Motor Capacitor FAQ

OverviewVoltage Capacitance Frequency (Hz) link Terminal Style situation Shape situation Size start vs. Operation Capacitors

Start Capacitors

Applications Specifications how do I recognize if my begin capacitor is bad? My engine is sluggish to start. Is my start capacitor bad? My begin capacitor has actually a resistor on it. Do I require a instead of capacitor through one? deserve to I usage a greater voltage rated capacitor 보다 the original?

Run Capacitors

How carry out I change a run cap in my air conditioner? Applications Specifications once to change Why did my run capacitor fail? exactly how long must my operation capacitor last? dual Run Capacitors If i can"t uncover a replacement for my dual run capacitor, can I usage two different run caps?



A capacitor will have actually a significant voltage denote its acceptable height voltage, no its work voltage. Therefore, friend can select a capacitor through a voltage rating in ~ or over the initial capacitor. If you"re making use of a 370 volt capacitor, a 370 or 440 volt one will work, despite the 440 volt unit will actually last longer. However, you cannot change a 440 volt capacitor with a 370 volt one without significantly reducing the lifespan.

You are watching: How to tell a start capacitor from a run capacitor


Select a capacitor through a capacitance value (given in MFD, uf or microfarad) that is equal to the original capacitor. Perform not deviate native the initial value, as it to adjust the operational features of the motor.

Frequency (Hz)

Select a capacitor through the Hz rating that the original. Virtually all capacitors will certainly be labeled 50/60.

Connection Terminal Style

Nearly every capacitor will usage a ¼" flag layout push-on connector. When replacing a capacitor, girlfriend will need to understand how many terminals per terminal post are needed for her motor. Most start capacitors have two terminals per post, and also most run capacitors will have actually either 3 or 4 terminals per post. Verify the the replacement has at least the variety of connection terminals per connection write-up as the original motor capacitor.

Case form (Round vs. Oval)

Nearly all start capacitors have actually a round case. Round instances are by much the most common, yet many motors still usage oval designs. Electrically speaking, there is no difference. If space in the mounting crate is no limited, the instance style does not matter.

Case Size

Just like case shape, in its entirety size provides no distinction electrically. Select a capacitor that will certainly fit within the an are provided.

Start vs. Run Capacitors

Start capacitors provide a large capacitance value necessary for motor starting for a really short period of time (usually secs long). Castle are just intermittent duty and also will fail catastrophically if energized as well long. Run capacitors are used for constant voltage and also current regulate to a motor"s windings and are therefore continuous duty. Lock are normally of a much reduced capacitance value.

In inexplicable circumstances, a operation capacitor might be provided as a start capacitor, yet the values obtainable are much reduced than the worths usually obtainable for committed start capacitors. The capacitance and voltage ratings would have actually to enhance the initial start capacitor specification. A begin capacitor have the right to never be provided as a operation capacitor, since it cannot not handle present continuously.

View our video tutorial below to learn much more about the differences in between start and also run capacitors.

Start Capacitors


Start capacitors are used to briefly shift phase start windings in single phase electric motors come create rise in torque. They possess very huge capacitance values for their size and also voltage rating. Together a result, castle are just intended because that intermittent duty. For this reason, begin capacitors will certainly fail after being left energized for too long because of a faulty starting circuit top top a motor.


Most start capacitors are rated because that 50-1200 uf and also 110/125, 165, 220/250, or 330 VAC. They room usually 50/60 Hz rated. Instance designs are typically round and also cast in black color phenolic or Bakelite materials. Terminations space usually ¼" press on terminals through two terminals per link post.

How carry out I understand if my begin capacitor is bad?

Most begin capacitor fail are one of two types. Catastrophic failure is usually resulted in by an electrical motor"s starting circuit being involved too lengthy for the intermittent duty rating of a begin cap. The top of the begin cap has actually literally been blown off, and also the insides have been partly or totally ejected. Similarly, a begin cap may just exhibit a ruptured push relief blister. In either case, it"s basic to tell the the start cap is in require of replacement.


My engine is slow-moving to start. Is my begin capacitor bad?

Your begin capacitor may have actually lost that capacitance rating because of wear and age, or friend may have actually other non-capacitor related issues that need to do with other motor components. You will want to measure the capacitance the your start capacitor to find out.

My start capacitor has actually a resistor ~ above it. Perform I need a instead of capacitor with one?

Most replacement begin caps will not encompass a resistor. Girlfriend can examine the problem of the old one by check the resistance value, or just replace it through a brand-new one. This have to read somewhere approximately 10-20k Ohms and around 2 watts. The resistors are usually one of two people soldered or crimped come the terminals. The purpose of the resistor is to bleed off residual voltage in the capacitor ~ it has been disconnected native the circuit after ~ motor start up. No all start capacitors will usage one, together there are other methods to achieve this. The important part is if your original capacitor had one, you"ll need to replace that on the new capacitor.

