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.
A poor PF due to an inductive load can be improved by the addition of power factor correction, but, a poor power factor due to a distorted current waveform requires an change in equipment design or expensive harmonic filters to gain an appreciable improvement. Many inverters are quoted as having a PF of better than 0.95 when in reality, the true power factor is between 0.5 and 0.75. The figure of 0.95 is based on the Cosine of the angle between the voltage and current but does not take into account that the current waveform is discontinuous and therefore contributes to increased losses on the supply.
Reactive current flowing in the supply is refered to as reactive power and is usually expressed in VARs or KVARs. A VAR is the product of the reactive current and the applied voltage. A KVAR is equal tp 1000 VARs.
Capacitive Power Factor correction (Power Factor Compensation) 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.
An induction motor draws current from
the supply, that is made up of resistive components and inductive components.
The resistive components are:
The current due to the leakage reactance is
dependant on the total current drawn by the motor, but the magnetizing
current is independent of the load on the motor. The magnetizing current
will typically be between 20% and 60% of the rated full load current of
the motor. The magnetizing current is the current that establishes the
flux in the iron and is very necessary if the motor is going to operate.
The magnetizing current does not actually contribute to the actual work
output of the motor. It is the catalyst that allows the motor to work
properly. The magnetizing current and the leakage reactance can be considered
passenger components of current that will not affect the power drawn by
the motor, but will contribute to the power dissipated in the supply and
distribution system. Take for example a motor with a current draw of 100
Amps and a power factor of 0.75 The resistive component of the current
is 75 Amps and this is what the KWh meter measures. The higher current
will result in an increase in the distribution losses of (100 x 100) /(75
x 75) = 1.777 or a 78% increase in the supply losses.
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. The resulting capacitive current is leading current and is used to cancel the lagging inductive current flowing from the supply.
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 Power Factor Correction".
The Power factor of the total current supplied to the distribution
board is monitored by a power factor controller which then switches capacitor
banks in a fashion to maintain a power factor better than a preset limit.
(Typically 0.95) Ideally, the PF should be as close to unity as possible.
There is no problem with bulk correction operating at unity, however correction
should not be applied to an unloaded or lightly loaded transformer. If
correction is applied to an unloaded transformer, you create a high Q
resonant circuit between the leakage reactance of the transformer and
the capacitors and high voltages can result. Bulk compensation systems
are usually incorporated with the switchgear supplying all or part of
As a large proportion of the inductive or lagging current on the supply is due to the magnetizing 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 magnetizing 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 severe 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 magnetizing current, which is essentially the open shaft current of the motor.
The magnetizing current for induction motors can vary considerably. Typically, magnetizing 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 magnetizing 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 under correction on most motors but can result in over correction in some cases. Where the open shaft current can not be measured, and the magnetizing 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 over correction under no load, or disconnected conditions.
Static correction is commonly applied by using on e 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.
must not be used when
the motor is controlled by a variable speed drive or inverter. The connection
of capacitors to the output of an inverter can cause serious damage to
the inverter and the capacitors due to the high frequency switched voltage
on the output of the inverters.
Static PFC 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. The capacitor contactor is only switched on 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
Static PFC must neutralize no more than 80% of the
magnetizing current of the motor. If the correction is too high, there is
a high probability of over correction which can result in equipment failure
with severe damage to the motor and capacitors. Unfortunately, the magnetizing
current of induction motors varies considerably between different motor
designs. The magnetizing current is almost always higher than 20% of the
rated full load current of the motor, but can be as high as 60% of the rated
current of the motor. Most power factor correction is too light due to the
selection based on tables which have been published by a number of sources.
These tables assume the lowest magnetizing current and quote capacitors
for this current. In practice, this can mean that the correction is often
less than half the value that it should be, and the consumer is unnecessarily
Harmonics on the supply cause a higher
current to flow in the capacitors. This is because the impedance of the
capacitors goes down as the frequency goes up. This increase in current
flow through the capacitor will result in additional heating of the capacitor
and reduce it's life. The harmonics are caused by many non linear loads,
the most common in the industrial market today, are the variable speed
controllers and switchmode power supplies. Harmonic voltages can be reduced
by the use of a harmonic compensator, which is essentially a large inverter
that cancells out the harmonics. This is an expensive option. Passive
harmonic filters comprising resistors, inductors and capacitors can also
be used to reduce harmonic voltages. This is also an expensive exersize.
Detuning reactors are connected in series
with power factor correction capacitors to reduce harmonic currents and
to ensure that the series resonant frequency does not occur at a harmonic
of the supply frequency.
Capacitive Power factor correction connected
to a supply causes resonance between the supply and the capacitors. If
the fault current of the supply is very high, the effect of the resonance
will be minimal, however in a rural installation where the supply is very
inductive and can be a high impedance, the resonances can be very severe
resulting in major damage to plant and equipment. Voltage surges and transients
of several times the supply voltage are not uncommon in rural areas with
weak supplies, especially when the load on the supply is low. As with
any resonant system, a transient or sudden change in current will result
in the resonant circuit ringing, generating a high voltage. The magnitude
of the voltage is dependant on the 'Q' of the circuit which in turn is
a function of the circuit loading. One of the problems with supply resonance
is that the 'reaction' is often well removed from the 'stimulus' unlike
a pure voltage drop problem due to an overloaded supply. This makes fault
finding very difficult and often damaging surges and transients on the
supply are treated as 'just one of those things'.
Harmonic Power Factor correction is not applied to circuits that draw either discontinuous or distorted current waveforms.Most electronic equipment includes a means of creating a DC supply. This involves rectifying the AC voltage, causing harmonic currents. In some cases, these harmonic currents are insignificant relative to the total load current drawn, but in many installations, a large proportion of the current drawn is rich in harmonics. If the total harmonic current is large enough, there will be a resultant distortion of the supply waveform which can interfere with the correct operation of other equipment. The addition of harmonic currents results in increased losses in the supply.
Power factor correction for distorted supplies can not be achieved by the addition of capacitors. The harmonics can be reduced by designing the equipment using active rectifiers, by the addition of passive filters (LCR) or by the addition of electronic power factor correction inverters which restore the waveform back to its undistorted state. This is a specialist area requiring either major design changes, or specialized equipment to be used.
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