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Electric motor start-run capacitor selection guide:

This article explains how to choose & buy an electric motor start capacitor, hard start capacitor, or run capacitor that is properly rated for and matches the requirements of the electric motor such as an AC compressor motor or fan motor where the capacitor is to be installed.

This electric motor capacitor article series explains the selection, installation, testing, & use of electric motor starter start and run capacitors used on various electric motors found in or at buildings such as air conditioner compressors, fan motors, some well pumps and some heating equipment.

These electric motors use a capacitor to start and run the motor efficiently. We explain the choice & wiring procedures for a hard start capacitor designed to get a hard-starting air conditioner compressor motor, fan motor, refrigerator, or freezer compressor or other electric motor (such as a well pump) going.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Green links show where you are. © Copyright 2017 InspectApedia.com, All Rights Reserved.

How to Find, Choose, & Buy a Replacement Electric Motor Starting Capacitor

Watch out: do not attempt electrical repairs if you are not trained and equipped to do so. Making a mistake can cause a fire or serious injury or death.

Definition of capacitance or capacitor

Capacitance is the ability of a device to store an electrical charge.

An electric motor capacitor is a device that first stores, then releases an electrical charge to help start an electrical motor (starting capacitor) or to keep it spinning (run capacitor) – the electrical charge or potential energy stored in the capacitor is used to give a power boost to the motor to help it overcome inertia – to start rotating.

How do you know that an electric motor needs a replacement starting capacitor?

If the electric motor hums without starting or has trouble starting, but will run if given a push (e.g. a spin of a fan blade – watch out for chopped-off fingers) the start cap is probably bad.

If the starting capacitor is bulged, burned, has a hole in it, or has leaked oil, it is shot and needs replacement.

[Click to enlarge any image]

Burst or Blown capcitor: A « catastrophic capacitor failure » of a motor starting capacitor is one in which the start capacitor literally « explodes » or bursts. This happens when the start capictor remained engaged too long – perhaps because a centrifugal switch inside the motor did not disengage the starting cap circuit when it should-have.

Blistered or bulged capacitor: this capacitor failure is similar to but not as dramatic as a blown-capacitor but here too, if you see a bulge or blister or rupture or burst-opening on your capacitor the cap has failed and needs to be replaced.

Hidden internal failure of a starting capacitor is shown as lost capacitance that can occur due simply to aging or « wear » from long service. An electrical test of the capacitor can reveal this failure.

How do you know that an electric motor needs a replacement run capacitor

If the electric motor draws high current when running or hums and won’t start the run cap is probably bad. If the motor hums but the thermal overload protection switch has not tripped, the start cap is probably bad.

If the run capacitor is bulged, burned, has a hole in it, or has leaked oil, it is shot and needs replacement.

Some motors use a combined start/run capacitor

A start/run capacitor combines the start capacitor and run capacitor functions in a single device. These caps will have three electrical terminals:

S = start wire connector

R = Run wire connector

C = common connector

Some equipment uses a dual capacitor

A dual capacitor – combines two capacitors in a single physical device, one to run a higher-amperage motor such as the compressor in an air conditioner compressor/condenser unit, and a second smaller capacitor to run a smaller motor such as the cooling fan motor in the compressor/condenser unit.

Where is the capacitor? If you can’t find the capacitor,

see LOCATE the STARTING CAPACITOR,

Some motors don’t use capacitors or use only a run capacitor

Watch out: not all electric motors, such as pool pumps, use starting or run capacitors.

For example a split-phase motor, often used in spas and hot tubs and at many above-ground swimming pools, uses a start switch and start winding but does not use a starting nor a run capacitor.

A « permanent split capacitor » motor uses a run capacitor but not a starting capacitor. Motors of this design, often found in in-ground swimming pool pumps, have a low starting torque and need only a run cap.

How to Choose a Replacement Motor Capacitor – 4 Methods

The best option if you are replacing a starting capacitor or a start/run capacitor is to match the existing device on your system.

