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.

 

[electricpete]

I should mention that the machine is critical in the sense that it has a safety function which must be performed reliably if needed (very infrequent).

The vast majority of time we operate 4 out of 6 machines and it would be no problem at all to shut down a machine for repairs IF we know there is a problem.

 

[SolDirJoe]

Are these motors used on dynamometers ? If so, inbalance of the coupling and lack of lubrication at critical speed could lead to destructive failure in seconds.

Are the motors AC or DC machines and what is the voltage ?

Are they variable speed ? If so how is speed varied [eg DC Thyristor Convertor, or AC Invertor ?

How is the load coupled to the Motor Shaft? - via gearbox, direct coupling, fluid, or torque limiting coupling etc.

What is the load type ?[continuous Torque, Fan Law, or cyclical] - All have a bearing on the type and severity of a potential failure.

Whatever the distance from Motors to computer, EMI and RFI could trigger false alarms so suggest you install signal conditioners as close to the motors as possible. Conditionaers to have 4 to 20mA output driving twisted pair screened cable.

Sorry for all the questions, hope this helps. SolDirJoe

 

[VWedelich]

You could monitor them, but if the bearings go bad they will avalanche, it is not like you will have time to do anything about it. The bearings will burn up and then you will replace them. If you are interested in at least taking the unit off line and minimizing shaft damage, then install a trip unit and trip the main breaker if they hit a certain temperature, this might still be to late. An accurate written record usually will give you all the information you need.

 

[electricpete]

SolDirJoe - These are 4kv horizontal 500hp sleeve bearing motors direct coupled to chiller compressors. No varirable speed drive, no electro magnetic interference concerns, no dyno's. I don't see coupling imbalance as an issue.

Thanks vw, that's good info. Do others have the same experience as VWedelich that an alarm of bearing temperature (or increasing trend) rarely serves a useful purpose? i.e. failures occur so fast there is not time to do anything anyway?

 

[automatic2]

I would tend to monitor your chill system more than your motor. Although motor current is a great story. Your unload cycle, temp differentials on your chill sys give you some idea of loads, efficiencies and loaming problems. Power in equals power out. I well balanced system has great durability. A system that swings or has hard loading will tire quickly.

 

[SolDirJoe]

For this type of application, differential temperature measurement can be useful.
[NB. Absolute overtemp alarms, if set to a point which gives adequate warning or alarm trip, are subject to false alarms due to excessive change in ambient temp sudden increase in demand, or load duty cycle]

Measuring bearing housing as well as bearing temperature and configuring the signals in the form of a Wheatstone Bridge compensates for environmental changes to both components, and comparing "change" with the alarm setting gives a much larger discrimination against 'false' or 'too late' signal levles. eg. 0 to 10 V = alarm signal level versus 0 to 10 Volts = actual temp + overtemp level.

Thats a poor explanation, but I hope the point stands out.

RTC's in the windings are set to trip at levles dependant upon the Class of Insulation of the copper windings [eg. Class B, Class E etc] They are reliable and have a very low slope at temperatures below the 'chosen' temperature, at which point the resistance increases almost vertically with further increase in temp. In the absence of current forcing Electronic controllers, they should be used mainly for trip protection. [ie. dirt, iol laden dust in cooling ducts and slots, poor ventillation etc etc.

If the windings overheat due to mechanical overload [such as the onset of bearing failure], then it is highly likely the sleeve bearings would would collapse before reaching RTC trip resistance.

I guess best thing is to flip the problem on its head. Determine the budget available versus risk of failure of a motor. Get quote for embedded thermocuples and differential amplifier with alarm output. If the cost falls within budget, then its cheap insurance.

Hope this help[s

SoldirJoe

 

[jbartos]

Suggestion: Visit

http://www.oilanalysis.com/secure/product_detail.asp?catalogid=21

for a book covering bearing analysis and predictive maintenance

http://www.coxmoor.com/images/pdfs/infrared.pdf

includes a graph that shows the bearing insipient thermal deterioration

 

[vanstoja]

What are the sleeve bearing materials , what coolant is employed and what are the running speeds and the bearing specific loading in psi? If you've got a impregnated carbon-graphite soft bearing material and speeds and specific loads are low enough (say 900 RPM and 18 psi), the bearings may be able to run dry for prolonged time periods. If not, then you may not be able to withstand loss of coolant to the bearings. For loss of coolant concerns, the bearing thermocouples might better be monitoring the outlet coolant temperature than imbedded in the wall of the bearing housing. Winding RTD's are no good for fast failure modes like locked rotor since they won't respond quick enough to take any corrective action. Winding RTD's should be set for alarming at the insulation temperature rating so as to warn of ongoing slow-rate deterioration. Frequently, there is a thermal loss through winding to RTD connections requiring an indicated alarm setting that may be substantially lower than the winding insulation rating temperature. If you've been monitoring both transducers periodically over a long period of time without incident then I'd suggest leave "well enough" alone.

