Can I derive A-B, A-C, B-C if I have all the phase to neutral figures?

Status
Not open for further replies.

Jezo

Member
I am monitoring a 75kva PDU but the issue is that the unit does not send out it's phase to phase figures, which are normally all 208 but I'd like to keep track of them.

I have all of the phase to neutral figures being sent out to a database, can I derive phase to neutral using them or are they a totally separate set of measurements? I also have the Phase to neutral THD figures but those I am not sure what to do with.
 

Jezo

Member
All I have from the unit is the phase to neutral and thd figures. I can try to contact the vendor to see if angles are on their list of monitored points, but they will take a few days to get a hold of. If they do what would the formula be to relate them together?

I did try multiplying a phase to neutral point by 1.72 and that brought me quite close to the around 208 figure I would expect to see, am I headed in the right direction? All three phases to neutral are currently right around 119. * 1.72 that brings me to 204. The unit is reading 207 for the phase to phase figures.

The issue is that the phase to phase is displayed on the screen but it doesn't get sent over the wire for me to view when I am not right in front of it.
 

Smart $

Esteemed Member
Location
Ohio
... I did try multiplying a phase to neutral point by 1.72 and that brought me quite close to the around 208 figure I would expect to see, am I headed in the right direction? ...
A better approximation can be calculated by sqrt(V1?+V2?+V1*V2) where V1 and V2 are the phase to neutral voltages of the lines comprising the phase to phase voltage. This formula assumes an exact 120? angle between voltages.
 

Jezo

Member
These two suggestions are great, thank so much for the assistance.

For the last one, is there reasoning behind the formula as to how or why it works? I'd like to learn more about it instead of just plopping numbers into an equation.
 

Smart $

Esteemed Member
Location
Ohio
These two suggestions are great, thank so much for the assistance.

For the last one, is there reasoning behind the formula as to how or why it works? I'd like to learn more about it instead of just plopping numbers into an equation.
Your line-to-neutral voltages (which you refer to as phase to neutral) can be represented by vectors having a magnitude (voltage) and direction (phase angle). When the vectors are arranged tail-to-tail, the line-to-line voltages are represented by the magnitude and direction from head to head of the vectors. Each of these magnitudes can be calculated using the law of cosines. Not knowing the exact angle of the line-to-neutral voltages, plug in 120? for the angle.
 
Status
Not open for further replies.
Top