The principle of operation of the magnetic starter and its technical characteristics

. In the article we will consider the principle of operation of the magnetic starter and its technical characteristics; we distinguish this group of devices with contactors. The articles of Runet are so vague definitions that even the information provided will already be useful. At the same time, we will consider the appointment of starters; we will explain why it is impossible to do without them in some cases. You will learn a lot of interesting things - not just listing the dry facts, but at the same time analyzing many things related to the topic.

How do magnetic starters differ from

The terminology is interesting: why the word "magnetic" is used. The reason is simple - inside there is definitely a magnetic starter coil, allowing you to perform a quick and error-free start. Moreover, this is not done by hand movement, but by means of a current pulse, which makes it possible to create remote control devices. Everywhere there are coils, what is the difference between contactors and magnetic starters? Consider first the reasons for the need for protective measures:

• Expensive

The engine is considered to be a complex mechanism, and in practice it is also expensive. Therefore, it is necessary to carefully handle the equipment so as not to spend extra money. This is the first reason. With the traditional direct start, a large torque develops, but at the same time sharp jolts are not always suitable for the specified type of devices. For example, with respect to pumps, a hydraulic shock can occur, which potentially leads to valve failure.

Any domestic water heater should be operated side by side with protection against such overloads. Partial blow is able to take hydroaccumulator. But the jumps still harm the protective enamel. As a result - cracks, in the future - the destruction of the protective coating. It hurts too sharp start and engine. Separate parts more quickly become useless. Thus, the magnetic starter is recognized as the necessary accompaniment for expensive equipment.

• Current consumption and overload

At the start the asynchronous motor consumes an extremely large current in the 220 V network, nothing can be done. At the factory, these engines are usually abundant, and unnecessary interference on the power bus is not needed. An additional compelling reason: the possibility of simultaneous launching of several devices, which in the future threatens with overloading of electrical wiring and triggering of protection systems. The parts will turn out to be false, but damage to the cable insulation is not welcome, replacing them is a long and complicated process, not to mention the price. The starting current can be reduced. The device class in question does that.

• . Multi-functional

. Simultaneously magnetic starters represent a number of other functions. For example, the reverse. If necessary, by changing the switching of the windings, the reversible magnetic starter changes the direction of rotation of the shaft to the opposite. Inside there is a protection circuit from simultaneous switching on of both circuits. As a result, the magnetic starter allows you to perform the reverse process painlessly. Other specific features are known, which are discussed below. Selected models stop powering when one phase disappears or even control voltage distortions.

It is clear from the above that the contactor simply closes and opens the circuit, while the magnetic starters simultaneously perform additional functions to protect or reduce the starting current. Conclusion: the contactor is geographically included in the starter and performs approximately similar functions( not always) together with other equipment.

How magnetic starters are arranged,

variants The main executive part of a magnetic starter is considered a contactor. This is a partially movable core coil. Due to the resulting magnetic fields at the right time, the contactor operates under the action of voltage. Magnetic induction is used, and in order not to work out, as in a hot plate, the core consists of a set of thin plates. Used special electrical steel. This ensures the division of the volume of the core into parts. Lacquer insulation is applied between the plates.

As a result, eddy currents are not induced along the thickness of the material, losses are reduced. In addition to the common part is attached a host of equipment. But before describing the mentioned pile, let us consider how the motor is started, eliminating network overload.

Re-switching of the type of coupling

The first method will be the re-switching of the type of combination of windings from a star to a triangle. The first is used during the launch period, and the second - when the engine accelerates. The effect of reducing the starting current is achieved by changing the voltage applied to the windings. In the first case, this is 220 V( the difference between phase and neutral), in the second - 380 V( line voltage of the network).As a result of such a turn, the power decreases, which naturally causes a smaller starting torque, and the starting current drops. When the shaft picks up speed, the magnetic starter will rewire the windings on a triangle, the equipment will enter the mode. In this case, the relay is inside two. And designed so as not to close at the same time( this blocks the occurrence of an emergency on the line).External power is only suitable for the relay responsible for the inclusion of a triangle.

