CT Sizing

[PLC Pie Guy]

Hey all. Not a PLC question but electrical in nature. I have a 200 H.P. Frick compressor for refrigeration. This MCC bucket is missing a CT used to tell the compressor what percentage of FLA it is running at. I am curious what some of you would do if sizing the proper CT for this application. The 200 HP motor states that the FLA is 192Amps. (based on ugly's and 575 Volt AC) However, I know that this motor will see much higher current when starting. Possibly up to 300%. The LRC for this motor is 1160 amps. Would I size the CT based on the FLA or more for the starting current that would be more in the 500 to 600 range?
Thanks for all your input!

 

[Tom Jenkins]

I would size for around 250 Amps. Most C/Ts can handle short term over-current at start-up without problem. Make your pick and then verify inrush capability with the vendor.

 

[jdbrandt]

Hey all. Not a PLC question but electrical in nature. I have a 200 H.P. Frick compressor for refrigeration. This MCC bucket is missing a CT used to tell the compressor what percentage of FLA it is running at. I am curious what some of you would do if sizing the proper CT for this application. The 200 HP motor states that the FLA is 192Amps. (based on ugly's and 575 Volt AC) However, I know that this motor will see much higher current when starting. Possibly up to 300%. The LRC for this motor is 1160 amps. Would I size the CT based on the FLA or more for the starting current that would be more in the 500 to 600 range?
Thanks for all your input!

"It all depends".
It may depend on what the output of the C.T. is going to.
If it is a hardwared 5AMP meter, and the meter is gradiated in "Percent load", perhap 0-150%, then you will need a 250:5 CT.
If, OTOH, the CT output is being received by and conditioned by <something.."the compressor">, the scaling of that something will need to be taken into account. In other words, what do you want the CT's 5 amp output to represent? IF that input expects the 5AMP to represent, as you say, 300% of FLA, then a 600:5 CT would be more appropriate, albeit a bit harder to source than a 500:5...