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Motor Protection


bob

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Hi,

 

We have on site one 250 kW sip ring motor 400 V 750 rpm driving a rock crusher. The motor is protected by a smart motor manager , the Rockwell CET4 relay. The overcurrent setting is set at 2.2 Ie for 1s and the relay is provided with other features such as overload assymetry etc. The thermal utilisation on this motor is around 40-50 %. But it happened, almost everyday, that we experienced peak for a very short period of time and the motor tripped on overcurrent. We never had a thermal trip on this motor.

These spurious trippings are affecting production and I am thinking of using the thermal image of the motor to trip it instead of the overcurrent one. I suggest to increase the setting of the overcurrent trip to 3 s. Could it be damaging to the motor ? I believe a motor should be able to withstand more than two times rated current for 3 s.

Any opinion ?

 

Bob

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Dear bob,

I believe a motor should be able to withstand more than two times rated current for 3 s.

Sure, if the motor is in good conditions.

 

What about power quality in that plant? Do you see some overvoltages?

Trips happens about in the same hour of the day, or randomly?

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hello Bob

 

I see no problem with lengthening and or increasing the overcurrent setting.

The major issue is the thermal setting and that is a long way away from the trip point. The overcurrent setting is primarily for load protection and indicates that the motor is heading towards a stall.

In crusher applications, it is common for the current to go well above 200% for short periods of time.

I would be setting it up around 300% for a couple of seconds.

 

Best regards,

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Hello Bob,

 

In addition to the suggestions made my marke, I would just like to say that the motor will still be adequately protected through the 'thermal model' overload protection provided by the CET4. That is, every time one of these overcurrent transients occur, the thermal model increases in value. Should the transients occur repeatedly, the CET4 would eventually trip on its overload function thereby protecting the motor. Therefore if the values for Locked Rotor Current and Locked Rotor Time have been entered correctly into the CET4, you should have absolutely nothing to worry about.

 

Hope that extra little bit of re-assurance helps.

 

Regards,

GGOSS

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Hi Marke and GGoss,

 

Thanks for your useful advice.The LRC is set at 6Ie and the LRT is set at 10 s. I have set the overcurrent trip to 3Ie for 3 s.

I am also thinking of setting the overcurrent trip at 1.5Ie but for a longer period, say 10 s.

Would it be detrimental to the motor?

 

Bob

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Hi bob,

if you calculate the I2t, you will found:

 

Actual I2t: 3 A x 3 A x 3 s = 27 A2s

Future I2t: 1.5 A x 1.5 A x 10 s = 22,5 A2s

 

22,5 < 27 , the motor will work in a safer mode.

 

Obviously, you cannot ask 5 overloads of 10 seconds each minute! There is the need to have enough cooling time between an overload and the next one.

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hello Bob,

 

I know that CET4 very well having worked with int and all forerunner products (CET1, CET2 & CET3) over the last 20 plus years. It would appear values for LRC and LRTime currently programmed into the CET4 have not been changed from factory default. If you are able to get actual values from the motor manufacturer you may in fact find that you are currently over-protecting the motor ie tripping earlier than you need to be.

 

The idea behid matching values of LRC and LRTime enetered into the CET4 is to fine tune the protection thereby enabling maximum productivity from the motor without fear of damage.

 

The calculations provided by mario are also very useful and should allow you to calculate the tripping time at any level of overload current. Bear in mind however that there are based on cold condition only.

 

Regards,

GGOSS

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Hi GGoss,

 

Thank you for your concern and time. You are right, the actual values for LRC and LRT are default values. The motor is a very old one > 25 yrs and no data is available on site. The new settings should work better.

I shall keep you all informed how the new settings work.

 

Bob

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  • 2 weeks later...

Hi Bob,

 

There are some excellent comments here to which I would like to add my opinion.

