Power Factor Definitions
Power Factor
The ratio between the KW of the load and the KVA of the load.
KW - KiloWatt
The real power in the load in thousands of watts.
Single phase KW = V x A x pf / 1000
Three phase KW = Vp x A x pf x rt3 / 1000
KVA - Apparent Power
The apparant power in the load.
Single phase KVA = Vrms x Arms
Three Phase KVA = Vrms x Arms x rt3
KVAR - Reactive Power - KVA Reactive
The reactive power is the apparent power in the reactive current.
This is essentially the KVA with the reactive current only considered.
The reactive power does not include any KW component.
Displacement correction is usually rated in KVAR.
Power Factor Calculations.
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.
There are two types of power factor, displacement power factor and distortion power factor. Only displacement power factor can be corrected by the addition of capacitors.
Displacement Power factor.
The Line Current comprises two components of current, a real component indicating work current, and a reactive component which is 90 degrees out of phase. The reactive current indicates either inductive or capacitive current and does not do any work. The Real current, or in phase current, generates Power (KW) in the load and reactive current does not generate power in the load. The effect of the reactive curent is measured in KVARs. The composite line current is measured in KVA.
The vectors can be represented as two equivilant triangles, one triangle being the real current, the reactive current and the composite (line) current. The cosine of the angle between the line current phasor and the real current represents the power factor.
The second identical triangle is made up of the KW KVA and KVAR vectors.
For a given power factor and KVA (line current) the KVAR (reactive current) can be calculated as the KVA times the sine of the angle between the KVA and KW.
Three phase calculations:
KVA = Line Current x Line Voltage x sqrt(3) / 1000
KVA = I x V x 1.732 / 1000
KW = True Power
pf = Power Factor = Cos(Ø)
KW = KVA x pf = V x I x sqrt(3) x pf
KVAR = KVA x Sine(Ø) = KVA x sqrt(1 -pf x pf)
Single phase calculations:
KVA = Line Current x phase Voltage /1000
KVA = I x V / 1000
KW = True Power
pf = Power Factor = Cos(Ø)
KW = KVA x pf = V x I x sqrt(3) x pf
KVAR = KVA x Sine(Ø) = KVA x sqrt(1 -pf x pf)
To calculate the correction to correct a load to unity, measure the KVA and the displacement power factor, calculate the KVAR as above and you have the required correcion.
To calculate the correction from a known pf to a target pf, first calculate the KVAR in the load at the known power factor, than calculate the KVAR in the load for the target power factor and the required correction is the difference between the two. i.e.
Measured Load Conditions:
KVA = 560
pf = 0.55
Target pf = 0.95
(1) KVAR = KVA x sqrt(1 - pf x pf) = 560 x sqrt(1 - 0.55 x 0.55)
= 560 x 0.835
= 467.7 KVAR
(2) KVAR = KVA x sqrt(1 - pf x pf) = 560 x sqrt(1 - 0.95 x 0.95)
= 560 x 0.3122
= 174.86 KVAR
(3) Correction required to correct from 0.55 to 0.95 is (1) - (2)
= 292.8 KVAR (= 300 KVAR)
To calculate the reduction in line current or KVA by the addition of power factor correction for a known initial KVA and power factor and a target power factor, we first calculate the KW from the known KVA and power factor. From that KW and the target power factor, we can calculate the new KVA (or line current). i.e.
Initial KVA = 560
Initial pf = 0.55
Target pf = 0.95
(1) KW = KVA x pf = 560 x 0.55 = 308 KW
(2) KVA = KW / pf = 308 / 0.95 = 324 KVA
=> KVA reduction from 560 KVA to 324 KVA
=> Current reduction to 57% (43% reduction)
Displacement Power Factor
What is Displacement Power Factor?
Displacement
Power Factor is caused by a reactive component in the load. If there is an inductive compinent in the load, then there will be an inductive current flowing in addition to the resistive current. The inductive curent follows the voltage waveform by 90 degrees. Likewise a capacitive component causes a capacitive current that leads the voltage waveform by 90 degrees. The vector sum of the reactive (capacitive and/or inductive) current(s) and the resistive current results in a single current with a phase angle before (leading) or after(lagging) the voltage waveform. The displacment power factor value is the cosine of the angle between the voltage waveform and the resultant current waveform.
Displacement power factor is typically decreased by inductive loads such as induction motors, transformers and lighting ballasts.
