Test Capacitor  |  Replacement Capacitor



Types of Capacitors


A motor 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 fact that most start capacitors fail after being left energized too long due to a faulty starting circuit on a motor.


Most electric motor applications use a rating of 50-1200 uf capacitance and voltages of 110/125, 165, 220/250 and 330 VAC. 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.


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 motor run capacitors are designed for continuous duty and as a result, have a much lower failure rate than start capacitors.


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



Start vs. Run Capacitors

Viewour run vs. starting capacitor page for more information.


How are Run Capacitors different from Start 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 motors windings and are therefore continuous duty. They are generally of a much lower capacitance value.


Can Start Capacitors be interchanged with Run Capacitors? Yes and no. In unusual circumstances, a Run Capacitor could be used as a motor 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).


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Capacitor Specifications


The specifications needed for selecting a replacement capacitor are:


Voltage of operation or original capacitor voltage. Capacitor Voltage should be at least the voltage that will be seen during operation. A motor being used on a 240 volt supply should use a capacitor rated above 240 volts. Using a capacitor with a rated voltage much higher than what will be seen during operation is an acceptable practice.


Capacitance value (uf, MFD or microfarrads) of original capacitor. This value must be the same as the capacitor being replaced.


Frequency (hz) of operation or original capacitor. Nearly all capacitors will have a 50/60 hz rating. The replacement capacitor must have the frequency in the rating that the motor will be operated on.


Connection terminal style of original capacitor. Most motor capacitors 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 capacitor have 2 terminals per post and most run capacitors will have either 3 or 4 terminals per post. Verify that your replacement capacitor selected has at least the number of connection terminals as the original capacitor.


Case Shape and overall size and 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, makes no difference electrically. Select a capacitor that will fit within the space provided.


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Choosing a Replacement Capacitor - Causes of Failure


Is it time to replace your Electric Motor's Start Capacitor? Depending on the frequency of starting of a piece of machinery, the relative load during start-up, and the temperature of environment that it is operated in, every motor's start capacitor will eventually have to be replaced.


Blown Start Cap Start Cap Ruptured Blister

How do I know if my Electric Motor's Start Capacitor is bad?

Most electric start motor capacitor 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 the start cap 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 start cap is in need of replacement.


"My motor is slow to start. Is my start cap bad?" The answer to this question is maybe. Your start cap 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.


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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. A motor 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 motor 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.


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


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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 is, 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 causing 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 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 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.


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How long should my Run Capacitor last?


The mid point for a good quality aftermarket (didn't come with your motor) run capacitor would be 30,000-60,000 running hours. Factory installed motor 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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Run Capacitor Selection

Need help with Run Capacitor selection?

We'll walk you through, it's EASY!

Voltage: Select a Run 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 capacitor will have a marked voltage indicating peak voltage acceptable - not operational voltage.


Capacitance: Select a Run 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 Run Capacitors 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 tabs. Nearly all run capacitors 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.


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Run Capacitor Product Selection

Replacement Capacitor - Run Cap

Round Run Capacitor, 370-440 VAC, ¼" Flag Terminal, 50/60 Hz

Capacitance
Value (uf)
1 Piece 5 Piece 25 Piece 50 Piece 100 Piece
5 RC0047
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RC0053
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RC0059
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RC0074
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RC0079
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Start Capacitor Product Selection

Replacement Capacitor - Start Cap
American Made in USA Start Cap

Round Start Cap, 165V, ¼" Flag Terminal, 50/60 Hz

Capacitance
Value
1 Piece 5 Piece 10 Piece
124-149 SC0092
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SC0093
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SC0094
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189-227 SC0095
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SC0096
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SC0097
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400-480 SC0098
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SC0099
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SC0100
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Round Start Cap, 330V, ¼" Flag Terminal, 50/60 Hz

Capacitance
Value
1 Piece 5 Piece 10 Piece
88-106 SC0101
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SC0102
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SC0103
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135-162 SC0104
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SC0105
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SC0106
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Starting Capacitor Help