 

In order to give the best balance between protection and functionality a motor protection relay needs to have the thermal model set correctly. This means finding the correct settings for locked rotor time and current. This information should be on the motor name plate. Given that the LRC and LRT on the CET4 default at 6 times and 10 s the CET4 is set to protect a class 10 motor. If the motor in your application is indeed class 10 then the thermal model is accurately set and increasing the instantaneous overcurrent should be fine. If the motor is not a class 10 then it is either over or under protected by the class 10 settings. I suggest checking the motor name plate for information on LRC, LRT or motor class, Google might also help if there is no information on the name plate. I would ensure that the thermal model is properly set before turning to the instantaneous overcurrent settings.

 

 

I have set the overcurrent trip to 3Ie for 3 s. I am also thinking of setting the overcurrent trip at 1.5Ie but for a longer period, say 10 s.

 

 

I would go in the opposite direction here. Once the thermal model has been set it will track smaller overcurrents like 1.5 Ie for 10 s. I would set the instantaneous overcurrent to 8 or 10 Ie with no time delay. If you look at the default settings of other motor protection relays in industry you will find that 10 is a fairly common default setting.

 

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Hello Don S,

 

There were a couple of items mentioned in your post that I do not follow completely. I have therefore listed them below for your consideration/comment.

 

I do not agree that Class 10 overload protection is the same as setting LRC at 6 x FLC and Maximum LR time at 10 seconds. In the case of CET4 at the settings stated the trip time will be 10 seconds at 6 x Ie however with a bi-metalic overload relay or an electronic overload relay, the characteristics of a class 10 are based on a In of 7.2 x Ie, and a trip time of anywhere between 4 and 10 seconds from cold condition.

 

The second thing that caught my attention was your recommendation for setting of the instantaneous overcurrent trip. A setting of 8 to 10 x Ie as you have proposed is in most cases well above the LRC of the motor and therefore that level of current would not be realised unless a short circuit developed in the motor. The correct setting for instantaneous overcurrent should be based on limiting run/stall motor torque to safe levels for the driven machine and/or the coupling between the motor and driven machine.

 

As indicated in my opening sentence, I would certainly like to hear your comments on the above.

 

Regards,

GGOSS

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Hello GGOSS,

 

Thank you for the opportunity to discuss these points. Perhaps I did not make myself clear; it is also possible that I am in error. A third possibility is that we are actually in agreement about the theory of motor protection but using motor protection terms differently.

 

I do not agree that Class 10 overload protection is the same as setting LRC at 6 x FLC and Maximum LR time at 10 seconds. In the case of CET4 at the settings stated the trip time will be 10 seconds at 6 x Ie however with a bi-metalic overload relay or an electronic overload relay, the characteristics of a class 10 based on a In of 7.2 x Ie, and a trip time of anywhere between 4 and 10 seconds from cold condition.

 

Concerning your first point; my understanding of numerical motor classes is that they are defined by the time to trip (when the motor is cold) at an overcurrent of 600% FLA, therefore 10 s and 600% is a class 10 motor. Do we have the same definition of numerical motor classes? Obviously we would want the time to trip to be reduced if a portion of the motor’s thermal capacity had already been used.

 

quote name='GGOSS' date='Jul 25 2006, 05:42 PM' post='4039']

The second thing that caught my attention was your recommendation for setting of the instantaneous overcurrent trip. A setting of 8 to 10 x Ie as you have proposed is in most cases well above the LRC of the motor and therefore that level of current would not be realised unless a short circuit developed in the motor. The correct setting for instantaneous overcurrenbt should be based on limiting run/stall motor torque to safe levels for the driven machine and/or the coupling between the motor and driven machine.

 

Concerning your second point; I think we might be using the term “instantaneous overcurrent” differently. The company that I work for designs and manufactures motor protection relays so I tend to think about motor protection terms in the way that we use them, perhaps this is an oversight on my part.

 

The way I think about motor protection is that in a motor that is operating normally, i.e. no fault conditions exist, the relay’s thermal model would adequately protect the motor. Protective relay settings, in addition to the thermal overload, are there to cover abnormal conditions.

 

I would use “instantaneous overcurrent” to describe a condition like a phase-to-phase fault. I would use the instantaneous overcurrent set point to protect against a phase-to-phase fault.