That means noting the data printed on the existing capacitor – if it’s legible and matching that.

Our capacitor tables given at the start of this article also provide general advice on matching a motor capacitor to the motor voltage and size ratings in Volts and kW or kilowatts.

[Click to enlarge any image]

Our photo illustrates a Mars-brand starting capacitor rated at 25uF and 370VAC – this cap is used ion AO Smith motors and is identified as 25MFD 370V – 628318-307.

Since there are just two terminals we know this isn’t a start+run capacitor.

How to Match a Replacement Capacitor for an Electric Motor

Options for matching a replacement motor capacitor include the following

1. Examine the original capacitor and match its ratings in Voltage and and capacitance, uF or micro farads.

   Look for and record all of the markings on the capacitor.

   Typically you’ll see a rating in microfarads like the 25uF in our capacitor photo above, and you’ll also see a voltage range rating such as the 370VAC (maximum voltage) rating also in the red circle of our photo above.

2. Take the capacitor or the whole electric motor to a motor rebuild-shop or to your local electrical supplier

   If the capacitor markings are legible,
   they’ll be able to match it.

   If the capacitor markings are not legible the electrical supplier or motor re builder will recommend a start, run, or combination capacitor based on the motor data tag information.

What if the Capacitor is Lost or Has No Legible Markings?

With the motor brand and model (and hopefully serial number in hand) you can also get the exact OEM part number – contact the motor manufacturer they can give you the exact OEM capacitor replacement part number or specifications.

3. Check the motor type & motor part number and data tag specifications and use a motor part substitution or OEM part catalog.

   See capacitor and motor repair part catalogs given

   at ELECTRIC MOTOR CAPACITOR CATALOGS

   For readers whose capacitor has no markings whatsoever, your electrical supplier will want to know the technical details about the capacitor’s intended use. Those can be found on the motor data tags for the motor served by the cap.

   Typical pool/spa motor capacitors: some pool pumps use a motor that gets a start-up boost from a starting capacitor.

   For example a typical aboveground pool, hot tub or spa motor will use a starting capacitor rated around 50-400 MFD and 125 or 250 VAC. And for the same motor, the run capacitor will be rated around 15-50 MFD and 370 VAC. 

   Capacitor Capacitance Value Limits

   Select a replacement capacitor whose capacitance value in µF, typically expressed as a range such as 30 µF – 50 µF matches the original capacitor and/or the data tag on your electric motor.

   Capacitor Voltage Rating Selection Limits

   The voltage rating of a capacitor indicates the highest nominal voltage at which it is designed to operate.

   Use of a capacitor at voltages below its rating will do no harm. For example, if your motor used a 370VAC rated capacitor you can replace it with a 370 or a 440VAC capacitor. The 440VAC capcitor will last longer. But because these voltage ratings are simply the peak voltage that the capacitor can handle, using a higher-rated cap (440) where a lower-rated one (370) was originally installed, is not harmful.

   Run capacitors must
   not be subject to voltages exceeding 10 percent of the nominal
   rating, and start capacitors must not be subjected to voltages
   exceeding 30 percent of the nominal rating.

   The voltage to which
   a capacitor is subjected is not line voltage, but is a much higher
   potential (often called back electromotive force or back EMF) which
   is generated in the start winding. – COPELAND ELECTRICAL HANDBOOK [PDF] from Emerson [PDF]

   Watch out: The voltage to which a capacitor is subjected is not line voltage, but is a much higher potential (often called back electromotive force or back EMF) which is generated in the start winding. On a typical 230 volt motor, the generated voltage may be as high as 400 volts and is deter-mined by the start winding characteristics, the compressor speed, and the applied voltage. – COPELAND ELECTRICAL HANDBOOK [PDF] p. A-9

   Capacitor Frequency (Hz) Rating Limits

   Most capacitors will be rated and marked as 50/60 Hz meaning that the capacitor can run at either Hz or frequency. But do not use a cap marked only 50Hz on a 60Hz circuit and vice versa.

