Run Capacitor Selection Guide
A run capacitor is used to continuously adjust current or phase shift to a motor's windings in an effort to optimise the motor's torque and efficiency performance. Because it is designed for continuous duty, it has a much lower failure rate than a start capacitor.
Dual Run vs. Run Capacitors »
Start vs. Run Capacitors »
Frequency (Hz) »
Case Shape »
Case Size »
Connection Terminal Type »
Replacing a Run Capacitor »
Causes of Failure »
Capacitor Lifespan »
Dual Run vs. Run Capacitors
The dual run capacitor design's only advantage is that it comes in a small package with only 3 connections. Aside from this, here is no other difference between run and dual run capacitors. If there is enough space for mounting, using two separate run capacitors in place of your original dual run capacitor is acceptable. They generally have connections marked "C" for "common", "H" or "Herm" for "Hermetic Compressor" and "F" for "Fan." They will also have two different capacitor ratings for the two different parts. See our guide on dual run capacitors for more details.
Start vs. Run Capacitors
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 catastrophically if energized too long. Run capacitors are used for continuous voltage and current control to a motor's windings and are therefore continuous duty. They are generally of a much lower capacitance value.
Are start and run capacitors interchangeable?
In unusual circumstances, a run capacitor could be used as a start capacitor, but the values available for them 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 never be used as a run capacitor, because it could not handle current continuously (only a couple of seconds).
Watch the video tutorial below to learn the differences between start and run capacitors.
Most run capacitor applications use a rating of 2.5-100 uf (microfarads) capacitance and voltages of 370 or 440 VAC. They are also usually always 50 and 60 Hz rated. Case designs are round or oval, most commonly using either a steel or aluminum shell and cap. Terminations are usually ¼" push on terminals with 2-4 terminals per connection post.
Voltage: 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, though the 440 volt unit will actually last longer. A run capacitor will have a marked voltage indicating peak voltage acceptable - not operational voltage.
Capacitance: 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.
Hz: Select a capacitor with the Hz rating of the original. Nearly all tun vapacitors will be labeled 50/60.
Case Style: Round or Oval? 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 style, overall size makes no difference electrically. Select a capacitor that will fit within the space provided.
Terminal Type: Most run capacitor terminal designs include 1-4 ¼" push on tab and will have either 3 or 4 tabs. Just make sure you have enough tabs per connection post to make the connections you need.
Round, 370 VAC
|Capacitance Value (uf)||Product ID|
Round, 370-440 VAC
|Capacitance Value (uf)||Product ID|
Oval, 370-440 VAC
|Capacitance Value (uf)||Product ID|
When is it time to replace a run capacitor?
As a general rule of thumb, a run capacitor will far out-last the same motor's start capacitor. A run motor capacitor will wear down differently, making them a bit more complicated when trying to determine if it needs to be replaced.
When a run capacitor begins to perform outside the allowable range,it is usually indicated by a dropping of the rated capacitance value. 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. If the capacitance drops outside of this range, the capacitor should be replaced.
Due to a defect in a capacitor's construction or a non-capacitor related motor issue, a run capacitor will sometimes bulge from internal pressure. For most modern run capacitor designs, this will open the circuit, disconnecting the internal spiral membrane as a protective measure to prevent the capacitor from popping.
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.
View the video below on how to replace a run capacitor in an air conditioning unit.
Causes of Failure
Depending on how close the run capacitor is to its design life, there may be multiple factors that determine why a run capacitor has failed.
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 occur because the capacitor is "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, causing the capacitor to run hot. If you can keep your run capacitor cool, it will last a lot longer.
Current - Motor failure causes the capacitor to be overloaded. This scenario is less commonly noticed, as it would usually 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 that should not be exceeded. Let's 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 less dramatic degree.
The mid point for a good quality aftermarket run capacitor (one that didn't come with your motor) would be 30,000 to 60,000 running hours. Factory-installed run capacitors sometimes have a designed lifespan that is much lower. 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.