Capacitor Part Two

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MaintenanceCircleTeam

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April 6th 2009

NEWSLETTER FOR MANUFACTURING COMMUNITY

Maintenance

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Continuing our journey in understanding the interesting world of capacitors……… If we can recollect our biology classes during school days, science teachers had repeatedly taught us that human body has millions of tiny cells that carry & supply energy for performing many tasks. These tiny “energy storing” cells are called MITOCHONDRIA which will “discharge” energy when needed and gets “charged” by digestion of food. In a way, our body itself has quite a lot of capacitors. More the work done, more will be the discharge of energy and sooner our body gets tired, demanding “re-charging.” Indeed, our body itself is a large capacitor bank!! Capacitors are used in applications ranging from a simple task of starting single phase fan to complex task of transmitting & receiving radio signals in a satellite network. Though this newsletter definitely cannot cover the vastness of all these usages, It will focus broadly on two simple & most common applications relevant to typical manufacturing industry: Power Factor Correction & Motor Power Factor Improvement, used on typical 415V 50 / 60 Hz three phase AC voltage system. Application-1: Power Factor (PF) Correction System: Before applying our thoughts on why’s & how’s of PF correction system, we must first turn few pages of electrical technology to understand the basics of power factor and how it affects the performance of an electrical equipment.

Power factor (PF), in its broadest sense, is a measure of “electrical efficiency” of a system or equipment using AC voltage. It shows how much of electrical energy “input” is actually converted into “useful” output. In electrical field, the “useful” power is termed as “true power” measured in Kilo Watts (KW) and “input” or total power is termed as “apparent power” measured in Kilo Volt Amperes (KVA). Theoretically, geometric (vector) difference between these two powers is the total electrical loss, measured in Kilo Volt Ampere Reactive (KVAr). Refer to the “electrical triangle” for understanding the relationship between these three parameters.
Sin 

PF or Power Factor is an electrical phenomenon which needs to be understood in detail. We will dedicate an article on PF soon. Here, we are interested in how a capacitor influences the PF.

KW

According to Pythagoras’ Theorem the formula for the right angle electrical triangle,

KVAr
Tan 

Cos 

Therefore,

KVA
Power Factor is defined as the ratio of “true power” (KW) to “apparent power” (KVA). Hence, From basics of geometry, we know that if  = 0°, then cos  = 1 and if  = 90°, then cos  = 0. Hence, to maintain lower PF, the cos  MUST GET CLOSER TO ZERO, which means KVAr component must be as less as possible.

From the fundamentals of trigonometry we know the COSINE, SINE & TANGENT angles

 Is called “phi”, a Greek word for representing angles.

MaintenanceCircleTeam

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April 6th 2009

NEWSLETTER FOR MANUFACTURING COMMUNITY

Maintenance
In any AC system, the electromagnetic induction plays a vital role in transmission of power between two mediums. For example, the step down transformer reduces voltage purely on this principle. Similarly, the rotor inside a 3-phase motor runs purely by the rotating magnetic field created by voltage passing thru stator windings. As the AC voltage is continuously passing thru conducting medium, apart from consuming energy for useful work, there will be many hidden losses which “waste” additional amount of energy. Iron loss, copper loss, heat dissipation are some common causes of wasting energy. These losses constitute “reactive” component of the total electrical system. Higher these losses, higher will be KVAr, which eventually reduces the Power Factor. This reactive component must be reduced as much as possible to increase power factor. We will explore more on these in
subsequent newsletter dedicated to PF.

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If KW is equal to KVA, then power factor is equal to one. This condition exists on resistive electric heaters, DC circuits and fully loaded large horsepower motors. THIS IS CALLED UNITY POWER FACTOR CONDITION. If KW becomes greater than KVA, power factor will be greater than one. For instance, this condition typically occurs when motor acts like a generator due to external mechanical inertia and when too many capacitors are installed in an electrical system. THIS IS CALLED LEADING POWER FACTOR CONDITION. If KW is less than KVA, which is usually the case, power factor will be less than one. This condition normally exists in all electrical systems. THIS IS CALLED LAGGING POWER FACTOR CONDITION

