Magnetic starters and contactors are devices designed for switching power circuits. By the way, about the name and characteristics of starters and contactors: you will not find such significant differences between a magnetic starter device and a contactor. It's just that in the Soviet Union there were starters that held current from 10 A to 400 A, and contactors that held current from 100 A to 4,800 A. After that, magnetic starters began to be classified as low-power and small-sized contactors. Next, we will tell you more about the device and the principle of operation of the magnetic starter.
What are magnetic starters used for?
The meaning of their use is different. For example, it is not recommended to install switching equipment in machine tools in paint shops, pumping units that pump fuel, and similar premises. Dangerconsists in the fact that whatever the device and principle of operation of the magnetic starter, breaking the load, it creates a spark and arc discharges that can ignite, like a spark in a lighter, flammable vapors. To do this, all starters are taken out to a separate, almost hermetically fenced off room. The operating voltage of starters is usually limited to 12 volts so that sparks do not occur in buttons that are located in the hazardous area. Starters are also used in various protection schemes, interlocking, reverse and the like. Below we give examples of some of these schemes.
Device
We will disassemble the magnetic starter device using the PME-211 model as an example. This type, although obsolete, is often found in Soviet-made equipment and machines. The PME magnetic starter device is quite simple and just right for mastering. Removing the protective cover, we see the contact groups.
They consist of contacts, which, in turn, are divided into movable (installed in a movable frame with an anchor) and fixed (installed on the head of the contactor). Please note that all contacts on the moving part are spring loaded. This is done for the best touch between the pads, that is, heat-resistant welding on the contact. Having removed the head of the contactor, we see that at the bottom of it there is an anchor directly opposite the magnetic circuit with the coil. A rebound spring is installed between them, which is necessary in the magnetic starter device in order to bring it to its normal state. This spring is strong enough tobring the starter into this state and break the load to reduce the time of exposure to the resulting arc. It is weak enough to overload the coil, as well as prevent the magnetic circuit from closing and fitting tightly together. Due to an incorrectly selected spring, the starter is quite noisy. When repairing and maintaining this feature should be taken into account. The coil is usually marked with information about it, operating voltage, type of current, number of turns, frequency.
Operation principle
The device of the magnetic starter implies work according to this principle: a supply voltage is applied to the coil, which is installed on the magnetic circuit. The magnetic circuit is magnetized, attracting the armature, and the armature, in turn, pulls the frame on which the contact groups are fixed. The device and operation of the magnetic starter are based on the action of an electromagnet. When the armature is retracted, the contact groups of the power contacts are closed.
Auxiliary contacts are divided into 2 types:
- normally closed, that is, those that, in the absence of voltage on the coil, open, turning off the power or forming a negative signal, depending on how and what it is connected to;
- normally open, which, on the contrary, close, thereby affecting the control circuit or giving a positive signal.
When the voltage is removed, the starter returns to its normal state, and the contacts are discarded under the action of the return spring. All contacts of the magnetic starter installed in a dielectric frame, as a rule, fromheat-resistant plastic, spring-loaded to ensure the best fit between moving and fixed contacts. The magnetic starter is quite simply arranged, and the principle of its operation is based on an electromagnet.
How to tell the difference between normally closed and normally open contacts?
On PME starters they are open and visible. But we will show, using the PML starter as an example, how to do this when the contacts are closed.
The multimeter is set to the continuity mode, and the starter is not energized. This is his normal state. Then the contact groups are called one by one. Those that do not ring are normally open, and those that do ring are normally closed.
Maintenance and repair
The device and the principle of the magnetic starter implies regular maintenance and repair. It is worth doing this as planned, since over time, carbon deposits appear on the contact pads. In this regard, the magnetic circuit can be oxidized under the influence of a damp environment, and exfoliated rust forms abrasive dust, which, getting into moving parts, leads to their excessive wear.
External inspection
It is done in order to detect cracks, chips, melted places. Also, over time, the integrity of the shell in which the starter was installed may be violated, and the presence of excessive dust or crystalline s alt growths will indicate this. It should be understood that the starter, when turned on andoff, it bounces a little, which means that the fasteners should not be cracked. Otherwise, the starter may simply fall off and turn on the load. Or turn on, for example, two phases out of three, which will certainly burn the engine.
