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49 Vs 51 Protective function in Motor Protection relay

49 Vs 51 Protective function in Motor Protection relay

49 Vs 51 Protective function in Motor Protection relay

I have seen all the below three protective functions set up in a motor protection relay for a Motor. Is it a must to set all these three functions though all these functions does act on overload?

49T Motor thermal overload protection, - RTD input
49M Motor overload protection, - motor thermal model based on the measured current
51 IDMT over current protection

RE: 49 Vs 51 Protective function in Motor Protection relay

51 (time overcurrent) can be used for other things besides overload protection of a motor. So roughly speaking overload protection is a subset of the functions that can be performed by 51.

For motor overload protection, I'm used to seeing 49 when there is an overload heater to trip a motor starter and 51 when there are CT's feeding a protective relay. It sort of makes sense that 49 senses temperature (even though that temperature may be a result of current through a heater) while the 51 senses current and uses non-thermal means to develop a time/current trip characteristic.

(2B)+(2B)' ?

RE: 49 Vs 51 Protective function in Motor Protection relay

RTD's protect winding against temperature rise due to overcurrent as well as insufficient cooling (clogged filters, non-working external blowers, insufficient cooling water etc.). The electrical protections like instantaneous OC/OL and IDMT OC will not sense insufficient cooling. I consider RTD's protection as a must, especially in MV machines.


RE: 49 Vs 51 Protective function in Motor Protection relay

Modern digital motor protection relays have sophisticated stator and rotor heating models that should provide superior motor protection compared to a simple time overcurrent (51) function.

In many of these relays, the motor model (49) can be biased by RTD inputs if available. This will help handle long term overloads and high winding temps - the RTDs make the motor model more accurate. But the RTDs are relatively slow to respond and won't be fast enough for some motor conditions. So I would not rely solely on RTDs. There should also be an instantaneous overcurrent (50) for short circuit protection.

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