Thermal Overload Relay

[hooverdale]

We have a few locally assembled motor controller from a particular supplier here in the Asia. I have all the details gather in every controller for reference purposes and i noticed that some of its thermal overload relay rating varies for the same horsepower. By the way, these are DOL and Wye-Delta motor controllers for 3Phase induction motors used in centrifugal pumps and blowers.

Is there a rule of thumb or particular standard that pertains to the proper selection of thermal overload relay?

Regards.

 

[burnt2x]

Any diagrams, pictures, or specs?

You mentioned wye-delta motor controllers and DOL controllers, so I guess that's the catch. Wye-delta controllers TOR can be arranged to monitor the phase current while DOL TOR's always measure the line currents.

Wait for others to chime in.

 

[waross]

Manufacturers instructions. Some overload sizing from one manufacturer depended on the size of the enclosure it was mounted in.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

ROT - 1.15 x FLA.
Standard - Typcially the local electrical code, such as the NEC.

Saying the ratings vary is very vague.

 

[jraef]

If the differences you notice are between the DOL and the Star Delta controllers, it is because in some SD controllers, the OLR is placed such that it only reads the phase current, not the line current, so it is set at 58% of the line current of the motor nameplate.

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[hooverdale]

Thank you for all your input guys

@Lionel - I was using that 1.15 x FLA to double check the rating of the TOR. For example: 3.5A for 1HP, 230V, 3Phase.
TOR = 1.15 x 3.5 = 4.025
Nearest TOR Rating is 2.8-4.2A Is this correct?
By the way these are FUJI thermal overload relays.

http://www.fujielectric.co.jp/fcs/eng/product/control/pdf/DEC1901a_P097_108.pdf

I checked each controller and compared it to past controllers that we had. Some of the ratings follows the 1.15 x FLA and others didn't.

@Waross - No sir, these are between DOL-DOL and SD-SD.

I will try to address it to the supplier and right now I'm just gathering ideas to back me up.

 

[burnt2x]

Each manufacturer makes TOR ranges that will cover most of the equipment ratings in the area of coverage. There is no standard range since there are lots of manufacturers.

What is important is that consumers can find TOR ranges where their equipment drives gets to be protected properly. You would like to use a TOR that has a middle range equal to your FLA. That way, you can tweak the setting higher or lower as the case maybe.

Remember that most TOR's will be set equal to the protected equipment's FLA and will trip your controls when the 115% overload persists for a specified period of time.

 

[LionelHutz]

That overload looks to be about right. Ideally, I'd want the FLA to fall somewhere around the 25% to 50% range on the dial to give some future adjustability. Sometimes, it's ends up close to the upper or lower limit because there isn't a range that is more suitable.

If these are small motors then the OL may not have been correct when supplied or the motor may have been changed some time after the overload was supplied. Small motors need to be sized by FLA since there can be such a range of possible motor FLA values for the same motor HP rating.

 

[waross]

Under the dictum of RTFM I find;

Dial amperage setting
The setting dial uses a RC (Rated
Current) marking which is set to the
motor full load current.
(page 3)
Some manufacturers require you to use a factor of 115% when sizing O/Ls, some don't. Apparently Fuji doesn't use the 115% factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter