For a person far from electrical engineering, a household device seems to be some kind of black box in which something is spinning. Everyone knows about the electric motor, but few know how it relates to the buttons on the panel. Meanwhile, any circuit in which there is an electric motor also contains a device that closes the circuit and connects the motor with the same power button. This device is called a magnetic starter, although its correct name is an electromagnetic starter.
Content
- Principle of operation
- Internal organization
- Connection diagrams
- Assortment and labeling of devices
- Contactors and starters
Principle of operation
In order for the appliance to work, it is necessary to ensure that the circuit is closed. This is not ensured by the button, but by the switching device behind it. There are many types of such devices, for example:
- contactor;
- switch;
- fuse;
- relay.
Moreover, there may be several of them in one chain. So, the fuse opens the circuit in case of overload, although after it there are simple switches in the circuit. Emergency opening can also be provided by thermal relays. But to find out what a magnetic starter is for, it is worth understanding its device.
Internal organization
Such a switch consists of two parts - movable and fixed. The fixed part is a coil on the armature, the stationary half of the core, and also contains fixed contacts. The movable part is the second half of the core and the movable contacts.
When you press the button, you complete the circuit and current flows through the coil. It attracts the moving part to itself and the button can be released: while the coil is powered, the contacts will be closed. If the circuit is opened with the shutdown button, the moving part of the starter will return to its original position thanks to the built-in spring. In a word, the principle of operation of a magnetic starter is simple.
Connection diagrams
The simplest circuit for connecting a three-phase electric motor according to the "turn on and off" principle looks like this:
This diagram indicates:
- Start - power button.
- KM-1 is a magnetic starter.
- P - thermal relay.
- C - off button.
- PR - fuse.
From the picture you can see that something, the place under which two letters are written - "BK" - will remain closed after you release the button. Pay attention to the fact that the engine is protected: a fuse and a thermal relay are included in the circuit. In case of overheating or short circuit, the circuit will open.
In practice, those circuits are more common that provide rotation of the engine in different directions - that is, with a reverse. Such a scheme can be equipped with both different switching devices and one reversing starter. The reverse circuit looks like this in a simplified way:
If you look closely, you will notice that when the engine rotates in one direction, the second chain is blocked - this can be seen by the KM-1 designation on the chain where the KM-2 stands, and vice versa. In electrician jargon, this is called foolproof.
If the motor is connected to a simple single-phase circuit, which is in any apartment, then the switching devices are put on phase, and resistance is added to them.
Assortment and labeling of devices
On the market of such switches, you can find their various modifications. This is due to both the variety of devices in which there are electric motors, and the parameters of the circuits where they work. Magnetic starters are found almost everywhere: in forced ventilation systems and air conditioners, washing machines and electric stoves with grill, elevators, and recently some electricity consumers began to put them in shields - they are much more convenient than simple ones knife switches.
To choose the right starter, you should pay attention to the following:
- what are the maximum currents in your circuit;
- do you need a reverse;
- where will you put your switching device.
The latter is important if you are going to install the starter in the shield near the house. There are now products on the market that are suitable for mounting on DIN rails.
Starters are completed in different ways. So, most of them connect the motor according to the "delta" scheme, so the starting current can be reduced. A number of products also contain thermal relays. They are worth paying attention to when your electric motor will run for a long time and overheat. To avoid breakage, it is a thermal relay that is installed. This is a simple bimetallic plate, which, when heated, bends to the side: metals, when heated, expand in different ways, and the circuit is opened.
Since the wiring is heated by current, the relay is selected so that the current in its marking is 10% higher than the nominal one. In the passport of the latter, the value of this denomination should be indicated, and sometimes put on the case. The value of the current on the magnetic starter is also indicated.
As a rule, starters are packed in an enclosure. It can be different and this determines the degree of its protection. When the starter is operating in a sealed case of the main device, this parameter is not so important, but if it is in the shield, where dust or precipitation gets, it is worth taking care of good protection. Contamination can lead to an unpleasant situation - the device will hum, or even completely fail.
Some starters are equipped with varistors to prevent power surges. It is advisable to put them in a chain when you live in a private house and during a thunderstorm all your equipment, especially your computer, may fail.
Marking
Domestic-made electromagnetic starters are marked in accordance with GOST 50030-4-1-2002. First of all, you need to pay attention to his contacts. Designations L1, L2, L3, etc. are supplied to the control circuit, and T1, T2, T3 and subsequent ones - to the load. The number of contacts can be different, and the diagram of their connection is contained in the passport and sometimes on the case. Contacts A1 and A2 come from the coil, and NO are auxiliary contacts, which are put into the device, which is called just in case. Some products can even be extended: a number of manufacturers produce contact attachments.
Most often, the starter marking begins with the abbreviation PML and four numbers.
If the device can operate in a 380 V circuit, then the value of the load current is set on it. This is the first figure after the PML, although the current value in direct form can also be put on the case.
- 0 - 6.3 Amperes;
- 1 - 10 Amperes;
- 2 — 25;
- 3 — 40;
- 4 — 63;
- 5 — 100;
- 6 — 160;
- 7 — 250.
The presence of a reverse and a thermal relay is also indicated by a number, it is the second:
- 1 - without reverse and without TL;
- 2 - without reverse with TL;
- 3 - with reverse without TL;
- 4 - with a reverse with TL;
The device has four degrees of protection: IP00, IP20, IP40, IP54, while the first of them assumes an open design, and the last one is a dust-splash-proof design. Depending on the degree of protection, the presence of buttons and indications, the product is marked with the third digit as follows:
- 0 - IP00 without buttons;
- 1 - IP54 with a "relay" button to return to its original state after triggering;
- 2 - IP54, start and stop;
- 3 - the same as 2, but with an indicator lamp;
- 4 - IP40 without buttons;
- 5 - IP40 with start and stop buttons;
- 6 - IP20.
Finally, the fourth digit indicates the number of contacts:
- 0 - 1 NO and 1 NC;
- 1 - 2 closing and 2 opening;
- 2 - 3 and 1;
- 3 - 4 and 1;
- 4 - 5 and 1.
The numbers 5 and 6 mark the devices for DC circuits as 1 NO and 1 NC, respectively.
Some factories indicate the possibility of mounting on a rail, the category of placement and wear resistance, but more often you can find exactly four numbers.
For starters of the PM type, the first two digits are the series number, and the next three are the current rating in volts. The sixth digit indicates the presence of a reverse and a thermal relay: 1, 2, 5, 6 mean the same as 1, 2, 3, 4 for PML, and the value of the seventh completely coincides.
PME are marked with three numbers: current value, degree of protection and the presence of a reverse with a relay. The designations on the PMA are approximately similar to those of the PML.
Such a variety of markings is explained by the fact that magnetic starters are long-used devices and some factories use the old marking, while others use a new one, while the order of numbers may differ. Therefore, it is worth focusing not so much on it as on various tables and instructions on the case, and also look at the product passport. This is especially true for foreign-made products.
Contactors and starters
These devices are not fundamentally different from starters. Purpose, device, principle of operation are the same. The difference lies in the fact that the contactors are designed to operate in circuits with high values of currents and voltages, so their dimensions are appropriate.
They do not have a protective case, therefore they are placed in closed rooms, protected from external influences.
Contactors are equipped with more powerful power contacts and arc suppressors; starters do not have them.
These devices are equipped with electric locomotives, trams, trolleybuses and industrial enterprises, where they close and open power circuits.
