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How can I select the right capacitors for my specific application needs?
How can I select the right capacitors for my specific application needs? (photo credit: mediarede.com)

Power factor correction

Once you’ve decided that your facility can benefit from power factor correction, you’ll need to choose the optimum type, size, and number of capacitors for your plant.

There are two basic types of capacitor installations: individual capacitors on linear or sinusoidal loads, and banks of fixed or automatically switched capacitors at the feeder or substation.

Individual vs. banked installations

7 advantages of individual capacitors at the load:

  1. Complete control // Capacitors cannot cause problems on the line during light load conditions
  2. No need for separate switching // Motor always operates with capacitor
  3. Improved motor performance due to more efficient power use and reduced voltage drops
  4. Motors and capacitors can be easily relocated together
  5. Easier to select the right capacitor for the load
  6. Reduced line losses
  7. Increased system capacity

3 advantages of bank installations at the feeder or substation:

  1. Lower cost per kVAR
  2. Total plant power factor improved – reduces or eliminates all forms of kVAR charges
  3. Automatic switching ensures exact amount of power factor correction, eliminates over-capacitance and resulting overvoltages

Table 1 // Summary of Advantages/Disadvantages of Individual, Fixed Banks, Automatic Banks, Combination

MethodAdvantagesDisadvantages
Individual capacitorsMost technically efficient, most flexibleHigher installation and maintenance cost
Fixed bankMost economical, fewer installationsLess flexible, requires switches and/or circuit breakers
Automatic bankBest for variable loads, prevents overvoltages, low installation costHigher equipment cost
CombinationMost practical for larger numbers of motorsLeast flexible

Consider the 5 particular needs of your plant

When deciding which type of capacitor installation best meets your needs, you’ll have to weigh the advantages and disadvantages of each and consider several plant variables, including load type, load size, load constancy, load capacity, motor starting methods, and manner of utility billing.

1. Load type //

If your plant has many large motors, 50 hp and above, it is usually economical to install one capacitor per motor and switch the capacitor and motor together. If your plant consists of many small motors, 1/2 to 25 hp, you can group the motors and install one capacitor at a central point in the distribution system.

Often, the best solution for plants with large and small motors is to use both types of capacitor installations .

2. Load size //

Facilities with large loads benefit from a combination of individual load, group load, and banks of fixed and automatically-switched capacitor units. A small facility, on the other hand, may require only one capacitor at the control board.

Sometimes, only an isolated trouble spot requires power factor correction. This may be the case if your plant has welding machines, induction heaters, or DC drives. If a particular feeder serving a low power factor load is corrected, it may raise overall plant power factor enough that additional capacitors are unnecessary.

3. Load constancy //

If your facility operates around the clock and has a constant load demand, fixed capacitors offer the greatest economy. If load is determined by eight-hour shifts five days a week, you’ll want more switched units to decrease capacitance during times of reduced load.

4. Load capacity //

If your feeders or transformers are overloaded, or if you wish to add additional load to already loaded lines, correction must be applied at the load. If your facility has surplus amperage, you can install capacitor banks at main feeders.

If load varies a great deal, automatic switching is probably the answer.

5. Utility billing //

The severity of the local electric utility tariff for power factor will affect your payback and ROI (return of investment). In many areas, an optimally designed power factor correction system will pay for itself in less than two years.

Reference: Power factor correction – A guide for the plant engineer – EATON (Download paper)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

5 Comments

  1. Capacitors have been used in many appliances all around the world. From basic appliances like TV sets and microwaves to large satellites in space, capacitors have a function everywhere. Understanding each application effects, will for sure help in selecting an ideal capacitor type.


  2. Milton
    Apr 03, 2015

    It is a good article, I am currently waiting for LED luminaires iran mounted Torres (4) nominally are 6000W per tower, the provision of energy in the area is through generators, so I hope instability in the voltage, the capacitors would use it more advisable to prevent failures.
    greetings To You
    Milton


  3. suresh
    Mar 07, 2015

    Please tell me about motor winding


  4. ENGINEER ALEX
    Feb 22, 2015

    am the best ever


  5. Rana
    Feb 19, 2015

    Can you please update an article upon the for which purpose we have to chose which type of capacitors according to its application….e.g. polar/un-polar capacitors…for charging discharging purposes… for AC /DC purposes….. thanks

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