Help With Trip Curves

Tim Ganz · Mar 24, 2014

Can someone help me understand the trip curves of these overloads listed here? http://www.ab.com/en/epub/catalogs/12768/229240/229254/229467/2442188/print.html

10,15,20. Is the curve showing the amount of time it takes to reset?

They have a line for hot trip and cold trip also? What do they mean hot trip and cold trip?

allscott · Mar 24, 2014

Tim Ganz said:
Can someone help me understand the trip curves of these overloads listed here? http://www.ab.com/en/epub/catalogs/12768/229240/229254/229467/2442188/print.html

10,15,20. Is the curve showing the amount of time it takes to reset?

They have a line for hot trip and cold trip also? What do they mean hot trip and cold trip?

No, the curve is trying to tell you how long the O/L will take to trip depending on the severity of the overload. As you can see when moving from left to right on the X axis the higher the degree of overload the faster the trip time.

An O/L will always trip sooner the warmer it is. The "hot" curve is telling you how long it will take to trip if the O/L is warm, the "cold" curve is if the O/L is cold...

jraef · Mar 24, 2014

The first time you start a motor when it is at "normal" room temperature, defined as 25C or below, is called a "Cold Start". Anything else after that is a "Hot Start". So these charts are telling you that once the motor is at operating temperature (or higher), it will take less time to trip on the same amount of current than it would if the motor were starting "cold".

milldrone · Mar 24, 2014

Tim Ganz said:
10,15,20. Is the curve showing the amount of time it takes to reset?

If you are asking "What is a class 10 trip vs a class 20 trip?"

A class 10 trip used to mean (back in the day when locked rotor was 6 to 8 times full load current) 10 seconds to trip under a locked rotor condition from a cold start.

A class 20 trip used to mean (back in the day when locked rotor was 6 to 8 times full load current) 20 seconds to trip under a locked rotor condition from a cold start.

Now that energy efficient motors are legislated to be the only motors available (locked rotor is 11 to 13 times full load). I'm not sure that the old times to trip are valid.

allscott · Mar 25, 2014

Lancie1 said:
Circuit breaker trip curves are used to do system breaker coordination, where you overlay the trip curves (on transparent sheets) for all the breakers from the Main plant breaker or fuse down to the last branch circuit breaker. No downstream breaker curve should have a trip point (at any point along the Time vs. Current curve) above that of any upstream breaker. If it does, there is a breaker coordination problem that should be resolved before a downstream breaker does not trip during a fault, allowing the full current flow of the upstream breaker that may burn up the local circuit.

I think I understand coordination but isn't a motor overload a different animal? IE: it's not there for branch circuit protection but simply to protect the motor?

I've never 100% understood the clearing ampacity of a fuse or circuit breaker either. I know enough never to replace a fuse or CB with a lower kA rating but I've never understood that if I have a 600A fuse with a 300kA rating how can I have a 60A breaker with only a 60KA rating downstream of that and still be protected?

thanks.

Lancie1 · Mar 25, 2014

I think I understand coordination but isn't a motor overload a different animal?

Yes, you are right, my mistake. I looked at the curves but not at what they were for. For larger motors, the overload trip curve is often coordinated with the long-time trip rating of the next upstream fuse or breaker (which should not trip before the motor overload durning a small but long-time current overload).

I've never understood that if I have a 600A fuse with a 300kA rating how can I have a 60A breaker with only a 60KA rating downstream of that and still be protected?

Each overload device only protects its own little turf (the stuff immediately downstream and before the next device). The idea is to have a fault cause a trip at the closest upstream overload device. If a device farther upstream trips first, then something is not right. Possibly the smaller downstream device is too slow and does not trip before the faster (but much larger) upstream breaker.

allscott · Mar 25, 2014

In your experience what would have been the cause of this failure? This happened a couple weeks ago. The breaker got so hot it melted the paint off the side of the cabinet (feeding a 60HP DC drive). When the breaker finally blew it took out two upstream 800A fuses.

Not a good day to say the least.

https://www.flickr.com/photos/121387664@N04/13405164225/

https://www.flickr.com/photos/121387664@N04/13405498004/

Tim Ganz · Mar 25, 2014

When setting these type of electronic overloads do you take motor FLA and do 125%?

I have been told to set the overload FLA to exactly what the motor name plate says as the 125% is already figured in on the overload?

Any truth to this?

Jeff23spl · Mar 25, 2014

Tim Ganz said:
When setting these type of electronic overloads do you take motor FLA and do 125%?

I have been told to set the overload FLA to exactly what the motor name plate says as the 125% is already figured in on the overload?

Any truth to this?

yes the usual overload ''knob'' take into acount some service factor and rule of thumb is to set it 1:1 to motor fla. I expect it is looking for a motor with 1.15 SF at normal altitude.
In some case you may have to reduce it for high elevation or lower SF but only few bother about it...Most of the time peoples crank it untill it stop tripping instead of looking at why it trip but this just makes more motor sells or rebuilds....

jraef · Mar 25, 2014

milldrone said:
... Now that energy efficient motors are legislated to be the only motors available (locked rotor is 11 to 13 times full load). I'm not sure that the old times to trip are valid.

