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Frequently Asked Questions

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frequently asked questions

All the data for calculating the accuracy of the current transducer is given in the datasheet. The calculation is done for the DS200UB-10V.

If you have a mixed signal the accuracy should be calculated for every frequency component. In this example we will calculate the accuracy for DC signal. Therefore, we use the accuracy data which is specified for the frequency below 10 Hz (<10 Hz). The total ratio error can be calculated by the following equation.

This formula is mentioned in the datasheet on page 3.

The data can be found in the datasheet as follows.

If we now calculate the accuracy of the amplitude for 10 A the equation is as follows.

If you measure 10 A primary current and your measuring device has taken into account the current transducer’s transfer ratio


you get a secondary voltage of

if you have an ideal current transducer without any amplitude error. But we have calculated an . This means that your measuring device shows a voltage between the following calculated values.

The percentage amplitude error at the instantaneous value of 10 A can be calculated as follows.

The ratio error can be decreased by compensating the offset error.

The ratio error can now be calculated for any primary current. Even for very small values. Please note that the ratio error increases for smaller primary currents.

The phase displacement is specified in the datasheet for frequency components below 10 Hz with 0.01 °.

Graphically, this results in the following diagram.

This depends on the resolution m in bits of your Analog Digital Converter – ADC. This determines the number of steps M which can be calculated with . The value of one step (least significant bit – LSB) is a parameter of the theoretical resolution of the ADC in relation to its analogue output range.
Let us assume, for example, that the measuring range of the signal input for the DS50UB-10V is specified as  . If a DC current of +50 A now flows through the DS50UB-10V, this results in 10 V on the secondary side of the current transducer The transformation ratio is defined as V/A = 1/5.

The use of higher-resolution ADCs generally increases the requirements for low-noise components, shielding and grounding concepts. These are therefore mathematical values.

Power analyzers usually use the following basic formula for active power calculation.

Thus, the digitized instantaneous values of the voltage v(t) and the current i(t) are multiplied together and the results are summed up over a defined time window. Basically, DC components, all harmonic and non-harmonic components up to the bandwidth limit or filter cut-off frequency of the power analyzer are taken into account.

The cut-off frequency of a power analyzer is the frequency characterizing a boundary between a passband and a stopband for the measurement signals which are applied to the input channels. Passband consists of the range of frequencies the filter lets through (minimal attenuation), and the stopband consists of the range of frequencies the filter rejects (high attenuation). The cut-off frequency is sometimes taken to be the point in the filter response where a transition band and passband meet, for example, as defined as the point at which the output level from the filter falls/rises by 50% (i.e., ±3 dB, since a fall/rise of 3 dB corresponds approximately to half power) of the in-band level, assuming a constant input level. It is also sometimes referred to as the half power or ±3 dB frequency.

The stopband of the filter is essentially the band of frequencies that is rejected by the filter. It is taken as starting at the point where the filter reaches its required level of rejection.

The input signals of the measurement channels are damped with -3 dB at the cut-off frequency. All frequency components or noise signals above this cut-off frequency are damped.

Due to the time delay which is generated by the low pass filter all interacting channels should have the same filter characteristics. Otherwise, the internal software of the measurement device has to compensate an additional time delay for a different type of filter.

You need a 50 Ohm resistor in the measuring channel that is equal to the characteristic impedance of the coaxial cable if reflections can occur on the connecting cable, i.e. when measuring high-frequency signals or steep-flanked pulses.

Without such a termination at one or both ends of the cable, a reflected wave can run back and forth several times in the cable. The measurement signal can be severely distorted in this way.

It is generally recommended to use high-impedance inputs from 1 MOhm for voltage signals. The unwanted current flow through a high-impedance resistor in the measuring device is kept very low, which reduces negative effects on our specified accuracy.

Nowadays, non-linear loads such as switching power supplies not only cause harmonic distortions in the power grid but are often also responsible for electromagnetic radiation. For example, magnetic stray fields are generated by winding components and electrical stray fields by conductors with high pulse voltages. These fields can couple into the current transducer. Oscilloscopes can be used to record these oscillations.

With a DS50UB-10V without a power supply connected and an unearthed housing, an oscillation of approx. 44 MHz can be detected via the BNC connection. This oscillation corresponds to a primary current of approx. 20 mA peak to peak. The measurement was carried out on a standard home office desk.

 

The interference is outside the specified bandwidth of the current transformer of 500 kHz. A low pass filter in the single-digit MHz range could significantly reduce this interference.

There are always power supply units in use in laboratory setups. In addition to the laptop, measuring equipment such as oscilloscopes also require a power supply unit that converts alternating current into direct current. To keep the internal transformer as small as possible, the mains voltage is inverted into high frequencies. This results in electromagnetic radiation, which can be qualitatively detected with a short-circuited oscilloscope probe, for example.

 

This radiation can, for example, pass through the inner hole of the current transducer and can therefore be recognized in the secondary signal.

 

 

An oscillation at approx. 3.33 MHz can be seen in the image above. The current transducers do not have a low-pass filter at the output, so that these signals are generally not attenuated. For oscilloscope applications, it is recommended to use oscilloscopes that can filter the input signal so that high-frequency interference is minimized.

Yes that is right. Normally the excitation frequency of the current transducer which is specified in the datasheet can be seen. For example for the DS50UB-10V the excitation frequency is specified with 31.25 kHz. This oscillation is part of the DC supply current.

Outer radius – Inner radius = Distance from the outer housing of the current transducer to the return or neighbouring conductor.

Due to the special core design of our current transducers, the fluxgate element which is responsible for detecting the DC current is shielded against electromagnetic fields. Changing the distance to the return conductor affects the accuracy in the ppm range. But of course, increasing the distance to the return conductor or other potential disturbance sources is always good.

In many cases, the specified bandwidth of the current transducer is below the bandwidth of the measuring device. Signal components transmitted to the secondary side above the specified bandwidth of the current transducer can be significantly amplified or attenuated. The bandwidth of the current transformers is not limited by a low pass filter with the corresponding cut-off frequency.

According to the Danisense datasheets all current transducers have a specified bandwidth. The value can be found on page 2.

Interference signals or higher frequency components in the primary signal can be amplified or attenuated by the current transducer in the range between the specified bandwidth of the current transducer and the cut-off frequency, which can falsify the measured values.

This is the expected behaviour of the Danisense current transducers. Due to the lack of the power supply, the current transducer can temporarily no longer regulate the magnetic flux density in the magnetic cores towards 0. That’s why those current transducers are also called zero-flux current transducers. If the primary current is not switched off, it continues to generate a magnetic field and therefore a magnetic flux in the iron cores. If the current transducer is put back into operation by plugging in the DSUB cable the green LED stays dark and the device remains in fault. The explanation is that the flux-gate detector is in saturation due to the existence of the primary current before transducer turn-on, so the transducer controls is not able to cancel/nullify the magnetic field inside the core. Consequence can be the larger offset error / magnetization of the core.



In order to achieve the specified error values again, the primary current should only be switched on if the power supply to the current transducer is correct. This is the only way to guarantee that no residual magnetization of the iron cores occurs.

 If the DSUB cable is accidentally disconnected from the current transducer during test operation and then reconnected, the primary current must be set to zero. Afterwards the current transducer should then work again as expected.