Single Phase Capacitor Sizing

Single phase capacitor sizing

21/03/2011byLemau13 CommentsWhen install amotorusing capacitor for starting or running methods,we must sizing theratedof capacitor suitable withmotorto get correct starting torque and avoid winding from overheating and can cause a damage.

This is basically a question of motor design.There is no straightforward regular relationship between capacitance and the motor size in kW.

When replacing these capacitors, the capacitancevalueand voltage should be taken from the manufacturers plate on the motor or from the old capacitor.This must be correct within 5% and is sometimes stipulated down to a fraction of a F.The choice of a running capacitor is even more limited than with a starting capacitor.

How to sizing the starting capacitor?

1) A rule of thumb has been developed over the years to help simplify this process. To select the correct capacitancevalue, start with 30 to 50F/kW and adjust the value as required, while measuring motor performance.

We also can use thisbasicformula to calculate capacitor sizing :

2) Determine the voltage rating for capacitor.

When we select the voltageratedfor capacitor,we must know the value of our power supply.For safety purpose,multiply the voltage of power supply with 30%.Factors that affect the selection of the proper voltage rating of the capacitor include: Voltage de-rating factor Safety agency requirements. Reliability requirements Maximum operating temperature Space available

How to sizing the running capacitor?

When selecting motor run capacitors all of the required parameters above need to be identified in an organized process. Remember that, not only are the physical andbasicelectrical requirements important.

But the type of dielectric material and the metallization technique should be examined.The wrong choice here can have adverse effects on the overall performance of the capacitors.Please refer to motor nameplate or contact supplier or manufacture to get a accurate value of capacitor.Safety First

- See more at: http://www.electricneutron.com/electric-motor/single-phase-capacitor-sizing/#sthash.2uIVJEqN.dpufhttp://www.temcoindustrialpower.com/Overview

Astart capacitoris used to briefly shift phase on a start winding in a single phase electric motor to create an increase in torque. Start capacitors possess a very large capacitance value for their size and voltage rating. As a result, they are only intended for intermittent duty. Because of this, start capacitors will fail after being left energized for too long due to a faulty starting circuit on a motor.

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 capacitorsare 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?

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 startcapacitor can not ever be used as a run capacitor, because it could not handle current continuously (only a couple of seconds).

View our video tutorial below to learn the differences between start and run capacitors.

What is a resistor and do I need one?

Most replacement start capacitors will not include aresistor. 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 motor start up. Not all start capacitors will use one, as there are other ways to accomplish this. The important part is if your original capacitor had one, you'll need to replace it on the new capacitor.

Learn how to install a bleed down resistor on your start cap.

Troubleshooting

How do I know if my 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 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 that have to do with other motor components.

Specifications

Most start capacitor applications use a rating of 50-1200 uf capacitance and voltages of 110/125, 165, 220/250 and 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 " push on terminals with two 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 one will work. The 440 volt unit will actually last longer. A capacitor will have a marked voltage indicating the accpetable peak voltage, 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 replacement capacitors will be labeled 50/60.

Connection terminal style

Nearly every capacitor will use a " 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 two 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 motor capacitor.

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.

Case size

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

Back to Top

Product Selection

110/125 VAC

CapacitanceValue1 Piece Pricew/ Shipping5 Piece Pricew/ Shipping10 Piece Pricew/ Shipping

108-130SC0005$8.63view SC0006$29.16view You Save $13.99SC0007$46.56view You Save $39.74











460-552SC0035$11.04view SC0036$42.28view You Save $12.92SC0037$66.84view You Save $43.5+






220/250 VAC














SC0004-25 Lot$347.60view You Save $161.64


165V





330V




Overview

Arun capacitoris 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

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 ourguide on dual run capacitorsfor more details.

Start vs. Run Capacitors

Start capacitorsgive 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).

View our video tutorial below to learn the differences between start and run capacitors.

Back to TopSpecifications

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 " 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 " 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.

Back to TopRun Capacitor Product Selection

Round, 370 VAC

CapacitanceValue (uf)1 Piecew/ Shipping5 Lotw/ Shipping10 Lotw/ Shipping60 Lotw/ Shipping

5RC0006$6.20view RC0021$25.02view You Save $5.98RC0036$47.60view You Save $14.40-

10RC0007$11.92view RC0022$36.10view You Save $23.50RC0037$50.20view You Save $69-

15RC0008$7.50view RC0023$35view You Save $2.50RC0038$52.16view You Save $22.84-



30RC0011$7.89view RC0026$34.95view You Save $4.50RC0041$60.90view You Save $18-




50RC0015$9.35view RC0030$40view You Save $6.75RC0045$55.05view You Save $38.45RC0046$375view You Save $186

Round, 370-440 VAC

CapacitanceValue (uf)1 Piecew/ Shipping5 Lotw/ Shipping10 Lotw/ Shipping25 Lotw/ Shipping50 Lotw/ Shipping100 Lotw/ Shipping

5RC0047$6.37view RC0053$12.35view You Save $19.50-RC0059$48.75view You Save $110.50RC0074$92.5view You Save $226RC0079$156view You Save $481

40RC0090$6.20view RC0091$28.13view You Save $2.95RC0092$47.53view You Save $14.90RC0093$105.73view You Save $50.75RC0094$193.03view You Save $120.50RC0095$363.75view You Save $264

100RC0096$15.04view RC0097$47.53view You Save $28.50RC0098$86.33view You Save $66RC0099$169.75view You Save $212.50RC0100$290.03view You Save $476RC0101$532.53view You Save $1001

