Although the contactor is not a complex device, you should consider several factors when selecting a contactor. Using the right product according to the requirements of the network and the load prevents mechanical and electrical failures that may occur. This article describes contactor selection parameters for the motors and other loads.
How to select a contactor?
The followings are the selection parameters of a contactor.
Number of poles
Power contactors have 3 poles or 4 poles versions. You should select the suitable contactor according to the number of phases of the load. For example, if you want to control a 3 phase motor, you should use 3 pole contactor. If you want to disconnect the neutral, your choice should be 4 pole version.
Load characteristics are another important selection parameter. Utilization categories of contactors are defined in IEC 60947-4-1 standard. First of all, you should know the application of the contactor and select the suitable product according to the characteristics of the load. For example, since the electrical motor draws an inrush current at the start-up, the contactor has to carry this excessive current during closing. That’s why you have to select the contactor according to AC-3 utilization category. If you want to switch a resistive heating circuit you should select it according to the AC-1 utilization category. For lighting circuits, AC-5 category is the option.
Current and power
A suitable contactor should be determined according to the current or power of the load. If you select a small contactor, the contacts wear and stick. This causes dangerous consequences.
It is the control voltage applied to the coil. When the coil is energized, the magnetic field changes the position of the main contacts. If excessive voltage is applied to the coil, it will burn. If less voltage is applied to the coil, the power contacts make a continuous vibrating pull-and-drop in a very short time. That’s why you have to supply the coil with the right control voltage.
Rated operating voltage (Ue), Insulation voltage (Ui), Impulse voltage (Uimp)
Rated operating voltage indicates the voltage value that the contactor can be supplied at nominal conditions. It is the phase-phase voltage in 3 phase application. Insulation voltage is a reference voltage for dielectric tests and leakage distance. The impulse voltage indicates the maximum voltage value that a contactor can withstand in impulses without deterioration. These values should be checked from the catalogs.
Number of auxiliary contacts
Internal auxiliary contacts work synchronously with main contacts. Auxiliary contacts provide information on the open or closed status of the contactor. Auxiliary contacts can be available internally such as 1NO, 1NC, 1NO + 1NC … etc. Besides, they can be mounted externally on the contactor.
The dimensions of the electrical products are getting compact day by day. The smaller the size, the better for the user. As the size of the product decreases, the dimension of the electrical panel decreases. Besides, the length of the consumable materials such as cables and cable ducts also decreases.
Mounting types and positions
Low voltage power contactors are generally suitable for Din Rail mounting. In models with base plate mounting, screws should be tightened according to the torque values specified on the product during assembly. Otherwise, the products may be out of warranty. Besides, the catalogs of contactors contain information about the mounting angles to be placed in the panel.
Contactors can be produced as screw and spring terminals. During cable entry, screwed models can be tightened with a screwdriver, and spring models can be mounted without tightening. If assembly time is critical, spring-loaded models can be preferred.
Standards and certification are other important factors. In particular, a contactor compliant with the UL standard may have different technical features. A product can also comply with IEC and UL standards. You have to check this from the catalogs.
A contactor has lots of electrical and mechanical accessories. Depending on the need, accessories such as mechanical interlock, time relay, varistor, terminal protector, terminal expander, connection busbars .. etc .. are available. The accessories should be compatible with the contactor type.
Heat dissipation at the poles
It is the value of heat emitted from one of the main contacts in nominal operation. This value should be taken into consideration when calculating the cooling and ventilation of the electrical panel.
Maximum operating and storage temperature
The maximum temperature of the contactor under energy and the maximum temperature at the time of storage is different from each other. These values should be considered and not exceeded.
Maximum working altitude
In applications above maximum altitude, power derating calculations should be made and the appropriate contactor should be determined.
Electrical and mechanical life
The electrical life is the number of switching cycles that the contactor can perform in the loaded condition. Operating current varies according to operating voltage and utilization category. The number of switching cycles of a contactor without energy is called mechanical life.
Shock resistance and vibration resistance
Shock resistance is a value determined for vehicles, cranes, in-ship applications, plug-in equipment. The contacts must withstand the specified “G” values without changing their position. In the case of vibration, devices at the vibration amplitude and frequency specified for boats and other vehicles must continue to operate. Especially in applications such as railways, these values are desired to be high.
Coil operating limits
These are the upper and lower limits of the nominal control supply voltage. If the coil is energized between these values, the contactor must pull. For example, a contactor normally operating at 110 VAC has a tolerance of 0.85 x Uc min … 1.1 x Uc. This means that any control voltage between 93,5V … 121V is applied, the contacts will change the position.
Coil consumption values
Coils consume energy at the moment of pulling and holding. If the consumption during pulling and holding is low enough, it may even be possible to switch the contactor directly from the PLC without using any additional interfaces.
It is the time elapsed after the coil is energized until the contacts are fully closed. In high-quality contactors this time is short.