Bi-metallic Overload Trip Curve
Bi-metallic Overload Trip Curve
(OP)
I have at hand a bi-metallic overload relay trip curve of the NEMA Class 10 type. The graph has both time (in seconds) against Multiple of Pickup Current (P.U.)axes. I will trace the curve on a standard log-log paper with Time (seconds) against Amperes axes.
Is this per-unit values based from Motor FLA as base current such that the Multiple of Pickup Current is equal to the O/L Level divided by motor FLA? Is my understanding correct?
Thanks in advance
Is this per-unit values based from Motor FLA as base current such that the Multiple of Pickup Current is equal to the O/L Level divided by motor FLA? Is my understanding correct?
Thanks in advance
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata






RE: Bi-metallic Overload Trip Curve
RE: Bi-metallic Overload Trip Curve
The heater is selected based on the motor full load amps or service factor amps, but will generally not be exactly the same.
When plotting TCCs, we generally plot in terms of motor FLA since the exact heater element size is often not known. It's a simple matter to convert from one "base" to the other.
RE: Bi-metallic Overload Trip Curve
The "Class 10" refers to the seconds to trip at 6X the current rating. You can plug in the time and current of this point into your time-current plot. The "curve" is generally I^2*T, so the 300% trip time will be approximately 40 seconds.
Remember that these are NOT precision devices.
old field guy
RE: Bi-metallic Overload Trip Curve
"Remember that these are NOT precision devices"
I wish that I had said that. And if the installer bent the heater when installing it,- you get the idea.
respectfully
RE: Bi-metallic Overload Trip Curve
I have been amused over the years when some of my clients 'upgraded' medium voltage motor protection from thermal overload blocks and CT's to the latest, bright, shiny microprocessor-based motor protection relays.
Whereas before, with the old protection, everybody was happy if the motor tripped within ten seconds of the expected point, they were suddenly greatly concerned with trip times that didn't match the overload curve within a tenth of a second. And suddenly there's this "current unbalance" thing that had been ignored for thirty years prior. And dozens of other settings that they never needed before, but NOW we had to address them.
That fluttering sound was good common sense flying out the window...
old field guy
RE: Bi-metallic Overload Trip Curve
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If we learn from our mistakes I'm getting a great education!
RE: Bi-metallic Overload Trip Curve
Maybe it's good to qualify when you posted:
Some manufacturers have produced thermal overload relays with innovative trip bars that actually takes care of "current imbalance". The bi-metallic elements are arranged to flex and push a common trip bar that if a current imbalance is present, the unequal travel of the bi-metallic strips creates a faster trip! These TOL's I have observed way back 70's, so it's not true that "current imbalance" thing has been ignored for 3 decades!
RE: Bi-metallic Overload Trip Curve
Thanks for reading...
I worked with motors from 480-13,800 volts, from fractional to 24,000 HP.
I was referring to medium voltage motors in the <1000 horsepower range, typically protected by thermal overload blocks of the simplest varieties up into the 1980's. Much past that horsepower range, the users went with current balance and negative sequence relays for the protection of the much more expensive motors.
Then along came the 80's and microprocessor relays hit the market. And things got fun...
Don't get me wrong. I'm not discounting the capabilities or needs for advanced protection now that it's available.
old field guy
RE: Bi-metallic Overload Trip Curve
If I understand you right.
Problem is not new relays, problem is right setting calculation.
Regards.
Slava
RE: Bi-metallic Overload Trip Curve
You're right. When the protection as a thermal relay, it was simple for people who were not necessarily versed in the various protection elements of a motor. They had ONE, choose the right thermal element heater for the motor current. When I upgraded these, then I had wo work with them on many other elements like unbalance, acceleration time, etc.
It's easy for you and I who deal with these things often, but when we're talking about a "maintenance engineer" for a samll petrochemical facility, and he's usually an ME or CE, there's more things to get confused over and to be frightened by.
I had to do a lot of hand-holding and comforting in those cases.
old field guy
RE: Bi-metallic Overload Trip Curve
It's common problem.
We use some EXCELL files with MACRO's for thermal image and unbalunce current calculation and with graph's. In many cases it's help also to ME and CE.
Regards.
Slava
RE: Bi-metallic Overload Trip Curve
In P.U. terms or Multiples 35A is 35/29.7 = 1.178 or 117.8%. Using the O/L P.U. curve for NEMA class 10 O/L relay, I started with 1.78, then 1.25, 1.4, 1.5, 2.0, 2.5, 3.0...8.0 in 1.0 P.U. intervals of O/L (with FLA as base) and get the individual time (seconds) equivalent. In my Log-log paper in terms of current, I simply multiply the P.U. O/L to the FLA and using the equivalent time as calculated earlier, I plotted the curve.
I have no software here to simulate easily, so everything i s done manually...I happen to get a hold of blank log-log graph only. Its like John Rambo in the middile of the forrest.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Bi-metallic Overload Trip Curve
RE: Bi-metallic Overload Trip Curve
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata