STPMC1 datasheet(26/77 Pages) STMICROELECTRONICS

26 / 77 page

Theory of operation

STPMC1

26/77

Doc ID 15728 Rev 6

Each ACx register contains a 20-bit accumulator of the relative phase current IX [Ah] and an

8-bit register carrying the information about phase delay between voltage channels.

The SWM mode is indicated by status bit

NAH =0:

●

Bit

NAH=0 (SWM on) happens when BFR=1 and RMS value of current signal is IX >

IXmax/4096 = 16 (IXmax = 2

16). In this case frequency is out of limits and RMS current 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 IX <

IXmax/8192 = 8, or BFR=0. In this case either voltage frequency is out of limits but RMS

current IX is too small to enter SWM mode, or voltage frequency is in the correct range.

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 UX is set to zero.

Example 1: Single wire operation with SYS = 0

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 IR, the RMS value of current

signal.

Example 2: Single wire operation with SYS = 0 and TCS = 1

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 IX.

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.

9.8

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

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