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Overload Trip Class - What does it really mean?


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#1 GGOSS

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Posted 25 February 2003 - 02:22 AM

Hi All,

Quite often we here of applications in which the motor protection/overload relay 'trips' during motor starting. The remedial action taken by many when faced with this problem is to select and install a replacement overload relay, generally one that provides a higher 'Trip Class' setting, for example, Trip Class 20 in lieu of the standard Trip Class 10.

Note: The appears to be a general misconception in industry that Overload Trip Class relates directly to the starting time of a machine. This is not the case.

All due care must be taken when selecting a protection relay offering higher Trip Class, as this can result in under protection and subsequent failure of the motor.

To select an overload relay with a more suitable trip class, you must in the first instance obtain data relating to the motor’s thermal withstand capabilities. That is, you need to know how many seconds (from cold condition) the motor can the sustain Locked Rotor Current before it is compromised.

This information is readily available from most leading manufacturers of motors and is generally provided in one of two formats.

a) Specific values for Locked Rotor Current and maximum Locked Rotor Time (from 'cold condition' ) are given.

B) A Motor Thermal Withstand Curve is provided.

With this information available to you, you can refer to the tables given in IEC 60947 to identify the most appropriate Overload Trip Class. This is defined as the one that provides a trip curve as close as possible to but below the overload curve of the motor. Adopting this process will ensure nuisance tripping is minimised and that the motor is adequately protected at all times.

Note: If the above processes are adopted but the trip conditions continue, there are 4 possible causes.

1. The motor (and overload relay) are not given sufficient time to cool between starts.

2. Assuming reduced voltage start (star/delta, auto-transformer, primary resistance, soft start etc., the motor is not delivering torque sufficient to accelerate the connected load to speed. That is the starting current and starting time under RVS conditions exceeds that permissible by the overload curve.

3. A more advanced protection strategy such as motor thermal modelling may be required. Motor thermal modelling allows the user to 'match' the curves of the protection device to the connected motor. This is of particular advantage when the motors thermal withstand capabilities and the start condition (starting current and starting time) fall between two curves defined by standard overload trip classes.

4. The motor is simply too small for the application.

Put simply, the most appropriate protection strategy is the one that allows the motor to be fully utilised without nuisance tripping or fear of motor burn-out.

Regards,
GGOSS

#2 marke

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Posted 25 February 2003 - 06:05 PM

Very well put!!

#3 GGOSS

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Posted 27 February 2003 - 03:11 AM

Thanks for the comments Marke,

When I get a chance I'll scan the tables referred to in my previous post, and post them here. Hopefully that will make life a little easier for anyone interested in the subject.

Regards,
GGOSS

#4 grobert

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Posted 08 October 2003 - 04:58 PM

The simple explanation is "class 10" will trip within 10 seconds of locked rotor condition. "Class 20" will trip within 20 seconds of locked rotor condition. "Class 10" should be the standard selection. If your application requires longer motor run up time you can use the "class 20".

In a separate but similar matter keep in mind overload selection per NEC is 125% of nameplate FLA (based on 1.15 SF and 40C temp rise). If you try this and your motor still can't start up you can go to a max of 140% of motor nameplate FLA(based on 1.15 SF and 40C temp rise).

#5 marke

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Posted 08 October 2003 - 06:20 PM

Hello grobert

Welcome to the forum.
You also need to bear in mind that the trip time is related to the Locked Rotor Current of the motor. If you have a motor with a high Locked Rotor Current, the trip time will be considerably shorter. For example, if you compare two motors, a) with a LRC of 600% and B) with a LRC of 850%, then the trip time with be will be 1/2 of the trip time with motor a)
The LRC can vary considerably between motors, but will generally lie in the range of 600% to 900% of the rated full load current of the motor.
I believe that the quoted trip time are for a LRC in the 600 - 650% range.

Best regards,

#6 grobert

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Posted 15 October 2003 - 12:36 AM

Hello marke. I am sorry for the over simplistic response. I concur with your reply. That was one of my first replies and I misunderstood the opening thread. I thought it was from a beginer asking differences of OL classes.

#7 GGOSS

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Posted 15 October 2003 - 01:38 AM

Hello Grobert,

Without trying to offend in any way, the statement "If your application requires longer motor run up time you can use the "class 20" in your post of 9/10/03, is a general misconception often resulting in under-protection and subsequent failure of motors.