Oval, 370-440 VAC

CapacitanceValue (uf)1 Piecew/ Shipping5 Lotw/ Shipping10 Lotw/ Shipping25 Lotw/ Shipping50 Lotw/ Shipping100 Lotw/ Shipping

5RC0048$6.37view RC0054$12.35view You Save $19.50-RC0060$48.75view You Save $110.50RC0075$92.5view You Save $226RC0080$156view You Save $481

7.5RC0049$5.60view RC0055$14.20view You Save $13.80RC0061$27.50view You Save $28.50RC0071$56.50view You Save $83.50RC0076$104view You Save $176RC0081$179view You Save $381

10RC0050$8.20view RC0056$18.05view You Save $22.95RC0062$31.10view You Save $50.90RC0072$62.25view You Save $142.75RC0077$115.50view You Save $294.50RC0082$200view You Save $620

15RC0084$5.33view RC0085$24.25view You Save $2.45RC0086$43.65view You Save $9.90RC0087$86.33view You Save $48.25RC0088$154.23view You Save $115.50RC0089$290.03view You Save $250

50RC0052$9.95view RC0058$29.95view You Save $19.80RC0064$59.95view You Save $43.55RC0073$129.95view You Save $118.80RC0078$249view You Save $248.50RC0083$449view You Save $546

Back to TopTroubleshooting

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.

Capacitor Lifespan

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.

Overview

Dual run capacitors are commonly used in HVAC (heating, ventilation, and air conditioning) units. Dual capacitors have three terminals, unlikeregular run capacitors, which have only two. Electrically, they are the same as run capacitors that have two terminals. Using a dual run capacitor allows you to save space if you have a small mounting area. The terminals are also clearly marked, making it easy to hook it up to the different components of your air conditioner.

Dual capacitors usually have connections marked "C" for "common," "H" or "HERM" for the hermetic compressor, and "F" or "FAN" for the fan in an HVAC unit. They also have two different capacitance ratings (for exampple, 35/5 uf). The larger value is always connected to the compressor (35 uf in the example), while the smaller value will always be connected to the fan (in this case, 5 uf).

Back to topSpecifications

Below is a list of specifications that must be taken into consideration when selecting a new capacitor.

Capacitance

Make sure the capacitance of your new capacitor is the same as that of the one you're replacing.

Voltage

Dual capacitors are usually rated for 370V or 440V. Use a capacitor that has the same or higher voltage as your system voltage. Never select one that has a lower voltage rating than your system voltage.

Frequency

Most capacitors will be rated for both 50 and 60 Hz.

Oval vs. Round Case Style

As long as the capacitance, voltage, and frequency of your capacitor is right for your application, the shape of the capacitor does not matter as long as it can fit within the provided mounting space in your motor or air conditioning unit.

Terminal Type

Most capacitor terminals include one to four " push on tabs. Most have 3 to 4 tabs, so make sure you have enough tabs per connection post to make the connections you need.

Back to topVideo - Replacing a Dual Run Capacitor

View our video tutorial on how to diagnose problems with your air conditioning unit and how to replace the run capacitors in it. For a more in-depth look at troubleshooting, check out ourTroubleshooting Guide.

Back to topProduct Selection

1 piece prices include shipping. All lots havefree shipping!

Dual Run Capacitor Selection Chart

Capacitance (uf)1 Piece5 Lot10 Lot25 Lot50 Lot100 Lot

35/5RC0102 $7.57RC0103 $29.10Save $9RC0104 $53.35Save $48RC0105 $105.73Save $165RC0106 $193.03Save $360RC0107 $363.75Save $750

40/5RC0108 $7.61RC0109 $28.62Save $9.75RC0110 $55.29Save $49RC0111 $107.67Save $166RC0112 $193.03Save $363RC0113 $367.63Save $755

45/5RC0114 $8.58RC0115 $29.05Save $14.30RC0116 $54.27Save $58.55RC0117 $126.05Save $191RC0118 $241.53Save $412RC0119 $435.53Save $855

50/5RC0120 $8.92RC0121 $31.04Save $14RC0122 $59.17Save $60RC0123 $134.83Save $198RC0124 $260.93Save $428RC0125 $445.23Save $888



Overview

Astart capacitor resistor(also called a "bleed down resistor") is used to bleed off residual voltage in a start capacitor after it has removed from a motor circuit after start up. Not allstart capsuse resistors, as there are other ways to accomplish the task of reducing residual voltage in a capacitor.

What is important to note is that if the capacitor you are replacing had a bleed down resistor, then you will need to replace the resistor in your new start capacitor. You can either check to see if the old resistor is still good, or you can replace it with a new one. The resistance value should read somewhere between 10 to 20k Ohms and around 2 watts. Resistors are usually crimped or soldered to the start capacitor's terminals.

Back to topSpecifications & Product Selection

Use for start capacitors:

Voltage:up to 330V

Capacitance:up to 1200 uf

Mounting type:Solder

Resistor body diameter:0.1255 in.

Resistor body length:Overall length (including leads):2.363 in.

Lead length:1 in. per side

Lead diameter:0.19 in.

View our selection of start capacitor resistors. All prices listed havefree shippingincluded.

QuantityPart NumberPrice

1SCR001 $3.95

5SCR002 $4.95

10SCR003 $6

25SCR004 $9

50SCR005 $15

100SCR006 $25

Back to topReplacing a Resistor

Most start capacitors do not have a resistor. But if you need to replace a start capacitor that does have one, you'll need to use a resistor on the new capacitor as well. You can either see if the old resistor is still good, or just use a new one.

View our video tutorial on how to install a start capacitor bleed down resistor.

Single Phase Capacitor Sizing

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