Changing the supply voltage

Often the adjustment of the starting current is made by variations in the amplitude of the supply voltage. The meaning is identical to that considered. It is required to reduce the value of the supply voltage, then the power will also fall. Winding without a difference, due to what changes occur. As a result, the simplest magnetic starters are performed on potentiometers, and more complex ones include thyristor switches. In the first case, a resistive divider is formed, on which a part of the voltage falls. From here the device is heated more strongly, but the construction is extremely simple. More advanced key schemes require complex organization. In the literature, they are sometimes called semiconductor magnetic starters.

Changing the Frequency

The principle of operation of a magnetic starter is sometimes based on a change in frequency. This control method is not suitable for all engines. Requires type with squirrel cage rotor. True, most of the equipment here and applies. With decreasing frequency decreasing the quality of field capture increases, the shaft rotation speed is lower. As a result, the desired effect is achieved - a reliable start( without interruption) in combination with a decrease in current. The implementation of the circuit requires the presence of an inverter. The input voltage is first rectified, and then the frequency decreases. In the case of complex electronic inverters, it becomes possible to gradually bring the parameters to the desired level.

Autotransformer

Starting through an autotransformer is often used to reduce the initial current of asynchronous motors. Usually, the process goes through a series of stages, during which different conclusions are successively used( this is the reason for using autotransformers directly, as a result, the number of switched contacts is reduced by half).Voltage steps increases gradually until the equipment is directly connected to the network.

We will explain the above methods. For example, how does a 380V magnetic starter with increased voltage work? The bottom line is that when a star is turned on, it is possible to use a voltage of approximately three times the root of the voltage than the nominal one. Of course, it is forbidden to include a winding triangle. But to do the opposite - to reduce the power to the root of three times - does not work, there will be a drop in power.

Due to the principle described, devices on autotransformers and dividers on potentiometers( rheostats) work. Consider the management of magnetic starters in terms of pros and cons:

1. Inclusion is directly used more often. In this case, the greatest torque is obtained at the start, but at the same time a current surge occurs, up to 10 times the nominal value. In addition, equipment is at greatest risk of overload.
2. Switching connections from a star to a triangle removes the first and second shortcomings of a direct start, but acquires others. First, the initial torque drops by one third, and secondly, it is impossible in this way to ensure reliable operation of devices with too low a load( for example, the idling of a small motor).The overcurrent will grow like an avalanche, and the effect of using the device is leveled.
3. The case with a potentiometer is characterized by similar moments: current surges occur when the resistance value is changed. This is removable if a smooth magnetic starter is used( see device description, technical documentation).It remains only a low starting torque.
   

   Modern equipment

4. Frequency magnetic starters, as stated above, are not suitable for all types of engines. There is a low starting torque. Adjustment is carried out without abrupt voltage surges. Due to the high cost of the product, smooth adjustment is possible, eliminating various jumps and gears.
5. The windings of the autotransformer change the input voltage always dramatically. Power surges can not be avoided, and the starting torque is reduced. Among the advantages is the possibility of a sharp decrease in current at the start of the engine.

So, the technical characteristics of magnetic starters in all cases are characterized by shortcomings. But for expensive equipment, this type of device will certainly go together.

Additional options for magnetic starters

According to the standard definitions, a magnetic starter provides specific protection, not just overheating. The classification according to GOST 2491 describes a number of parameters:

1. Unprotected protection devices.
2. Bimetallic or other thermal relay.
3. Measurement circuit on a posistor( thermistor).

Let it be said that there is no protection, after all, the current is regulated, already implying a more careful attitude to the supply network. Remember, protection can be internal( from overheating of the engine, as in the start-up relay of the refrigerator) or functional( reduction of current to prevent the operation of automata or other safety devices).

We hope that the readers now understand the expression of the magnetic starter. The outlined information will help to understand how the start of a three-phase asynchronous 220V motor. In this case, it is permissible to change the speed only by feeding the required amplitude. For the same reason, a switching magnetic starter 220V is usually not used. He simply has nothing to control. The windings are constantly connected in the same way. But the reverse is possible to provide, but this is a new story.

Of the characteristics, we note the number of cycles of operation. This magnitude of the magnetic starter directly determines the life of the device in most cases.