Capacitor for Generator

A generator capacitor is an important part of the generator, and it’s essential to know how it works and what to do if there are any problems.

A blown capacitor can cause your generator to stop working, so it’s important to be able to test and replace them when necessary.

This guide will teach you everything you need to know about your capacitor for generator, including how they work, types of capacitors, their failure modes, how to test them, and how to fix them if they go bad.

What is a Capacitor?

A capacitor is a two-terminal passive electronic component that stores electrical energy in the electric field. A basic capacitor consists of two isolated electrical conductors, often in the form of two parallel plates of area “a” separated by a distance “d,” with a dielectric material of uniform thickness such as glass, paper, mineral oil, ceramics, or plastic filling the in-between space.


Their effect is characterized by the term capacitance, denoted by C. The dielectric materials increase the capacitance by a factor of K over the value with no other medium except air between the conductors. This factor K is known as the dielectric constant of the insulating material.

When a potential difference is applied across its terminals by connecting a battery, one of the conductors acquires the positive charge while the other acquires an equal but negative charge, causing an electric field to develop across the dielectric.

Capacitance is the amount of charge that must be put on the conductors to produce a 1V potential difference between them or C = Q/V. The unit of capacitance is Farad, which is a very big value for normal capacitors. Hence, in practical applications, their value is typically measured in Microfarads (uf rating), which is one-millionth of a Farad.

How Do Capacitors Work?

As stated in the previous section, a capacitor will acquire an electric charge when connected to a battery. If you disconnect the battery and connect the capacitor to a load, it will feed the load till it is completely discharged. Hence, they store energy during the charging process and release it during the discharge process.

The difference between a capacitor and a battery is that the battery can charge and discharge at a slow and fixed rate only, while the capacitor can charge and discharge within a short period, making them suitable for many applications where a battery can’t be used.

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When connected to an AC supply, the capacitor gets charged and discharged in a given interval; the plates change polarity in different half-cycles, and the ease with which they pass electricity increases with the increase in frequency.

One of the most important properties of the capacitor is that the capacitor voltage cannot change instantaneously, and the voltage change rate defines the current flowing through it. The capacitive reactance of any capacitor is inversely proportional to the frequency.

Main Functions of A Capacitor in Any Electronic Circuit

A capacitor is installed in an electronic circuit for the following reasons:

  1. Storing and releasing the charges – Thereby functioning as a short-term power supply for certain circuit parts on disconnection from its charging circuit. For example, the camera flashlights utilize this feature. They maintain the power supply while the battery is getting charged.
  2. Providing voltage regulation – They function to maintain the voltage at a certain level. We will discuss this for the generator in detail in the next sections.
  3. It blocks the flow of DC currents but allows alternating currents.
  4. Power Conditioning – They are used to condition the output of half-wave and full-wave rectifier circuits by reducing the pulsation in the voltage. The capacitors get charged when the voltage increases and try to maintain the voltage level of the output when the incoming voltage from the rectifier falls in the second portion of the half-cycle.
  5. Power Factor Correction – In electrical power distribution, they are used to improve the power factor, as the current in the capacitors leads the voltage, allowing them to supply the reactive power.
  6. Coupling Capacitors – These are mostly used to separate the DC components of the signal from AC ones.
  7. Decoupling Capacitors – They help in protecting a part of the circuit from the effects of the other.
  8. Filters – They are used to create low and high-pass filters.
  9. Motor Starters – They get utilized in many motor starting circuits, particularly the single-phase ones.

Types of Capacitors

The capacitors are of two main types – polarized and non-polarized- further subdivided based on the dielectric material.

Polarized Type

A polarised capacitor is designed to have an inherent polarity, so each terminal has an assigned fixed polarity, positive or negative. This allows them to be connected in only one direction in the circuit. The capacitance of this type changes with the temperature, voltage, and frequency.

Aluminum Electrolytic capacitors

The capacitance is given by the formula C = K (A/d),

where K = Dielectric Constant of the dielectric material

A = Area of the plates

d = distance between the plates.

Since K and A can’t be varied much, to pack capacitance in a small size, the plates are made thinner to reduce the distance between plates. This led to constructing the capacitor with thin metal foils deposited on both sides of the dielectric and rolling them and, ultimately, the origin of electrolytic capacitors.

They are constructed from an aluminum foil ribbon, having a film of aluminum oxide deposited on it, which acts as a dielectric. The second plate consists of a water-based electrolyte covered by another foil plate.


