Setting Current in Thermal Overload Relay

Consult the applicable NEMA or IEC documents.

Look for trip curves.

Don't take short cuts, do the job properly by getting the applicable specifications.

Factories may burn when you get it wrong.

..."Standard tripping classes are 10 A, 10, 20, 30. The tripping class indicates according to IEC 60947-4-1 the maximum tripping time in seconds under specified conditions of test at 7.2 times the setting current and specifics tripping and non tripping times for 1.5 and 7.2 times the setting current."...

https://www.platt.com/CutSheets/ABB/ThermalOverloadRelays.pdf

https://library.e.abb.com/public/a9cd1c3f07dc1870852573a300738595/1SXU106043B0201.pdf

Study the directions that come with the overload relay. The selection is not done entirely by calculation, nor is there necessarily any single formula.

You set it to the full load current on the maker's rating plate attached to it, if you've got an ounce of sense between your ears. If you haven't, then you take the serial number and the manufacturer's phone number down on a bit of paper, and write down the correct setting on the paper at the end of the call. It ain't rocket science, Mildred!

Not quite that simple. There may be adjustments if ambient temperature at the motor differs widely from that at the overload relay, for service factor, and possibly other reasons.

It seems that if you are setting a thermal overload relay the temperature should be your concern. An electric motors current causes heating of the windings and if the motor is loaded intermittently the current can be quite high without the temperature becoming excessive.

IEC Relays are calibrated as per 60947-4. The setting dial/knob/lever is marked as per the rated current that the relay will carry indefinitely. When it is set to some current,(a) it will not trip for 1.05 times the set current for 2 hours and (b) it will trip for sure at 1.2 times the set current within 2 hours. How quickly it will trip on locked rotor and other overloads is decided by its class, 10 / 10A / 20 / 30 as someone else has already mentioned. So you need to set the dial at the rated current of the motor, so that it is properly protected.

However, when star-delta starting is involved, the relay is often wired within the delta. So, the current through it will be phase current and it has to be set to In /√3 .

This is all theoretical. One reality is that the actual motor current will be slightly different from a theoretical value. So you need to actually measure the current drawn by the motor and then set the relay.

setting the dial at 1 times or 1,15 times the rate current of the motor?

For IEC relays, you set it at the rated current of the motor. (The currents is probably given on the motor nameplate, but it is better that you actually measure the current sice there may be a slight varion from the nameplate data). As per IEC, the relay (a) must NOT trip for 2 hours at 1.05 x set current and (b) MUST trip within 2 hours at 1.20 x set current.

(i did give this comment earlier in the day, but was surprised that it wasn't appearing. Strange. Must be my mistake)

Kvsridhar, I respect your experience and knowledge imparted on this site, and I consider you as being even better than myself when it comes to issues on motor control and component design.

But I respectfully disagree with the part of your answer espousing setting of an OL relay based on a measured value. Unless you have a dynamometer available to put exactly 100% of rated load on that motor, you are going to end up setting the OL to the LOAD current, not the motor RATED current. There are two problems I have with this concept.

1. If, by chance, the LOAD demands more current than the motor is CAPABLE of, you have just set up your motor for self destruction. Although we can ASSume that a reasonable person would be smart enough to not adjust the relay to a value OVER that on the nameplate, my experience in this industry tells me that my use of capitalization in the word "assume" is based on reality.

2. If we get past that and the user sets the current level to a value BELOW the motor nameplate FLC, then essentially they are "wasting" capacity in that motor. While not inherently dangerous to the motor in and of itself, again, my experience has taught me that this rarely ends well. It is not the fact of "nuisance" tripping being harmful to he motor, it is the human response to nuisance tripping tht leads to danger. People tend to get annoyed when overload relays trip. Untrained or uncaring operators will, given the opportunity, "make the problem go away" in a manner that presents the least amount of effort. If you have an adjustment dial, this results in them turning it up to cease that annoying trip. Eventually, they turn it up to the point of allowing the motor to fail. Yes, this SHOULD be preventable through proper traoning, but that is going by the wayside more and more now as owners want the lowest operating cost they can get. And yes, it could happen at any time, but if you set the relay for the stated CAPACITY of the MOTOR, not the measured load, you significantly reduce the likelihood of nuisance tripping. This is especially important in areas with wide voltage floctuations. If you set the OL based on a measured value when voltage is perfect, then the voltage drops a little, it triggers that nuisance trip scenario.

When a motor manufacturer puts the Full Load Current on the nameplate, it is Zsupposed to be a value they determined by connecting that motor to a dyno tester and measuring the current at 100% rated mechanical loading. If you do not trust that value, find another motor supplier.

You are quite right as always. Your experience is far more comprehensive than mine, and i have learnt quite a lot from you.

i must qualify that my comment was referring to the un-organised sector in India. These guys make motors suited to the horrible voltage conditions prevalent in the agricultural market. Some of these motors are quite good, and do their job adequately. Obviously, the current drawn is considerably more than normal due to low voltage. Since the farmer is some distance away from the pump, nuisance tripping is more of a problem than in a factory. Hence the local electrician sets the relay to such a current that eliminates frequent tripping. This is an application which, while very large in its market size, gives a skewed picture.

You are quite right in recommending setting the relay as per the nameplate value. Good manufacturers will ensure the correctness of the nameplate data.

Overload relay settings are determined by equipment design specifications.

1. Winding insulation class and allowed temperature rise rating.

2. Equipment maximum and/or "locked-rotor" current rating.

3. Equipment Service Factor.

4. Equipment ambient operating conditions. (Ambient temperature, number of full load starts within a defined time period, ambient humidity.)

A good place to start is by obtaining the equipment default protection parameters and guidance from the Original Equipment Manufacturer (OEM) service department or the provided technical Installation & Operations Manual (IOM).

After the equipment is put into operation it is a good idea to monitor the temperature for several operation cycles to determine if the settings will provide adequate protection and/or if the equipment design meets the application requirements.

It's not complicated, as already expressed earlier.

1. READ the motor nameplate. The OL relay is there to protect THAT motor. THAT motor has ALREADY been designed by the motor mfr. to handle the current as denoted on the NAMEPLATE of THAT motor. Nothing else matters.

2. RTFM (Read the Factory Manual) of the OL relay. EVERY OL relay mfr approaches the design philosophy slightly differently, within the standards they are addressing. You CANNOT generalize ANYTHING about how an individual product is configured, but the MANUAL for that product MUST tell you what you need to know to adjust it for THAT motor in THAT application.