No, nothing about trip curves changes with regard to energy efficient motors. It's a problem for Short Circuit Protective Devices, but thermal OL curves are based on the long time thermal damage curves of the motors.

allscott · Mar 25, 2014

Jeff23spl said:
yes the usual overload ''knob'' take into acount some service factor and rule of thumb is to set it 1:1 to motor fla. I expect it is looking for a motor with 1.15 SF at normal altitude.
In some case you may have to reduce it for high elevation or lower SF but only few bother about it...Most of the time peoples crank it untill it stop tripping instead of looking at why it trip but this just makes more motor sells or rebuilds....

But since IEC motors don't have a service factor does that mean you should turn it down from the FLA rating?

I just replaced a soft start on a compressor motor, the motor is rated 89A with a SF of 1.2. It's drawing about 105A constantly but one leg is only drawing about 96A. With the FLA, SF, and class 20 OL set in the soft start it seems perfectly happy to run it 24/7 like that but I suspect something isn't right with the motor?

jraef · Mar 25, 2014

Jeff23spl said:
yes the usual overload ''knob'' take into acount some service factor and rule of thumb is to set it 1:1 to motor fla. I expect it is looking for a motor with 1.15 SF at normal altitude.
In some case you may have to reduce it for high elevation or lower SF but only few bother about it...Most of the time peoples crank it untill it stop tripping instead of looking at why it trip but this just makes more motor sells or rebuilds....

With NEMA overload relays, you factor in the motor Service Factor only if you intend on running the motor into it, which shortens the motor life. For IEC overload relays, you set them at the motor nameplate FLC, there is no such thing as Service Factor for IEC motors, and the pick-up point of the OL curve is ALREADY factored into the design.

In the case of the E1 SSOL you (Tim Ganz) are using, they are designed to emulate an IEC bi-metal OL relay. The pick-up point is ALREADY set to 120% of the dial setting, so DO NOT add another 25% or you will lose the motor.

RTFM...

jraef · Mar 25, 2014

allscott said:
I just replaced a soft start on a compressor motor, the motor is rated 89A with a SF of 1.2. It's drawing about 105A constantly but one leg is only drawing about 96A. With the FLA, SF, and class 20 OL set in the soft start it seems perfectly happy to run it 24/7 like that but I suspect something isn't right with the motor?

You have about an 11% current imbalance going on, definitely NOT good for the motor! Your rotor is heating up disproportionally (due to negative sequence currents) to your stator current values and will fail in a relatively short time. But because your overload protection (via the soft starter) is only looking at the stator current, it will not protect it unless it has Current Imbalance protection built-in. If it does, then someone either disabled it so that it would run, or set it too high to do any good.

If I had to guess (and I do) I'd say someone used one of those cheap "2-phase" soft starters that has SCRs on only 2 of the 3 phases, and a straight-through bus bar on the center phase. If the low current is on the center phase, that most likely proves it. Those things are really dangerous for motors. The only people who like them are OEMs who can put them in cheaper and pass on the long term cost of replacing the motor to the end user, because it will likely last out the warranty.

allscott · Mar 25, 2014

jraef said:
You have about an 11% current imbalance going on, definitely NOT good for the motor! Your rotor is heating up disproportionally (due to negative sequence currents) to your stator current values and will fail in a relatively short time. But because your overload protection (via the soft starter) is only looking at the stator current, it will not protect it unless it has Current Imbalance protection built-in. If it does, then someone either disabled it so that it would run, or set it too high to do any good.

If I had to guess (and I do) I'd say someone used one of those cheap "2-phase" soft starters that has SCRs on only 2 of the 3 phases, and a straight-through bus bar on the center phase. If the low current is on the center phase, that most likely proves it. Those things are really dangerous for motors. The only people who like them are OEMs who can put them in cheaper and pass on the long term cost of replacing the motor to the end user, because it will likely last out the warranty.

It's a Benshaw and I believe it has 3 sets of SCR's but to be honest I didn't really check. I didn't know anyone made a soft start that only controlled 2 phases.

jraef · Mar 25, 2014

allscott said:
It's a Benshaw and I believe it has 3 sets of SCR's but to be honest I didn't really check. I didn't know anyone made a soft start that only controlled 2 phases.

yeah, unfortunately they do. But I didn't think Benshaw did, however it's been a long time since I have looked at their product.

If it is a full 3 phase (6 SR) design, and it has a built-in bypass contactor, it may be that the bypass contactor has failed on one pole. They did a thing where instead of using one 3 pole contactor, they used one contactor on each pole (it was cheaper). The down side to that is that if one contactor fails, it causes a current imbalance.

If you have no bypass contactor, then there may be a bad termination somewhere, or a bad firing board. Stuff like that only gets worse.

Help With Trip Curves

Tim Ganz

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