Current measurement

The current measurements can be divided in two categories:

  • high currents with two principles Rogowsky coil or current clamps

    Rogowsky coil can be used for AC current measurements. Directly it measures the derivative of current, therefore an integrator circuit or software filter module must be used.

    Current clamp works on the Hall effect principle and it outputs the voltage proportional to current.

  • low current measurements is often done with shunt resistors, where we need to cut the wire to include the shunt in series. We have to be very careful not to exceed maximum current of the shunt.

Both principles include a phase shift of the output.

Scale the current clamps

This can be done from Dewesoft Setup → Analog tab Channel setup for the current measurement used channels.

For information about Calibration analog input channel see → Channel setup - Calibration.

NOTE: First, as usual, we enter the ‘Units of measurement’. In the channel setup we enter that we measure current I in unit A. It is very important that the measurement range is chosen according to the expected signal. If we choose too high range, the inaccuracy of the current clamps will be too high to make correct readings.

Scale the shunt resistor

Module setup can be done from Dewesoft Setup → Analog tab Channel setup.

For information about Modules shunt resistor settings see → Channel setup - Module.

Sensor correction

When we are using the current clamps, we have amplitude and phase errors. So the current clamp is in this case the main source of calculation errors. Obviously we need to tell Dewesoft that we have a sensor which is not perfect and enter somehow the transfer curve of our sensor. Transfer curve gives information about amplitude and phase for sensors at certain frequencies and from this information Dewesoft can compensate the errors.

When we go back to analog setup after saving the sensors with Save file button and then choose the sensor for current channel, open the Sensors tab and choose in Sensor field the serial number of the sensor previously entered in the sensor editor we can’t enter the normal scaling or sensitivity anymore. However we do have a chance to enter a Transducer scale, which we can use for reversing the polarity of the sensor by entering a value of -1.

For the next setup we don’t have to define a sensor anymore, but can just choose it from the sensor list.