Page 106 - Current & voltage sensors catalog

Calculation guide
Electronic technology current sensors NCS sensors

1 - Reminder of the key elements
Formulas Abbreviations

I P : primary current
I PN : nominal primary current
V M1 = R M1 x I S1
I PMAX : maximum primary current
I S1 : secondary current at I PN
1SBC146045V0014 with 0 Ω < R M1 or R M2 < 350 Ω I S2 : secondary current at I PMAX
V M2 = R M2 x I S2
V S1
: secondary voltage at I PN
4 V S2 : secondary voltage at I PMAX
:
supply voltage
V A
V M : measuring voltage
R M : measuring resistance
NCS125-10
R MMIN : minimum measuring resistance
R MMAX : maximal measuring resistance
Reminder of the sensor electrical connection

NCS sensor Power supply
+ V A + V A
0V 0V
- V A - V A
I s1
I s1 R M1
V M1
I s2
I s2 R M2
V M2
V s1 R M3
I p
V M3
V s2 R M4
V M4 G0220DG
0V
2 - Measurement circuit calculation (current output)
Example with NCS125-4 sensor
I PN = 4 000 A I S1 @ I PN = ±20 mA
I PMAX = 20 000 A I S2 @ I PMAX = ±20 mA
R M = 0 - 350 Ω (I S1 & I S2) V S1 @ I PN = ±10 V
R M > 10 kΩ (V S2 & V S2) V S2 @ I PMAX = ±10 V
V A = ±15 V ... ±24 V
The design of the sensor requires that 2 operating points are respected on the outputs I S1 and I S2:
• A maximum measuring voltage of 7 V DC (V MMAX ≤ R MMAX x I SMAX)
• A maximum output current of ±20 mA DC.
The supply voltage does not have any influence on the output signals.
2.1 - What load resistance (R M) is required to obtain a 5 V measuring (V M = 5 V) when the current I P = 6000 A peak?
The measured current is greater than I PN (4000 A for a NCS125-4), I S2 is therefore used as the measuring signal.
Firstly the output current on I S2 must be calculated when I PN = 6000 A DC
I S2 = I PN / I PMAX x I SMAX = 6000 / 20000 x 20 = 6 mA (correct because I S2MAX = ±20 mA DC)
Now determine the value of the resistance R M
R M = V M / I S2 = 5 / 0.006 = 833.33 Ω
Conclusion
The NCS125-4 sensor can measure 6000 A peak on the signal output I S2 with a resistance of 416.67 Ω
(greater than 350 Ω) because the output current is smaller than I SMAX i.e. 20 mA DC
The product of R M x I SMAX must always be smaller than or equal to maximum output of 7 V DC

3 - Measurement circuit calculation (voltage output)
No special calculation needs to be made. This NCS sensor range supplies a voltage directly proportional to
the primary current I P between -10 V and +10 V. A load resistance of a value greater than or equal to 10 kΩ 1SBC140072S0201
adapts the impedance of the measured output (V S1 or V S2) to the acquisition system.

104 | ABB