Platinum vers Copper RTDs for stator temperature detection 1

[kh2]

This subject is about the practice of using six wire dual element RTDs, copper (10 ohms) and platinum (100 ohms) Resistance Temperature Detectors (RTDs) placed at two different axial locations in the stator slots between the top and bottom bars.

The new trend is to use only platinum because of its higher accuracy than copper, and the increasing scarcity of finding transducers for the copper RTDs.

Appreciate you input on the advantages of having both the copper and platinum RTDs to detect the stator temperature, and your experiance with the new trend.

Thanks in advance.

 

[electricpete]

I have a hard time believing accuracy of a few degrees F is critical.

We have copper RTD's on all our large motors. The problems we have:

1 - Problems sometimes affected by loose and corroded connections. It would seem platinum RTD is less susceptible since a 0.1 ohms connection resistance (for example) is less percentage change for 100 ohm then fo 10 ohm.

2 - After 20 years of service, we have some motors with up to 4 of 6 RTD circuits open-circuited somewhere inside the motor (not inside the term box). I'm not sure if it's the connections at the RTD or the RTD itself.


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[jraef]

We provide an RTD input on our products which can be programmed to accept any kind of RTD material and ohm value. If the customer is going to use platinum RTDs, we are confident in pre-calibrating the inputs at the factory and sending it out ready to go. If it is anything else, we instruct them on calibrating the inputs in the field because there are too many variability issues, especially with copper. I don't know details as to why, this is just based on our field experience.

"Venditori de oleum-vipera non vigere excordis populi"

 

[electricpete]

Now I re-read your post and the terminology trips me up.

Six wire? I never heard of it. Do you mean six separate RTD's in a motor (that's what most people use).

Dual element RTD? I never heard of dual element RTD. Although dual element thermocouple (for bearing temp) is common.

Maybe you are mistaken on the terminology? If not I would be interested to know because it may just be something I never heard of.

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[electricpete]

The number of wires per RTD can be 3 or 4. We use 3 wire RTD's.

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[kh2]

The follwoing is an explaination of the three wires in the RTD and a description of the six wire dual element.

(1) The three leadwire RTD connections are recommended for the RTDs having low sensing element resistance, where a small leadwire resistance can have a large effect on the readout accuracy. In the two wire RTD, the resitance of the leadwire is in series with the sensing element, and the readout is the sum of resitance of the sensing element and the leadwire. The lead wire resistance can be high and must be compensated for. Compensation is achieved with a three-lead configuration. The three leadwire Copper RTD, for example, has three wires marked R1, W1, W2 or L1, L2, L3. W1 and W2 (L2 & L3) are connected to one side of the element and R1 to the other side. The measurement of the resistance is between R1 to W1, R1 to W2, and W1 to W2. Take the average of R1 to W1 & R1 to W2. Subtact from the average the resitance W1 to W2. By doing so the resultant resistance is close to the sensing element resistance. Platinum RTD is the same with three lead wires marked B1, Y1, Y2 (L1, L2, L3). R is Red, W is White, B is Blue, and Y is Yellow.

(2) The six leadwires ( 3 for the copper & 3 for the platinum) dual element stator winding RTDs provide extra protection and backup (in case one element failed). Dual elements can be one copper and one platinum, or both platinum. Most of the large hydro generators stator windings require a minimum of 12 dual elements with an even number of RTDs in each circuit of the three phases. The elements are placed at two different axial locations in the slot. Assume a 60 inches stator core length and the dual element length is 40 inches long (20 inches Copper RTD & 20 inches Platinum RTD). Half of the copper elements, in the dual element RTD assembly, will be at the top of the stator core while the platinum element is at the center of the stator core. The other RTD assemblies per phase shall have the platinum at the center of the core while the copper element is at the bottom of the core. This means that every other RTD, around the circumference of the stator shall have the copper element at the top. This arrangement will provide a full spectrum of the temperature distribution along the slot from top to middle to bottom.

Hope the above discussion clarifies the termenolgy used.

The question is still why the copper RTDs are not favored anymore and the Platinum RTDs are teking over on the rewind and new winding projects. The price is the same, therfore it is not a factor.

 

[PowerfulStuff]

I agree with a point of electricpete's. If the Platinum RTD gives better accuracy for same cost, why use anything else?

 

[electricpete]

Thanks kh2 for explaining the 6-wire dual element. Thanks powerstuff for noticing something that made sense among my rambling.

Some more thoughts. From what I see in our plant, platinum rtd's are widely used for instrumentation accross a wide range of applications and copper rtd's are used only in electric machinery. I am wondering why that is.

Here is a quote from Instrument Engineer's Handbook, Revised Edition, Liptak and Venczel, page 306:

"Last in usage [least frequently used] is copper, which is generally available only at 10 or 100 ohm ice point resistance values due to low specific resistance of the winding wire. Copper's temperature coefficient is almost the same as platinum and it is very linear above hte ice point. Copper in bifilar windings is used in electrical machinery due to the very low inductive or capacitive reactance, but platinum can also be used. ... There is no internationally recognized standard curve for copper, although some national standards exist."

I am trying to understand what they're saying about why copper is used in electrical machinery (high-lighted in bold). I see from other parts of the instrument handbook that the rtd can be film or coil. If it is a coil, then I can see there may be a somewhat high inductance. Copper I'm pretty sure has lower resistance and therefore less turns, particularly if 10 ohms copper vs 100 ohms platinum. Less turns would decrease inductance proportional to turns squared.

But why would inductance be important? The measuring circuit is dc. The only ac would be from external noise. If the purpose has to do with minimizing noise pickup from external ac fields, then it makes sense this might possibly be a bigger concern in electrical machinery. But still:
1 - shouldn't the dc measurement circuit be relatively immune to ac noise
2 - if the detector is enclosed in a steel probe connected to ground, shouldn't this prevent any noise pickup at the detector coil anyway?

If anyone has any other ideas to interpret the quote above regarding why copper finds useage in electrical equipment I would be interested to hear your opinion.

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