Also, if AC equipments are started simultaneously, there will be a sudden dip in the voltage level due to increased starting currents. This also applies to equipments which have sudden & cyclic loads, like welding machines, stamping presses, bending machines and material transport conveyors. The cyclic or sudden increase in “load,” though for a short duration, creates an increase in input energy level for that duration. This input energy requirement is called “demand” and each manufacturing set-up will have a “maximum demand” as its limit. If more machines are installed, these intermittent, but sure-shot sudden load requirements should be taken into consideration before deciding on the “maximum demand.” This will result in overdesigning of an electrical system. By installing suitably rated capacitors in proper locations, both scenarios described above can be almost eliminated. One, reduce the reactive component of total power and increase the power factor. Second, reduce the “maximum demand” required for a set-up which subsequently reduces fixes costs & total electricity bill.
Reactive component supported by capacitor, reducing total demand

ILLUSTRATION

Based on historical data & technical capabilities of various components, state electricity boards require a typical manufacturing unit – with HT or with LT connection – to maintain PF greater than or equal to 0.92. In fact, if the unit maintains a constant power factor beyond 0.95, they will even get certain discount on their electricity bill. For our illustration purpose in this newsletter, let us focus on a PF of 0.95.
Reactive component supplied by incoming electricity

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April 6th 2009

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As we know by now, the PF can be increased by reducing KVAr or reactive component in an AC system. So, the capacitors used for PF compensation are also rated in KVAr only. The actual capacitance in micro farads can also be measured, but we will discuss this point little later. There are two methods of calculating capacitor rating for power factor compensation.

Method-1: Historical Data Analysis
In this method, the PF value recorded for past four to six months will be noted from electricity bill. A correction factor is applied and capacitor with almost suitable rating can be selected. This method is very simple & quick to implement. It will be useful for calculating PF correction capacitor for ENTIRE electrical system. Remember, each installation will have a “restricted contract demand” which should be exceeded. Let us look at an example & calculate capacitor rating.
Historical Bill Details

Month January February March April

Contract Demand, KVA 100

Recorded Demand, KVA 100 60 120 100

Recorded average PF, 0.8 0.6 0.8 0.7

Active Power, KW, Average 80 36 96 70

Target PF, lag

0.95

There is a fluctuation in the demand usage changing PF every month. For calculation purpose, let us target 0.6 as the actual power factor that needs to be corrected to achieve 0.95. With a contract demand of 100 KVA and PF of 0.6, true power, KW = KVA x PF = 100 x 0.6 = 60 Kilo Watts

A group of capacitors connected together to an electrical load system is called “capacitor bank.” Interestingly, like a bank where we “deposit” & “withdraw” money, in capacitor bank, we “charge” (deposit) & “discharge” (withdraw) electrical energy!!

So, required capacitor rating in KVAr = KW x Correction Factor (from table) = 60 x 1.005 = 60.3

KVAr  61 KVAr

61 KVAr 3-phase capacitor will improve the power factor from 0.6 to 0.95
Table for selecting correction factor is attached in the end for easy reference and easy printability. This method of calculation can be used as a first step of getting correction capacitors. Depending on how much improvement PF shows in subsequent month(s), further analysis can be done to add or remove capacitors. As a thumb rule, this method is applied for set-ups with contract demand up to 250 KVA. But, for most practical applications, this method can be a pretty good starting point.

Method-2: PF AUDIT
In manufacturing set-ups where the electrical equipments are a combination of conventional motors, solid state electronic equipments, welding machines and different type of loads (stable, fluctuating, cyclic) it is quite difficult to measure the capacitor rating required for PF correction using historical data method. A thorough analysis of either each equipment or the load center (a group of machines, usually connected to one intermediate distribution panel) for measuring current PF should be carried out.
The “trivector” electronic meter which continuously monitors PF does NOT measure & record it instantly. It will derive an average value from the 30 day reading for billing purpose. For example, if the PF was 0.6 for 28 days and 0.95 for 2 days, the average PF recorded for billing purpose will be LESS than 0.95, but a little more than 0.6 and hence penalty will be imposed.