 

I would use the term “short time overcurrent” or “jam” to describe an overcurrent of 2 to 3 x FLA. It seems to me that what you would call an instantaneous overcurrent I would call short time overcurrent or a jam. I know that the CET5 has a jam setting, does the CET4?

 

I feel that in, an ideal situation, a trip on any protective relay would provide the user with as much information about what happened as possible. An overcurrent of 8 to 10 times FLA would trip as an overcurrent, a ground fault would trip as a ground fault, and a jam would trip as a jam.

 

Again thanks for the opportunity to discuss this, please let me know what your thoughts are.

 

Best regards,

 

Don S

 

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Hello Don S,

 

I don’t think there are any right or wrong answers here, just a difference in understanding and possibly in terminology as you say.

 

Trip Class vs Thermal Model

 

In recent years there has been a change (and improvement) in the way that numerical trip classes are defined. According to IEC 60947, the characteristics of an overload relay offering Class 10 protection is that it will trip in no less than 4 seconds and no more than 10 seconds at 7.2 x FLC from the cold condition.

 

The easiest way to illustrate the difference between overload trip class 10 and a CET4 thermal model setting of 6 x FLC, 10 seconds is through a simple I2t example.

 

Trip Class Protection

7.2 x 7.2 x 4 = 207.36 amp squared seconds (earliest trip condition)

7.2 x 7.2 x 10 = 514.4 amp squared seconds (latest trip condition)

 

CET4 / Thermal Model Protection

6.0 x 6.0 x 10 = 360 amp squared seconds

 

To determine max cold tripping time for a class 10 overload at 6 x FLC, a reverse calculation can be made.

 

514.4 / 6 / 6 = 14.27 seconds.

 

It is therefore fair to say that a CET4 setting of 6 x FLC, 10 seconds ‘fits’ within the IEC specified range for Class 10 protection devices, but as you can see from the above they are clearly not the same. In case of the CET4, you are working with a thermal model and have a defined & fixed I2t, in the case of a bi-metalic or electronic overload relay offering trip class protection you have an I2t which can vary between 207 and 514 amp squared seconds.

 

Instantaneous Overcurrent / Ground Fault Protection.

 

I agree with your comment that providing the user with as much information about what has happened is important, however the name of the game here is motor protection not alarm annunciation!

 

If a phase to phase short or ground fault develops, the damage is already done! Under these conditions the current drawn from the supply will rise very rapidly and so it doesn’t matter all that much whether you have the trip point set at 8 x FLC, 10 x FLC or even 15 x FLC. Some would argue that instantaneous overcurrent functionality can be used to limit the damage to the motor under these conditions, and whilst I agree with that in part I firmly believe response time is far more important.

 

As I have always believed it is the responsibility of the circuit breaker to isolate power to the motor under these conditions, I have used the instantaneous overcurrent functionality to protect the coupling & load (not the motor) against excess torque conditions particularly in those cases where stall and jam protection are not offered.

 

Different manufacturers will use different terminology for these type of functions. Overcurrent protection may be referred to a stall protection, excess start time protection may be referred to a jam protection etc etc and so for a maintenance electrician who may not be overly familiar with the product in front of him/her fault anunciation is not always sufficient without documentation that clearly describes functionality. NB; The following is a general comment and is not pointed at anyone here. In my opinion if a circuit breaker trip is not sufficient to provide the user with a clear indication that the motor and/or power cabling has suffered a major fault, then they probably shouldn't be involved in troubleshooting/fault rectification processes. I get the feeling that manufacturers are being driven to increase the amount of information provided to compensate for a decreased level of knowledge/experience amongst people that may find themselves in front of a product.

 

CET5

 

Adding 'Bells & Whistles' is great however it is more important for a product to perform its core functionality well. I will therefore refrain from making any more comment on the CET5 until the manufacturer (not Sprecher + Schuh or Rockwell Automation) get that part right.

 

Regards,

GGOSS

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