Can I use a greater voltage rated capacitor 보다 the original?

Yes. Click right here for much more detail.

Run Capacitors


Run capacitors are offered to continuously change current or phase transition to a motor"s windings in an initiative to optimise the motor"s torque and also efficiency performance. They are designed for constant duty, and as a result, have actually a much reduced failure price than start capacitors. Castle are typically used in HVAC units.


Most run caps are rated because that 2.5-100 uf (microfarads) with voltage ratings of 370 or 440 VAC. They space usually 50/60 Hz rated. Instance designs space round or oval, most commonly using either a stole or aluminum shell and cap. Terminations space usually ¼" press on terminals through 2-4 terminals per link post.

When come Replace

As a general ascendancy of thumb, a operation capacitor will far outlast the very same motor"s begin capacitor. A operation cap will also fail or wear in different way than a start cap, do troubleshooting slightly much more involved.

When a operation capacitor starts to perform external the allowable range, that is most frequently indicated through a dropping that the rated capacitance worth (the microfarad value has gone down). For many standard motors, a operation capacitor will have actually a "tolerance" stated describing how close come the rated capacitance worth that the actual value may be. This will certainly be normally +/- 5 to 10%. For many motors, as long as the actual worth is is within the 10% mark of the rated value, you"re in good shape. If the drops exterior of this range, you"ll must replace it.

In some cases, as result of a defect in a capacitor"s building and construction or sometimes caused by a non-capacitor associated motor issue, a run capacitor will certainly bulge from inner pressure. For most modern-day run capacitor designs, this will open up the circuit, disconnecting the inner spiral membrane together a protective measure to avoid the capacitor indigenous popping open.

If its bulging, time to replace. If you measure no continuity across the terminals, that is additionally time come replace.

Why did my run capacitor fail?

Below space some common reasons that operation capacitors fail, however depending on how close the operation capacitor is come its architecture life, it might be an overwhelming to accurately decide the factor on a single factor.

Time - every capacitors have a architecture life. Several components may it is in interchanged or combined to rise or alleviate the life the a run capacitor, yet once the architecture life is exceeded, the internals may begin to much more rapidly decay and also drop in performance. Merely put, a failure might be attributed to it gift "just old."

Heat - Exceeding the style limit of operating temperature have the right to have a big effect on run capacitor life expectancy. In general, motors that space operated in hot atmospheres or with little ventilation will experience a dramatically decreased lifespan on their capacitors. The same have the right to be resulted in by radiated warm from a generally hot running motor that reasons the capacitor to operation hot. In general, if you deserve to keep your run capacitor cool, it will certainly last a lot of longer.

Current - as soon as a engine is overloaded or has actually a fail in windings, it reasons the present to climb, which deserve to overload capacitors. This script is less generally noticed, as it would commonly be attach by a partial or complete failure the the motor.

Voltage - Voltage have the right to have one exponential effect in shortening the design life that a capacitor. A operation capacitor will have a significant voltage rating that should not it is in exceeded. Because that example, a capacitor is rated because that 440 volts. At 450 volts, the life might be diminished by 20%. In ~ 460 volts, the life may be decreased by 50%. At 470 volts, over there is a 75% life reduction. The same can be applied in reverse to assist increase the style life by using a capacitor with a voltage rating significantly greater than needed, though the impact will be less dramatic.

How lengthy should my run capacitor last?

The life the a an excellent quality aftermarket run capacitor (one the didn"t come through your motor) would be 30,000 to 60,000 running hours. Factory-installed operation capacitors sometimes have a much reduced designed lifespan. In very competitive sectors where every part can have actually a significant impact top top cost, or wherein a motor"s intended use would most likely be intermittent and infrequent, a lower grade of run capacitor may be selected through a style life of as tiny as 1000 hours. Additionally, all the determinants from the section over ("Why did my run capacitor fail?") may substantially modify the reasonable expected life that a operation capacitor.

Dual operation Capacitors

Dual operation capacitors are two run capacitors in one case. They have nothing else that makes them electrically special. They generally have connections significant "C" for "common", "H" or "Herm" because that "Hermetic Compressor" and "F" for "Fan." castle will also have two various capacitor ratings because that the two various parts. You can see 40/5 MFD, an interpretation that one next is 40 microfarads (measurement the capacitance) and the various other side is 5 microfarads. The smaller value will always be connected to the fan. The larger link will constantly be connected to the compressor.

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If ns can"t find a replacement because that my dual run capacitor, deserve to I use two different run caps?

The dual run capacitor design"s only advantage is that it come in a small package with only 3 connections. There is no other difference. If over there is enough room for mounting, utilizing two separate run capacitors in place of your original dual run capacitor is an acceptable practice.