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From: Jerry Avins on 18 Nov 2009 12:19 martind wrote: >> martind wrote: >>> Hi All, >>> >>> I am a newbie to DSP, and near the bottom of the learning curve. I am >>> currently designing a 3-phase IDMT protection relay, which monitors >>> voltage, current, earth leakage etc. >>> >>> The question I have is related to the monitoring of the current. I am >>> sampling 3-phases of current and need to convert each phase to True > RMS, >>> and in it's simplest form, output a signal when the any phases true > RMS >>> current reaches either a predefined trip point, or IDMT output level. >>> >>> I am using 32-bit fixed point hardware, with MAC. >>> >>> My confusion is the True RMS conversion itself. One way is to take 1 > cycle >>> of samples, square each sample as received, then divide by total number > of >>> samples then sqrt. But this has a disadvantage as the result if the >>> calculation is given at the end of the cycle. And the response time > would >>> not meet the spec. ... Why do you need square roots at all? When I > I_trip, it is also true that I^2 > (I_trip)^2. Given I_trip as a spec or input, calculate (I_trip)^2 once and compare against that. Such simple transformations are what make embedded systems practical. (At Siemens, we did what you propose with a 68HC11 that also calculated the motor's thermal model and would trip on winding excessive temperature. Jerry -- Engineering is the art of making what you want from things you can get. ����������������������������������������������������������������������� |