The very reason I started this thread was to provide an understanding of what 'trip class' actually means thus further assisting people with the selection of motors for the more difficult industrial applications and then the correct selection & set-up of overload relays.

If you believe the original post can be re-written to make it clearer/easier to understand, I would certainly appreciate your input and would be happy to do the necessary work.

Thanks & regards,
GGOSS

#8 grobert

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Posted 18 October 2003 - 05:33 PM

GGOSS
no offense taken.
QUOTE

If your application requires longer motor run up time you can use the "class 20" in your post of 9/10/03, is a general misconception often resulting in under-protection and  

That quote was near verbatim from an explanation on class selection of adjustable overloads from a manufacturers book (Sprecher & Schuh). We are always told, and it is an NEC requirement, to follow manufacturer's documentation. As far as rewriting the original submittal it is apparent to me that you are more than qualified and I can offer no further technical information. I do have to wonder about the statement "resulting in under-protection " The whole point of doing anything other than a setting above 125% of motor nameplate FLA is if you have a situation that has been tried and failed to allow the motor to start up. You would use, for IEC components, class 10 set to the motor nameplate FLA first. Its trip characteristics would allow for the 115% over the face setting. There would be no reason to go further if your motor started up. However if needed, and the HP is not undersized for the work to be done, you are allowed to take steps. NEC allows for a maximum of 140% of nameplate FLA if you have tried the 125 and it would not allow startup. The values I am listing are for motors with a 1.15 service factor and minimum 40 temp rise in Celsius. Obviously you will be giving up some overload protection but you must do what is necessary to allow the motor to start up. The application should be, and I believe required to be, set up per NEC requirements. Upsizing class or OL face settings can only be done when that has been tried and failed to allow full starting. I appreciate the opportunity to discuss these matters with individuals such as yourself.

#9 GGOSS

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Posted 22 October 2003 - 07:35 AM

Hello Grobert,

Great post worthy of considered & detailed response.

Let me start by saying INTERESTING! Having had a direct involvement with the S+S motor protection products since 1982, I am very surprised that THEY would make such a statement in their book. S+S established themselves as a leader in the field of electronic motor protection many years ago ie when they released their CET1 electronic motor protection relay. Now at CET5, the CET range has always offered a motor thermal modeling function, which, in essence is the next level up from adjustable trip class. Devices that offer the motor thermal modeling feature allow you to set a protection curve that most closely resembles the thermal withstand curve of the connected motor. This provides for maximum motor utilization (production) without fear of nuisance tripping or motor burnout.

If it could be put in relative terms, an overload relay providing adjustable trip class may provide curves for say trip classes 10 and 15, but the thermal withstand curve of the connected motor may fall somewhere between the two. If you set the overload to trip class 10, then you are in fact over-protecting the motor and not able to make full use it its thermal capacity. If you set to trip class 15, then you are under-protecting the motor (allowing it to operate beyond its thermal capacity) thus compromising its service life. A product that provides motor thermal modeling may allow you to set for example the equivalent of trip class 12 which as we know is not a valid trip class, but most suitable for the motor in this example.

In your post you make the comment “There would be no reason to go further if your motor started up”. That statement holds true for industrial applications that do not experience repeated transient overloads during normal operation (eg jaw crusher) and/or where restart capability is not a priority (eg long cool down time between starts). Being able to fully utilize the motors thermal capacity is essential in many applications and therefore we need to think beyond the starting function.

The next statement that I find interesting if not frightening is “NEC allows for a maximum of 140% of nameplate FLA if you have tried the 125 and it would not allow startup”. Man...this is scary stuff! The need to do this would not be necessary if one had reviewed the motor thermal curves and selected an overload relay with the correct trip class or installed a protection relat with programmable thermal model in the first instance. Irrespective of the selected curve, the ‘ultimate trip point’ of both would always be less than 1.15 times the face setting and therefore the motor would never be compromised. It would appear some updating of NEC is required!

Grobert, I too appreciate the opportunity to discuss these matters with individuals. Lets face it if we are not either contributing or learning then we're as good as dead!

