Function of stator winding RTD in a motor 7

[krisys]

The motor in question is 11kV, 3,300 kW Ex”d” 3-phase, squirrel cage induction motor. It has stator winding RTDs. Two RTDs per phase, thus total six (6) RTDs. Now one of the RTDs (say RTD#4) is found to be faulty. Presently the RTDs are connected to RTD module of the motor protection relay. This motor winding temperature is communicated to DCS and temperature is logged in the DCS. The trend can be plotted in graph.

When the RTD measured temperature reaches a set limit there will be an alarm. If the temperature exceeds higher value, the motor will trip instantaneously.

Now we have to disable RTD#4 (faulty) to run the motor. I want to know, what is the risk arising by keeping one of the RTDs (RTD#4) disabled. Do we need to replace the motor or rectify the faulty RTD to make it fit for running or it can be in service in this condition for the prolonged duration.

The following are my own views:
[ol ]
[li]RTD is basically a condition monitoring device. It detects the sample temperature of a winding. It cannot detect the hot spot temperature.[/li]

[li]One RTD per phase is technically sufficient to detect the winding temperature. So the second RTD in the same phase can be considered as spare. So as long as one RTD is available per phase, there is no problem.[/li]

[li]The motor temperature rise has a time dalay to develop. Hence in order to avoid any false tripping, a time delay, (say 5 to 10 minutes) can be given to the RTD trip. This will help the operator to take the necessary precautions before the motor trips.[/li]

[li]The motor trip by RTD temperature, is only a back up for the motor protection through the current based protection.[/li]

[li]Conventionally the motor RTD temperature was a good guidance for the operator to control the load. This was significant, when the motor was sized close to the load (no design margin). Hence controlling the load is important (specifically during the peak summer) to adjust the load to limit the motor temperature based on the ambient temperature.[/li]
[/ol]

Is there any problem, if we provide a time delay for the motor tripping? What can be the time delay? Is the time delay of say 5 minutes, will be risky?

Alternatively, can we keep only the alarm and disable the trip?

 

[edison123]

pete - Not sure about the instrumentation wiring grounding. But I have often measured RTD resistances right on their termination board while the machine is running. And I am still here.
I know you are dictionary of various tech studies and articles. Do you have any article / study on induced voltages on RTD's ? Regardless of machine voltage, the voltage stress is fairly constant at around 2.5 KV/mm across insulation.

Scotty - My point was up to 5 KV, there is no stress grading tapes/paints applied. So the coils/bars outer surface is not 'grounded'. Embedded RTD's in such machines are working fine and safe.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Got you - no problem.

 

[electricpete]

edison - Yes, you convinced me there is not a hazard in terms of induced voltage, even if the instrument circuit is ungrounded. First by analogy to lower voltage machines which you pointed out. That got me thinking and I can easily convince myself of the same thing:

We could estimate the floating 60hz voltage of the leads by capacitive impedance voltage divider using estimated capacitance to RTD-to-ground and estimated capacitance RTD-to-coil as
Vrtd = Vlg * Cc/(Cc+Cg)

If you measured that voltage with RTD determinated at junction box and motor running, I wouldn't be surprised to see a non-negligible fraction of the Vlg when measureing the RTD voltage to ground using a high impedance volt meter. But even if you measured that voltage and came up with something like 1kv, it still comes in series through such a high impedance (small capacitance Cc) that it probably couldn't deliver enough current to be dangerous to people.

If we measured that voltage with RTD terminated to plant wiring, it would probably be far lower (even assuming ungrounded instrumentation circuit) because the wires run adjacent to steel conduit or shield wire the entire way increasing the capacitance to ground.

I may be facing this decision (rewind or install extra RTD outside the slot) for one of our 13.2kv motors that has 5 out of 6 RTD circuits open-circuited! We have plans to pull the motor and send it out. It may be that the shop can fix the wiring to restore those 5 RTD's once they get access, but if not then I'll have some decisions to make. I'm still not exactly comfortable with the idea of putting rtd on the endwinding. thanks to you, I'm no longer concerned about hazard from voltages induced into the circuit. but I'm still amconcerned about effect on voltage stress distributions and potentially causing partial discharge or winding failure. I'm also thinking about just putting one or more extra RTD's directly on the core end (finger plate) to measure iron temperature. that may not be a perfect indication, but at least I know it won't do any harm. And at least I can see how it compares to that one remaining slot rtd to build my own fudge factor.

=====================================
(2B)+(2B)' ?

 

[edison123]

pete - 5 out of 6 RTD's failing indicates fairly old winding on its deathbed? If yes, then I suggest preemptive rewind to ensure another 25 years of winding life. And you get to put in as as many RTD's as you like.

If rewind is not an option, then I recommend RTD's under the slot wedge, which is closer to the original in-between top and bottom layers location so that the 'fudge' factor is reduced. Even if the winding is global VPI, removing a few wedges is not problem if properly done.

