motor overloads

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mull982

Senior Member
I have a 480V 200hp motor with a FLA of 230A. The motor is being overloaded and keeps tripping. The overloads that are presently on there are Furnal E44 overloads rated for FLA between 230-248A. We keep tripping these overloads and damaging motor.

I notice the NEC recommends using 125% of FLA for motors rated at 1.15 SF or 40deg ambient which this motor is. Based on this the new overload setting would be 287A. Do you recommended getting the overload size range that corrosponds with this value? Maybe a size or two up?

I have a feeling that although this motor is tripping, people keep reseting it and restarting it with the starting making things worse.
 

Cold Fusion

Senior Member
Location
way north
I have a 480V 200hp motor with a FLA of 230A. The motor is being overloaded and keeps tripping. The overloads that are presently on there are Furnal E44 overloads rated for FLA between 230-248A. We keep tripping these overloads and damaging motor.

I notice the NEC recommends using 125% of FLA for motors rated at 1.15 SF or 40deg ambient which this motor is. Based on this the new overload setting would be 287A. Do you recommended getting the overload size range that corrosponds with this value? Maybe a size or two up?

I have a feeling that although this motor is tripping, people keep reseting it and restarting it with the starting making things worse.
I agree that the practice of resetting and restarting repeatedly, shortens the life of the motor. From what I have read, and one personal example, usually around the fourth start within one hour, lets the smoke out.

The NEC lets you go to 140%. That's pretty well where I tend to set them.

For your case, I'd be monitoring\recording the running current. If the motor has rtds, check the winding temp. If it is consistently above the rated winding temp, or service factor there's nothing you can do to save the motor - except fix the mechanical load on the motor:
Dragging bearings (motor or driven device)
Gearing
Bigger motor

Setting the overloads up or down won't fix anything - except, I think setting them up to the NEC max is better, it lessen the number of hot starts.

One other thing. If you are stuck with an installation where the motor is in, or above, the service factor, consider installing an overlaod with a thermal model - say like an AB E3+. They are inexpensive and can be programmed to lock the motor out when the thermal damage curve is exceeded. It won't fix the overloaded motor, but it will cut down on the hot starts.

cf
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
mull982,
First off, I would NOT assume the OLR is defective or set incorrectly. You even said "The motor is being overloaded and keeps tripping." How does that relate to being a problem with the OLR? I would say it is doing it's job! If there is damage occurring to the motor, the constant over loading would be my chief suspect, not the "nuisance" tripping of the OLR. If the motor is constantly over loading, maybe the motor is too small, or maybe the load needs to be reduced. I don't think there is any magic here. But if premature resetting is a problem for the operators of this equipment, you may want to consider using an Electronic Overload that has a programmable cool-down time that you can set longer. There are several out there that provide this feature.

Secondly, what the NEC says and what the starter manufacturer say are not to be construed as additive! The NEC says 125% as the overload current allowed, but that is the BASIS of the design of the OL heater element (or the Pick Up Point of an adjustable OL relay) when the OLR manufacturer makes the selection chart. In other words, that 125% IS ALREADY FACTORED IN to the selection chart that you see. Read the instructions, it clearly tells you to select the heater element for the nameplate FLA, NOT the FLA x 1.25.

Here is how it reads in the Siemens (ex Furnas) OL selection section:
"Heaters shown on the following pages provide a maximum trip rating of 125% of minimum motor amperes for 40?C motors (service factor 1.15). For other motors (service factor 1.0), select the next lower listed heater catalog number within the designated table which provides a maximum trip rating of approximately 115%."

As far as I know, this is true of all other motor control manufacturers. The wording may be different and maybe not as informative (AB just tells you to select based on nameplate FLA without specifically stating why), but that is how the industry bases OL Relay device selection or settings.

Thirdly, the 140% issue is NOT the best way to protect all motors, it is an ALLOWANCE for special circumstances. But motor rewinders love you guys who do that...
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The overload elements should be chosen based on the requirements of the manufacturer of the overload relay - not based on the NEC.

For example, a Square D motor starter with a 1.15SF motor and the controller in the same ambient temperature, has its melting alloy element chosen based on 100% of the FLA - no multipliers are used.
 

a.bisnath

Senior Member
start with simplicity

start with simplicity

have you run the motor uncoupled? what is the no load amps? recording values are important
what about starting torque?,application of motor -fan,compressor ,pump? all these will detrermine the o/l setting,any vibration testing done any temperature profiles done,vertical or horizontal mounting?in practice I,once, had to rate an overload at 185% of FLA(oil dashpot type),is it a hollow shaft with oil bearing at top?
a motor especially at that size which to me is small can only be started a certain number of times per hour ,do you have an anti-restart timer in your control circuit -PUT ONE
insulation resistances?,surge tests to overload-as found readings?could the overload just be doing its job?
 

Besoeker

Senior Member
Location
UK
I have a 480V 200hp motor with a FLA of 230A. The motor is being overloaded and keeps tripping.
What current is the motor actually taking when on load?
Are all three phases balanced?
Do the checks and you might find the answer right there.
 
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