PTC & RTD PT100 4

[natee]

I'm purchasing MV motors which right now some suppliers propose to use winding temperature detection and protection as PTC, but some propose RTD PT100. Could you please anyone can clarify to me what is different of these elements? Which one have the advantages more than the other one?

Thank you in advance

 

[jbartos]

Comment:
PT100 principle of operation is based on thermoresistance (Platinum Resistance) (Pt100 or Ni 100). It is RTD type. Possible parameters are:
temperature range: from –200 °C to +850 °C – class of accuracy: A or B – reference regulations: IEC 571
PTC (Positive Temperature Coefficient) principle of operation is based on thermistor characteristics that have different characteristics, and parameters, e.g.
Thermistor (PTC kty 81 types)
temperature range: from –50 °C to +150 °C
Thermistor is an electron device that makes use of change of resistivity of semiconductor with change in temperature.
It cannot work without a control device.

Visit

http://www.usmotors.com/PB500/2001/Mods_T-W.pdf

etc. for more info

 

[edison123]

In my experience, PTC type thermistors are less reliable than RTD's. PT 100 RTD's have a longer life with good repeatability and better accuracy.

As a rewinder, I have always replaced thermistors with PT 100 RTD's and my clients are happy (though it involved a little bit spending on new RTD temp controller) till date.

Since thermistors are not standardized (as jb, says different thermistors have different charactersitics), one has to have original specs when ordering for replacements (during rewinds).

One advantage of thermistors is they are tiny as compared to RTD's so they can be fitted anywhere in the windings (normally they are in overhangs).

 

[aolalde]

RTD = Resistance temperature detector
The temperature is proportional to the ohmic resistance change. It measures temperature accurately into its working range.

PTC = Positive Temperature Coefficient thermistor.
This is a semiconductor device that increases the resistance when the trip temperature range is reached. Regularly it is part of an electronic trip off circuit.

The control devices are totally different for each other.

 

[jraef]

In addition to the above, there is a considerable difference in the way they are used. As mentioned by aolalde, the PTCs will work with a "trip relay", in that they allow the motor to run up to their trip point, then shut it down. That is it.

With RTDs on the other hand, a good RTD relay system can be programmed for responses at different temperature levels and provide warnings or alarms before you end up with an unscheduled shutdown. This can be very helpful in critical path equipment. In addition, Motor Thermal Management systems such as GE/Multilin relays will use the RTD inputs to bias the "thermal register" used to model the motor performance. This means that instead of calculating the motor temperature based just on current and time, the system compares it to actual values read from the RTDs in order to detect trouble spots that might have otherwise been missed.

The downside to this is that if it is a 20HP motor, it does not make much sense to use a $6000 relay on it! Hence the appeal of PTCs.

"Venditori de oleum-vipera non vigere excordis populi"