Underload trip setting for motor

[Isaiah]

We have a 3ph, 460V TEFC motor, 2.5 nameplate FLA, 2HP, 3490RPM 1.15 SF – for the Low-Low Amp setting, the vendor set the trip at 3A on the electronic OL device – obviously the motor tripped once energized.

Is there mathematical formula or chart to determine typical underload trip settings?

 

[sameguy]

Have the vendor come back, it should be in his documents, is the motor being overloaded?

 

[Jraef]

There is no absolute answer to that, every motor is different. No load amps are mostly reactive current so can be anywhere from 20 - 60% of FLC. The best way is to measure the current with the motor uncoupled and use that value to start from, because that is what will tell you that a shaft or belt or chain has broken.

 

[Isaiah]

Have the vendor come back, it should be in his documents, is the motor being overloaded?

No overload is occurring and the motor is essentially fine; its the Low-Low amp trip that needs to be adjusted. The overload is electronic and settings came from the factory (Siemens) at the upper range of the LOW amp trip - which is 3A - thus the motor tripped when fully engaged at FLA of 2.5A.
(The electronic OL is part of size 1 combination starter installed in a Siemens 'Tiastar', 480V MCC).
We set the trip down to 2A and now the motor is running without trip. But I need paperwork to back up the Low amp trip setting. I'm sure the motor manufacturer has a different setting based on each individual motor characteristics as noted by JRaef.

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[topgone]

No overload is occurring and the motor is essentially fine; its the Low-Low amp trip that needs to be adjusted. The overload is electronic and settings came from the factory (Siemens) at the upper range of the LOW amp trip - which is 3A - thus the motor tripped when fully engaged at FLA of 2.5A.
(The electronic OL is part of size 1 combination starter installed in a Siemens 'Tiastar', 480V MCC).
We set the trip down to 2A and now the motor is running without trip. But I need paperwork to back up the Low amp trip setting. I'm sure the motor manufacturer has a different setting based on each individual motor characteristics as noted by JRaef.

​

Crazy that the TOL tripped when set lower than the FLA! Your case might be the first in the history of motor protection!

 

[Jraef]

Crazy that the TOL tripped when set lower than the FLA! Your case might be the first in the history of motor protection!

No, not crazy. The UNDER load trip setting was 3A, which means at any current value UNDER 3A it will trip. So his motor was 2.5A FLA and was likely set for that as the OVER load pick-up point, but that means even if the motor was fully loaded, the relay would trip on UNDER load. It was a silly setting, likely a default left in place and forgotten about.

 

[topgone]

No, not crazy. The UNDER load trip setting was 3A, which means at any current value UNDER 3A it will trip. So his motor was 2.5A FLA and was likely set for that as the OVER load pick-up point, but that means even if the motor was fully loaded, the relay would trip on UNDER load. It was a silly setting, likely a default left in place and forgotten about.

So, in this case the "overload (OL) means "underload"!

 

[Isaiah]

No, not crazy. The UNDER load trip setting was 3A, which means at any current value UNDER 3A it will trip. So his motor was 2.5A FLA and was likely set for that as the OVER load pick-up point, but that means even if the motor was fully loaded, the relay would trip on UNDER load. It was a silly setting, likely a default left in place and forgotten about.

Exactly. We're finding more motors doing the same thing. This is probably an 'across the board' default error from factory for Low-Amp trip set too high.

 

[Isaiah]

So, in this case the "overload (OL) means "underload"!

The OL is actually looking for both - overload and underload. An underload could occur from failure in drive belts, pump intake or discharge, control valves in process lines etc., basically anything that affects flow rate GPM, in which case you'd want the motor to stop.

 

[Sahib]

An underload could occur from failure in drive belts, pump intake or discharge, control valves in process lines etc., basically anything that affects flow rate GPM, in which case you'd want the motor to stop.

So perhaps you may have to get the underload setting of the motor by trial and error because motor underload current may be different from drive belt failure to discharge valve failure, for example.

 

[kwired]

Methinks someone mistakenly set it for what overload setting needed to be, or didn't realize it was an "underload" setting.

Where to set it depends on what you are trying to detect and how the equipment responds to certain underload conditions. Broken belt may yield different motor load than dry pump, blocked air on a fan, etc. Some cases may even need time delay to allow load to stabilize before sensing underload condition.