500hp motor - monitoring of temperature for windings and bearings 15

[electricpete]

We have 6 500HP horizontal sleeve bearing motors which were purchased with winding RTD’s and bearing thermocouples installed in the motor.

These temperature sensors were not connected to any plant computer and are only monitored manually on an infrequent basis. The motors have been in operation for 14 years with no known problems. Bearing vibration is monitored approximately every 3 months and we try to take oil samples annually.

Standard practice in new construction would be to monitor and alarm these temperatures by computer.

Now I am faced with a question. Should we go through the trouble of hooking these temperature sensors to our computer. I would like to see it done but the cost to run cables will be high and I would have to justify the need to spend that money.

** Are there any standards that cover the subject of whether sleeve bearing and winding temperatures should be alarmed for this size motor?

What is the likelihood of a rapidly developing fault that will wipe out the machine before we have a chance to detect and act. (I can think of an easy one… loss of oil due to leak… we have no indication until the machine trips). Any comments on other possible failure scenario’s.

Any other comments about the pro’s and con’s?

Thanks.

 

[jbartos]

Suggestion: Visit

http://engr.oregonstate.edu/~hori/WhiteP1_RG.doc

for: Design Guide for Electric Power Service Systems for Generating Systems
IEEE Standard 666 1991

 

[electricpete]

Thx jbartos - I will get a copy of IEEE666

 

[electricpete]

jbartos' referenced IEEE Standard 666 - "IEEE Design Guide For Electrical Service Systems for Generating Stations" has some relevant stuff:

SECTION 11.11.1.2 "Overtemperature Portection. Overtemperature conditions in motor windings due to moderate overloads should be detected and investiagated......conditions other than persistnetn overloads can cause a motor to overheat... poor ventalation, unbalanced voltages etc.... For large motors the principal means of protecting the stator windings is the RTD. .... can be connected to temperature monitoring devices or relay."

11.11.2.1 "Sleeve Bearing Overtemperature"... Temperatures above normally acceptable values should be annunciated".

The cost benefit arguments are good and most relevant for decision making, but are also subject to criticism due to subjective estimates of probabilities of failures etc. Having a standard on my side will go a long way to help me make my case.

 

[induction]

This may sound like a step back from todays equipment. I would set it up with a meter that would show the readings of each R.T.D. Digital or analog does not matter.in the steel industry I have wittness analog meters that have saved motor or generator with this simple design some with alarms like sirens this were very old. Others just had the meter and you just looked at them. The older motor were built better more lamanations better rotors "copper bars".The only fear I would have is if you developed a leak in your oil system. You could set it up with something to track the oil levels. Some of the simple designs I have seen work the best. They used to make A oil, dirt detection system for D.C. motors. Mounted on the brush hole it had a pair of little metal strips mounted aprt from each other on a piece of glasstic. It had a voltage going to it and when that gap closed it set off a alarm simple but effective. The rtds do this now dirt cloging air flow will cuase heat and set them off. But not oil or other contaminates. If you do go with a computer alert systems make sure you grab at least three rtd`s from the winding one from each phase skiping around the motor evenly then your readings will be better. If a bearing starts to go and your rtd`s are from one side it may not show till damage is done. Good luck

 

[jbartos]

Suggestion: Visit

http://www.maxim-ic.com/appnotes.cfm/appnote_number/270

for:
1. Analog-Signal Data Acquisition in Industrial Automation Systems
2. Integrated Data Acquisition System (DAS) solutions
3. A circuit that provides current to an RTD sensor and digitizes the resulting output
e
4. Etc.

 

[GusD]

Hello electricpete

You have the bearing temperature detectors,by not having the relaying in place to make use of these data is not the best option.Thermal detectors do work most of the time.
I understand your concern about having to spend some money to do this work, on the other hand ,my poor math tells me that one 500 Hp motor costs approx $50Grand,if the bearings wipe you are looking at a repair job of $10Grand.If the bearing fails and the rotor contacts the stator,you could be looking at a new rewind and a possible restacking of either rotor or stator or both.The cost of such a repair is about the same as a new motor.My math did not take in consideration the cost of an unplanned shutdown or restrict production.I am sure you know what the cost would be.
ElectPete,I don't know what motor protector relays you use,most of these relays accept temp inputs.There are reasonable cheap stand alone relays that accept inputs from RTDs ,Tcouples or whatever signals you have .they can be wired to alarm or trip as you wish.
All our motors over 100hp are equiped with RTDs or Tcouples.I also do vibration analyses on all of these motors,and I am very happy that we do have temp detectors that can save us from extremely costly repairs and some embarassement when we don't make the right call.10 years ago or so we went through the same discussions and I am sure that no one would go back to the old way.
If you have PLCs,with TCouple input cards ,they will accept these signals as well.
There are still some people that believe that failures will happen anyway,either due to fast bearing temp rise or any other reason.It is true that any protection can fail.We continue to use Overload relays /overcurrent relays and what have you,even so,sometimes motors do fail.
These devices have to be set properly.If you have journal bearings or any bearings on any particular motor,and these bearing run normally between 50 C and 70 C don't set your alarms at 100 C.I would set'em at 5 or 10 Deg over normal operating temp.A large margin of safety without nuisance trips.
A little history on a 1500hp sleeve bearing motor that I tested 3 months ago.Motor had new seals replaced and when I test run these type of motors,I always wait till bearing temp stabilize,which it did at around 55 DEg C in this case.I tagged motor and sent it to stores as a spare.
During the weekend,maintenance decided to use this same motor.
Mechanics installed motor ,aligned same and coupled it up.
After running for 1/2 hour the DEnd RTD tripped.The oil in the reservoir was quite black.They called me, and after I mention that I had tested this motor with no problems ,I suggested they look for a problem with coupling or alignment.The crew installing the motor, was not familiar with mechanical centers /magnectic centers concerns on these type of motors.They had shved the rotor against one thrust collar and didn't take long to over heat same.
We washed reservoir,replaced with new oil and restarted motor.No problem whatsoever.
My point>If we didn't have RTDs on the Bearings,it would overheat and destroy bearing at a minimum.That was our spare as well.On the final analyses it was a "very cheap RTD,Thermocouple or whatever device you care to use.

