Ammeter connected through parallel CT's?

[fijensen]

First of all; I'm a tech not an engineer. This doesn't look right to me at all. I'm thinking the ammeter (ACA) is only going to see the unbalance between A and C phases (jumper on right of PFFR relay). I don't know what the formula for this would be, but in the past I've always seen an ammeter on a single phase, or on 3 phases with a switch. I'm thinking it's a drafting mistake, but it is connected like that. I think the meter, and transducer should be on B phase, and the C0/ground jumper on A and C. Your thoughts would be appreciated. See link for drawing.
Operators use this meter to set their flow, and I think they are overheating the motor.

http://files.engineering.com/getfil...78fe-4e40-9241-3e1bbed8db0b&file=Expander.jpg

 

[rbulsara]

The ammeter ACA will read the vector sum of A and C phases, which will be equal to B phase (or T2) current, in normal conditions.

This must have some diagnostic value, vs putting the ammeter in just one of the phases. I haven't given much thought yet.


Rafiq Bulsara

http://www.srengineersct.com

 

[rcw retired EE]

With the jumper as shown, the transducer and ammeter read the vector sum of A + C = -B unless there is a ground fault.

(Remember that Ia + I b + Ic = 0. All current going to the motor on one phase has to go back on the other two.)

The connection is esentially averaging A and C phase currents. It should work if all of the CT's are wired correctly. If one CT is backwards, the reading will always be near zero.

 

[fijensen]

Thanks for the replies. I guess it's OK then. I've never come across a connection like that. I'm still not sure why they didn't just run the Ammeter through one phase by itself? One reason I questioned it; the motor has been replace quite a few times due to winding failures.