SCR Current measurement
SCR Current measurement
(OP)
A basic question - the line current at the output of a 3 phase SCR heating controller (phase angle fired) is being measured with a standard digital clamp-on multimeter.
Will the non-sinusoidal waveform cause any measurement errors by the meter? I am thinking the reading should be reasonably accurate.
Will the non-sinusoidal waveform cause any measurement errors by the meter? I am thinking the reading should be reasonably accurate.





RE: SCR Current measurement
Google: fluke true-rms
RE: SCR Current measurement
The Fourier series of a partial half-sine as found in a SCR circuit is going to have harmonics that the meter won't see. But the meter will see the lower harmonics which is where most of the current [power] is at. Since you indicate it is a heating controller, the load is resistive and will not contribute to creating additional harmonics. My guess-itmate is that your readings will probably be off only 5% to 7% max.
RE: SCR Current measurement
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: SCR Current measurement
I think comcokid is referring to the harmonic components of the distored waveform - the meter will need a reasonable bandwidth to measure the distorted waveform although the higher harmonics do drop away fairly quickly.
I'm not sure how Fluke specify their instruments though - does it state the maximum fundamental frequency at which it can maintain acceptable measurement accuracy for (say) a square wave, or does it state the actual -3dB bandwidth?
RE: SCR Current measurement
RE: SCR Current measurement
Bill
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"Why not the best?"
Jimmy Carter
RE: SCR Current measurement
RE: SCR Current measurement
RE: SCR Current measurement
RE: SCR Current measurement
A bandwidth spec is typically given at the -3dB points. So it'll be significantly inaccurate before the defined frequency bandwidth limits.
The waveform you've described should be, almost has to be, predominately low frequency. You could just listen for any higher frequency harmonics, as you'd probably hear something whining if any were of significant amplitude.
RE: SCR Current measurement
RE: SCR Current measurement
Does anyone remember the "Hot Wire" motor starting relays?
With a suitable resistive shunt and a DC calibration the heating effect should give a true RMS indication at any frequency or complex waveform.
Even if this method is never implimented, the suggestion may be used as justification for accepting a less than perfect reading from another instrument.
Bill
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"Why not the best?"
Jimmy Carter
RE: SCR Current measurement
RE: SCR Current measurement
In practice, any decent quality true RMS clamp meter will give an accurate enough reading almost every time.
The testing of utility metering probably requires something better. Offhand, I can't really think of any other reason you'd need more accuracy.
RE: SCR Current measurement
For me this discussion turned into a good review of fourier series and AC waveform theory.
RE: SCR Current measurement
RE: SCR Current measurement
Z
RE: SCR Current measurement
Some of the 324 opamps could handle capacitive loads and others could not. Siemens never understood what was happening. I think that the fix (extra pull down resistor) is standard now.
Yes, I was a local hero, too.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.