Motor Capacitor FAQ

Basic Information

Replacing a Capacitor



Basic Information

How long does it to ship my capacitor?

All orders less than 50 pieces are shipped USPS First Class or Priority Mail and will arrive 1-3 days from the day of shipment. Same day shipping on most stock items. All other orders are shipped UPS or FedEx.

What is a start capacitor used for?

A start capacitor is used to briefly shift phase on a start winding in a single phase electric motor to create an increase increase in torque. Start capacitors possess a very large capacitance value for their size and voltage rating, and as a result, are only intended for intermittent duty. It is because of this that start capacitors fail after being left energized too long due to a faulty starting circuit on a motor.

See the video below to learn how to replace a start cap in a single phase motor.

What are typical start capacitor ratings?

Most start capacitors have ratings of 50-1200 uf capacitance and voltages of 110/125, 165, 220/250 or 330 VAC. They are also usually always 50 and 60 Hz rated. Case designs are typically round and cast in black phenolic or Bakelite materials. Terminations are usually ¼" push on terminals with 2 terminals per connection post.

What is a run capacitor used for?

A motor run capacitor is used to continuously adjust current or phase shift to a motor's windings in an effort to optimise the motors torque and efficiency performance. All run capacitors are designed for continuous duty, and as a result, have a much lower failure rate than start capacitors.

What are typical run capacitor ratings?

Most electric motor run capacitor applications use a rating of 2.5-100 uf (microfarrads) capacitance and voltages of 370 or 440 VAC. They are also usually always 50 and 60 Hz rated. Case designs are round or oval using most commonly either a steel or aluminum shell and cap. Terminations are usually ¼" push on terminals with 2-4 terminals per connection post.

How is a run capacitor different from a start capacitor?

Start capacitors give a large capacitance value necessary for motor starting for a very short (seconds long) period of time. They are only intermittent duty and will fail if energized too long. Run capacitors are used for continuous voltage and current control to a motors windings and are therefore continuous duty. They are generally of a much lower capacitance value.

What is a dual run capacitor?

Dual run capacitors are 2 run capacitors in one case. They have nothing else that makes them electrically special. They generally have connections marked "C" for "common", "H" or "Herm" for "Hermetic Compressor" and "F" for "Fan." They will also have 2 different capacitor ratings for the 2 different parts. You might see 40/5 MFD, meaning that one side is 40 microfarads (measurement of capacitance) and the other side is 5 microfarads. The smaller value will always be connected to the fan. The larger connection will always be connected to the compressor.

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Replacing a Capacitor

Round vs. Oval

The only difference between round capacitors and oval ones is the case. There is no electrical difference. As long as the replacement will fit within the space provided, the case shape is irrelevant.

"If I can't find a replacement for my dual run capacitor, can I use two separate run capacitors?"

The dual run capacitor design's only advantage is that it comes in a small package with only 3 connections. There is no other difference. If there is enough mounting space, using two separate run capacitors in place of your original dual run capacitor is an acceptable practice.

"Can a start capacitor be interchanged with a run capacitor?"

Yes and no. In unusual circumstances, a run capacitor could be used as a start capacitor, but the values available are much lower than the values usually available for dedicated start capacitors. The capacitance and voltage ratings would have to match the original start capacitor specification. A start capacitor can not ever be used as a run capacitor, because it could not handle current continuously (only a couple of seconds).

"My start capacitor has a resistor on it. Do I need a replacement capacitor with one?"

Resistor on Start Capacitor Terminals

Most replacement start capacitors will not include a resistor. You can check the condition of the old one by checking the resistance value, or just replace it with a new one. This should read somewhere around 10-20k Ohms and around 2 watts. The resistors are usually either soldered or crimped to the terminals. The purpose of the resistor is to bleed off residual voltage in the capacitor after it has been disconnected from the circuit after start up of the motor. Not all start capacitors will use one, as there are other ways to accomplish this. The important thing to note is that if your original capacitor had one, you'll need to replace it on the new capacitor.

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Select a capacitor with a voltage rating at or above the original capacitor. If you're using a 370 volt capacitor, a 370 or 440 volt capacitor will work. The 440 volt unit will actually last longer. A capacitor will have a marked voltage indicating peak voltage acceptable - not operational voltage.

Can I use a capacitor with a higher rated voltage than the original one?

Yes. You can use a capacitor with an equal or higher rated voltage than the original, but you cannot use a capacitor with a voltage rating that is lower than the one you are replacing.

Capacitance value (uf, MFD or microfarads)

Select a capacitor with a capacitance value (given in MFD, uf or microfarad) that is equal to the original capacitor. Do not deviate from the original value as it sets the operational characteristics of the motor.

