Instructions for choosing a thermal relay to protect the motor

Continuous operation of the mechanism at maximum causes overheating of the windings and damage to the insulation, as a result of which there is an interturn circuit, which develops into extensive burnout of the motor poles and costly repairs. To prevent this from happening, a relay is installed in the power circuit, which is called a thermal or "heating coil". Through the power circuit, this unit monitors the amount of current and with a long deviation from the installation rating, opens the contacts, depriving the power of the control circuit, opening the starting device. In this article, we will tell you how to choose a thermal relay for an engine in terms of power and current.

Selection technique

To select the thermal relay rating correctly, we need to find out its In (operating, rated current) and already relying on these data, we can select the correct setting range of the device.

The rules for the technical operation of the PUE stipulate this moment and it is allowed to set up to 125% of the rated current in explosion-proof rooms, and 100%, i.e. not higher than the rating of the engine in explosive.

How to find out In? This value can be found in the motor certificate, plate on the housing.

Characteristics AIR80

As you can see on the plate (to enlarge, click on the picture) two ratings are indicated 4.9A / 2.8A for 220V and 380V. According to our switching scheme, you need to select the amperage, focusing on the voltage, and select the relay from the table to protect the electric motor with the desired range.

Technical characteristics of the RTL series relay

As an example, consider how to choose thermal protection for an AIR 80 induction motor with a power of 1.1 kW, connected to a three-phase 380 volt network. In this case, our In will be 2.8A, and the permissible maximum current of the “heat” is 3.5A (125% of In). According to the catalog, RTL 1008-2 is suitable for us with an adjustable range of 2.5 to 4 A.

What to do if passport data are not known?

For this case we recommend use current clamp or a C266 multimeter, the design of which also includes clamp meters. Using these devices, it is necessary to determine the current of the motor in operation by measuring it in phases.

In the event that the data is partially read on the table, we place a table with the passport data of asynchronous motors widely used in the national economy (type of AIR). Using it, it is possible to determine In.

AID Summary Table

By the way, we recently reviewed principle of operation and arrangement of thermal relays, with which we strongly recommend that you familiarize yourself!

Depending on the current load, the response time of the protection will also vary, at 125% it should be about 20 minutes. The diagram below shows a vector curve of the current multiplicity versus In and the response time.

Diagram

Finally, we recommend watching a useful video on the topic:

We hope that after reading our article, it will become clear to you how to choose a thermal relay for the motor according to the rated current, as well as the power of the electric motor itself. As you can see, the conditions for choosing the device are not complicated, becausewithout formulas and complex calculations, you can choose a suitable face value using the table!

We also recommend that you read:

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2 comments

  • Yury

    A
    In my opinion, you did not calculate the motor current correctly. It turns out that you divided the power of 1100 into a voltage of 380 and you got a current of 2.8A. But for 3 phase motors in the current calculation formula there seems to be a square root of 2 and therefore the current will be different

    To answer
    • Vladimir

      open the formula and carefully read it. the current in this case is 2.84 to be more precise. not only divided by the root of 3, but also take into account the efficiency of the engine and cosine.

      To answer

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