motor insulation temperature rise

[superbee]

What is the difference between motor 'Measurement of Temperature Method' when designated on motor data sheet as 'resistance' vs. 'RTD'? I understand that the RTD method of testing will better eliminate possible hot spots in the windings of a new motor.

 

[ScottyUK]

Resistance could be a thermistor bead.

An RTD has a well defined characteristic from which there is minimal variation due to manufacturing so a system using an RTD is capable of a fairly high level of accuracy 'out-of-the-box'. Thermistors have a greater manufacturing tolerance around the nominal response, so the overall measurement is less accurate unless the instrument is calibrated for a specific thermistor.

Both methods can only measure the temperature at the point they are measuring, so both methods rely heavily on the choice of location and the installation method to provide protection.



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Sometimes I only open my mouth to swap feet...

 

[controlnovice]

I thought the resistance method was an actual resistance reading of the motor windings during (or directly after) the run test. The heat rise is deferred from the resistance reading.

RTD method is reading the temperature with the RTDs (usually permanent) in the windings.

It's been a while though for me. I think RTD method is preferred.

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This is normally the space where people post something insightful.

 

[ScottyUK]

Hi controlnovice,

Could well be.


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Sometimes I only open my mouth to swap feet...

 

[edison123]

The resistance method (of measuring winding resistances) will provide the average copper temp. The RTD's, if located in theoretical hot spots, will read hot spot temperatures and hence preferred.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[Hoxton]

I guess that the "data sheet" refered to is the motor manufacturer's test sheet. As controlnovice says, the tester measures the winding resistance (and the ambient temperature) before test and immediately after the test. The change in resistance is used to determine the temperature rise of the winding above ambient, actually the average rise. Large motors can have RTDs fitted for monitoring of winding temperature. These can be used on test, in which case "RTD" is entered on the test sheet in the 'Measurement of Temperature Method' space. Standards usually allow either way of measurement. You have to safly disconnect the power source for the motor and this can take a little while on a large, high voltage motor. During this period the motor could cool slightly. So you use the RTDs.

RTDs only measure the temperature where they are! On the large (1000+ kW) I used to be involved with, we put them in the slot separator, that is the strip of insulation between the top and bottom layers of the winding. That is the place you expect to be the hottest. As they were about 10 to 20cm long, they will again only measure the average.

 

[rotman]

Some additional information:
Since resistance method gives average winding temperature, and ETD method - (theoretically) temperature in hottest poits of the winding, for the latter method limits of temperature rise are higher by 5-10 K (depending on machine output, acc.to IEC60034-1).
Acc to a/m IEC standard: "In general,(...) the resistance method (...) shall be applied (...)"; however, with several exceptions, eg. for machines of output 5 MW(MVA) and more the ETD method shall be used.
(ETD=embedded temp.detector is more general term, and acc.to IEC it can include not only RTD(as Pt100) but also thermocouples etc.).
ETD (RTD) method is much more practical to use in practice (to monitor winding temperature during machine's operation).

 

[superbee]

I guess the only remaining confusing point is if a Motor is purchased with 2 embedded RTDS per phase winding, are these the RTDs used in 'Method of Temperature Measurement' or are specific RTDs located temporarily in various winding areas to perform the factory test?

 

[oftenlost]

In general, my company tends to install RTDs based on one RTD in the middle of the phase group and one in the intersection with an adjacent phase group so that the loss of one RTD will still provide two potential sources of temperature detection. Resistance rise is problematic from the user's standpoint as many motor connections are heavily taped and as a result, are too much trouble to check for most users. In my experience, motors with temperature rises based on resistance tend to have high surface temperatures which attract the attention of the owners. I do not mean to imply that they are inferior but they do tend to operate on the edge of published temperature limits.

 

[superbee]

Oftenlost, I think you can best explain my question, but I think Controlnovice is closer to understanding my question.
During the factory 'Method of Temperature Measurement' when using the RTD method are they using the (if specified & purchased)permanent embedded RTDs 2 per phase winding during the test or are they installing temporary RTDs for the test temperature rise measurement? The resistance measurement is clear.

 

[aolalde]

Testing Induction Motors, we followed NEMA standard MG1 and IEEE Std 112. If the customer did not request the instalation of RTD's we installed them anyway to monitor constantly the windings temperature during a load test. Load test is not a rutine type test, it is for prototypes or under customer request (and charged). When the RTD's temperature gets steady, we stop the motor and measure the windings resistance to get the final temperature by resistance (average). Then the temperature rise is get by substracting the ambient temperature from the total average temperature. The RTD's could be left or removed but they are great to monitor overtemperature alarm or trip off when the motor is finally coupled to the real load. The RTD's are located on the three phases and on the expected hotter spots, upper slots for horizontal motors.