4. Choose a capacitor based on Electric Motor Type, Motor Horsepower or Motor Kw or Kilowatt ratingIf you cannot find any data giving the motor’s brand, model, specifications, and you have no markings nor even the actual old start/run capacitor from an electric motor, you might get into the right ballpark by choosing a capacitor based on the motor’s horsepower and voltage and application.

   If you don’t even have that data, we’re stuck. Take the motor to an electrical motor repair expert who might find markings you missed.

What if the Motor has No Data Tag? Ballpark Start-Run Capacitor Values

If the motor has no legible markings nor data tag, first check again. Some electric motors such as oil burner motors have data stamped right into the metal casing of the motor itself. The data may be there but only visible under good lighting.

But if your electric motor truly has no legible information about its specifications then you might consider these very basic capacitor ranges that can get a small electric motor running again:

Notes to the tables

In general, starting capacitors for electric motors will be in the range of 50 – 1200 µF and will be rated for voltages of 110/125, 165, 220/250 or 330 VAC and will be labeled as functioning at 50Hz or 60Hz. Most capacitors use a 1/4″ flat bladed push-terminal connector with two terminals per post.

Run capacitors may have 3-4 terminals pwer post.

While most starting capacitors wear a round case, some are oval in design: the shape does not affect the capacitor’s function but the capacitor’s shape might affect it’s ability to fit into the original capacitor’s location or case on your equipment or motor.

Notes to the tables above

Watch out: as a general rule of thumb, electric motor start capacitors can be replaced with a micro-farad or µF or mfd rating equal to or up to 20% higher µF than the original capacitor serving the motor. On the replacement capacitor the voltage rating must be equal to or greater than the original.

Watch out: do not touch an electric motor capacitor nor any other electrical components before you have removed electrical power and SAFELY discharged the capacitor or you can get a serious shock or injury.

Replacement capacitor for-sale listings may be described leaving you to interpret the numbers, such as this example: 35+5 uF +5%, 370VAC, 50/60Hz – this is a 35µF Start + 5µF Run capacitor, capable of handling voltage up to 370 volts alternating current (so you’re ok with 120 volts or 240 volts).

1. Watch out: these are only « ballpark guesstimates » – check with the manufacturer of your specific motor and application.

For example among the motor type « pool pump » horsepower range may mean different capacitor values.

If your capacitors burn out quickly there are several common reasons: wrong capacitor size (too small), motor is damaged, or mis-wiring.

2. The KW and VAC data are excerpted from the small motor capacitor table based on motor voltage and horsepower in

ELECTRIC MOTOR CAPACITOR RATING TABLES given at the start of this page

3. Typical replacement sold for a Goodman brand air conditioner at online parts suppliers – Ed.

4. Typical replacement sold at WallMart June 2018
Watch out: Wallmart capacitor prices on 20187/06/30 were observed to be up to ten times or more higher than other vendors of the same or equivalent parts. Check also with your local electrical supplier.

5. Watch out also: not all motors use start, run, or combination capacitors, and some motors such as those sporting a switchless motor or « PSC » (Permanent Split-Capacitor) motor requires a run capacitor that improves motor efficiency and stays in-circuit when the motor is starting or running.

6. Hobart commercial restaurant mixer motor starting capacitor, sold by Heritageparts.com

7. Using the Hydromatic single phase capacitor pack fdor single phase pumps such as from Pentair® or Hydromatic® provided by R.C. Worst & Co., rcworst.com included these submersible sewage grinder pumps: S3HRC300M7-2
HPGH/F300M7-2
HPGHF/H300M7-2
HPGH/FX300BD
HPGH/FHX300BD

Capacitor Ratings in Farads or micro-Farads µF and Voltage VAC

Capacitor energy ratings are expressed in farads – the amount of electrical power stored in the capacitor, where µF or µF = microfarad (10^-6 farads) and is the same as mfd (microfarads) written in other references.

