Generator Embedded Thermocouples
Generator Embedded Thermocouples
(OP)
I am refurbishing a 50year old generator which has embedded type K thermocouples in the stator. We attached thermocouple/4-20mA transducers direct onto thermocouples at stator junction box. We find that the temperature reading on all 6 is fine when there is no generation, but raises by some 30 deg once the generator is on line and generating.Certainly not a heat effect as it rises as fast as we raise current (MW or MVAR). Tried earthing one side of thermocouple with no effect. Have tested and verified no pickup in the transducer or 4-20mA leads when generating. Only conclusion is pickup in thermocouple leads in stator. Any suggestions?






RE: Generator Embedded Thermocouples
Generators and motors can operated normally at temperatures high enough that you cannot keep your hand on the outside of the machine.
RE: Generator Embedded Thermocouples
T/Cs require such a small voltage for a given temperature I can easily see this problem. I'd rather RTD's or thermistors but..
Can you disconnect your 4-20mA converter and stick the leads directly to a DMM? This will tell you if the converter or the T/C is where your problem is occurring.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator Embedded Thermocouples
RE: Generator Embedded Thermocouples
To give you some idea of the energy available in the stray field, we managed to get an old Philips Scopemeter running for a couple of seconds from its wall-wart plugged into an extension cable maybe 10' away from the outer casing of the machine. Only thing was, the extension cable wasn't plugged in. After the initial bemusement we figured the smoothing capacitor in the supply was being charged up by the stray field while the instrument was off and held enough charge to allow it to switch on for a couple of seconds before fading out. The battery wasn't even in the instrument.
We have no problems of the type you're experiencing with t/c's mounted in the stator of big 16kV generating sets and those t/c's are routed directly to DCS I/O cards a couple of hundred metres away. I honestly don't think the t/c is the guilty party. Can you install comp. cable and get the transducer away from the generator? Further the better - distance is usually the most cost-effective way to reduce magnetically coupled interference.
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Sometimes I only open my mouth to swap feet...
RE: Generator Embedded Thermocouples
The original scheme used a switched milivolt meter (no auxiliary supplies at all) with one side of the thermocouples commoned, ie a 7 wire system.
RE: Generator Embedded Thermocouples
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator Embedded Thermocouples
I would also try previous recomendation to move one of the 4/20 transducers far away and connect it with extension lead and see if that helps
RE: Generator Embedded Thermocouples
Please explain why you do not think this is valid
RE: Generator Embedded Thermocouples
Yes we used a metal box and it is a complete enclosure
RE: Generator Embedded Thermocouples
Aside from checking the wiring routing, twisting and shielding to remove the induced voltage, there might be some filtering options. Maybe your transducer has some kind of an input filter to remove ac noise? If not, maybe try a cap across the transducer input (acts like an open circuit to dc but a shorting impedance to ac).
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RE: Generator Embedded Thermocouples
It's rather like having a pressure transducer that is not measuring small changes in pressure because the head pressure is way too high and the transducer is pegged. It's not the 'pressure's fault'.
You will get a very strong 50/60Hz magnetic coupling in that environment with wire twisting being able to help only so much. If your instrument can't handle that correctly you will get erroneous readings.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator Embedded Thermocouples
The t/c is a low impedance source which is quite difficult to couple noise into, as is the surrent output of the transducer. The rest of the circuit within the signal conditioner will be typical op-amp electronics, lots of small semiconductors with high impedance signals, all prone to pickup of interference.
Screening LF magnetically coupled noise is virtually impossible at a practical level. Earthed metal enclosures are great for reducing capacitively coupled noise, but have negligible effect reducing magnetically coupled noise, unless the earthed metal just happens to be a mu-metal box. At 50Hz or 60 Hz it would be a very thick walled mu-metal box to be effective. Keep away from it remains the easiest solution.
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Sometimes I only open my mouth to swap feet...
RE: Generator Embedded Thermocouples
I would have said the t/c is a low-voltage signal source connected to a high-impedance circuit which would be very easy to couple noise into. Would I be wrong?
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RE: Generator Embedded Thermocouples
A t/c is definitely source of very low voltage, but it's a source with a very low internal impedance. It's not easy for an external interfering source to impress anything onto the t/c signal, unless the t/c cable forms a loop antenna by virtue of not being a twisted pair. That would be a very poor installation and asking for problems. The signal conditioning circuit itself is likely to have a lot of fairly high impedance nodes internally within the circuit and thus be susceptible to external interference, hence my suggestion to move it away from the source of the interference.
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Sometimes I only open my mouth to swap feet...
RE: Generator Embedded Thermocouples
Back to the converter, it better cope correctly with the 50/60Hz and it should be non-susceptible to the fields,(somehow), directly. Speaking of which, the best way to deal with the 50/60Hz in a converter like this one is to to synchronously read the T/C voltage with the noise source, that is, the 50 or the 60Hz. This means any 'noise' is always the same magnitude and can be calibrated out. I say this because if the converter is expecting 60Hz but is in 50Hz service (or vis-verse) you will lose that feature.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator Embedded Thermocouples
RE: Generator Embedded Thermocouples
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator Embedded Thermocouples
Are these 1500 rpm machines (1800 rpm in the 60Hz world)? I'm curious as to the origin of the 2nd harmonic frequency.
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Sometimes I only open my mouth to swap feet...
RE: Generator Embedded Thermocouples
RE: Generator Embedded Thermocouples
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Sometimes I only open my mouth to swap feet...
RE: Generator Embedded Thermocouples
RE: Generator Embedded Thermocouples