Two Overload relays 2

[nerb31]

Hi,

I would like to be "enlightened" on the basic Code requirements about motor overload protection.

1. What is the rationale with the now NEC requirement of having 3-Overload relays for 3-phase motors, versus the then allowed 2-overload relays for 3-phase motors.

2. What's the reason behind the 115% factor in sizing overloads? Was this derived via calculation?

Appreciate your inputs.

 

[waross]

Three overloads versus two overloads;
You must have some very old equipment. It became apparent about 50 or more years ago that in the event of voltage unbalances the resulting current unbalances may cause the current on one phase to be higher than the current on the other two phases and two overload relays would not provide adequate protection.
For the younger posters, time was in NEMA land when many overload relays were single pole devices.

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

 

[jraef]

It officially changed in 1978, the debate about it probably lasted 20 years prior to that. All for the cost of one OL heater element per starter, which if memory serves me, was worth about $1.28 at the time.

Bill's explanation is dead on though.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[jraef]

Sorry, didn't address the 115% part.

Don't be confused here. You select the OL heater or relay setting based on their motor nameplate FLA. The 115% is almost always already built-in to the selection charts for the heater elements or the internal settings of adjustable OLRs. Read the manuals for them, they tell you that. Most people don't by the way, so there are a LOT of motor heaters installed out there where someone multiplied the FLA x 1.15, and then picked a heater, which already was sized for 1.15. Then when the motor burns up, everyone wonders what happened...

But back to your question. NEC Article 430.32(A)(1) states that the OL protection must be no MORE than 115, or 125% for 1.15SF motors (unless under some very specific circumstances it can be 140% under pp C). But this is a MAXIMUM. There is nothing technically stopping anyone from using 100%, it's just a bad idea. The rationale behind using 115% for the maximum pick up point, as opposed to 100%, is to allow for short duration overload conditions; so that you don't have nuisance tripping that will annoy someone to where they jump around it altogether.
Motors then are designed with that pick up point in mind. In other words the thermal damage curve for the motor is taking that pick-up point into consideration.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[nerb31]

thanks waross & jraef for your replies. We do have some old equipment and I was surprised to know that two overload heaters were used then. Actually had some "discussions" with the "seasoned" engineers here but no one could explain the theory behind the two-overloads scheme. They are all saying the two overloads are adequate as two remaining phase will draw so much current when one phase was lost...

I was hoping to understand the theory behind. Bussman SPD handbook has a short discussion about it, but I am little lost with its explanation.

Appreciate more inputs.
Thanks

 

[waross]

Hi jraef;
I came across a 1972 Canadian Electrical Code book and it looks like we got three overloads in Canada in 1972.
Remember when the first three pole overload relays had a jumper in the center pole? I saw my first differential protection in an IEC starter. Wow, what a great idea!
nerb31;
Loss of a phase may not be the worst case.
An induction motor acts also as an induction generator. That is the source of the back EMF. The difference between the applied voltage and the back EMF is what determines the current. The greater the difference between the applied voltage and the back EMF the greater the current. But the back EMF is equal for all three phases. That is the reason that a small unbalance in phase voltages may cause a much larger unbalance in currents. With the old two relay protection schemes, if the phase without an overload relay had above average voltage then the current in that phase could be so great that the motor could burn out without the other two phases going over current.
On paper in the classroom, it looks as if two O/L relays could provide adequate protection but that assumes balanced voltages and currents. You can't count on that in the real world.

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

 

[jraef]

You Canadians are always so out-in-front of us. I think it's because for so much of the year, you are sitting around snowed in thinking about stuff while we are getting sun tans at the beach...

Like Scandinavians too.

"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