STPMC1 ST Microelectronics, STPMC1 Datasheet - Page 26

• Manufacturer Part Number: STPMC1
• Description: Programmable poly-phase energy calculator IC
• Manufacturer: ST Microelectronics

www.DataSheet4U.com
Theory of operation
9.8
26/76
The SWM mode is indicated by status bit NAH =0:
●
●
When bit BFR is set, for a certain phase, its energy registers (active, reactive, fundamental)
are blocked. Then, if RMS value of current signal is big enough, bit NAH is cleared (0) and a
SWM operation is entered. In this case the RMS value of current signal is accumulated in
ACx register and the value of voltage RMS U
Example 1: Single wire operation with SYS = 0
Example 2: Single wire operation with SYS = 0 and TCS = 1
The accumulators ACx can be read by means of SPI.
To retrieve energy information, RMS value of current signal accumulated in registers ACx
can be multiplied by a constant representing the value of RMS voltage. This operation must
be executed by a microcontroller.
Usually the supply voltage for the electronic meter is taken from the line voltage. In SWM,
since the line voltage is not present anymore, some other power source must be used in
order to provide the necessary supply to STPMC1 and the other electronic components of
the meter.
Load monitoring (status bit BIL, configuration bit LTCH)
The STPMC1 includes in each phase a no-load condition detection circuit with adjustable
threshold. This circuit monitors the voltage and the current channels and, when the
measured voltage is below the set threshold, an internal signal BIL becomes high. The
information about this signal is also available in the status bit BIL, one per each phase (see
Table
The three phase status bit BIL is the AND of each phase status bit BIL.
Bit NAH=0 (SWM on) happens when BFR=1 and RMS value of current signal is I
I
is big enough, so it is accumulated in the corresponding ACx phase register.
Bit NAH=1 (SWM off) happens if BFR=1 and RMS value of current signal is I
I
current I
SYS = 0 (3-phase system) is set and in the R-phase the voltage signal is too low
(status bits of phase R BFR = 1 and LOW = 1).
Because of the too low voltage signal the frequency can't be calculated and energy
registers related to the R-phase are blocked.
If RMS value of current signal is big enough, the device enters SWM and clears NAH of
R phase. The ACR register is incremented by adding I
signal.
SYS = 0 (3-phase system) and TCS = 1 (CT sensor selection) are set and in all phases
(R, S and T) the voltage signal is too low (status bits BFR = 1 and LOW = 1 for all
phases).
Because of the too low voltage signal the frequency could not be calculated and all
energy registers are blocked.
Since when TCS = 1, a frequency value is needed to calculate the RMS value of the
current signal, the default value of 50 Hz or 60 Hz (if bit FRS=1) is taken. If the RMS
value of current signal is big enough, the device enters SWM and clears NAH of all
phases and ACR, ACS and ACT registers is fed with the correspondent I
Xmax
Xmax
30).
/4096 = 16 (I
/8192 = 8, or BFR=0. In this case either voltage frequency is out of limits but RMS
X
is too small to enter SWM mode, or voltage frequency is in the correct range.
Xmax
= 2
Doc ID 15728 Rev 1
16
). In this case frequency is out of limits and RMS current I
X
is set to zero.
R
, the RMS value of current
X
.
X
STPMC1
<
X
>
X