Contact groups
Opening the protective cover, we can see the contact groups. Depending on the purpose and device of the magnetic starter, they can be of different sizes and with solderings of different metals. Minor soot is removed with a rag or needle file. It is impossible to use the skin here, since it is difficult to keep track of the angle of inclination, the plane will not be sustained. Because of this, the contact will be loose, which means that the contact pads will heat up. Fusions and shells are removed with a file, and then with a fine file.
Anchor, magnetic circuit and coil
The armature and the magnetic circuit must not have traces of rust, and the plates from which they are assembled must be securely riveted. The coil, in turn, must be dry and not have any traces of soot (in the case of using paper as an external insulation) or melting if it is filled with plastic. If such signs are found, it is better to replace it.
Moving parts fastening, grooves
The grooves must be free of cracks, chips and dust. Otherwise, this can cause biting and a slow rejection of the moving contacts from the fixed ones. Elements installed in grooves should have slight play and move freely along the groove. It is also worth noting that the armature, like the magnetic circuit, is not rigidly installed. This is done so that the magnetic circuit can easily magnetize the armature tightly and reliably. A slight wiggle of the anchor in its groove is normal. If there is no wiggle, this means that a lot of dust has accumulated there or the mount is deformed. This should certainly be eliminated in order to uninterruptedly perform the functional purpose of the device.
Magnetic starter devices according to the principle of action performed in the circuit
Usually, such a scheme is used when the voltage loss in a particular equipment is critical. For example, a household single-phase pump with a starting winding. If the power suddenly disappears and reappears after a few seconds, the engine will simply burn out. For such protections, the following scheme exists.
The self-switching protection circuit works as follows: the voltage to the starter coil passes through the normally closed contact of the “stop” button, which is designated as KNS in the diagram, to the normally open contact of the “start” button. Between the "stop" and "start" buttons, a wire is output that goes to the normally open auxiliary contact on the starter. On the other side of the contact, 2 wires are supplied: the output after the “start” button and the power wire to the coil. When the “start” button is pressed, power is supplied bypassing the normally open contact to the coil, as a result of which the contact closes. When werelease the “start” button, the starter provides power to itself through the auxiliary contact. When the stop button is pressed, the coil loses power, causing the contact to open.
Deadlock scheme
Usually this circuit is used with two starters in a pair to turn on the motor reverse or, for example, to limit the operation of one function while another is on.
Power for the control circuit is supplied to the normally closed contact of the stop button (SNC). Then there is a branching into normally open contacts KnP “right” and KnP “left”. Moreover, power comes to the normally open contact KnP “right” through the normally closed contact KnP “left”. And vice versa. This is done in order to avoid the simultaneous activation of both starters, as a protection against accidental pressing. If the starters turn on at the same time, then since the reverse works due to the change of two wires, in some places a short circuit will occur, which will cause significant damage to the contact groups.
Then the wire that goes to the normally open contact of the KnP “right” goes to the auxiliary normally open contact of the starter. Then, on the other side of this starter, the output from the KNP “right” is connected and a jumper is installed leading to the coil contact. The second contact of the coil is passed through the normally closed auxiliary contact of the second starter. This is done for reinsurance, in order to exclude the possibility of simultaneously turning on the starters. The power supply of the second starter is arranged in a similar way. Before you come onnormally open contact KnP “left”, it is passed through the normally closed contact KnP “right”. Then, in a similar way, it is connected to the second starter. On one side of the normally open contact group, a wire is connected that goes to the KnP “left”, and on the opposite side - which goes after the KnP “left”. A jumper is installed leading to the coil contact. The second contact of the coil is passed through the normally closed contact of the first starter.
In conclusion, we can say that there are a lot of methods for using starters. We have given the most widespread, which are used in production, and can also be useful in everyday life. In any case, no matter how you use the device of the contactor, magnetic starter, before buying, you should calculate the current that will pass through its power contacts, set the operating voltage of the coil, the type of current. It is also worth considering dust and moisture protection of the starter from harmful environmental factors. It is imperative to inspect the starters on a scheduled and unscheduled basis, when the equipment that it feeds has become unusable. Sometimes the starter is the cause of equipment failure.
