starter with three or four pairs of power contacts. Three phases of the network are connected to the input terminals, the wires go from the output terminals to the load. If the engine rotates in the opposite direction after starting, then any two phases at the input or output of the starter are reversed.
The wiring diagram for the starter control circuit is a little more complicated. When choosing a starter, it is necessary to consider which coil is used in it. The choice of coils for voltage is great - in order not to complicate the circuit, it is better to immediately take it for 220 V or 380 V. The pick-up coils are also available for direct current. When they say that this magnetic starter is 220 V, they mean that the coil used is rated for 220 V.
In this case, the circuit will look like this: phase, fuse, stop button, start button (these buttons can be on the starter itself or on remote control panel), in parallel with the "start" button, normally open interlocking contacts of the starter, control coil, zero the wire.
When you press the start button, a current flows through the coil, creating electromagnetic forces in it, which attract and close the power and normally open interlock contacts. This happens very quickly and the start button is still compressed. At this time, the interlocking contacts create their own circuit, which bypasses the button. When the button is released, the starter remains on due to the already closed interlock contacts.
If a thermal relay is used, it also has latching contacts, which are normally closed. The normal condition is when the device is not working. If the thermal relay is triggered, the contacts inside it open. Therefore, they are placed in an open circuit between the coil and the neutral wire. The same is observed in the wiring diagram of the 380 V magnetic starter. The only difference is that the coil is not connected between phase and zero, but between two phases.
Reverse application
The word reverse itself means "reverse, opposite". In relation to the engine, it means turning it on in the opposite direction. To change the rotation of the motor rotor in the opposite direction, it is necessary to change the phasing. The easiest way to do this is with a second magnetic starter. Ready-made reversing starters are produced. They differ in that there are two contactors in one housing and an electrical and / or mechanical interlock is already provided.
Interlocking is necessary to prevent simultaneous activation of both starters, otherwise it will cause a phase-to-phase fault. If there is no reversing starter, two conventional ones can be used. A three-phase voltage is supplied to the terminals of the power contacts in such a way that at the output of the starters, two phases of the same name are reversed. It is important to remember that when one of the starters is turned on, the output of the other will also have voltage.
Reverse MPs are also used when it is necessary to reduce the starting current. During starting, the engine is connected according to the "star" scheme, and after picking up speed, it switches to the "delta".
Protection methods
Magnetic starters are used not only to connect and disconnect loads, but also to protect motors. For three-phase AC motors two things are dangerous:
- Short circuit (it does not matter, to the case, between the windings or interturn).
- Phase imbalance or loss of one or two of them.
A thermal relay helps fight the first occurrence. Its main element is a bimetallic plate. In a cold state, it has one shape, in a heated state, another. A working current is passed through it, going to the electric motor, which heats it. The stronger the current, the more it heats up. In order for the plate not to change its shape ahead of time, it is deformed.
Through the insulating material, a movable normally closed contact is attached to it, which is included in the control circuit of the MP coil. When the current is exceeded, the plate changes its shape and opens the contact, which leads to the operation of the MP and the engine stop. In total, such relays are placed two per MP, one per phase. The third phase will in any case be associated with these two.
Voltage safety
When it comes to tension, things are more complicated here. You can, of course, put a voltage relay for each phase, but this will complicate the circuit, which, in turn, will lead to an increase in the cost of the structure. Partially this problem is solved by the coil itself. If this is a 220 V coil, then it takes power from one of the phases. When the voltage on this phase disappears, the coil is de-energized, and the MP is turned off.
It is even better if the coil is 380 V - then two phases are protected, but when the voltage on the third one disappears, the protection will not work. An additional relay can be supplied by powering it from an unprotected phase, and its normally open contacts included in the control circuit of the MP coil. Then, with a loss of voltage in this phase, the relay will turn off, and the power supply circuit of the MP coil will be broken.
This solution has a significant drawback. In order for the MP to turn on, it is necessary that this relay has already been started, and this will not happen until the MP turns on, because the relay is powered from the phase that follows the MP. It is impossible to connect the relay to the "start" button, there will be a phase-to-phase short circuit. In this case, you can use the double start button, taking the voltage from the phase of the same name in front of the MP. Then, after switching on the MP, the relay will operate normally.
There is another, more original way. As you know, on a time scale, the voltage between three phases at any time interval is equal to zero. If you connect a 20 μF capacitor to all phases at one end, and connect the other ends to each other, you will get a "star", in the center of which there will be 0.
Connect a relay designed for a voltage of 220 V between the center of the "star" and the neutral wire. When voltage is present in all phases, the relay is disconnected. When the voltage disappears in one or two phases, a voltage appears in the center of the "star", in this case the relay is activated. Its normally closed contacts open (and they are included in the control circuit of the MP coil), interrupting the circuit in the MP coil.
It is a very sensitive circuit that even reacts to voltage surges. To reduce the sensitivity, it is necessary to lower the capacitance of the capacitors. The voltage of the capacitors must be at least 400 V. Even if any capacitor fails, the circuit will work, since the symmetry will be broken.
