In choosing the correct contactor, you will need to consider the load type that the contactor will be used with. Special consideration for the 3 phase Squirrel-cage inductive motor load, which is one of the most common load types in the industry, where the motor draws a significant amount of current (inrush current) when starting. The load type and duty cycle of the load are known as the Utilization Category and is defined by the IEC standards. Other factors to consider when choosing a contactor for your motor is the site electrical supply. In this article, we will discuss these two factors in details so that you can correctly select the right contactor for your motor application.
What to Consider when choosing a contactor?
1. The Utilization Categories
IEC standard defind the Utilization Categories where the Induction Motor used in the industrial application belong to the AC-3 utilization category (Squirrel-cage motors). As you can see from the utilization categories table, there are many types of load, and the electrical characteristic that a contactor are design for varies between ecah categories.
2. Motor’s Inrush Current
The inrush current during the start of the motor is significantly higher than the motor running current. This can be seen in the chart above. The contactor will have to be able to witstand this current.
When selecting a contactor, please make sure that the contactor’s rated current is specified for the AC-3 load type. Ensure that you are looking at the right table.
You can see the rated current for the AC-3 load is lower than the rated current for the AC-1 load. This is because the manufacturer has factored in the high starting current of the motor that is in the AC-3 load type.
3. The site electrical supply voltage
Most industrial sites utilize the 3 phase 400V supply, however, some sites use 110V supply, especially for machines that were made in Japan, and up to 690V for heavy industry. The contactor rated current varies according to the voltage and therefore please ensure that you are selecting the right contactor according to the voltage being supplied.
As you can see from the picture above. The contactor can withstand the current up to 7A for 440V supply but is reduced to 6A for 550V supply
3 Steps to selecting the right contactor for your motor application
Step 1: Gather the right information from your motor’s nameplate
The electrical properties that you will need are the motor’s Voltage, Power (either Hose Power or Kilowatte), and the Current. Below is an example of an ABB motor’s nameplate.
If the site supply is a 3 phase 400VAC then the motor’s power is 30kW and the operating current is 55A.
If the site supply is a 3 phase 690VAC. The motor’s power is still 30kW but the operating current has changed to 32A.
Step 2: Find the right manufacturer’s datasheet and selection guideline for the load type.
Finding the correct manufacturer’s catalog and the datasheet is essential. The example below is from Mitsubishi Electric contactor catalog.
The figure in the square brackets indicates the rated current shown on the rating plate of the product at which the category AC-3 opening/closing durability is 2,000,000 times
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Step 3: Make the selection
Make sure that you are looking at the selection table for the AC-3 Utilization Categories. Assuming that the supply voltage is 400VAC at 50Hz, for 30kW motor that operate at 55A current; Mitsubishi S-T65 looks to be the correct choice.