Rules for choosing a magnetic starter
Functionality
The following are typical functions performed by magnetic starters, far from exhaustive scope of their application:
- Control of asynchronous electric motors in industrial machinery drives.
- The inclusion of outdoor (street) city lighting, exterior and interior illumination of industrial facilities.
- Switching of electric heaters (electric heaters or infrared heaters) of electric heating systems.
- Use as starting organs in industrial automation circuits.
The choice of magnetic starters is made when designing control and automation circuits, or in the process of their repair, when to replace an obsolete or missing device, you need to select its analogue.
Criterias of choice
When choosing the necessary electrical apparatus, its technical characteristics and design features are considered. Let us dwell on the main ones.
Switching circuit rated voltage. Most often, magnetic starters are used to start squirrel-cage induction motors for an industrial voltage of 220/380 Volts. It is for this choice that the majority of manufactured models of switching devices are designed. When using devices for electric motors on 380/660 volts, which are much less common, it is necessary to choose a starter for the corresponding voltage.
Rated current of main contacts. Comparison of the current of the connected load with the rated current of the switching device is one of the first actions when choosing the latter. Magnetic starters manufactured conditionally are classified by the values corresponding to the rated current of the device. Below is a table of ratios of values and rated currents. According to it, you can correctly select the magnetic starter for current, or for power, by recalculating according to the formula.
| Value | O | I | II | III | IV | V | VI |
| Inom | 6.3 A | 10 A | 25 A | 40 A | 63 A | 100 A | 160 A |
Products of foreign manufacturers are represented by a wide selection of contactors of various versions for various rated currents.
Switching wear resistance. This characteristic displays the number of responses guaranteed by the manufacturer. There are 3 classes of wear resistance: A, B and C. Class A is the highest and guarantees from 1.5 to 4 million cycles of operation of the magnetic starter. Class B models are guaranteed to operate from 0.63 to 1.5 million cycles.Class B is the lowest and is characterized by 0.1 to 0.5 million response cycles.
Mechanical abrasion resistance. An equally important characteristic that displays the number of on / off cycles of the device without repair or replacement of its parts. In this case, switching on and off should be carried out without load (when there is no current in the circuit). Mechanical durability can be from 3 to 20 million cycles of operation.
Number of poles. To power three-phase electric motors, devices having three poles are used. It is such a performance that is most common. However, there are a number of situations when you need to choose a device with a different number of poles. For example, when the load is lighting circuits or electric heaters. In this case, it is convenient to choose a switching device from the line of contactors of foreign manufacturers, represented by a wide variety of designs.
Coil Rated Voltage. Magnetic starters used in electrical control circuits are most conveniently used with coils for the same voltage as the switched load. For this reason, versions with 220 or 380 Volt coils are most common. When constructing various kinds of automatic circuits, for a number of reasons, it may be necessary to use control coils to a different voltage level. This is due to the use in these circuits of relays, sensors or other components designed for a specific supply voltage. For this case, in the lines of domestic and foreign manufacturers there is a choice of options for supplying coils with any voltage from a nominal range of 9 Volts and above (9, 12, 24, 36, 110, 220 or 380 V).
Number and characteristics of auxiliary contacts. In addition to the main power contacts, switching the main electrical load circuits, magnetic starters are equipped with auxiliary contacts that operate synchronously with the main. These contacts are intended for switching control circuits, interlocks, power supply for signal lamps, relay coils and other auxiliary devices. Auxiliary contacts can be of two types - normally open and normally closed. The former are open when the control coil is de-energized and are closed when the electromagnetic starter is triggered, in the latter, everything happens the other way around. The need to select a certain number of additional contacts of one type or another is determined by the circuit in which the device is used.
For example, to organize the simplest control of the mechanism using a two-button post, it is enough to choose the option with one pair of normally open auxiliary contacts that pick up the control coil when the Start button is pressed. There are versions of closed magnetic starters equipped with start and stop buttons on the housing. If necessary, to signal the state of the mechanism, you need to select a starter that has two more pairs of contacts. Normally closed energize the “Off” signal lamp, normally open energize the “On” lamp.
The presence of reverse. If you need to choose a magnetic starter for controlling a reversing motor, give preference to a reversing model, in the case of which there are two separate starters, interconnected.
Availability of protection. In the basic version, the magnetic starter is not equipped with protection for the connected electrical equipment. The protection module with thermal relay is optional and can be selected based on the required characteristics. More about what is a thermal relay, you can find out from our article.
In addition to the criteria listed above, it is necessary to choose the right climatic version and IP degree of protection products. The methodology for this selection is the same as for any electrical equipment.For example, if the starter is placed in a protected cabinet, you can choose the degree of protection IP20. If the placement conditions of the device are unfavorable (high dust, humidity, etc.), we recommend choosing a magnetic starter in the housing, the degree of protection of which is IP54 or IP65.
Finally, we recommend watching a video that details how to choose a magnetic starter in terms of power, current and other parameters:
These are all the most important criteria for choosing a magnetic starter. If you have questions or you did not find the necessary information, write in the comments under the post, we, in turn, will try to help you find the right answer!
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