Regards,
GGOSS

PS: I have developed a powerpoint presentation that greatly assists to further explain the above and highlight the functional differences between overload relays with adjustable trip class and thermal modelling. Will discuss with Marke the possibility of posting a non-downloadable version on LMPhotonics. Not sure if that's possible but he seems to be able to achieve the impossible on occassions.

#10 Raka

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Posted 31 October 2003 - 08:27 AM

I'm interested in this power point presentation, is it available?

#11 GGOSS

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Posted 02 November 2003 - 10:33 PM

Hello Raka,

There is a possibility that a non-downloadable version will be added in future. Can't promise anything at this stage.

I will consult Marke and advise in due course.

Regards,
GGOSS

#12 jraef

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Posted 19 November 2003 - 06:18 AM

Something worth noting in your excellent discussion of OL class issues. The Class number, i.e. 10, 20 30 etc., denotes the tripping time at 600% current, as previously pointed out. What most people don't know however is that that is only the MAXIMUM tripping time. A relay which trips earlier than the desired Class rating can still be labled at that class. For instance if, during testing, a Class 10 relay trips in 6 seconds, then 7 seconds, then 9 seconds, it is considered good as a Class 10 relay. As long as it never takes 11 seconds or more.

We did some extensive testing of both bimetal OLRs and electrnic OLRs from 20+ manufacturers for inclusion in one of our products. We found that most Bimetal relays tripped very early, as in 6-8 seconds, at 600%. Eutectic melting alloy relays were a little more consistant, but presented other problems for us. Most electronic OLRs were much more accurate and repeatable so that is what we ended up using.
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#13 GGOSS

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Posted 19 November 2003 - 11:17 PM

Hello jraef,

You are absolutely correct, up until a few years ago all trip time references were made against an overload current of 6 x the face setting on the overload relay. More recently however that has been changed, the reference current now being 7.2 x FLC, most probably because the Locked Rotor Current of motors these days is much higher than in years gone by.

The problem with that however is that it has added a lot of confusion to what was once quite simple to comprehend. That is, whereas previously trip class 10 was defined as a trip within 10 seconds at 6 x FLC, it is now defined as a trip within 7 seconds at 7.2 x FLC. When you do the calculations however you will find the I2t is pretty much the same and therefore the protection performance is also the same.

That leads me to the second point which is to briefly outline the performance differences between electronic and bi-metalic overload relays. As you quite rightly stated in your post, tests conducted on several bi-metalic overload relays of different manufacture will show that they trip in much less than 10 seconds and that there is no consistency between brands. Multiple tests on a single bi-metalic overload relay will also show an inconsistency in trip time for the same reference or overload current. A difference of +/- 2 seconds between trials is certainly not unheard of, and I guess the overall performance would be very much dependant upon the level of ambient temperature compensation and a number of other design and material tolerance factors.

This inconsistency in tripping time was also very noticeable on earlier electronic overload relays which made us of RC networks to emulate the transfer of heat into the motor windings (copper temperature), the transfer of heat from the windings to the frame (iron temperature) and finally the transfer of heat to the frame to the atmosphere which needless to say is different for both the run and standstill conditions. You can image component tolerances coming into play here generating inconsistencies. You can also imaging that it would be difficult if not impossible to produce an I2t curve using discrete components.

The more recent electronic motor overload relays are digital by design. Their trip curves are accurate and repeatable and follow a fixed I2t characteristic, thus making it possible to calculate the tripping time at any level of overload current, from a cold condition.

For the above reasons, where $ permit, I will always offer/use electronic overload relays over bi-metalic ones. However with the falling price of electronics, I don't think it will be too much longer (3 - 5 years) before bi-metalic relays become obsolete.

Regards,
GGOSS

#14 jraef

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Posted 20 November 2003 - 02:21 AM

GGOSS,
You are right about that. We were disappointed in the lack of flexibility we found available in SSOLs, so we are making our own. Because of the microprocesser power available, our engineers went crazy and put in lots of extra features etc., which makes it more expensive than a bimetal (naturally!). Now we are starting to work on a stripped down version which will rival their current selling price. Knowing what I know about the cost of manufacturing electro-mechanical devices like that, they will probably respond by lowering prices even further, but at least the marketplace benefits. In the long run, the inaccuracies of bimetal OLRs will be relegated to the "how cheap can I get away with and still meet minimum standards" market, which will ALWAYS exist.
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