RTD for MV motor

[Jed057]

Anyone please me,

The problem now we encountering RTDs of the11kV motor 280 kw EExn fail(disconnected ,resistance between active and common is open,actually should be 100 omh.)
please advise what can we do to solve this problem without send it back to manufacturer.
And atualy per IEC Standard,required for 280 kw motor or not incase we have overload and unbalance protection relay for this.

 

[petronila]

Hello Jed

Usually the manufacturers install 6 RTD, 3 to work and 3 for spare, the motor is 11 kV so the RTD are installed on the slot.In most acses the RTD´S are PT-100 type.(100-112 Ohms)for (0 to 20 grades C).

First check if all RTD´s are damage is not try to identify to wich phases belongs (A,B,C)and if you have at least 3 in good conditions then check if the 3 belongs to different phases, if yes you can use it for your propouses .

If not you could install a 6 new RTD´s in this case not in the slots,you most do it in the winding heads, take account the slot numbers and how you will space each one , be carefull with the 120 electric grades spacing for each phase.The RTd´s could be fixed with resin.

I think you will need to contact with one electric motor repair shop EX certified.

Regards

Carlos

 

[jraef]

To answer your implied question as to whether or not the RTDs are even necessary when you have an overload relay and unbalance protection, the answer may be more code related that technical.

From a technical standpoint, the answer would be no. An OL relay is the only thing absolutely necessary. But on very big very expensive motors, relying only on an OL relay for motor protection is like saying you are willing to drive a very expensive car by looking only at the "idiot lights" on the dash board, not the gauges that can give you an idea of what is ABOUT to happen by watching a trend. RTDs, fed into a Motor Protection Relay, accomplish that extra degree of protection that can help prevent a very costly repair and often more importantly, extremely expensive down time. The general rule is that the bigger the motor, the more important it is going to be in the process of your plant doing what it does. So having the best protection system for it provides a higher degree of protection against unexpected shutdowns.

That said, what Carlos said is right. Generally there are at least 6 RTDs, and although the MPR may be looking at all 6, we usually enable a feature called "voting" in which only 3 of them need to agree with each other to guard against the exact scenario you are experiencing. Remember, it's all about the down time. So check your MPR programming and make sure that "voting" is enabled, it may allow you to get up and running again until your next scheduled maintenance shutdown where you can fix the bad RTD.

The one caveat I alluded to above in it being a code issue has to do with it being an explosion proof motor. I don't know for sure because we don't need RTDs in Exp. motors in the US, but I've heard that some countries require them. Worth checking into.

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[rhatcher]

If there are no spare RTD's then any qualified motor repair facility should be able to install them for you. This may be able to be performed in the field depending on the motor construction.

There is one difference between a replacement RTD and one that is installed in a new motor or a newly rewound motor. For a new motor or a rewound motor the RTD's are installed in the bottom of the stator slot, near the center of the bore length, and beneath the windings.

Obviously you cannot put a replacement RTD in this location without rewinding the motor. The usual practice is to attached the RTD to the bottom (outside) of the coil extensions as close to the stator core iron as possible. Keep in mind that RTD's in this location will read a lower temperature than RTD's installed in the stator slot of the same motor.

 

[stephenw22]

We had several RTDs fail on a 5 year old, 6000HP 13.8kV motor at our site. The manufacturer came and installed 6 additional RTDs on the end of the windings. The RTDs and leads were epoxied to the windings.

Their thermal modelling showed that the temperature difference between the embedded RTDs and the replacement RTDs would only be about 5C. Actual comparisons after the motor start-up showed a maximum difference of 6-7C at the hottest operating conditions, and 3-4C normally (the new RTDs ran cooler than the embedded ones). We adjusted the motor protection to allow for this.

Our only concern was that the failures were a sign that something bigger was failing inside the motor, but extensive inspection and testing showed nothing.