ALUMINIUM ELECTROLYTIC CAPACITORS Technical Information

Transcription

1 The capacitance o the anode oil will depend on the etching pattern and the orming voltage. The cathode oil is etched and has a thin oxide layer on it which is caused due to atmospheric oxidation. onstruction An aluminum electrolytic capacitor consists o two electrically conductive aluminum layers separated by a dielectric layer. One o the electrodes (the aluminum oil called anode) undergoes a Process called orming by which a dielectric layer o aluminum oxide (AlO3) is electrochemically coated on it. The other electrode is a conductive liquid called the electrolyte. The second aluminum oil the cathode acts as a large suraced contact area or passing current to the electrolyte. The basic principle o the capacitor is to store electrical charge and is deined as: Q V Q charge in oulombs capacitance in Farads (between the plates) V potential dierence between the plates Based on the ormula given above it can be said that the unit o capacitance the Farad is the capacitance between the plates across which appears a potential dierence o I Volt when it is charged by I oulomb o electricity. The value o capacitance in a capacitor is directly proportional to the area o the plates and is inversely proportional to the distance between them. Hence capacitance is expressed by the equation: Eo. Er. A d A surace area o the plates in m d distance between the plates (or dielectric thickness) in metres Eo. permittivity o ree space 8.885x0 F/m Er. relative permittivity o the dielectric (9.5 or AIO3) The surace area o the anode is enlarged (up to 00 times) by an electrochemical etching process. Similarly the cathode is also etched to increase the surace area. The thickness o the dielectric layer is very small (in microns) and increases in proportion to the orming voltage (approximately.nm/v) making the distance between the two plates very small. This construction o aluminum electrolytic capacitors allows or very high capacitance per unit area in comparison with capacitors which use other dielectric materials. An equivalent circuit o an aluminum electrolytic capacitor is: Manuacturing Process The main stages o the manuacturing process are: Anode oil Etching Forming Slitting athode oil Etching Slitting Winding apacitor paper Slitting Ageing Assembly Impregnation Assembly Sleeving Testing Packing Super pure aluminium oil is etched to increase the surace area. The anode oil undergoes an electrochemical process called orming by which a dielectric layer is ormed on it. The anode and cathode are interleaved with support and/or absorbent papers and wound into a cylinder as shown in the ig. below. During winding aluminum tabs are attached to the oil or electrical contacts by a cold welding process. Anode athode Tissue Paper Extended athode The capacitor element is impregnated with an electrolyte under vacuum. In the assembly process terminals are riveted and/or welded to the tabs and housed in an aluminium can without anchoring material as shown: Terminal Terminal RL RESR RESR RL LESL old weldedtabs tabs Multiple Ultrasonic tab to terminal Wind element welding & revi ng Aluminium canaluminium with insulacanng sleeve LESL equivalent series resistance (ESR) capacitance resistance due to leakage current equivalent series inductance (ESL) apacitor Spigot element Spigot aluminium can Extended cathode Rib Ribbed aluminium can July 0

2 Selection o current limiting resistor: apacitors are then sealed and aged. The aging process repairs any damage to the oxide layer that may have been caused during the process o assembling the capacitor into the aluminium can. A thorough test is carried out or seepage by placing the capacitors in an oven. A visual check is carried out on each capacitor or any sign o electrolyte leakage. Next capacitors are tested or the ollowing electrical parameters. i ) apacitance ii) ESR iii) Leakage current iv) Tan δ The capacitors are then sleeved and packed. Ater completion o the production process the company s Q.A. Personnel carry out a sample test. A current limiting resistor which is to be connected in series with the capacitor may be chosen as ollows: a) or capacitors o rating up to 500µF the limiting resistor will be o 000 ohms. b) or capacitors o rating higher than 500µF the value o current limiting resistor will be determined on the ormula: 6 R.5 x 0 capacitance in µf R resistor value in Ohms apacitance: apacitance can be measured by: i) measuring its A impedance ater taking into account amplitude and phase ii) measuring the charge it will hold when Dc voltage is applied D capacitance is approximately equal to. to.5 times A capacitance Notes: Measurement o capacitance is made at requency o o 00 Hz and ambient temperature o 5. The value is in microarads (µf or MFD) and is indicated on the capacitor. apacitance increases with temperature and decreases with increases in requency. Dissipation actor (Tan δ): This is the ratio o ESR to capacitive reactance in the equivalent series circuit. Alternatively it could be deined as the ratio o eective power (dissipated power) to the reactive power or a sinusoidal voltage: Electrical haracteristics Rated voltage: The rated voltage is the D voltage or which the capacitor has been designed. The capacitors can be operated continuously at the ull rated voltage within the operating temperature range. Surge voltage: The surge voltage is the maximum D voltage that a capacitor can be subjected to or a very short duration not exceeding 30 seconds. This includes transients and peak ripple at highest line voltage. apacitors are designed to withstand 6 such surge in an hour at a minimum interval o 0 minutes. The capacitor will withstand the ollowing surge test: The capacitor is connected in series with a current limiting resistor. The rated surge voltage is applied at room temperature or a period not exceeding 30 seconds. The capacitor is then discharged through a suitable resistor. This cycle (charge discharged) may be repeated or a maximum o 6 cycles in one hour each being at an interval o 0 minutes. R i p p l e v o l t a g e : T h e r i p p l e v o l ta g e i s t h e superimposed A voltage that may be applied to the capacitor provided that: i) the sum o D voltage and superimposed A Voltage dose not exceed the rated voltage. ii) rated ripple current is not exceeded. Reverse voltage: Aluminium electrolytic capacitors are polar capacitors. Reverse voltage <.5V can be applied or a duration o less than second but not continuously or repeatedly. The reverse voltage o.5 V is the voltage at which the breakdown o the oxide layers on the cathode takes place. Where necessary a diode may be connected to prevent any reverse voltage rom appearing on the capacitor. RESR LESL R ESR Tan R ESR R ESR Equivalent series resistance (ESR): The equivalent series resistance is the resistive component o equivalent series circuit. it is related to dissipation actor by the ormula: δ RESR Tan ω s RESR equivalent series resistance in Ω Tanδ dissipation actor s series capacitance in Farads ω π (requency) ESR values are measured by the bridge method (to eliminate the resistance o lead wires) at a requency o o 00Hz and ambient temperature o 5. ESR July 0

