Induced EMF in a half vs. whole ferrite toroid

[GaryLS]

Hello all -

I've stumbled upon what seems to be a pretty basic magnetic materials question that's got me stumped. Maybe one of you can help me figure this out.

I did the following experiment (see figure below):

• Put a ferrite toroid around a current carrying wire carrying 12 Amps RMS at 120V, 60 Hz.
• Toroid has a wire coil around it that's wrapped around a small angular section of the toroid
• Measured 90 mV RMS potential difference between the coil terminals on the voltmeter, which is about what I expected based on what I learned about this problem in college physics
• Cut the toroid in half, and put the half-toroid with the coil around it right next to the current carrying wire, but in this case measured nearly 0 mV RMS potential difference between the coil terminals

Can anyone explain why I measured nearly zero mV when the toroid was cut in half? Even though the toroid is cut in half, it's still made of ferromagnetic material, so I would have expected the field strength to be boosted inside the half-toroid (by a factor of α, where α is the ferrite's relative permeability), and therefore would have expected to see a much higher induced voltage in the coil. I would have expected to see more like 50 mV RMS in the coil in this case, but not zero.

Thanks in advance for any insight you can provide!

Gary

 

[davenn]

Hello all -
......

• Cut the toroid in half, and put the half-toroid with the coil around it right next to the current carrying wire, but in this case measured nearly 0 mV RMS potential difference between the coil terminals

Can anyone explain why I measured nearly zero mV when the toroid was cut in half? Even though the toroid is cut in half, it's still made of ferromagnetic material, so I would have expected the field strength to be boosted inside the half-toroid (by a factor of α, where α is the ferrite's relative permeability), and therefore would have expected to see a much higher induced voltage in the coil. I would have expected to see more like 50 mV RMS in the coil in this case, but not zero.

Thanks in advance for any insight you can provide!

Gary

do some reading up of current transformers and how they work
here is a link or two to start you off

https://en.wikipedia.org/wiki/Current_transformer

https://www.quora.com/How-do-current-transformers-work

they will explain why you cannot have a broken or only a section od a coreDave