Datasheet

ADE7953 Data Sheet
Rev. B | Page 38 of 72
POWER FACTOR
The ADE7953 provides a direct power factor measurement
simultaneously on Current Channel A and Current Channel B.
Power factor in an ac circuit is defined as the ratio of the active
power flowing to the load to the apparent power. The power
factor measurement is defined in terms of “leading” or “lagging,”
referring to whether the current waveform is leading or lagging
the voltage waveform.
When the current waveform is leading the voltage waveform,
the load is capacitive and is defined as a negative power factor.
When the current waveform is lagging the voltage waveform,
the load is inductive and is defined as a positive power factor.
The relationship of the current waveform to the voltage wave-
form is illustrated in Figure 59.
ACTIVE (–)
REACTIVE (–)
ACTIVE (+)
REACTIVE (–)
+60° = θ; PF = –0.5
I
V
I
–60° = θ; PF = +0.5
ACTIVE (–)
REACTIVE (+)
ACTIVE (+)
REACTIVE (+)
CAP
ACITIVE LOAD:
CURRENT LEADS
VOLT
AGE
INDUCTIVE LOAD:
CURRENT LAGS
VOLT
AGE
09320-028
Figure 59. Capacitive and Inductive Loads
As shown in Figure 59, the reactive power measurement is
negative when the load is capacitive and positive when the
load is inductive. The sign of the reactive power can therefore
be used to reflect the sign of the power factor.
The mathematical definition of power factor is shown in
Equation 37.
Power Apparent
Power Active
PowerReactiveofSignFactorPower
||
)( ×=
(37)
The absolute value of active power is used.
The power factor measurement includes the effect of all
harmonics over the 1.23 kHz bandwidth.
The power factor readings are stored in two 16-bit, signed
registers: PFA (Address 0x10A) for Current Channel A and
PFB (Address 0x10B) for Current Channel B. These registers are
signed, twos complement registers with the MSB indicating the
polarity of the power factor. Each LSB of the PFx register equates
to a weight of 2
−15
; therefore, the maximum register value of
0x7FFF corresponds to a power factor value of 1. The minimum
register value of 0x8000 corresponds to a power factor of −1.
By default, the instantaneous active and apparent power
readings are used to calculate the power factor, and the register
is updated at a rate of 6.99 kHz. The sign bit is taken from the
instantaneous reactive energy measurement on each channel.
USING THE LINE CYCLE ACCUMULATION MODE
TO DETERMINE THE POWER FACTOR
If a power factor measurement with more averaging is required,
the ADE7953 can use the line cycle accumulation measurement
on the active and apparent energies to determine the power factor
(see the Active Energy Line Cycle Accumulation Mode section
and the Apparent Energy Line Cycle Accumulation Mode section).
This option provides a more stable power factor reading.
To use the line cycle accumulation mode to determine the power
factor, the ADE7953 must be configured as follows:
• The PFMODE bit (Bit 3) must be set to 1 in the CONFIG
register (Address 0x102).
• The line cycle accumulation mode must be enabled on
both the active and apparent energies by setting the
xLWATT and xLVA bits to 1 in the LCYCMODE register
(Address 0x004).
When using line cycle accumulation to determine the power
factor, the update rate of the power factor measurement is an
integral number of half line cycles. The number of half line cycles
is programmed in the LINECYC register (Address 0x101). For
complete information about setting up the line cycle accumula-
tion mode, see the Active Energy Line Cycle Accumulation Mode
section and the Apparent Energy Line Cycle Accumulation
Mode section.
POWER FACTOR WITH NO-LOAD DETECTION
The power factor measurement is affected by the no-load
condition if no-load detection is enabled (see the No-Load
Detection section). The following considerations apply only
when no-load detection is enabled and a no-load condition
occurs:
• If the apparent energy no-load condition is true, the power
factor measurement is set to 1 because it is assumed that
there is no active or reactive power.
• If the active energy no-load condition is true, the power
factor measurement is set to 0 because it is assumed that
the load is purely capacitive or inductive.
• If the reactive energy no-load condition is true, the sign of
the power factor is based on the sign of the active power.