3 values decrease with increase in temperature and requency: resistor must be connected in series with the capacitor under test. Measurement is carried out at an ambient O O temperature o 5 ± 3. The rated voltage is applied or 5 minutes beore the leakage current measurement are taken: ESR 5o 55o 85o 0 03 Frequency Hz 04 Leakage urrent 05 Impedance: Impedance is given by the ormula: 0 ESR + ( XL Xc ) impedance in Ohms equivalent series resistance (Ω) π L Xc π Impedance is dominated by capacitive reactance (Xc) at lower requencies and by inductive reactance (X L ) at higher requencies. Resonance occurs when: Z Z ESR XL XL 0 Impedance O Ripple current: The ripple current rating o a capacitor 06 Frequency Hz Leakage current: Leakage current is the residual current which continues to low through the capacitor even ater the capacitor has been charged to the set voltage or rated voltage. Ater the capacitor has been ully charged to the set voltage the leakage current will continue to ail with time until a steady state has been reached. Leakage current is a measure o the quality o the dielectric layer and is dependent on capacitance voltage and temperature. Measurement o leakage current is made at the rated D voltage o the capacitor which is applied rom a steady source like a regulated power supply. A current limiting 0 Temperature ( ) Typical curve or : 35 Φ capacitor 50 Φ capacitor 63 Φ capacitor 76 Φ capacitor 05 5 Leakage urrent X at which Z ESR 04 0 Time (minutes) Impedance below resonance decreases with increase in temperature and requency. However impedance above resonance decreases with temperature but increases asrequency increases is the rms value o A current that lows through a capacitor due to the presence o ripple voltage. Ripple current generates heat inside the capacitor which is: P Ir xesr p power loss in watts Ir rms value or ripple current in Amperes ESR equivalent series resistance in Ohms The maximum ripple current that a capacitor can handle depends on : i) the winding design ii) aluminium can design iii) surace area o the can iv) ESR Since ripple current increases the temperature o the capacitor it has a signiicant eect on the operational lie o the capacitor. Ripple current handling capacity is dependent on requency and temperature. Heat sinking and orced air cooling will aid heat transer and allow higher ripple currents to be applied. Shel lie: Shel lie is deined as the times or which a capacitor can be stored without voltage being applied. Normally the capacitance ESR and impedance o a capacitor do not change signiicantly ater extended storage period. However the leakage current can slowly increase. The shel lie versus storage July 0

