L.M.Photonics Ltd
P.O. Box 13 076
Christchurch
New Zealand
Fax ++64 3 332 5220 [New Zealand 03) 332 5220]
Tuesday, April 28, 2009
Static Power Factor Correction
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 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 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 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.
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 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.
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.
The power factor correction required can be calculated from either a known power factor, load and required power factor, or from known KVA and KW of the load.
The power factor correction required can be calculated from either a known power factor, load and required power factor, or from known KVA and KW of the load.
Introduction to Power Factor Correction
Power factor is the ratio between KW and KVA and is a measure of the usefulness of the current applied to the load. A poor power factor can result due to a significant reactive component to the current, or due to a high level of harmonics in the current flowing. A lagging power factor is common and is due to an inductive load such as induction motors, chokes, lighting ballasts and transformers. A lagging power factor can be corrected by the addition of power factor correction capacitors. Poor power factor due to a high level of harmonic currents as caused by variable speed drives, rectifiers and discharge lighting can not be corrected except by the use of expensive filter circuits.
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".
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".
Acceleration
The rate of acceleration of the motor and driven load is a function of the effective load inertia and the acceleration torque. The acceleration torque at any speed, is the difference between the load torque at that speed and the torque produced by the motor.
Electrical Calculations will plot out the speed time curve for a motor and load provded that you have entered in the load torque cure and inertia, and the motor characteristics. You can select different starters and see the effects of these starters on the acceleration time.
If the load torque is close to the torque developed by the motor, the rate of acceleration will be reduced considerably and the start current should be increased to ensure that there is always a good margin between the two torque curves.
Electrical Calculations will plot out the speed time curve for a motor and load provded that you have entered in the load torque cure and inertia, and the motor characteristics. You can select different starters and see the effects of these starters on the acceleration time.
If the load torque is close to the torque developed by the motor, the rate of acceleration will be reduced considerably and the start current should be increased to ensure that there is always a good margin between the two torque curves.
Slip Ring Resistors
Slip ring motors, or wound rotor motors, need to have resistors in their rotor circuit to enable them to develop high slip torque. If the rotor is shorted out, the motor will have a very highLocked Rotor Current and a low Locked Rotor Torque.
If you then apply a reduced voltage starter to the stator, the shaft torque will be much lower than for a standard cage type motor.
The resistors in the rotor circuit modify the start torque curve of the motor. As the resistance is increased, the slip at which the maximum torque occurs is increased. At zero ohms, the maximum torque is at very close to full speed. By selecting a number of stages with the torque occuring across the slip range from zero to 100%, you can design a start system where the motor can produce maximum torque for minimum current fromzero speed to rated speed. In order to prevent a very high current surge when shorting the last resistor stage, it is important to position the final stage close to full speed. If you position the final stage maximum torque at half speed, there will be a big jump in torque when the last contactor is closed.
When selecting resistors for a slip ring motor, you must select a resistor that has the correct resistance and also is capable of absorbing enough energy.
When starting a machine, the full speed kinetic energy of that machine is dissipated in the secondary resistors. This is usually a significant amount of energy.
To determine the values of the rotor resistances, you need to know the rotor voltage and rotor current of the motor, and the number of steps required.
The resistors can be connected in a star configuration, or in a delta configuration. If you are going to apply a soft starter to the stator of the slip ring motor, you select 1 stage only and use that value. This will result in a lower torque and higher start current than would be achieved using a proper multistage slip ring starter.
If you then apply a reduced voltage starter to the stator, the shaft torque will be much lower than for a standard cage type motor.
The resistors in the rotor circuit modify the start torque curve of the motor. As the resistance is increased, the slip at which the maximum torque occurs is increased. At zero ohms, the maximum torque is at very close to full speed. By selecting a number of stages with the torque occuring across the slip range from zero to 100%, you can design a start system where the motor can produce maximum torque for minimum current fromzero speed to rated speed. In order to prevent a very high current surge when shorting the last resistor stage, it is important to position the final stage close to full speed. If you position the final stage maximum torque at half speed, there will be a big jump in torque when the last contactor is closed.
