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Run Capacitor

Run Capacitor

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. Most electric motor run cap applications use a rating of 2.5-100 uf (microfarads) capacitance and voltages of 370 or 440.

Viewall run capacitors here »

How to Select a Motor Run Capacitor

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. The 440 volt unit will actually last longer. A run cap 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.

Frequency (Hz): Select a capacitor with the Hz rating of the original. Nearly all will be labeled 50/60.

Case Style: Round or Oval? Round 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 capacitor terminal designs include 1-4 ¼" push on tabs. Nearly all run caps that TEMCo offers have either 3 or 4 tabs. Just make sure you have enough tabs per connection post to make the connections you need.

Run Capacitor Selection

Choosing a new Run Capacitor for Your Air Conditioner

Is it time to replace your Motor Run Capacitor?

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

When a motor run cap 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, the capacitor will have a specified "tolerance" 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 the capacitor is used and its 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 a non capacitor related motor issue, a capacitor will bulge from internal pressure. For most modern capacitor designs, this will open the circuit, dis-connecting the internal spiral membrane as a protective measure.

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.

For details on how to test capacitor condition, click here. »

Electric Motor and Run Capacitor

Why did my Run Capacitor fail?

The answer may be simple, but depending on how close to the design life the capacitor is, 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 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 it being "just old."

Heat - Exceeding the design limit of operating temperature can have a big effect on capacitor life expectancy. In general, motors that are operated in hot environments or with little ventilation will experience a dramatically reduced lifespan on their capacitors. The same can be caused by radiated heat from a generally hot running motor, causing the capacitor to run hot. In general, if you can keep your 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 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 cap 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 run cap (one that didn't come with your motor) would be 30,000-60,000 running hours. Factory installed motor run caps 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 capacitor may be selected with a design life of as little as 1000 hours. Additionally, all of the factors from the section above (causes of failure) may dramatically modify the reasonable expected life of a capacitor.

What's the difference between a run cap and a start cap?

See our run vs. starting capacitor page for a more in-depth explanation of the differences between a run cap and a start cap. Viewour video below to see why you can and cannot use them interchangeably..

Product Selection

Round, 370-440 VAC

Value (uf)
1 Piece 5 Piece 25 Piece 50 Piece 100 Piece
5 RC0047
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Run Cap Special Volume Discount

Starting Capacitor Help