selecting correct overload relay for motor 2

[jrick]

I have 3-ph,240 motor, label list amp at 4.8, when motor is idling am on each leg is 2.3, 2.0,2.3; when cycle runs amp rises to 2.5,2.3,2.5. on start up jumps to somewhere close to 5 but happens so fast cant tell. I have three allen bradley relays, 2500 (20-25amp) 1500 (10-20amp) and 1000 (6.5-10amp) one of them came from here but Which one, or is none appropriate. How would I determine. One last thing, how would i know if I have delta or wye configuration ..just curious)

 

[rbulsara]

The norm and Code is to set the OL trip setting to no more than 125% of the rated full load amps of the motor.

For large or oversized motors running well below rated current, some prefer to set the OL to 125% of the normal load current.

To determine type of connecitons, it depends on what is accecible (leads). If all six leads are brought then it is easy to determine. If it has six terminals (2 rows and 3 columns) and all shorting links are veritcal then it becomes delta connected. If one of the rows (normally bottom) is shorted, if becomes star connceted.

If only 3 leads are brought out you may have to determine by inspection.

I am sure you will see additional posts form experts in motors.

 

[jrick]

thanks, i suppose what I would like to know is when looking at overload, do you consider each line or are they added together. in above 125% of 4.8 = 6
2.5+2.3+2.5 = 7.3

 

[rbulsara]

Rated current is always for each phase (line).

You set at 1.25*4.8=6A.

Also look at thread238-96044

 

[buzzp]

Keep in mind that 125% is the maximum setting as defined by the NEC. NEMA motor-generator standards (protection of equipment not personnel) generally have a suggested lower setting. The larger the motor, generally, the smaller the difference between FLA and the OL setting. If the motor has SFA on the nameplate then set it here or if you expect to run at the lower amps then you might set it lower so you are alerted to anything out of the ordinary as far as operating conditions (amps go up then something has changed even if still below FLA).
I dont think any of the relays you have are suitable for this installation except the 6.5A one is marginal. I would not use it.

 

[jraef]

I gave buzzp a star because he mentioned the issue of 125% being MAXIMUM, not recommended. So many people read it the wrong way and end up with burned out motors. An OLR set to 125% will allow 125% of the rated FLA before BEGINNING its IsqT trip cycle. If the motor had a 1.15 service factor you were already 10% over current by then and, according to a class 20 trip curve, it should have tripped in roughly 20 minutes. The 125% comes from the fact that elsewhere in the NEC you are required to size the conductors for 125% of FLA minimum, and in many cases it is only the OLR that is protecting the conductors from long time overcurrent. The NEC is not directly concerned with protecting the motor from thermal damage short of a fire. Remember, it is written by the NFPA, not NEMA or EASA.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

Thanks Jraef for the star.

 

[advidana]

motors over 1 hp per NEC for sizing overloads is 115 percent, motors marked with not less than 1.15 are 125 percent and motors marked temperature rise not over 40 degree c are 125 percent. Also 230 vac motors is standard voltage used with 240 vac systems. Suggest you look are the code book article 430.

 

[MVRockwell]

Which AB relays are you using?