Instructions for choosing a thermal relay for motor protection
- Selection technique
- What to do if passport data is unknown?
Selection technique
In order to correctly choose the thermal relay rating, we need to know the In (operating, rated current) and already relying on this data, we can determine the correct setting range of the device.
The rules of technical operation of the PUE stipulate this moment and are allowed to set up to 125% of the rated current in explosion-proof rooms, and 100%, i.e. no more than the motor rating in explosives.
How do you know that? This value can be found in the engine certificate, plate on the housing.
As you can see on the board (to enlarge it, click on the image) two ratings 4.9A/2.8A are specified for 220V and 380V. According to our switching scheme, you need to select the current, focusing on the voltage, and select the relay from the table to protect the electric motor with the required range.
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 V network. In this case, we will have In 2.8A, and the maximum permissible current of the heating unit is 3.5A (125% of In). According to the catalog, the RTL 1008-2 is suitable for us with an adjustable range from 2.5 to 4 A.
What to do if passport data is unknown?
For this case we recommend using a current clamp or a C266 multimeter, the design of which also includes clamping counters. Using these devices, it is necessary to determine the current of the running motor by measuring it in phases.
In case the data is partially read on the table, we place the table with passport data for asynchronous motors widely used in the national economy (AIR type). Using it, it is possible to determine In.
By the way, we recently reviewed the principle of operation and arrangement of thermal relays, which we highly recommend that you familiarize yourself with!
Depending on the current load, the protection response time also varies, at 125%, it should be about 20 minutes. The graph below shows a vector curve of current multiplicity versus In and response time.
We hope that after reading our article, it will become clear to you how to choose a motor thermal relay according to the rated current, as well as the power of the electric motor itself. As you can see, the conditions for choosing a device are not complicated, because without complex formulas and calculations, you can choose a suitable nominal value using the table!
If you have any inquiry, please contact us freely and we will be happy to serve you.
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