[SOLVED] Current Flowing through 3P3W (Delta connected system)

[Shanmukesh]

Hi everyone,

In an delta connected system. I have the value of current flowing through R-Phase and Y-Phase.
How do I calculate the Current through B-Phase?

The 3 phase voltages and Load connected to them are unknown.

 

[KlausST]

Hi,

for instantaneous current: IB + IY + IR = 0.
--> IB = - (IY + IR)

But:
there is no possibility to calculate RMS current for unknown unsymmetric load.
(you need to know two RMS currrents and additionally two phase shifts to calculate the third RMS current)

but for symmetric load: IB(RMS) = IY(RMS) = IR(RMS)

Klaus

 

[Shanmukesh]

Thank you. It helped me.

My application is a microcontroller based motor protection device. I m using 2 ADC to monitor IR and IB.
Since, a 3 phase induction motor is a balance load; IY (the unknown) should be = IR = IB
Is this assumption going to affect the performance of the motor protection device in case of insulation failure of Coil of the motor between Y-B and/or R-Y.
As per my understanding, this fault condition would result in difference in IR(being monitored) and IY(being monitored). Hence, this can be sensed and protective action can be tiggered.

 

[Shanmukesh]

I suppose that such type of connection between CT and ADCs should serve the purpose

 

[FvM]

The circuit can't actually monitor insulation faults, respectively can only under the assumption of balanced motor currents.

IY can be calculated as -IB-IR (for the instantaneous value, not averaged value). But the calculation is only valid if not ground current exists. In contrast, to monitor ground currents, you'll use a current sensor around all three wires.