When selecting resistors for a slip ring motor, you must select a resistor that has the correct resistance and also is capable of absorbing enough energy.
When starting a machine, the full speed kinetic energy of that machine is dissipated in the secondary resistors. This is usually a significant amount of energy.
To determine the values of the rotor resistances, you need to know the rotor voltage and rotor current of the motor, and the number of steps required.
The resistors can be connected in a star configuration, or in a delta configuration. If you are going to apply a soft starter to the stator of the slip ring motor, you select 1 stage only and use that value. This will result in a lower torque and higher start current than would be achieved using a proper multistage slip ring starter.
Selecting a Starter
1) Identify the driven load and if possible, obtain the starting requirements of this load.
If this load has not been used in earlier calculations, you will need to enter the speed / torque data into the table, otherwise the data can be recalled.
2) Identify a potential motor and if possible, obtain the starting characteristics of the motor. Of particular interest are the speed / torque curve and the speed current curve for that particular motor. If this motor has not been used in earlier calculations, you will need to enter the speed / torque and speed / current data into the table, otherwise the data can be recalled from disk.
3) Open the "Motor Starting" section of the program and fill in all the data on the motor and load. Where the full data is not available, enter in the locked rotor characteristics only. Make sure that the correct units are selected. When the units are changed, you are given the option of changing the values or leaving the values as entered. For specific load and motor data, it is preferable to save them as absolute units to avoid confusion. Much of the data available is quoted in percentage only. This refers back to the motor rating. If the data has been previously entered and saved, use the "File" "Open Motor" and/or "Open Load" menu options to select and open the relevant data file.
When the data is correctly displayed in the table, the starter options are shown in a results panel at the bottom of the page. This panel includes the minimum starter setting, i.e. auto transformer on the 80% tap, of soft starter at 380% current.
4) Save the data for future usage using the "File" "Save ...." menu options. Use a file name that is very descriptive to make it easy to identify this information next time. Save the data in absolute units, (NM or lbft) unless you wish to save the data as a generic curve that can be applied indicatively across a range of KW sizes. NB using generic curves, particularly for motors can be very erroneous due to the massive variation between motor designs.
5) The starting current and starting torque curves for a range of starter and starting conditions can be displayed using the "Graph" menu option.
If this load has not been used in earlier calculations, you will need to enter the speed / torque data into the table, otherwise the data can be recalled.
2) Identify a potential motor and if possible, obtain the starting characteristics of the motor. Of particular interest are the speed / torque curve and the speed current curve for that particular motor. If this motor has not been used in earlier calculations, you will need to enter the speed / torque and speed / current data into the table, otherwise the data can be recalled from disk.
3) Open the "Motor Starting" section of the program and fill in all the data on the motor and load. Where the full data is not available, enter in the locked rotor characteristics only. Make sure that the correct units are selected. When the units are changed, you are given the option of changing the values or leaving the values as entered. For specific load and motor data, it is preferable to save them as absolute units to avoid confusion. Much of the data available is quoted in percentage only. This refers back to the motor rating. If the data has been previously entered and saved, use the "File" "Open Motor" and/or "Open Load" menu options to select and open the relevant data file.
When the data is correctly displayed in the table, the starter options are shown in a results panel at the bottom of the page. This panel includes the minimum starter setting, i.e. auto transformer on the 80% tap, of soft starter at 380% current.
4) Save the data for future usage using the "File" "Save ...." menu options. Use a file name that is very descriptive to make it easy to identify this information next time. Save the data in absolute units, (NM or lbft) unless you wish to save the data as a generic curve that can be applied indicatively across a range of KW sizes. NB using generic curves, particularly for motors can be very erroneous due to the massive variation between motor designs.
5) The starting current and starting torque curves for a range of starter and starting conditions can be displayed using the "Graph" menu option.
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