End windings are better cooled (I consider them as heat sinks) due to the flow of more cooling medium over them and no iron loss contribution to their temperature. Hence, end windings are not a great place to monitor winding temps.

Muthu

http://www.edison.co.in

 

[electricpete]

Thanks Muthu.

It is an epoxy global VPI winding about 15 years old from a shop we use routinely. I don't think the winding is in bad condition, I suspect (and hope) that it is the RTD wiring deteriorating for one reason or another.

If for some reason we do recognize a wiring problem but can't fix it (maybe too close to slot or within the slot) I'm still thinking about options.

Pulling the wedges out of the VPI winding sounds difficult to my thinking in this global vpi winding.

It sounds like you consider the core end a better location than the endwinding. I think that will be my plan if needed (install at core end).



=====================================
(2B)+(2B)' ?

 

[krisys]

Excellent points have come out during the discussion. Below points are what I have to say at this stage.

Site safety regulation will not permit to do any experiments and not allow to check the RTD when the motor is running.

Regarding the shielding of RTD:
Grounding (earthing) of the shield is required for the following reasons.
a) To protect the measuring circuits of RTD modules. As the surge energy is low, it may not be harmful to the personnel, but can damage the circuit components.
b) Many places, the RTD trip setting may not have any intentional time delay. In such cases, there will be nuisance tripping of the motor due to the RTD surge.
c) EMC compliance needs the shield to be grounded, fir the safety reasons.

In view of the high voltage gradient in the end winding, the non-embedded RTD shall be placed with due diligence. May be placed preferably close to star point.

Update on the subject motor:
The motor is about 24 tonnes, in explosion proof (Ex”d”) enclosure. Installed in an offshore platform. Hence taking the motor out to rectify RTD issue is not an option.
Vendor has endorsed to operate the motor with three (3) RTDs out of service condition. But suggested some additional safeguards like,
a) Frequent monitoring while running
b) Monitoring the cooling air temperature
c) Reducing the alarm settings of the remaining healthy RTDs for early warning

As the motor is new (only two years old), and the insulation class-F, but the temperature rise limited to Class-B insulation. Hence the margin of safety is very high.

 

[dArsonval]

Explosion Proof Enclosure changes the dynamics of the discussion.
Would have been helpful to know this at the very beginning.

John

 

[edison123]

dArsonval

Check the very first line of the OP's first post.

How does Ex classification change the dynamics of the discussions?

Muthu

http://www.edison.co.in

 

[krisys]

Muthu,
Any input/view on the proposed strategy?

Any value addition on the above strategy by the esteemed panelists is welcome.

 

[dArsonval]

Granted, re-reading the original post, a term labeled... Ex"d" is used.

I missed it.

That abbreviation is nowhere to be found in

FOLLOW-UP SERVICE PROCEDURE issued by Underwriters Laboratories Inc.

for Motors and Generators, Rebuilt for Use in Hazardous Locations.

Acknowledging the forum is on a world stage here...

So, I'll plead guilty and recognize it didn't mean anything to me.

It changes the dynamics of the discussion because the motor left
the manufacturer with all its "innards" and now it doesn't have them.

Information on the insulation system materials should show that all materials
used in the item have at least the specified hot spot temperature rating...
and there is simply no way anyone sitting at a keyboard reading this forum
could authenticate this or all of the many other details required for such a motor.

If it were just a regular motor, I'd stand by my original comment on the failed RTD.

Now that I understand it to be an "Explosion Proof" motor...
it changes things to something outside the scope of this forum.

We've talked about Explosion Proof motors here before. They are a different animal.

A Competent Re-builder should be consulted.

John

 

[edison123]

krisys

The idea behind embedded winding RTD's is 24x7 monitoring (via a temperature controller or DCS), not just frequent monitoring.

I don't see how the other solutions (b & c) given by the vendor would compensate for the loss of RTD's, which I might add have failed prematurely. Did you ask the vendor why the RTD's failed in just two years?

Muthu

http://www.edison.co.in

 

[ScottyUK]

You will need to find the ATEX certificate for this motor. If it has an 'X' as the final character then there are special conditions for use over and above those which apply to all flameproof installations, which may include the use of the RTD's. It's commonplace for embedded temperature sensors to be a requirement for submersible Ex 'd' pump motors, perhaps less so on general purpose Ex 'd machines for surface applications. At this size and power output the requirements will be highly vendor-specific.

 

[krisys]

Items a)
I totally agree with you.

Other solutions (Item b & c) are not the real solutions. To make it precise, the irreparable problems is identified. Now these are only the mitigations, so that the facility can continue to operate till the next motor scheduled overhaul. At the time of overhaul, we will use the spare motor and keep this motor as a spare.