Take care

GusD

 

[jbartos]

Suggestion to the previous posting: The overload with their 20% manufactured accuracy do not necessarily trip when they should because of that inaccuracy.
Also, some RTDs applied to motors annunciate only. They do not trip.

 

[GusD]

JBartos
we can use the annunciation for whatever purpose we want.
We use mostly Multilins ,PLCs or simple stand alone RTD or TC Relays. All the Multilins ,PLCs or RElays come with integral relays or output contacts that can be used for whatever function you want.Normally we set bearing alarms at 80 Deg C and Trips at 90 DegC.Believe me they do trip!
If you only want to annunciate they will do that too.
JBartos ,in any case,even if the only thing I could get is annunciation it would still be better than nothing at all.

GusD

 

[induction]

Pete the bigest thng is there are standards I call them bonderies. Make sure whatever you do you try to taylor them to your in inviorment.

 

[jbartos]

Suggestion to GusD (Electrical) Apr 27, 2003 marked ///\\we can use the annunciation for whatever purpose we want.
///True.\\We use mostly Multilins ,PLCs or simple stand alone RTD or TC Relays. All the Multilins ,PLCs or RElays come with integral relays or output contacts that can be used for whatever function you want.
///True.\\Normally we set bearing alarms at 80 Deg C and Trips at 90 DegC.
///True.\\Believe me they do trip!
///Never disputed. That is why I indicated that some RTDs annunciate only. This is convenient to know if the temperature stabilizes a little above 80C for the mentioned setting above. It can also drop below 80C and there is no problem. However, if there is some catastrophic failure, temperature will climb quickly from 80C to 90C. There may or may not be much what can be done about it.\\

 

[BJC]

Electricpete
measuring the winding temperature with an RTD is to late.
I would recommend reading this.

http://www.selinc.com/techpprs/6024.pdf

The SEL 701 and 749M relays have a pretty good model for measuring heat in a motor by intergrating the I squared T over time. It's different than that on the Multins and some other relays I looked at.
It has inputs for bearing and stator RTDs but I think measuring the heat in the motor is better.
The relay has an output that can run a meter as well as control. I have an application where I want to install a "temperatrue gague" on an opperators panel. The analogy to driving a car fits pretty close. In this case the operator controls the process. He can decide to feed more stock into the crusher which will load the motor more. If he sees the motor is hot he can slack off a little. It's just like driving your car up a hill, when you see the temperarue rise you slow down or shift gears.
The relays have an input for an ambient temperature RTD and adjust the model as the temperature.

I am not pimping for SEL. It just happens this relay was a perfect solution we just ran into.

 

[electricpete]

Thx all.

For my particular situation, I am not looking to upgrade the sensitivity/protection for tripping funcitons. These are safety-function motors and the fail-safe situation is to not trip.

As far as modeling motor temperature, I'm sure those guys at SEL are very smart, but is their model supposed to be better than reality? RTD measures actual temperature after all...not a prediction. But it's a peripheral issue that doesn't seem to affect my present situation.

I believe that for most situations of this large a motor it would be a no-brainer as Gus has suggested that existing bearing and motor temperature devices should be monitored (alarm and indication). However the switchgear and computer are both a large distance away up to 300' and 3 floors. Also this is happens to be a facility where the cost of even simple modications is very large. Off the top of my head I believe that for all 6 motors to simply connect these to process the paperwork and complete the connection to our plant computer system would cost somewhere from $20,000-$50,000.

 

[electricpete]

I would like to suggest to draw this thread to a close.

Thanks to all for a lot of good advice. I think I have what I need to make my case. The most valuable piece of new information I gained to help me make my case was the industry standard IEEE666 (thx jbartos).