Rated capacitance for a capacitor is the capacitance value (electrical energy storage value) for which the capacitor is designed.

The rated capacitance has to be sufficient to give the energy boost or kick to the electric motor to start it spinning (a start capacitor) or to help keep it spinning (run capacitor).

Capacitors are also rated for the voltage range in which the capacitor can safely operate, such as 220V or 440V.

The starting capacitor photo shown here, provided 2018-07-10 by reader Robert, is marked as rated 35 +5 uF 440V 60HZ.

That’s 35 microfarads +5% and is the capacitor rating, along with voltage, that the reader will need to specify when buying a replacement.

We also see a specific part number: SH 8720 1238 80-197, but I in replacing this capacitor you’d attend first the rated capacitance and voltage.

[Click to enlarge any image]

Rated voltage for a capacitor is defined as the highest continuous voltage that can be applied to the capacitor at its rated temperature without damage.

Depending on the application, the micro-farad range of starting capacitors varies according to the motor size.

Start capacitors typically range from 20-30 µF up to 250-300 µF. The example capacitor charts at the start of this article are adapted from AFCAP. [2]

The voltage range for a start-up capacitor typically ranges from 250VAC to 450VAC.

Watch out: When you are replacing an electric motor capacitor, never put in a lower rated capacitor. If you cannot get an exact size match to the original motor capacitor, it is acceptable to use a capacitor rated one step higher in µF. The substitute capacitor must be able to handle the voltage.

Run capacitors typically range in micro-farads from 1.5 to 50µF. or 50 microfarads or MFDs.

For example you can’t sub a 110V-rated capacitor into a 220V system.

Robert’s run capacitor shown here is rated at 7.5 µF +/- 5%, 370 Volts, and 50/60~ or cycles. The cap also sports it’s part number.

You can also check a capacitor to compare its performance with its microfarad rating by using an ohmmeter. In a working capacitor ohms will build-up and then fall off (when the capacitor discharges).

Details of that procedure are

at TEST a MOTOR START or RUN CAPACITOR

HVAC suppliers sell general-use starting capacitors that are intended for use across a range of electrical motors and motor sizes.

But at least some industry sources (the Sealed Unit Parts Company or Supco) make a more careful argument explaining that it’s best not to install a significantly oversized starting capacitor on an electric motor.

According to Supco, [quoting]

If the start capacitor is too large for the application, the capacitor can actually mask the developed voltage in the start windings and keep the start capacitor in the circuit continuously. …. The … run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs.

A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit.

… A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start windings in the compressor. … If the capacitor is never removed from the start windings, premature winding failure could occur. As such, care should be taken when selecting capacitor sizes for an application.

Care should also be taken regarding products that tout a « bigger capacitor is better » approach to compressor starting. SUPCO E-Class devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode. [Italics are ours-Ed.].

Electric Motor Capacitor Working Class

Capacitors are also rated in working classes which describe the expected service life of the capacitor in hours, assuming the cap has been properly selected.

Notes to the table above

Sources:

Kemet, Capacitors for AC Motor Applications, [PDF] Kemet Electronic Components, 941 Linda Vista Dr, West Chester, PA 19380 USA, Website: http://www.kemet.com/ retrieved 2018/06/18, original source: http://www.kemet.com/Lists/ProductCatalog/Attachments/158/F9000_GenInfo_MotorCaps.pdf

WEG, Specifications of Electric Motors [PDF], retrieved 2018/06/18, original source: http://ecatalog.weg.net/files/wegnet/WEG-specification-of-electric-motors-50039409-manual-english.pdf

