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Problem with unbalance detection in three phase system

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Hello! I want to check voltage unbalance in a three phase system. I have used the below circuit for checking unbalance. But in normal operation that all three phase are approximately equal there is'nt zero voltage difference between points N and N'. I have near 5 Vrms in normal operation, while in some weeks ago there was near 0 voltage difference. Do anyone know what is the problem??

**broken link removed**
 

Do you mean the problem is with power grid that supplies 3 phase?! When I measure phase voltages there is little difference between them! and 3 resistors also are equal.
 

There are two distinct unbalance conditions: The three phases have unequal peak voltages AND/OR the phase relations between the phases are not equal.

In general, we expect the load too be symmetric on the three phases. This condition is often not fulfilled. The effect depends on the source impedance.

Your load is not the only load on the system; other large loads on the same circuit can affect the phase and amplitude. If the line voltages are 220/440V, then 5V on the neutral due to unbalanced current is rather small and nothing to worry.

If the voltage gets over 10% of the line voltage, you need to redistribute the other loads so that some symmetry is attained.
 
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