Thermal Overload Relay

Before damage is done...

It should be set at a point that protects the equipment from thermal damage.

I"d consult with the maker of the device you want to protect. I'm having trouble divining the make and model number to give to the manufacturer, you'll have to do that.

In a DOL starter, i would set it at the rated full load current of the motor.

In a ΥΔ starter, IF THE RELAY IS WITHIN THE Δ, i would set it to to the FLA/√3 = 58.3% of the full load current of the motor.

This is what i would do.

Up to you what you want to do....

Generally true, but as pointed out, we don't know the mfr name and model. Some (most) require setting at nameplate FLC and the pickup point is already factored in, others require the user to factor in the pickup point from the nameplate FLC and set the trip point there. You don't know until you RTFM. (Read The F###ing Manual)

I'll give you a GA because most of the questions here relate to starters and motor drives. Just because the OP doesn't know what they want doesn't mean you cannot provide a valid response.

"Just because the OP doesn't know what they want doesn't mean you cannot provide a valid response."

It appears to me that you have never been dragged into court by someone who couldn't communicate clearly what they wanted or needed, but you provided information that seemed appropriate for the application anyway. The individual (foolishly) acted on what is now known to be inappropriate advice and suffered a subsequent loss.

... not that I am bitter or anything.

Ooops. Calculation error. 1/√3 = 57.7%

Some expansion on the answer, taking into account the more recent responses....

i assumed you are from India, so i referred to the practice here. If you are not from India, then service factor may be involved, and my response may not be appropriate.

If you are located in India, you may need to find the actual full load current of the motor, which may be somewhat different from the nameplate data. You need to set the relay to the actual current. You may end up with nuisance trips otherwise.

There are different opinions to set the over load relay .Normally 105% is recommended considering the service factor , 5 % more than the FLC to meet the load fluctuations. But I do the following :

Set it at 105 % and if the machine is running in perfect load conditions , measure the actual load current . Then if the current drawn is less than FLC , then I change the OLR setting to 105 % of the actual current drawn . This way you are protecting the motor for any over load above the normal current at the time of commissioning . Also this will help to monitor the deterioration of the conditions of the drive , in case unexpected overload gets reflected on the motor. From this you can finalise the OLR setting . At any cost don't cross 105% of the FLC.

IEC relays already have the 1.05 factor built in. In other words, if one sets an IEC relay at 10A, it will (a) not trip at 10.5A and (b) will definitely trip at 12A. So, designers target 1.125 as the aymptotic ordinate.

Before setting the O/L relay motor FLA must be checked carefully( not depending on Name plate data only).

Manindra

i said this in #7 : _{If you are located in India, you may need to find the actual full load current of the motor, which may be somewhat different from the nameplate data. You need to set the relay to the actual current. You may end up with nuisance trips otherwise.}

What you are now saying in response to my post...is it something different?

Per NEC (Article 430.32 A)

This overload device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater 125%

Motors with a marked temperature rise 40°C or less 125%

All other motors 115%

In some cases, it may be (Article 430.32 C):

Motors with marked service factor 1.15 or greater 140%

Motors with a marked temperature rise 40°C or less 140%

All other motors 130%

- MS

85 % of KW for example 5 KW motor thermal over load setting is 8.5 A

Sir,

Many answers...all helpful. Let's step back a second:

The thermal overload relay is attempting to mimic or approximate the thermal conditions within the winding of the motor and cause the power to be shut off before the winding temperature exceeds that at which the insulation of the windings is damaged or destroyed. Over many years, the accumulated experience of engineers, designers, users, insurers, etc. have resulted in the rules quoted in the many answers.

You will see that motor designs which include a higher load tolerance (higher temperature insulation and higher service factors) allow a higher thermal overload relay setting. No one mentioned that if the motor and the controller are in different locations and the temperature difference between these locations is significant then the thermal overload relay will have to be set lower or higher according to that difference between the local temperatures. Also, some design's of thermal overload relays will respond quickly to single-phase conditions while the motor is running (and also respond to very unbalanced currents due to voltage imbalance), while other designs will not, (A single-phased motor, if lightly loaded will often draw less current than its nameplate, but will still burn-up because of the excessive heating caused by the imbalanced magnetic currents within the motor.)

Go to the codes or regulations adopted by your local authority. If there is none, then the NEC (National Electrical Code in the USA), article 430, will be a good starting point because it has over 100 years of experience to guide its rules.

Finally, remember that a thermal overload relay is only a good guess at the actual motor's temperature. The best approach (required on large motors) is a sensor embedded within the winding, which is used to interrupt current if/when the winding gets too hot.

--John M.