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power factor correction
Hi
I downloaded a shareware program called BusBar Calculations 2.22 by
Photonics Ltd P.O. Box 13-076 Christchurch, New Zealand.
http//home.clear-dot-net.nz/pages/lmphotonics I thought it might
be of some use to a friend who is an electrician. Didn't find the program to
be of much use. But I found this in the help files.
In light of some recent posts on power factor correction for NSTs and RSGs,
etc. I thought I would pass it along as it has some points to ponder.
"Introduction to Power Factor Correction
Power Factor correction is applied to circuits which include induction
motors as a means of reducing the inductive component of the current and
thereby reduce the losses in the supply. There should be no effect on the
operation of the motor itself.
Power factor correction is achieved by the addition of capacitors in
parallel with the connected motor circuits and can be applied at the
starter, or applied at the switchboard or distribution panel.
Capacitors connected at each starter and controlled by each starter is
known as "Static Power Factor Correction" while capacitors connected at a
distribution board and controlled independently from the individual starters
is known as "Bulk Correction".
Bulk Power Factor Correction
The Power factor of the total current supplied to the distribution board is
monitored by a controller which then switches capacitor banks In a fashion
to maintain a power factor better than a preset limit. (Typically 0.95)
Ideally, the power factor should be as close to unity as possible. There is
no problem with bulk correction operating at unity or even over corrected.
Static Power Factor Correction
(this is where it starts to get interesting)
As a large proportion of the inductive or lagging current on the supply is
due to the magnetising current of induction motors, it is easy to correct
each individual motor by connecting the correction capacitors to the motor
starters. With static correction, it is important that the capacitive
current is less than the inductive magnetising current of the induction
motor. In many installations employing static power factor correction, the
correction capacitors are connected directly in parallel with the motor
windings. When the motor is Off Line, the capacitors are also Off Line. When
the motor is connected to the supply, the capacitors are also connected
providing correction at all times that the motor is connected to the supply.
This removes the requirement for any expensive power factor monitoring and
control equipment. In this situation, the capacitors remain connected to the
motor terminals as the motor slows down. An induction motor, while connected
to the supply, is driven by a rotating magnetic field in the stator which
induces current into the rotor. When the motor is disconnected from the
supply, there is for a period of time, a magnetic field associated with the
rotor. As the motor decelerates, it generates voltage out its terminals at a
frequency which is related to it's speed. The capacitors connected across
the motor terminals, form a resonant circuit with the motor inductance. If
the motor is critically corrected, (corrected to a power factor of 1.0) the
inductive reactance equals the capacitive reactance at the line frequency
and therefore the resonant frequency is equal to the line frequency. If the
motor is over corrected, the resonant frequency will be below the line
frequency. If the frequency of the voltage generated by the decelerating
motor passes through the resonant frequency of the corrected motor, there
will be high currents and voltages around the motor/capacitor circuit. This
can result in sever damage to the capacitors and motor. It is imperative
that motors are never over corrected or critically corrected when static
correction is employed.
Static power factor correction should provide capacitive current equal to
80% of the magnetising current, which is essentially the open shaft current
of the motor.
The magnetising current for induction motors can vary considerably.
Typically, magnetising currents for large two pole machines can be as low as
20% of the rated current of the motor while smaller low speed motors can
have a magnetising current as high as 60% of the rated full load current of
the motor. It is not practical to use a "Standard table" for the correction
of induction motors giving optimum correction on all motors. Tables result
in undercorrection on most motors but can result in over correction in some
cases. Where the open shaft current can not be measured, and the magnetising
current is not quoted, an approximate level for the maximum correction that
can be applied can be calculated from the half load characteristics of the
motor. It is dangerous to base correction on the full load characteristics
of the motor as in some cases, motors can exhibit a high leakage reactance
and correction to 0.95 at full load will result in overcorrection under no
load, or disconnected conditions.
Static correction is commonly applied by using one contactor to control
both the motor and the capacitors. It is better practice to use two
contactors, one for the motor and one for the capacitors. Where one
contactor is employed, it should be up sized for the capacitive load. The
use of a second contactor eliminates the problems of resonance between the
motor and the capacitors.
Inverter. Static Power factor correction must not be used when the motor is
controlled by a variable speed drive or inverter.
Solid State Soft Starter. Static Power Factor correction capacitors must
not be connected to the output of a solid state soft starter. When a solid
state soft starter is used, the capacitors must be controlled by a separate
contactor, and switched in when the soft starter output voltage has reached
line voltage. Many soft starters provide a "top of ramp" or "bypass
contactor control" which can be used to control the power factor correction
capacitors."
later
deano