Examples & Sources of Start / Run Motor Capacitors

• Copeland Industrial Capacitors such as the Copeland Stoelting 231047 run capacitor shown here for use in a Copeland brand HVACR compressor motor system.
• Packard 370V 45+5 MFD Round Run Capacitor
• Supco™ 30+5 MFD 440V 440 Volt 50/60 Hz Round Motor Capacitor (typical application for Amana, Carrier, Rheem, Trane equipment)
• Supco™ 45+5 MFD 440V 440 Vol 50/60 Hz Round Motor Capacitor (typical application for Amana, Carrier, Rheem, Trane equipment)
• Supco HS6 (SPP6) Hard Start Relay Capacitor (230V) gives a starting torque boost of up to 500%. Quoting from product sales literature:

  The SUPCO SPP6 Super Boost hard start capacitor increases starting torque up to 500%. Features: Installs in seconds across run capacitor terminals Contains specially designed relay and large start capacitor for severe hard start problems.

  Applications: Room A/C units, Residential and commercial PSC A/C units and heat pumps, For all PSC A/C units from 4,000 thru 120,000 BTU’s (1/2 thru 10 H.P.) Can be used on 120 thru 288 VAC units, For severe low voltage and hard starting compressors.

  SPP6 Specifications: Voltage: 115V – 230V. Increased Torque: 390 ounce inches. – retrieved 6/16/14 Amazon.com sales

• Supco™ Universal (broad application) Capacitor, 10 MFD at 370V

Start & Run & Dual Capacitor Guidebooks, Catalogs, Cross-References, Sources, Specifications

• AO Smith Electric Motors, CENTURY POOL & SPA MOTOR MANUAL [PDF] AO Smith Electrical Products Co., 531 North Fourth Street,
  Tipp City, OH 45371
  USA Website: www.aosmithmotors.com
  (937) 667-2431 – includes troubleshooting, repair, replacement, capacitor testing
• AO Smith ELECTRIC MOTORS for the FARM [PDF] Op. Cit.
• Century Electric Motors (Previously A.O. Smith Electric Motors), CENTURY POOL & SPA MOTORS REPLACEMENT PARTS & MOTOR CROSS REFERENCE MANUAL (2012) Distribution Marketing
  1325 Heil Quaker Blvd.
  LaVergne, TN 37086
  PH: 866-887-5216
  FAX: 800-468-2062
  centuryelectricmotor.com
• Century Electric Motors (Previously A.O. Smith Electric Motors), SINGLE & THREE PHASE INTEGRAL HORSEPOWER MOTORS [PDF] (2013)
• Century Electric Motors (Previously A.O. Smith Electric Motors), V-GREEN 165 VARIABLE SPEED ELECTRIC MOTOR MANUAL [PDF]
• Conis Elco Ltd., CAPACITORS for MOTOR APPLICATIONS, [PDF], Website: http://www.conis-bg.com/EN/, retrieved 2018/06/18, original source: http://www.elcomp.net/conis.pdf
• COPELAND ELECTRICAL HANDBOOK [PDF], Emerson Climate Technologies, White-Rodgers US
  8100 West Florissant Avenue
  Saint Louis MO 63136 USA, Customer Support: email: wr.techsupport@emerson.com, Tel: 1-888-725-9797 U.S. & Canada
• Eaton, POWER FACTOR CORRECTION: A guide for the plant engineer [PDF] (2004) Technical Data SA02607001E, Capactior banks and passive harmonic filters, Eaton
  1000 Eaton Boulevard
  Cleveland, OH 44122
  United States
  Eaton.com retrieved 2019/10/08, original source https://www.eaton.com/ecm/groups/public/%40pub/%40electrical/documents/content/sa02607001e.pdf
• Essex, Brown: « Motor Repair Supplies » (Catalog), Essex Group, Inc., 1601 Wall St., Fort Wayne, Indiana 46801, Tel: 219-461-4633, Website: www.superioressex.com, retrieved 6/20/14, original source: http://www.essexbrownell.com/uploadedFiles/Content/Products/MR%20Supplies%20Catalog-s.pdf – see pp. 86-89.
• Galco SIZING LOW VOLTAGE CAPACITORS at MOTOR LOAD [PDF] Galco Industrial Electronics, 26010 Pinehurst
  Madison Heights, MI 48071 USA Website: www.galco.com, Tel: 800-575-5562, 800-521-1615 or Emergency Service:
  888-988-8086 retrieved 2019/10/08 original source: https://www.galco.com/techdoc/abbc/c485g30-u_cp.pdf
  Excerpt:

  A capacitor is properly sized when its full load current rating
  is 90% of the no-load current of the motor. This 90% rating
  avoids overcorrection and the accompanying problems such
  as overvoltages.