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The analysis will be done using sophisticated meters, usually by an external vendor. Based on the values derived from the audit, lot of inferences can be drawn, some of which are listed below. Separation & re-grouping of loads based on their type, load cycles and frequency of operation Capacitors need to be installed at multiple locations Difficulties in improving PF due to equipment type Re-routing of distribution cables to reduce amount of capacitor needed Installing harmonic filters & surge suppressors Relocation of certain equipments to improve power factor

We will understand more about these methodologies in newsletter on PF
If capacitors are installed at ONE location to improve power factor, it is known as BULK CORRECTION. Instead, if capacitors are distributed closer to electrical loads for various reasons, it is known as DISTRIBUTED CORRECTION. Also, when the entire rated capacitor is switched on independent of load condition, it is known as STATIC CORRECTION. On contrary, if capacitors are switched on & off responding to load, which is done using an APFC control system, it is known as DYNAMIC CORRECTION. Interesting details in the newsletter on PF

Understanding a standard 3-phase 415 V AC capacitor
The phase capacitors used for 415V system are usually connected in delta mode with three terminals for three phase – R Y B – connections. The capacitors are enclosed inside a round or rectangular metal enclosure for safety against physical damages and fire. Let us understand few specifications – printed on nameplate – which are essential to understand, maintain & troubleshoot a capacitor bank. 1. KVAr: The rating of capacitor – Remember: The KVAr increases with increase in input voltage. Hence it is essential to select KVAr rating for lower voltage. For instance, if a 5 KVAr capacitor rated for 415V AC is selected, its capacitance will increase to 5.62 KVAr at 440V AC 2. AC Voltage: It represents typical voltage at which capacitor can deliver rated capacitance. If voltage fluctuates, capacitance also changes. Normal allowable tolerance is ±1% 3. Insulation Strength (or Dielectric Strength): The voltage at which dielectric material breaks down. The minimum value will be usually 22,000 Volts AC 4. In, the current rating of capacitor. For all practical purposes, a thumb rule can be followed: A good (or new) capacitor will draw 1.33 amps per KVAr on each phase at 440V. And, at 415V, it draws 1.39 amps per KVAr. 5. Type of conducting & dielectric material – With technological advancement in the field of plastics & metal, most power capacitors are built with MPP abbreviation for METALLIZED (the conductor part, usually aluminum or copper sheet with less than 30 microns thickness) POLYPROPYLENE (the dielectric material, usually less than 5 microns thickness)

6. Type of healing of dielectric material – SH or NH – SH type means Self Healing dielectric material is used in the capacitors. When SH type is used, the capacitance gradually decreases over a period of time. The dielectric material ruptures and “heals” itself in that area. It does not SHORT the entire capacitor. Power factor compensation systems use SH type. With SH type, it is essential to check some parameters regularly at fixed interval. Self-healing types are used in locations where voltage levels fluctuate more & also on specific

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April 6th 2009

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applications. On contrary, in the NH type, the dielectric material does not heal and capacitor will be short instantly. NH type capacitors can be used in distributed type of compensation method, including three phase induction motors. They are also widely used as backup capacitors on drives, to make the fault evident instantly warranting replacement. 7. Power loss – every item that stores energy will have its own internal losses. We discussed about dielectric losses in previous newsletter on capacitors. This value will also be printed on capacitor shell. Normally, the internal loss is around 0.3 to 0.8 watts per KVAr. So, on a 50 KVAr capacitor, the loss will be between 1.5 to 4 watts. For all practical purposes, power loss value can be ignored, since it is not very high. 8. Working temperature – Capacitors generate heat during charging & discharging process. Hence good ventilation is necessary for satisfactory operation. The working range of 3-phase power capacitor is between -30° and +50° C. Provision for sufficient natural or forced ventilation must be given in design stage itself. Do not tuck the capacitor in some corner of a room or panel. Capacitors need sufficient breathing space. 9. Filling material – About a decade back, POLYCHRONATED BIPHENYL – PCB – was used as filler (similar to the transformer oil) for increasing dielectric strength and heat dissipation properties. But due to health & environmental regulations, majority of power capacitors manufactured today are “dry” type without any PCB. “NON-PCB” or “PCB” type will be printed on the capacitor shell. Today, all available power capacitors are dry type, though there are a few exceptions. 10. In-rush Current – When a fully discharged capacitor is put into circuit first time, there will be a sudden flow of current (similar to starting currents of standard motor). Based on the rating, each capacitor has limited in-rush current withstanding ability. To prevent accelerated damage of dielectric material due to repeated inrush current levels, capacitors must not be switched ON & OFF too frequently. 11. Discharge Resistor – When capacitors are pulled out of circuit, it will retain the electrical charge for longer duration based on its design. This electrical charge must be discharged before attempting any work on the capacitors. Discharge resistors should be fitted to automatically discharge capacitors whenever it is out of circuit. These resistors can be fitted “internally” or “externally.” The discharge resistor value depends on capacitor rating. For 415 volt system, THE CAPACITOR VOLTAGE MUST DROP CLOSE TO ZERO VOLTS WITHIN 40 SECONDS. IF THE TIME IS MORE, DISCHARGE RESISTOR MUST BE REPLACED as early as possible. Three Phase Terminals 12. Earthing – The outer shell of capacitor – whether the shell is rectangular or round – must be grounded properly to avoid electric hazards. The power capacitors are available in cylindrical or rectangular type. Each has its own merits & demerits. And also each has its one unique application areas. Larger rating power capacitors (beyond 75 KVAr) are usually made in rectangular metal enclosures. Having understood the nitty-gritty of power capacitors, now let us concentrate on two more aspects: (i) Maintenance aspects to improve capacitor(s)’ life (ii) Measuring various parameters

