Q: measuring thyristor chopped AC with clamp amperemeter?

[KLP]

Hi,

I have tried to measure 50 Hz AC current (5-50Amp) , which comes from a
thyristor regulator and do not get the expected values. I suspect this to be
because the AC current is chopped by the thyristor (power control) and
therefore will have a high content of harmonics. My clamp voltmeter is an
old analog instrument with a pickup coil and rated for 50/60 Hz.

So how to measure high AC current (5-50 Amp) which is chopped by a thyristor
circuit?

Best regards

Kasper Paasch

 

[Robert Baer]

KLP wrote:

Hi,

I have tried to measure 50 Hz AC current (5-50Amp) , which comes from a
thyristor regulator and do not get the expected values. I suspect this to be
because the AC current is chopped by the thyristor (power control) and
therefore will have a high content of harmonics. My clamp voltmeter is an
old analog instrument with a pickup coil and rated for 50/60 Hz.

So how to measure high AC current (5-50 Amp) which is chopped by a thyristor
circuit?

Best regards

Kasper Paasch

Ahhh...What you need is for the current probe transformer to drive a
scope.
This means that the current transformer must be "terminated" and
calibrated.
What you will see then, are partial sine waves; a sharp step from zero
current (at a time after line zero crossing) and the balance of the sine
wave until it gets to zero.
Depending onthe configuration, this "pulse" may be half-wave (only one
half of the cycle) or full-wave (two pulses per cycle).
The peak of that waveform will be the peak current, and an analog
meter with a current clamp would read the average.
A digital meter is more likely to miss the pulse or othewise "alias"
the sampling.
To make a current transformer for this, one can get a 50 amp filament
transformer and use the secondary in series with the thyristor; the
primary is loaded witha resistor as follows:
Start with the voltage ratio of the transformer; say 240VAC in, 2.5VAC
out) and run it backwards, 50 amps in for 500mA out ==> 1 ohm load would
give 500mV for that 50 amps (fairly close and might be good enough for
the first approximation).