CT sensor inaccuracy due to triac

A ct (current transformer) sensor is a transformer whose output is dependent on current in the primary (the wire you clamp it on), the geometry and composition of the device and, presumably, the frequency of the AC.

We think of mains AC as 50Hz, but surely if you are using a triac there will be high frequency components to the current which will have an effect on the coupling of the current transformer.

I hope I am wrong.

Forgiveness, please, for any duplication of another thread due to difficulty of searching.

Robert Wall's picture

Re: CT sensor inaccuracy due to triac

Yes, I'm afraid you are wrong. CTs in general (and I tested the standard Yhdc one to 2.5 kHz) are good to quite high frequencies and so should give a fairly faithful reproduction of the current waveform. The limitation is likely to be in the sampling rate of the microprocessor - the Nyquist criterion - which limits the highest frequency that can be digitised to half the sample rate. The calcIrms( ) library function works out to about 117 samples per cycle at 50 Hz, i.e. roughly the same as the upper frequency that I tested, and the calcVI( ) function limitation is about half this.

See here: http://openenergymonitor.org/emon/buildingblocks/report-yhdc-sct-013-000... for the details.

nigeldodd's picture

Re: CT sensor inaccuracy due to triac

Would the current transformer (CT) work for DC?


alco's picture

Re: CT sensor inaccuracy due to triac

as mentioned in treat: http://openenergymonitor.org/emon/node/265

You can't use an COIL based ampere sensor for measuring Direct Current flows.

You need an sensing resistor for this.

nigeldodd's picture

Re: CT sensor inaccuracy due to triac

Sorry, that was a rhetorical question. So there is a relationship between the efficiency of a transformer and frequency since at 0Hz the efficiency is zero.

What is the nature of the relationship between efficiency of the transformer versus frequency above 0HZ?

(efficiency might be defined as closed circuit current in the secondary given fixed current in the primary)

Robert Wall's picture

Re: CT sensor inaccuracy due to triac


People have tried, usually by accident, and the result is blown fuses (hopefully) or a burned-out primary winding.

In the case of a small c.t. like the yhdc split-core type, the result would be no output - because of course a transformer works on the rate of change of flux in the core. And with d.c in the primary, the flux would be unchanging.

If you need to sense d.c. (your P.V. output?) you either need a shunt as mentioned above, which with the voltage needed to drive the Arduino input will result in a lot of wasted power, or you need a Rogowski Coil <edit> or a Hall Effect sensor </edit> and the associated electronics.

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