 

[TheDOG]

As vanstoja points out in the above post, RTD's used by themselves do not provide adequate protection of motor windings. When used in conjuction with a motor overload, the RTD's certainly have merit. The main advantage is that the RTD's are embedded in the windings and are therefore affected by high ambient temperatures and a loss of cooling (which the overload is not), which will increase the winding temperature.
They are quite common in applications outside of North America.

 

[electricpete]

Thanks for the good comments.

These are oil bath (not forced oil) sleeve bearings. I don't know the loading. Is that rotor weight divided by area of the lower half?

I don't intend to provide any automatic temperature trips. These motors have a safety function, which must be performed when called upon. (auto trip could jeapardize that). The alarm would allow manual intervention under conditions when it is not imperative that machine operation continue.

 

[THooper]

Good input from all, Here is my 2cents, If you make temperature measurements on any motor equipped with embedded resistance detectors, make sure you know how to read the output from these useful sensors. Whereas the thermocouple reads at a single point, and the change of resistance method responds to the entire winding, the RTD sees temperature change averaged over a length of core slot, usually 10 to 20 inches.

First find out which basic type of RTD is involved. The two most common versions are:
The 10 ohm using a copper wire resistance element. It measures exactly 10 ohms at a reference temp of 25 C (77F)

The 120 ohm element, using nickel wire. Its resistance is 120 ohms at oC (32F)

To get detailed calibration tables for them, request tables
16-9 and 120-7 from MINCO Products, Inc.

Make it a habit to measure the ristance of each RTD in the motor carefully at ambient temp, with the motor cold. Don't use any of the RTD having a cold resistance that doesn't closely match the calibration table value for that ambient. The great advantage of RTDs they permit continuous temp monitoring.

Here are some considerations affecting the choice of control circuit settings:

How critical is process continuity in the plant? It has long been recognized throughout the motor industry that most machines can be overloaded for a short period not exceeding SF,

Alarm and shutdown temp too close together can lead to unexpected shutdown without prior warning. The motor mfg knows that thermostats, for example, may operate at any temp from 5C below to 5C above their nominal setting. But only the user knows what tolerances may apply to his monitoring equipment in an RTD circuit.

Will the motor be subject to suddenly applied overloads, or are they only likely to come gradually, If a slowly rising winding temp is more probable, a fairly late alarm will still provide time for corrective action. But a suddenly applied overload will result in a high rate of increase in winding temp, so circuits should trip sooner.

Can the motor be unloaded by closing a valves, switching to backup equipment, and so forth? That also would justify setting a fairly high alarm setting, because trouble can be compensated in a hurry. But if unloading is lengthy or difficult, then a lower alarm setting will provide an earlier warning.

NEMA standards for 1500hp and below assume that RTD rise for class B or F insulated machines will be 10C above the resistance rise. For class H machine, the standard difference is 15C instead of 10C.

High cost monitoring equipment may be hard to justify, but RTDs are the most reliable hot spot temperature sensors available.

Kind Regards,
motorhead

 

[jraef]

electricpete
If you already have controllers and protection for these motors, here is a stand alone RTD monitor that is low cost and simple to implement. I have used them several times in the past and have been satisfied with performance and reliability so far. They use a mux on the inputs and so are not quite as fast acting as the traditional Multilin RTD input option, but I too question the responsiveness with regards to bearing failures anyway. As mentioned by others though, the winding temperature can be very usefull. Since you already have the sensors imbeedded anyway, this would be a simple way to get information into a computer system for tranding etc. w/o having to wire each RTD individually.

http://www.springercontrols.com/page4.html

Quando Omni Flunkus Moritati

 

[snpyon]

My suggestion after reading through all the messages here, and most are fine recommendation, but i see we fail to answer the question you originally ask,are there standards?