Frequency (Hz)

Select a capacitor with the Hz rating of the original. Nearly all replacement capacitors will be labeled 50/60.

Terminal style

Nearly every capacitor will use a ¼" flag style push on connector. The next question is, "How many terminals per terminal post are needed for the application motor?" Most start capacitors have 2 terminals per post, and most run capacitors will have either 3 or 4 terminals per post. Verify that your selected capacitor has at least the number of connection terminals per connection post as the original one.

Case Shape

Nearly all start capacitors have a round case. Round run capacitors are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference; fit is the only question here. If space in the mounting box is not limited, the case style does not matter.

Overall Size

Just like case shape, overall size makes no difference electrically. Select a capacitor that will fit within the space provided.

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How do I know if my start capacitor is bad?

Blown Start Capacitor Start Capacitor Ruptured Blister
Most failures are one of two types.

"The start cap blew its guts out!" This is what we call catastrophic failure. It is usually caused by an electric motor's starting circuit being engaged too long for the intermittent duty rating of a start cap. The top of it has literally been blown off, and the insides have been partially or fully ejected.

Similarly but not quite as dramatic, a start cap may just exhibit a ruptured pressure relief blister. In either case, it's easy to tell that the capacitor is in need of replacement.

My motor is slow to start. Is my start capacitor bad?

The answer to this question is maybe. Your start capacitor may have lost its capacitance rating due to wear and age, or you may have other non capacitor related issues related to other motor components.

Is it time to replace your run capacitor?

As a general rule of thumb, a run capacitor will far out last the same motor's start capacitor. A run capacitor will also fail or wear differently, making them slightly more involving when trying to determine if the time has come for replacement. Start capacitors will commonly fail catastrophically, making the decision on when to replace obvious, limiting time spent troubleshooting.

When a run capacitor begins to perform outside the allowable range, it is most often indicated by a dropping of the rated capacitance value (the microfarad value has gone down). For most standard motors, a run capacitor will have a "tolerance" specified describing how close to the rated capacitance value that the actual value may be. This will be usually +/- 5% to 10%. For most motors, as long as the actual value is is within the 10% mark of the rated value, you're in good shape. As a run capacitor is used, and the capacitance drops outside of this range, you’ll need to look at a replacement.

In some cases, due to a defect in a capacitor's construction or sometimes caused by a non capacitor related motor issue, a run capacitor will bulge from internal pressure. For most modern run capacitor designs, this will open the circuit, dis-connecting the internal spiral membrane, as a protective measure to prevent explosion.

The test in this case is simple; if its bulging, time to replace. If you measure no continuity across the terminals, it is also time to replace.

How to test capacitor condition ».

Motor and Run Capacitor
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"Why did my run capacitor fail?"

The answer may be simple, but depending on how close to the design life of the run capacitor, it may also be difficult to nail it down to a single factor.

Time - All capacitors have a design life. Several factors may be interchanged or combined to increase or reduce the life of a run capacitor, but once the design life is exceeded, the internals may begin to more rapidly decay and drop in performance. Simply put, a failure may be attributed to being "just old."

Heat - Exceeding the design limit of operating temperature can have a big effect on run capacitor life expectancy. In general, motors that are operated in hot environments or with little ventilation will experience a dramatically reduced lifespan on their run capacitor. The same can be caused by radiated heat from a generally hot running motor, which causes the capacitor to run hot. In general, if you can keep your run capacitor cool, it will last a lot longer.

Current - Motor failure causes the capacitor to overload. This scenario is less commonly noticed, as it would usually would be accompanied by a partial or complete failure of the motor. The motor is overloaded or has a failure in the windings, causing the current to climb. This can have an effect on the capacitor.

Voltage - This single factor can have an exponential effect in shortening design life. A run capacitor will have a marked voltage rating not to be exceeded. Lets use 440 volts as an example. At 450 volts, the life may be reduced by 20%. At 460 volts, the life may be reduced by 50%. At 470 volts, there is a 75% life reduction, and so on. The same can be applied in reverse to help increase life by using a capacitor with a voltage rating significantly higher then needed, although to a lesser dramatic degree.

How long should my run capacitor last?

The mid point for a good quality aftermarket (a capacitor that didn't come with your motor) run capacitor would be 30,000-60,000 running hours. Factory installed run capacitors sometimes have a designed lifespan of much less that this. In highly competitive industries where every part can have a significant impact on cost, or where a motor's intended use would likely be intermittent and infrequent, a lower grade of run capacitor may be selected with a design life of as little as 1000 hours. Additionally, all of the factors from the section above (run capacitor causes of failure) may dramatically modify the reasonable expected life of a run capacitor.

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