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Maintenance aspects:

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1. Select capacitors with: a. Correct KVAr rating b. Type of dielectric material – NH or SH c. Voltage Rating d. AC frequency 2. Install capacitors complying these conditions: a. Surrounding area is well ventilated b. Cable of correct size is used – do not squeeze too many cables into the terminal c. The capacitor is held firmly in place d. Surrounding area is free of moisture, excessive vibrations, heat & sunlight e. All cables – if a capacitor bank is installed – are properly identified with proper routing f. Take into consideration the distances between various equipments before installing 3. Maintain the capacitors: a. Regularly check for any external damages to capacitor & its terminals b. Measure voltage, current, capacitance regularly (follow second aspect to understand) c. Do not engage & disengage capacitors frequently

Measuring various parameters:
Four important parameters – which should also be measured regularly for monitoring & maintenance – should be within the tolerance limits for efficient functioning & ensuring long life of capacitors. Also, remember that if capacitors are used in PF compensation system, measuring / monitoring parameters is very critical to take preventive measures & avoid PF penalty. Let us also understand simple formulae that will help in making day to day calculations. 1. Voltage a. AC input voltage: It is important to note that the capacitance increases with increase in voltage up to limit. The voltage must be measured across terminals using a “TRUE RMS” digital multimeter or a conventional analog meter. The reading should be taken after multimeter stabilizes at a particular reading. Do not hurry. b. Discharge voltage: When the capacitor is disconnected from electrical system, it should discharge the voltage, usually within 40 seconds. Measure using same meter to judge the condition of discharge resistor. 2. Current a. The clamp-on meter (tong-tester) can be quickly used to measure the current. The current must be measured when connecting a fully discharged capacitor to the Measure voltage after few electrical system. In a bank type system, an ammeter will seconds of placing the probe to be fitted on panel with CT (Current Transformer) avoid recording peak readings connected, BUT it may NOT measure the current of individual capacitors. It is imperative to measure the current of each capacitor regularly. In a 415 Volt system, as a thumb rule, a fully discharged capacitor draws 1.39 Ampere per KVAr and in a 440 Volt system, it is 1.33 Ampere per KVAr.

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3. Capacitance a. Although capacitors are designated by KVAr, its actual value is measured in micro farads. LCR meter (Inductance-Capacitance-Resistance) or special Capacitance meter should be used to measure the capacitance. The capacitor must be discharged completely before measuring. It is suggested to use a suitable resistor for “externally” discharging the capacitor or wait till it discharges itself. b. Whether it is SH or NH type, a log-book (well, MS excel sheet or equivalent spread sheet in today’s technological world) keeping a neat record of all values, especially capacitance, will go a long way in preventing damage control exercise later. 4. Temperature a. If a capacitor bank is installed, it is essential to monitor the temperature at a specific location using external measuring device. A thermocouple-temperature display system can be installed if capacitor bank is installed warm / hot locations.

