Unbalance currents

[electrolitic]

Can I consider Pt100 sensors at induction motor stator windings a good protection against overheating caused by unbalance currents (max unbalance voltage=5%).RTD has a relatively slow reaction time. Can it respond adequately to the heating process? Thanks

 

[squeeky]

In my opinion, yes, but the control equipment is more expensive. If you want to monitor all three phases you will require 3 x PT 100 controllers on a more expensive PT 300 controller. The latter will not allow you to discern which phase is over heating so it will be the same result from thermistors. Modern soft starters and VSDs have inputs for both.

What size is the motor and how many poles?

Watch for retro fitting them and running the probe conductors parallel the motor windings as mutual induction may blow the controllers.

 

[electrolitic]

Thanks Squeeky
The replaced (4P) motor is new, the Pt100 available. The problem is the voltage, 13,8kV, the low nominal current 45 Amps and the existing CTs ratio (too high to perform a good ANSI 46 protection.

 

[dpc]

I would think the RTDs should provide adequate response for low magnitude negative sequence current heating. For gross problems (loss of phase), a negative sequence overcurrent function will improve your odds. This is generally going to be available in any microprocessor-based motor protection relay. You say the CT ratio is too high, but the newer relays are very sensitive.

 

[squeeky]

OK so it'its a large motor. At that size you should have a decent motor protection relay. These should have PT100 inputs. Which one do you have?