The energy storage capacity and the power factor of electrolytic capacitors are quite high, but they have certain limitations in performance.

  • The operating voltage is limited to 450 V only.
  • The oxide in the dielectric has rectifier-like properties, allowing the flow of current only in one direction, so they are suitable only for DC applications.
  • They are unsuitable for voltage reversals, damaging the film and the capacitor. For AC applications, you need to go in for a non-polarised type.

They are mostly used in lowfrequency DC filters, low-frequency coupling, and bypass applications where more capacitance is required in small sizes.

Tantalum capacitors use Tantalum as anode and tantalum pentaoxide as dielectric material, providing larger capacitance than aluminum ones. It has improved characteristics but comes at a substantially higher cost.

Non-Polarized Type

The terminals on these capacitors do not carry positive or negative polarity. Both terminals can be connected in any direction to the circuit at random. This makes them suitable for AC applications. In particular, they are used in compensation, decoupling, coupling, and feedback applications.

The different types include Ceramic, Silver Mica, Polycarbonate, Polyester, Polystyrene, Polypropylene, Teflon, glass capacitors, etc.

Types of Capacitors in Generator

Generators mostly use electrolytic capacitors. Some manufacturers do use polypropylene capacitors.

function of Any capacitor For Generator

As the design of the generators, particularly the brushless ones, has evolved over the years, so has the use of capacitors in them. In some generators, you will find a different start-and-run capacitor, while others will have a single capacitor.

In addition, they have been used in multiple configurations, even for the same function. We will highlight some common uses in the current range of generators to highlight the concepts. One or more of these features may be available in the generator set.

Assist in reaching the full output of the Generator.

You might already be familiar with the working of brushless and normal portable generators and have seen their excitation circuit diagram.

When the engine starts rotating, a small voltage (4 to 12V) is induced in the stator due to the residual magnetism of the rotor core. Capacitors are placed across the stator excitation windings, which are placed at 90° from the main stator windings.

Due to the leading current in the excitation circuit due to the presence of the capacitor, a pulsating magnetic field is established. This magnetic field equals two rotating magnetic fields in the opposite direction. If the rotor moves clockwise, the anticlockwise rotating field has twice its relative speed and induces voltage at twice the frequency.

These double frequency field and voltage components are instrumental in raising the alternator’s output from about 8V to 125V, making it ready to power the rated generator load. This feature is found in brushless generator sets.

Voltage Regulation System

The capacitor is continuously charged and discharged in the loop and acts as the voltage regulation system for many of the generator sets, particularly in smaller ratings. The rate of charging of the capacitor is proportional to the induced voltage in the stator excitation winding. Once fully charged, it discharges, providing a constant voltage., which is provided to the Automatic Voltage Regulator, rectified, and then fed to the rotor.

For machines producing more power output, more elaborate systems are used that also protect against normal failures.

How Much Is A Generator Capacitor?

The price of a portable generator capacitor is usually less than $ 50.

Capacitor Failure Modes

Let us quickly run through the possible reasons and the mechanism for any capacitor failure.

Electrolytic Capacitor – Possible Reasons for Failure

  1. They age over time, losing the ability to perform.
  2. The failures of the electrolytic capacitors are due to the thermal and voltage stress on the dielectric metal oxide film. Electrolytic capacitors exhibit poor resistance temperature characteristics, so as the temperature increases, the leakage current increases, and the breakdown voltage decreases. The electrolyte gets vaporized and escapes through the end seal regularly throughout the life of the capacitor. The loss increases under the conditions causing the thermal or voltage stress or if there is an internal fault. When the total electrolyte loss reaches about 40% of the initial electrolyte volume, the electrical properties get degraded, and it is time to replace it.
  3. The electrolyte loss also happens during long periods of storage and when the alternator is allowed to operate at lower voltages for long periods.
  4. The materials like paper, aluminum foil, and electrolytes degrade physically and chemically. The degradation is accelerated by high temperatures and high currents.
  5. Since electrolytic capacitors are polarized, they should only be used in DC circuits. If AC components are present in the circuit, the sum of DC and peak AC voltage should not exceed the DC voltage rating of the capacitor. The peak AC voltage should be less than the DC voltage to avoid overheating, which may damage the capacitor.
  6. The current values of the capacitor must be within the permissible ripple ranges. Generally, any type of capacitor will deteriorate if frequently exposed to currents higher than its rating.
  7. Utilizing it for duties more than what it is designed for. If it is made to filter unusual levels of voltage transients or noise, it is likely to get damaged faster.
  8. If everything goes well during the capacitor’s lifetime, it can continue in service for about ten years. However, it is recommended to provide a replacement in 6 to 8 years if the generator is used frequently. Depending on the specific duty, some capacitors may require a change after four years of running.