Capacitance value at different 440V & Capacitance values in micro farads – TABLE 1

Capacitance value at 415V & Capacitance values in micro farads – TABLE 2

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Formula-1

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Since the capacitance value changes with voltage & frequency within certain limits, a simple formula can be used to calculate the actual value. Let us understand with two examples. Name plate details: 15 KVAr / 440V AC / 50Hz Measured voltage: 425V AC Measured frequency: 48.5 Hz

, where
Fm = Measured Frequency = 48.5 Hz in this example Fr = Rated Frequency = 50 Hz Vm = Measured Voltage = 425V AC in this example Vr = Rated Voltage = 440V AC Rated KVAr = 15 in this example Substituting the values,

= 13.57 KVAr
Name plate details: 15 KVAr / 415V AC / 50Hz Measured voltage: 425V AC Measured frequency: 48.5 Hz Substituting the values,

= 15.26 KVAr
Formula-2
The thumb rule of 1.33 & 1.39 amps per KVAr for 440 & 415 V capacitors give fairly accurate current value, the theoretical current can be calculated using following formula. The clamp-on meter / tong tester should measure actual current and the capacitor condition can be judged by looking at the deviation value.

I = KVAr x 103 / (

x VL), VL is the line voltage.
x 440) = 19.68 Amperes x 415) = 20.97 Amperes

Example: For a 15 KVAr capacitor rated to 440V AC, I = 15 x 103 / ( For a 15 KVAr capacitor rated to 415V AC, I = 15 x 103 / (

Formula-3
To calculate the capacitance of a capacitor if voltage, frequency & KVAr ratings are known and the micro farad is the unit commonly used.

C (µF) = KVAr x 109 / (4 x π x f x VL2), where VL = Line Voltage, f = frequency

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For a 15 KVAr capacitor rated for 50 Hz, 440V AC, capacitance will be

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C (µF) = 15 x 109 / (4 x π x 50 x 440 x 440) = 123.31µF
At 415V the capacitance will be equal to 15

x 109 / (4 x π x 50 x 415 x 415) = 138.62µF

As discussed before, LCR or Capacitance meter can be used for measuring the value as well. A log-book, preferably with following parameters must be maintained and readings be taken either every Monday morning or on fortnight basis. FIRST OR INSTALLATION DATA Installation Date Rated Voltage REGULAR OR MONITORING DATA Date Measured Voltage Measured Current Measured Frequency Measured Discharge Time, Seconds Temperature

Capacitor Rating Rated Frequency

Capacitor Current Discharge Time,

CAPACITOR RATING

VOLTAGE READING

This log book serves as a quick guide to understand any deviation. Alternatively, to analyze the trend quickly, pre-set charts can be printed on A4 sheets for each or two set of parameters and reading be marked at the agreed intervals.
DATES / WEEK NUMBER / DAY

DATES / WEEK NUMBER / DAY

CURRENT READING

CAPACITANCE DATES / WEEK NUMBER / DAY DATES / WEEK NUMBER / DAY

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POWER FACTOR CORRECTION FACTOR TABLE

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Target PF

Present PF

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Application-2: Motor Power Factor improvement: If a 3-phase induction motor is not part of the centralized PF correction system or if there are few motors located far away from each other, it is essential to fix individual capacitors for motors. If the motor runs on a star-delta system, the capacitor must be fitted in delta side. The thumb rule is 1/3rd or 33.33% of HP rating is capacitor required in KVAr. For example, on a 10 HP motor, a 10 x (1/3) = 3.33 KVAr capacitor can be fitted for PF improvement. Also remember, DO NOT FIX ANY CAPACITORS FOR MOTORS WHICH HAVE soft starters, speed drives or other type of electronic switching system is installed for controlling motor. And, now time to see whether this newsletter made any sense to you. Try solving this simple Techuzzle on the aspects elaborated in previous ten pages. Happy Solving…….
1 3 2

4

5 6

7

EclipseCrossword.com

Across
3. Reduce this component to improve power factor

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6. 7. Increase this also increases capacitance value Total power is a sum of this & true power

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Down
1. 2. 3. 4. 5. Normal condition, power factor is less than unity Condition wherein PF is more than unity This is very essential to discharge a capacitor This meter measures power factor and many parameters This contract......should never be exceeded to avoid penalty

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