I have an issue when measuring AC current using CT the issue is very strange
I have induction cooker current rate is 7.4A then when measure current of it via CT then get very accurate result as 7.25A
I tried another load as heater then rated current of it is 8A but the result via CT is as 4A
could you please tell me what is the problem & how to solve it?
Without data on the CT, Circuit, and software I can only take a SWAG and suggest it is the Burden Resistor. Please post a schematic, not a frizzy thing, and what you are using for a current standard and how accurate is it?
gilshultz:
Without data on the CT, Circuit, and software I can only take a SWAG and suggest it is the Burden Resistor. Please post a schematic, not a frizzy thing, and what you are using for a current standard and how accurate is it?
CT: 5A/2ma - 2500:1
Burden Resistor : 100ohm
int peakPower = 0;
int current = A1;
void setup()
{
Serial.begin(9600);
}
void loop()
{
int maxCurrent = 0;
int minCurrent = 1000;
for (int i=0 ; i<=200; i++)
{
current = analogRead(A1);
if(current >= maxCurrent)
maxCurrent = current;
else if(current <= minCurrent)
minCurrent = current;
}
if (maxCurrent <= 517)
{
maxCurrent = 516;
}
double RMSCurrent = ((maxCurrent - 516)*0.707)/15.5;
int RMSPower = 220*RMSCurrent;
if (RMSPower > peakPower)
{
peakPower = RMSPower;
}
delay (1000);
Serial.print(RMSCurrent);
Serial.println("A");
}
If it’s a 5A current transducer , then it may well saturate at higher currents giving a non sinusoidal output and or your bias circuit may then be incorrect .
hammy:
If it’s a 5A current transducer , then it may well saturate at higher currents giving a non sinusoidal output and or your bias circuit may then be incorrect .
according to specifications of CT with 100 ohm burden resistor can measure from 0 to 70A
To measure true RMS you can't just take max and min(*) - you have to sample many times per
waveforms, square and average and take the root: root-mean-square. Rated currents for
ac appliances are RMS as that's the value that matters for loading wiring.
(*) that only works for purely resistive loads when the mains voltage is a pure sinuisoid - an
induction heater is very much not such a load, and these days mains voltage isn't usually
a very good sinusoid either due to all the SMPS running from it.
the issue that I am facing with capacitive load like (CFL Bulbs, switch mode power supplies, LED's) reading not correct, is there any suggestion or example to do tests?
Current sensors only read current, not power or power factor.
If you need the later you have to know the voltage waveform too, so the phase information
can be factored in.
True power is easy, just sample current and voltage regularly, take the product and accumulate
the results over time - the total accumulation over an integral number of cycles can be
used to calculate power.
If you measure rms current, rms voltage and true power together, you can compute
power factor from them.
MarkT:
Current sensors only read current, not power or power factor.If you need the later you have to know the voltage waveform too, so the phase information
can be factored in.True power is easy, just sample current and voltage regularly, take the product and accumulate
the results over time - the total accumulation over an integral number of cycles can be
used to calculate power.If you measure rms current, rms voltage and true power together, you can compute
power factor from them.
is there any example code & schematic to start tests?
The open energy monitor might be a good place?
You could always use an electricity meter with a pulse output - accurate and gives you isolation and an output that is easy to deal with .
Btw - when listing the spec for a CT the first number is usually given as the maximum current such as 100A/1A.
hammy:
You could always use an electricity meter with a pulse output - accurate and gives you isolation and an output that is easy to deal with .Btw - when listing the spec for a CT the first number is usually given as the maximum current such as 100A/1A.
The CT I have is as 5A/2ma 2500:1 I got from another meter but that meter can measure from 0 to 70A could you tell me how that meter can measure 70A using 5A/2ma also burden resistor used as 100 ohm
Found this
“ The rating factor is used when determining what size CT to use in a particular installation. Some CT’s have a rating factor of 4, 3, 2, or 1.5. What this means is that the manufacturer says the CT is accurate beyond its nameplate value. For example, a 200:5 CT that has a rating factor of 4 will accurately measure a service up to 800 amps. So, if that particular service were to have 800 amps on it, there would 20 amps coming out of the secondary side of the CT and in the meter base. This is important because we want to size our CT’s so that they are fully saturated. Meaning that we want a 200:5 CT to be sized so that the amps flowing through the primary side has as close to 200 amps as possible. When the core of the CT is fully saturated it is the most accurate. CT’s tend to lose some of their accuracy at lower amp levels.”
Magnetic saturation is not something you want in a current transformer used for measuring:
I think the point you are trying to make is that you'd size a CT just big enough for the load it
has to measure to avoid spending unnecessarily.
Page 6 of the linked paper discusses accuracy and saturation. The graphs at the end of page 3 show
how badly a saturating CT will behave.
Yes I noticed the saturated bit - my text was from a manufacturers site copied “ as is”, so weird . My point was really just about how CT’s are specified ( the Op’s 5A/20mA spec for a 70A device) .......maybe that manufacturer uses something clever in the time space continuum with a flux capacitor to negate the saturation by taking current with a phase shift
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