Overloads 1

[jenkinscody]

I have a 3 HP three-phase 480V squirrel cage motor. I have selected a sz0 motor starter and have never had much to do with component selection. I have a control transformer size 75VA making my step down from 480-120 for my controls. I am trying to size some overloads for my motor starter and dont even know where to start. Can someone help me here?

 

[Gorcan]

Overloads are to protect the motor, General advice to take full load current of the motor, alarm at +10% and trip (shutdown) at +20%. Adjust these figures according to source as you would shutdown the whole system.

 

[jraef]

Most Overload Relay manufacturers will instruct you to set the OL (or purchase the "heater elements) based on the FLC read from the motor nameplate; no factors, multiplications , percentages etc. Any setting higher than that will risk shortening the life of your motor, anything lower risks nuisance tripping.


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[jenkinscody]

Ok thank you

 

[DickDV]

You might want to check the electrical code requirements in your local jurisdiction. Usually, they are specific as to how to determine overload settings.

 

[flexoprinting]

Hello all

jraef has far more knowledge here than I do so I am questioning my procedure for selection. I would look it up in NEC manual and what I get is the following.

Per NEC 430.32
Continuous duty Motors

A separate overload device that is responsive to overload current. This device shall be selected to trip or be rated at no than the following percent of motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater - 125%

Motors with a marked temperature rise 40 degrees C or less - 125%
All other motors - 115%

Normally this is the electricians responsibility and I don't need to concern myself with it. What am I missing here?

 

[jraef]

The biggest problem with using the NEC reference above, is that some manufacturers of overload relays already account for those issues inside their product. If you then ADD to the settings, you are putting the motor and wiring at risk. That's why the only valid setting instruction must come from the manufacturer of the relay itself.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[flexoprinting]

That is good to know! Thank you.

 

[waross]

One type of O/L relay on the market has an adjustable current range. (such as 3.2A to 7.6A) If you are using this type of O/L relay, you select a size that brackets your motor full load current.

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

 

[buzzp]

Just some food for thought:
Most manufacturer's tell you to set in the FLC because the SF is fairly standard across the manufacturer's selling to the US(NEMA again), based on motor horsepower, type, phases and voltages (all required to size OL's) and would aid confusion in the field.
Thermal OL's will allow the motor to run past SFA for a short time. There is a trip curve in NEMA's publication which will tell you when it should trip. Most manufacturer's use this as there standard. I am not up to speed with the NEC to know what they say specifically. There primary concern is fire and safety hazards, not so much of protecting our motors.
Electronic OL's are more adjustable to trip before these times specified. Thermal's operate on temperature alone, and in my mind, the damage has already occurred before they trip. It is a matter of what you want to protect or what is more important, the motor or the process? Sometimes the process far out ways the protection of the motor.