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I am going to measure the AC current(three phases), with a current probe, of a large machine, with an oscilloscope. The scope I am using has the following two options:

  • RMS: RMS (Root Mean Square) value of the voltage of the complete displayed waveform.
  • AC: RMS value of the AC part of a periodic signal, calculated over all periods on the display. The AC result is is derived from the DC and AC+DC results.

Can someone tell me the differences of the two measuring types?

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  • \$\begingroup\$ The scope I'm using is a Rohde & Schwarz R&S®Scope Rider RTH \$\endgroup\$ Sep 25, 2017 at 12:17
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    \$\begingroup\$ If you are using current transformers for the "current probes", then the current signal is AC coupled anyway. \$\endgroup\$ Sep 25, 2017 at 12:21

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The difference of the two measuring types is as described. One takes into account the DC component as well as the AC component. The second only takes the RMS of the AC component. In most cases, only the AC component is of significance. In a large AC machine, the DC component should be small and can be ignored. Your current probe probably can only respond to AC signals anyway.

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  • \$\begingroup\$ Thanks a lot, I didn't see any differences indeed! \$\endgroup\$ Sep 25, 2017 at 12:18
  • \$\begingroup\$ But there may a big difference in power if you don't measure mean phase shift since you are not computing 3phi V*I for each sample then computing RMS power.. just current at the moment. \$\endgroup\$ Sep 25, 2017 at 13:12

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