4 temperature graph is shown below: I capacitors are mounted with the saety vent at the lowest Position (shown below) a small pool o electrolyte may orm near the saety vent. When the vent punctures the electrolyte may spray out on to other components causing damage. Alternatively the electrolyte may dry and crystallize inside the saety vent over a period o time making it non unctional Storage Temp (O) Shel Lie (thousands o hours) Saety vent Region Leakage current remains unchanged Region Leakage current increases In both cases capacitance ESR and impedance do not change signiicantly. I the capacitors are in region capacitors should be preconditioned prior to use. The procedure ollows under preconditioning : Preconditioning: preconditioning is carried out by applying the rated working voltage across the capacitor. The power source should be a regulated power supply. A suitable current limiting resistor should be connected in series with the capacitor. The voltage should be maintained or one hour ater its value has becomes equal to the rated working voltage applied +3%. Ater this the capacitor should be discharge through a resistor o suitable value. The capacitor can now be stored idle or to 4 hours. Ater this period the capacitor can be tested or any o the speciied parameters. apacitor bank design: capacitor may require parallel or series connections or both. This depends on the application. Parallel connection 3 n In a bank o 'n' capacitors connected in parallel each with capacitance rating o...n and voltage rating V V...V n respectively the eective capacitance and voltage o the bank will be: bank n V bank minimum voltage rating o any capacitor in the bank. It is advisable to use capacitors o the same nominal capacitance value and voltage rating to avoid excessive stress on any one capacitor in the bank. In this circuit i any capacitor in the bank ails due to an internal short circuit then all the other capacitors in the bank will discharge through this particular capacitor leading to an extremely abrupt and severe discharge phenomenon. Hence it is advisable to connect these capacitors through a use: Application Notes Mounting positions: During operating the leakage current o the capacitor will cause electrolysis o the electrolyte. The oxygen produced during electrolysis helps in Sel Heating o the dielectric layer. The minute quantity o hydrogen released at this time may increase the internal pressure in the capacitor over an extended period o time. All capacitors are provided with a saety vent which punctures when the pressure inside the capacitor increase beyond the sae limit o 70 psi. Thereore it is recommended that the capacitors be mounted upright or horizontal with the vent on top as shown: Saety vent 3 n use For impulse discharge circuit where it may not be easible to use the uses the capacitors can be 4 July 0

5 protected during charging by means o a suitable current limiting resistor and then connected in parallel at the time o discharge: The value o the shunt resistance can be computed as ollows: nvr Vb A) R L..max {(Vb / Vr) ( (n+9) /0)} R : Shunt resistance value (minimum) in ohms Vr : Rated voltage o each capacitor n : Number o capacitors in( n>) Vb : Bank voltage L.. max : Maximum leakage current o one capacitor(in amp.) B) Suggested wattage o resistor Vr / R ombined series parallel connections apacitors may be connected as ollows: Series onnection 3 n Fig In the bank o 'n' capacitors connected in series each with capacitance rating o... n and voltage rating V V...V n respectively the eective capacitance and voltage o the bank will be: bank Fig V bank V + V Vn I one capacitor in the series bank (Fig.) ails due to a short circuit the other capacitors will be subjected to the total voltage. This may lead to an excess voltage on the other capacitors causing all capacitors in the bank to ail. Hence it is recommended that capacitors are connected as shown in Fig where only one "series bank" suers the risk o ailure in the event o a short circuit o one capacitor. Lie Expectancy : During the working lie o capacitors certain physical and parametric changes occur. These changes eventually make the capacitor unusable either due to "thermal runaway" leading to catastrophic ailure or an excessive parametric drit. At a higher temperature degradation o the material used to manuacture the capacitor accelerates these eects. There are many reasons or these changes. Some perormance aspects o the capacitors cannot be predicted. Hence evaluation o the capacitor's long term behavior must be determined by endurance' tests. Useul lie (service lie or operational lie) is the lie It is advisable to use capacitors o the same nominal capacitance value and voltage rating to avoid excessive stress on any one capacitor in the bank. When capacitors are connected in series the voltage o any individual capacitor should not exceed the maximum permissible voltage. The total D voltage applied is divided among individual capacitors in proportion to their insulation resistance value (leakage current). Hence to avoid any imbalance during charging o the bank it is recommended that a shunt resistor be connected with each capacitor: R R 3 R3 4 R4 5 July 0

