Three phase phase angle between phase to phase

[heiko58]

Hi Guys,

I need to ask something here.
I know for a three phase system, each phase is separated by 120 degree.
But now i have a scope meter from fluke (fluke 123) to measure a star network, that means no Neutral reference.
What i did was measuring phase to phase on 2 channel, A and B.

This is my setup.
1)
Channel A= L12 (red probe to L1 black probe to L2)
Channel B=L23 (red probe to L2 black probe to L2)
I get a 60 degree separation angle between them.

2)
Channel A= L12 (red probe to L1 black probe to L2)
Channel B=L31 (red probe to L3 black probe to L1)
I get a 60 degree separation angle between them two.

Can anyone confirm to me whether phase to phase spearation angle is 120 degree? If so, why?

Thanks

 

[-Sean-]

You need your black probe for both channels connected to the same phase conductor. Otherwise you are applying a 180° phase shift to one waveform (viewing waveform upside down). Hence 180-120=60° viewed phase shift.

 

[heiko58]

Thanks for your clarifications! it helps a lot!

 

[heiko58]

You need your black probe for both channels connected to the same phase conductor. Otherwise you are applying a 180° phase shift to one waveform (viewing waveform upside down). Hence 180-120=60° viewed phase shift.

Hi again sean, i had tried ya advice but i still get a 60 degree reading.Can u teach me how to properly measure the phase angle different between phase in a star network that i dont have connection to neutral and earth? I am using scopemeter fluke 123, which i think both channel -ve are common.How do i make a display of 120 degree phase angle difference with the fluke scopemeter?