True RMS protection
True RMS protection
(OP)
For motor protection, how many people look for true RMS vs. RMS devices(for voltage and current protection)? Or do you care? Thank you.
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RE: True RMS protection
RE: True RMS protection
Some types of thermals also require ambient temp compensation in one form or another.
RE: True RMS protection
If heavy harmonics are present, the true rms detector is better than not since it takes some account of harmonic heating... but even the thermal (true rms) relay is not going to properly compensate for the complex resulting heating effects in the motor. (for example increased hysteresis and eddy current heating, rotor skin effect for high-frequency). Perhaps the digital guys try to model these more accurately. I'm sure most competent experienced relaying engineers don't pay much attention to these small distinctions.... at some point you have to say it's close enough.
RE: True RMS protection
I think a much better approach is to install an embedded thermal detector in the motor, either an RTD or a thermistor.
RE: True RMS protection
Interesting comment about the temperature. In our plant all the large motors have rtd's but none have trips from rtd. Until now I never wondered why we don't trip on the more direct indication of temperature (we do alarm though). Perhaps relay engineers are more familiar with ct's. Perhaps rtd's are less reliable.... would electronic switch actuated by processing circuitry.... a little more complex than a simple relay contact. Although a simple temperature switch certainly could spec'd into the motor.
RE: True RMS protection
What kind of temperature trip functions do the new relays offer? Are they being used?
RE: True RMS protection
I am not sure many people trip a motor based on temperature only or at all. It would seem this would be the best way to protect from an overload conditon. A lot of electronic OL mfg's don't offer this in a standard overload device. It is usually an add-on. I do know of motor mfg's who embed these in their motors.
I think alot of people don't trip on temp is because of the cost of the electronics to do this(at least RTD's). It is not that high but the development time, etc, is longer. And you have to worry about ambient temp so there is another measurement. About four years ago I think I remember stand alone RTD monitoring boxes being $150 per RTD. This didn't include a suitable relay for a motor control circuit or a user adjustable trip point. I am sure the cost has come down some since then though.
RE: True RMS protection
There must be a point where sampling the higher frequency components would not add much to the actual trms value. Given a carrier freq of say 20kHz on a PWM drive, is it necessary to sample frequencies as high as this? My guess is no but doing so would produce more accurate results. Also, the most energy is concentrated in the lower harmonics so sampling anything above the 7th harmonic say, would not produce that much more accuracy. I can't recall where the SIGNIFICANT energy is in the harmonics on a PWM (isn't it the third, fifth, and sometimes the seventh?).
Electricpete, Most digital overloads are not TRMS devices. But there are some available in a variety of price ranges. They involve DSP's or are performing complex math (at least for a microcontroller) or they may use an off the shelf IC like maxims line of TRMS converters. The limitation with these seems to be the crest factor (5 is highest I have seen). I haven't checked into the bandwidth of these either but I would imagine it is well above the highest energy concentration of the waveforms of most drives.
RE: True RMS protection
Induction Motors: Part II - Protection http://www.selinc.com/techpprs/6024.pdf
Induction Motors: Part I - Analysis http://www.selinc.com/techpprs/6023.pdf
Thermal Models in Power System Protection http://www.selinc.com/techpprs/thrmodel.pdf
Using Thermal Limit Curves to Define Thermal Models of Induction Motors http://www.selinc.com/techpprs/6120.pdf
Appendix E: Motor Thermal Element - 145 KB {manufacturer-specific information} http://www.selinc.com/instruction_manual/701/701_0E_20010719.pdf
RE: True RMS protection
Most of the new digital motor protection relays do accept RTD inputs that can be used for tripping. We see this used in many facilities.
The embedded temperature detector is much superior to any overload relay. An RTD or thermistor system will provide protection against overheating from any cause, including single-phasing, ventilation problems, high ambient, etc.
dpc
RE: True RMS protection
For motor protection, how many people look for true RMS vs. RMS devices(for voltage and current protection)?
///In many instances, the true RMS are mandatory. With ever more increasing application of electronic circuits and signal processing, the true RMS are also possible to obtain by electronics rather then over the thermal element. The complex RMS converter produces the RMS result for any AC waveform. Visit
http://cns-web.bu.edu/pub/dorman/true-rms.html
http://www.sestechno.com/ses/acm102.htm
http://www.testproductsintl.com/440.html
http://www.traceengineering.com/technical/tech_notes/tn2.html
etc. for more info.\\\
Or do you care?
///The manufacturers do care, e.g. motor, meter, etc. manufacturers and whoever is highly dependent on the manufacturer product proper applications or implementations.\\\