6 achieved by the capacitor without exceeding a speciied ailure rate. Useul lie can be prolonged by operating the capacitors at load actors below the rated values speciied like lower operating voltage ripple current and ambient temperature apacitor lie can also be prolonged by appropriate cooling methods. No. o components used in the ield N 000 No. o operating hours to hrs No. o ailures n n Failures% 0.% F (%) N Failures rate λ Failure percentage is the ratio o number o ailures to total number o inspected capacitors. Failure rate(or long term ailure)is the number o components ailing per unit time. The characteristic curve is as ollows: (A) (B) All capacitors are designed to ensure that heat rom the core is transerred quickly and eiciently to the outer aluminium can. It is recommended that eicient air low is created to enable the capacitor bank to remain at the lowest possible temperatures. This would increase the lie expectancy o the capacitor. From the table given below it will be seen that the ripple current carrying capacity can be enhanced by increasing the air low rate. () Time ( t ) Region A is the early ailure period (or inant mortality) this can be decreased by improvement in the manuacturing process Region B is the useul lie (operational lie or service lie) where ailure rate is nearly constant Region is the 'wear out' period. This occurs due to the end o lie o the capacitors and occurs when capacitor properties deteriorate. End o lie o can be due to: i) atastrophic ailure like short circuit open circuit or operation o the saety vent ii) Parametric ailure like ESR increases to more than thrice the initial speciied limit leakage current greater than speciied maximum limit capacitance changes o more than ±30% a combination o these Ripple urrent Multiplier Air Flow Rate metres per second (m/s) an Size (DxL) mm 35X6 35X80 35X05 50X80 50X05 50X0 63X05 63x0 63X45 76X05 76X0 76X45 76X77 76X0 90X05 90X0 90X45 90X0 Reliability is the probability that the capacitor will perorm satisactorily under given set o conditions or a given length o time. For calculation o useul lie the components are taken rom a normally manuactured batch. The components are tested under controlled conditions and the data or long term reliability is based on a conidence level o 55%. The igure can be taken only as a guide or reliability since actual working conditions are likely to deviate signiicantly rom those used in routine testing. Meantime between ailure (MTBF) is the inverse o ailure rate MTBF F(%) 0.% to % /000hours Forced Air ooling Failure Rate ( ) 000 Note : <.0 m/s Free onvection.0 m/s.5 m/s 5.0 m/s Forced Forced Forced air cooling air cooling air cooling Above Ripple urrent Multipliers are or lamp Mounted apacitors λ e.g. For given set o conditions 6 July 0

7 Operating conditions: During operating capacitors Precautions Polarity: Aluminium electrolytic capacitor are polar. Thereore the capacitors should be connected accordingly. I the polarity o a capacitors is reversed the capacitor will heat up and normally the saety vent will operate. In extreme cases there is possibility o an explosion and ire. Mechanical stress: During installation capacitors should not be mechanically damaged. i) apacitors have been designed with the can being negative. Hence damage to the insulation sleeve may causes a short circuit. ii) The terminals o screw terminal type capacitors (AEST) are made o highly pure aluminium the screw are made o brass or stainless steel which are a hard material. Hence mismatch o the threads during itment can cause damage to the threads o the aluminium terminals. Also while connecting the screw terminals the tightening torque should not exceed Nm. iii) Vibration resistance test : To IE test Fc : Displacement amplitude 0 75 mm requency range Hz acceleration max. 0 g duration 3 x h. apacitor mounted by its body which is rigidly clamped to the work surace. leaning agents: Halogenated hydrocarbons i in contact with capacitor may cause serious damage. These solvents may decompose the insulation sleeve and reduce insulating properties below permissible levels. Moreover these may penetrate the capacitor through the capacitor seal leading to premature ailure. ommonly used halogeneted hydrocarbons and other solvent which should not be used are reon t r i c h l o ro et hy l e n e m et hy l c h l o r i d e ca r b o n tetrachloride acetone methyl ethyi ketone. leaning agents which normally do not have any detrimental eects are methanol ethanol propanol and isopropanol. may ail due to the ollowing: i) Operating in very high ambient conditions. ii) Surge voltage exceeds or surge voltage is applied or longer periods than speciied. iii) Voltage on the capacitor exceeds rated voltage iv) Ripple current exceeds the speciied values. v) Reverse polarity. These can lead to catastrophic ailure with the possibility o an explosion and ire. Hence care should be taken during use. Also capacitors should be used in a well ventilated enclosure. Exposure to electrolyte: When an electrolyte comes in contact with skin wash throughly with water. I electrolyte comes in contact with eyes wash throughly with water and immediately seek medical advice. Storage: The ollowing conditions or storage are recommended: i) When not in use capacitors should be kept in their original packing. ii) apacitors should be stored indoors away rom o direct sunlight at a temperature o 5 to 35 anda humidity level o less than 70%RH. iii) apacitors should be stored in an environment ree rom water oil salt water and gases like hydrogen sulphied. Keep away rom other chemicals like suluric acid hydrochloric acid chlorine ammonia or any corrosive environment. iv) On storage capacitors should not be subjected to severe mechanical shock or vibration beyond speciied limits. Saety: Due to the characteristics o electrolytic capacitor there can be a rebound voltage o up to o 40 to 50v even ater the capacitor is discharged or a brie period. Thereore it is necessary to ensure that the capacitor is totally discharge beore using them so that other sensitive components will not be aected. atalogue No. AEPL APP Note July 0 The speciication shown in this catalogue pertain to the current manuacturing range o the ompany. The ompany reserves the right to change and /or modiy any part o or whole o the speciications as a result o research and development and as may be necessary without prior notice 7 July 0