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Rogowski Coil Low-Frequency and Droop Characteristics

Source: Probemart 2024-12-11 Views: 1665

In principle a Rogowski coil cannot measure DC, so its detection sensitivity falls at low frequency; the -3 dB point is usually taken as the low-frequency cutoff. When the signal contains low-frequency components, the measured result can deviate from the actual current - take care in use. The cutoff and its roll-off specification vary by model; the roll-off (droop) characteristic refers to the response to a step pulse and is tabulated in Table 1.

1. Overview

Description

In principle, a Rogowski coil current probe cannot perform DC testing. Therefore, it has the characteristic that detection sensitivity decreases at low frequencies. Generally speaking,

The low-frequency cutoff frequency is defined as the frequency at which detection sensitivity drops to -3 dB.

Affected by this low-frequency characteristic, when testing signals containing low-frequency components, the actual current and the result measured by the Rogowski probe will differ to some extent; pay close attention during use.

Low-frequency cutoff frequency and roll-off specifications for each model

Another low-frequency characteristic specification related to the low-frequency cutoff frequency is roll-off (Droopping). The roll-off referred to here is the characteristic when testing a step pulse waveform,

The basic specification values are shown in Table 1 below.

Table 1 Basic specifications of roll-off

Model	Low-frequency cutoff frequency [Hz]	Roll-off [%/ms]
SS-281A	110	80
SS-282A	65	50
SS-283A	32	25
SS-284A	9	9
SS-285A	6	6
SS-286A	3	3
SS-287A	2	2
SS-293S/L	1	0.8
SS-294S/L	0.8	0.7
SS-295S/L	0.6	0.5
SS-296S/L	0.4	0.35

Definition and characteristics of roll-off

Roll-off (Droopping) is the amount of attenuation after a certain time when the pulse waveform rises to the top.

When a Rogowski coil probe is used, it is defined as the attenuation after 1 ms; the unit is [％/ｍｓ].

This ratio increases as the low-frequency cutoff frequency increases.

2. Effect on the measured waveform

The following is a comparison of test results between the DC current probe (SS-240A) and a Rogowski current probe.

It can be seen that the lower the low-frequency cutoff frequency, the smaller the effect of roll-off; at the same time, the lower the sensitivity (the larger the detection range), the more noise is caused

the greater the effect. Therefore, be sure to select the appropriate model according to the actual test purpose.

2.4. Switching waveform (pulse adjustable)

Items ①–③ below are switching waveforms simulated using a variable-frequency circuit (fixed pulse width); the switching frequency is set to 1.8kHz and the AC signal frequency to 50Hz.

The switching frequency is already sufficiently high relative to the low-frequency cutoff frequency, but because an AC signal component (50Hz) is included, its effect on the measured waveform can be seen.

① UsingCase of SS-283A (low-frequency cutoff frequency 32Hz)