Effects of Different Conditions on the Capacitor

  1. High Voltage conditions: The presence of high voltage conditions results in a decrease in capacitance and an increase in Equivalent Series Resistance (ESR). A high ESR causes more noise, heat, and voltage drop.
  2. Transients will increase the leakage current, and the chances of internal short circuits increase in their presence.
  3. Reverse Bias: As the electrolytic capacitors are polarised, reverse bias causes an increase in leakage current and ESR, along with a decrease in capacitance value.
  4. High Vibrations: They can result in internal shorts or open circuits with higher capacitance losses, increased leakage currents, and ESR.
  5. High Ripple Current: They result in internal heating, increasing the core temperature, and aging of the capacitor.

How Do You Know If A Capacitor Is Bad?

You should always look out for the following tell-tale signs of any capacitor going bad.

  1. Deformation indicates internal fluid leaks or excessive temperatures.
  2. Wires scorching at the connection points to the capacitor indicate a much higher current flow than the design.
  3. Burnt Valve Cap Protrusion is another common sign of a likely short-circuit in the capacitor leading to excessive gas formation and stresses due to pressure build-up that may eventually lead to protrusion breakage.
  4. Visible oil leakage – This may lead to excessive temperatures in the capacitor and high impedance.
  5. Increase in temperature – It is recommended to measure and note down the temperature of the capacitor in the initial period of the generator’s life. You can measure the normal temperatures after a few years of service every time you run your machine. The regular increase will indicate an internal problem in the capacitor.
  6. Decrease in Capacitance – You may note the capacitance value (with a multimeter) along with the temperature. A decrease in the value after many years of service will indicate that the capacitor is now going weak.
  7. Poor Voltage Regulation – Many new generators deploy the capacitor for voltage regulation. If your voltage output continues to be low, the capacitor might be losing its value. If the voltage produced is zero or less than half to three-quarters, the capacitor is the most likely cause.

How To Test A Capacitor in Generator?

The capacitor can get open-circuited (nonconductive) or develop a short-circuit due to electrolyte leakage, resulting in direct contact between the plates. The alternator will not provide any electricity to the loads in both conditions. Do a visual check first and try to observe if,

  1. The terminal end of the capacitor has bulged out, indicating its failure.
  2. If the gel has leaked out, exercise caution and do not touch it with bare hands.
  3. The terminal connections are loose from the body,
  4. The capacitor wobbles and does not sit on a flat surface.

You need to replace them immediately if any of the above signs are noticed. If there are no visible signs, you need to test the capacitor at the site.

Start by bridging the terminals of the capacitor with a screwdriver with an insulated handle to discharge any residual charge.

Disconnect the capacitor wiring and ensure that the engine is shut down.

Measure the capacitance value with a multimeter. Check it against the value noted on the capacitor. The variation of +- 5 µF is acceptable.

If marginally more than the above tolerance, connect a multimeter or an analog ohmmeter (with a needle) across the terminals. Set the scale at R X 100. If the needle moves towards the right side, it indicates that there is an increase in resistance as the capacitor charges. Once fully charged, the resistance will remain constant, signaling a healthy capacitor.

Several manufacturers use small-sized and metal-encased capacitors. They are designed to suppress the radio frequency noise in the output circuits. These capacitors will not be able to produce any output if there is a short to ground. Test them in the same way as above.

Capacitor Replacement

Generator capacitors have a voltage rating of 450V and a capacitance between 18 to 100 µF. Check the details in the customer manual and buy one of the same ratings. It should be designated for generator service. The starting capacitors used on electric motors tend to fail in a short period of time when used on generators. The below video will guide you.


We’ve covered a lot of ground in this article on capacitors for generators. Hopefully, you now have a good understanding of how capacitors work in generators and the different types that are available. You should also know how to test them and what to do if they fail. If you have any questions or need help with capacitor replacement, please comment below, and I will get back to you as soon as possible. Thanks for reading!

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