How To Choose a Capacitor?


Capacitors for a three-phase motor need a sufficiently large capacity – we are talking about tens and hundreds of microfarads. However, electrolytic capacitors are not suitable for this purpose. They require a unipolar connection, that is, especially for them, you will have to fence a rectifier from diodes and resistances.

In addition, over time, the electrolyte dries out in electrolytic capacitors and they lose capacity. Therefore, if you put one on the engine, you need to make allowances for this, and not believe what is written on them. Well, one more thing is listed behind them: electrolytic capacitors tend to explode sometimes.

Therefore, the problem of how to choose a capacitor for a three-phase motor is often solved in several stages.

What Is a Capacitor?

It is an electrical charge storage device. It consists of a pair of conductive plates, located at a small distance from each other and separated by a layer of insulating material.

The following types of electric charge storage devices are widely used:

  • Polar: They work in circuits with constant voltage, they are connected in accordance with the polarity indicated on them.
  • Nonpolar: Work in circuits with alternating voltage, you can connect as you like
  • Electrolytic: The plates are thin oxide films on a sheet of foil.
  • Non-polar capacitor
  • Electrolytic ones are better than others for the role of an ac capacitor for starting an electric motor.

Quick Tips

The most important thing with a “star” connection is to determine the winding path because if you didn’t guess at least one pair of windings and, say, start-end, start-end, end-start, then the work will be bad and it will be immediately visible, there is also the opportunity to burn engine in this case.

Not all engines have terminal markings, most often marked with “mass”, the rest need to be “ringed” with a multimeter, or read the instructions, manufacturers often indicate this information there.

Choosing a Starting Capacitor for an Electric Motor

The modern approach to this issue involves the use of special calculators on the Internet, which carry out a quick and accurate calculations.

To carry out the calculation, you should know and enter the following indicators:

  1. Connection type of motor windings: delta or star. The capacity also depends on the type of connection.
  2. Engine power is one of the determining factors. This indicator is measured in Watts.
  3. The mains voltage is taken into account in the calculations. As a rule, it can be 220 or 380 volts.
  4. The power factor is a constant value, which is often 0.9. However, it is possible to change this indicator when calculating.
  5. The efficiency of the electric motor also affects the calculations. This information, as well as others, can be found by examining the applied information by the manufacturer. If it is not there, you should enter the engine model on the Internet to search for information about what efficiency. Also, you can enter an approximate value, which is typical for such models. It is worth remembering that the efficiency may vary depending on the condition of the electric motor.

Such information is entered in the appropriate fields and an automatic calculation is carried out. At the same time, we obtain the capacity of the working condensate, and the starting one should have an indicator 2.5 times greater.