Yes and no.
But to make this a short but effective reply, i would like to suggest this.
Any type of sensor of critical components, (bearings, temperature, trip, lube oil loss, air ventilation loss), is a useful tool in preventing Downtime, failures, and maximizing in scheduling Preventative maintenance to these motors, it has been my experience with northfield chilling system similar to what you have, all the sensors from the pressure, loaming, temperatures (feeds and return) ammonia detections, water pressure,temp,ETC) it all can assist you to predict the behaviour of your equipment, the standard is this:
All sensors are tied to a central computer a/d interface board and the software interprets the values to a usable variable that the user can understand, at any given time the input from the keyboard can map your sensors layout, and the health of the system, (that's the previous technology), the new technology utilizes wireless transmitters attached to the addressable modules on the the sensors wherever they may be, this information is transmitted back to the computer which will display the information to you in regard to the health of the equipment, in my plant, we had cable wunning everywhere for each sensors, something to the order of 4000 inputs ranging from signals from transducers, to power line "doughnuts" current clamps, to temperature of the incoming water and pressure, to bearing temperature, lube oil pumps, pressure , temp, you get the picture, and we under took a monumental task to add in more sensors that were not being utilized, and our Downtime decreased, our efficiency increased by 98.7 percet ( a hard number to maintain in a harsh environment that we produce in), and our overhead initially was high, but the overhead came down when we notice the components we normally order to make repairs, suddenly decreased and actually put money back into our budget in the maintenance dept.
But to answer your question, yes it would be advisable to use these sensors, and to chart a history on these motors for future planning or scheduling of PM, this is the only way to reduce the cost of maintenance, cost of ownership, and the cost of downtime, to give you an example, one machine we use, for every 1 hour of downtime, it cost the company on the order of 5010.00/hr, and if a system could have been seen earlier, and a history is known, we would have known that the bearing was about to fail, and the pm crew could have had it replaced on a scheduled base, but since it failed out right with no knowledge of the impending failure, we had to resort to ordering the part special delivery, (thousand of dollars), pull 3 12 hour shift to tear down the affected equipment, (overtime) and reassemble, then restart .. so do the math, 5000. times 36 hours.. 180,000 dollars.... then theres the risk that the director of operation will want to know if this could have been prevented.. yes it could have been prrevented... the information is there, the technology is there, and so are the engineers..

 

[jsummerfield]

Many 5 kV motors and related switchgear include something like a GE Multilin multifunction protection relays. Other manufactures exist.

 

[d23]

electricpete

14 year old motors without any known problems mean someone made an excellent design. The problem is that your motors are like me they are not getting any younger. Having to replace all six motors over a one or two year period would be very expensive as compared to intervention / repair cost. I would tend to run wires or install a memory device that could be downloaded manually. Tracking changes and improving control will eventually save you money and headaches.

Actually on a forum named “ENG-TIPS” is there ever such a thing as having too much data?

Good Luck!

 

[GusD]

If you have Winding RTDs and Bearing T/couples in your motors,it would make sense to put them to work.
Winding RTDs may not save your windings,but it will alert you to a motor that may be Overheating for whatever reason.As for the bearings thermocouples,it may very well save your motor from catastrophic failure.(Rotor/Stator wipeout)
For about 10 years we have used bearing RTDs or TCs on all our motor bearings and windings on all motors over 250 hp `We have over 100,000 hp of connected drives,from 5 hp to 15,000 hp.I would not operate a motor without RTDs/TCs on the bearings.
A simple RTD/TC relay can be purchase to alarm/trip your drive when temperature rises abnormally.We have Multilins,
motor protector relays,which accept 10 or more RTD inputs.
The expense of an RTD/TC relay will pay for itself in a very short time. GusD

 

[electricpete]

Thanks, all good comments. Does anyone know of any standards addressing this?

 

[Kawartha]

electricpete,

Sorry, can't answer your standards question.

But I just wanted to add a little confirmation to your original comments when you initiated this thread. We recently designed and installed several 1250 hp, 3600 rpm, 4,000 volt induction motors driving intgral geared, high speed blowers (about 12,000 rpm). The units have been in operation for several months.

Last week one of the motors alarmed on the Non Drive End RTD, and the unit was shut down. Although this failure is related to a workmanship issue, the RTD's saved this motor from far more serious damage. The motors are also sleeve (babbit) bearings with "ring-oil" lubrication. There was a failure of the inner labyrinth seal which allowed the oil from the bearing reservoir to leak into the motor stator.

Just a reminder that things do go wrong.

 

[GusD]

HI electricpete.

I know of no standards that regulate the use of Winding or bearing RTDs.or Thermocouples.
The Alarm levels or trip levels we use are mostly Industry accepted values .Your motor winding class (B/F/H) would determine what alarm/trip level to use (eg 130/145 C).
As for motor bearings we use 80 C for alarm and 90 C to trip.This values are OK for most of your motors with Roller/Ball bearings.On our Journal (Babbitt) bearings we use a lower level of 70 C Alarm and 80 C trip .

GusD