Power Factor of an Induction Motor
Reference to the equivilent circuitof an induction motor, shows that the induction motor draws current from the supply that is made up of resistive components and inductive components. The resistive components are:
1) Load current.
2) Loss current.
and the inductive components are:
3) Leakage reactance.
4) Magnetizing current.
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.
Distortion Power Factor
What is distortion power factor?
Distortion power factor is caused by the presense of harmonics in the current waveform. The harmonics are caused by a non linear load which is commonly a solid state rectifier of SCR based controller.
The major sources of harmonics in industry are the input rectifiers of AC and DC drive systems and switchmode power supply systems.
Distortion Power Factor Correction
Distortion power factor can only be corrected by reducing the harmonic currents, This can be achieved by the use of
passive harmonic filters, active filters or active rectifier circuits.
Like displacement power factor, distortion power factor indicates the potential losses in the supply that can be reduced by the appropriate correction. Additionally, a poor distortion power factor can have a serious affect on other equipment connected to the supply. Hence, a poor distortion power factor is far more damaging and less desirable than a pooor displacement power factor.
Detuned Power Factor Correction
Why detune?
Displacement power factor correction systems are made up of switched banks of capacitors adding capacitive reactive current to neutralise the inductive reactive current of the load.
In the presence of high
harmonic currents, the woltage waveform becomes distorted with the distorted voltage applied to the terminals of the capacitors.
Harmonic voltages increase the currents flowing through the capacitors and can cause premature capacitor failure.
The impedance of the capacitor reduces with increasing frequency. The addition of a series reactor causes the impedance to rise with increasing frequency and reduces the harmonic currents through the capacitors.
Detuned Capacitors
The addition of detuning reactors to power factor correction capacitors causes the terminal voltage of the capacitors to rise in the presence of harmonics.
It is important to increase the voltage rating of the capacitors condsiderably.
Capacitors designed for use with detuning reactors are typically rated at 525V or 565 volts for a 380/400 volt system. The KVAR rating of the capacitors must be based on the actual supply voltage and the continuous applied voltage rating must be much higher.
Detuning reactors.
The detuning reactors are designed to be used with special high voltage capacitors and are rated in
KVAR where the KVAR rating is the KVAR of the capacitor that they are designed to be used with. You use a 25KVAR detuning reactor to detune a 25KVAR capacitor.
The series circuit of reactor and capacitor forms a resonant circuit. It is important that the resonant frequency of this tuned circuit is not near a harmonic frequency. This is why the detuning reactor must match the detuned capacitor.
Power Factor Correction - Static Correction Capcitor Selection
400V 50Hz - not detuned
| Motor(KW) | 2 Pole | 4 Pole | 6 Pole | 8 Pole |
| 5.5 | POLT400-2 | POLT400-2 | POLT400-2.5 | POLT400-4 |
| 7.5 | POLT400-2.5 | POLT400-2.5 | POLT400-2.5 | POLT400-5 |
| 11 | POLT400-4 | POLT400-4 | POLT400-5 | POLT400-6.2 |
| 15 | POLT400-5 | POLT400-5 | POLT400-6.2 | POLB400-8 |
| 22 | POLB400-8 | POLB400-8 | POLB400-8 | POLB400-10 |
| 30 | POLB400-8 | POLB400-8 | POLB400-10 | POLB400-12.5 |
| 37 | POLB400-12.5 | POLB400-12.5 | POLB400-12.5 | POLB400-15 |
| 45 | POLB400-15 | POLB400-15 | POLB400-15 | POLB400-15 |
| 55 | POLB400-15 | POLB400-15 | POLB400-15 | POLB400-20 |
| 75 | POLB400-20 | POLB400-20 | POLB400-25 | POLB400-25 |
| 90 | POLB400-25 | POLB400-25 | POLB400-25 | POLB400-25 |
| 110 | POLB400-30 | POLB400-30 | POLB400-35 | POLB400-37.5 |
| 132 | POLB400-30 | POLB400-30 | POLB400-35 | 2 X POLB400-20 |
| 160 | 2 x POLB400-20 | 2 x POLB400-20 | 2 x POLB400-20 | 2 x POLB400-25
|
Simon Fan Sales Manager
Shenzhen VTdrive Technology Co.,Ltd.
Skype:simon.fan0611 E:simon.fan@vtdrive.com W:www.vtdrive.com T:+86-0755-23060667 M:+86-15920046216
