Domestic Power Factor Correction
Q. Can power factor correction save KWHrs in a
domestic environment?
No, The addition of pwer factor correction may reduce the current drawn,
but will not cause your meter to run slower.
Q. Does power factor correction make motors run
more efficiently?
If you connect the correct value of capacitors in the supply to an induction
motor, you will reduce the current flow from the supply to the point where
the capacitors are connected. If you measure the current in the supply
between the capacitors and the motor, you will find that the current does
not change. The current into the motor is independent of the connection
of the capacitors. The efficiency of the motor is unchanged.
Q. Does power factor correction make motors run
cooler?
No, the motor losses are not changed so the temperature rise of the motors
remains the same.
Q. Does power factor correction reduce energy
consumed?
No, the addition of power factor correction will reduce the current drawn
from the supply in situations where there is an inductive current flowing
and the correction is equal to or less than the inductive current. The
reduction in current can reduce the losses in the supply, but no appreciable
loss reduction will occur within the residence.
Technologies.
Power Factor Definition : Power factor is the ratio
between the KW and the KVA drawn by an electrical load where the KW is
the actual load power and the KVA is the apparent load power. It is a
measure of how effectively the current is being converted into useful
work output and more particularly is a good indicator of the effect of
the load current on the efficiency of the supply system.
All current will cause losses in the
supply and distribution system. A load with a power factor of 1.0
results in the most efficient loading of the supply and a load with
a PF of 0.5 will result in much higher losses in the supply system.
A poor power factor can be the result of either a significant phase
difference between the voltage and current at the load terminals,
or it can be due to a high harmonic content or distorted/discontinuous
current waveform.
Poor load current phase angle is generally the result of an inductive
load such as an induction motor, power transformer, lighting ballasts,
welder or induction furnace.
A distorted current waveform can be the result of a rectifier, variable
speed drive, switched mode power supply, discharge lighting or other
electronic load. 

The current drawn by an inductive load such as an induction
motor, comprises two components: the resistive current that actually does
the work, and the inductive or magnetic current that does no work. The
resistive component of the current is in phase with the supply voltage
and draws KW as measured by the power meter. The inductive current, commonly
referred to as the reactive current is at 90 degrees to the supply voltage
and does not contribute to the KW measured by the power meter.
When you connect an ameter in series with an induction motor
to measure the current, you measure both the resistive current and the
reactive current at the same time, and due to the phase angle between
them, they do not directly add. For those with a mathematicaal bent, the
resultant current measured, is the square root of the sum or the square
of the resistive current and the square of the inductive current.
KW = V x I_{resistive}
KW = V x I_{motor} x pf
where pf = Cos(ø) and ø is the angle between the resistive
current and the motor current.
Any change in I_{magnetising} does not alter the KW in the formula
above.
Adding power factor correction to a circuit that has an
inductive current flowing, will add a capacitive current. The capacitive
current is also a reactive current, but it is in the opposite direction
to the inductive current. It is 180 degrees out of phase. In effect, the
capacitive current subtracts from the inductive current. If they are equal,
they wil cancell out to zero. If the capacitive current is less than the
inductive current, the total reactive current will reduce. If the capacitive
current is greater than the inductive current, the resultant current will
be capacitive.
If capacitive current is added to a resistive load, the net current will
increase.
To correct the power factor, the amount of capacitive current
needs to be constantly adjusted to match the amount of inductive current
drawn. This can not be done by adding a fixed amount to the supply. An
automatic switching unit is required.