• Kaiser, Cletus J., The Capacitor Handbook: A Comprehensive Guide For Correct Component Selection In All Circuit Applications. Know What To Use When And Where, 2d Ed., [at Amazon.com] C.J. Publishing (2011), ISBN-10: 0962852538, ISBN-13: 978-0962852534 – product description

  This book provides practical guidance and application information when using capacitors in electronics and electrical circuit design. This easy-to-use book covers the following capacitor types: Ceramic, Plastic Film, Aluminum Electrolytic, Tantalum, Glass, Mica, and others.

  This book also has a very comprehensive Glossary and Index. The Selection Guidelines and the Symbols and Equations sections have the answers to all of your daily application questions. This book is one in a series of component handbooks.

• Kemet CAPACITORS for A.C. MOTOR APPLICATIONS [PDF] Kemet Electronics Corporation, 1 East Broward Blvd.,
  Fort Lauderdale, FL 33301 USA, Tel: (954) 766-2800 Email: info@kemet.com retrieved 2019/10/08 original source: http://www.kemet.com/Lists/ProductCatalog/Attachments/158/F9000_GenInfo_MotorCaps.pdf
• Kemet CAPACITOR CATALOG [PDF] (2015) KEMET Corporation
  2835 KEMET Way
  Simpsonville, SC 29681
  USA
  864.963.6300
  www.kemet.com retrieved 2019/10/08 original source https://www.avnet.com/

  Over 300 pages, although titled a « Capacitor Selection Guide » this is a catalog and not an explanation of how to choose the correct capacitor for your application.

• MOTOR START and RUN CAPACITORS [PDF] AFCAP (African Capacitors Limited), Metallized polypropylene film capacitors for
  motor running applications web search 08/05/2011, original source: http: //www.afcap.co.za /manual/Part2.pdf; 2018/07/11 update: the original source link is no longer valid – Ed.
• Sealed Unit Parts Co., Inc., PO Box 21, 2230 Landmark Place, Allenwood NJ 08720, USA, Tel: 732-223-6644, Website: www.supco.com, Email: info@supco.com, Website: http://www.supco.com/ , Supco Catalog, retrieved 6/20/14, original source: http://www.economicelectricmotors.com/cdrom/catalogs/Supco_catalog.pdf – see pp. 2-6.

  Supco
  provided the compressor starting capacitor and packaging information (purchased by the author from an air conditioning parts supplier in New York) – our example uses a Sealed Unit Parts Company Solid State part No. RSC 10 115V starting capacitor which was designed for installation on refrigerators and freezers. See www.supco.com/

  « The E Class Advantage », Supco (op cit), describes the company’s advanced start/run capacitor products. Web search 08/04/2011, original source: http://www.supco.com/eclassadvantage.htm Quoting from that article:

  The SUPCO E-Class Series comprise the most advanced developments in start device technology:

  1. Voltage sensing technology that monitors for motor start (current sensing devices require internal fuse protection).

  2. A 2-wire connection that simplifies installation

  3. A secondary timing circuit that ensures that the capacitor is not permanently left in the start winding circuit

  4. A fully electronic device – minimizing the limitations of mechanical devices and secondary fusing associated with triac devices

  5. A start device matched with an appropriately sized capacitor to cover the range of compressors for the intended application (one size does not fit all)

  The use of compressor start devices results from a need to ensure that a compressor (usually air conditioning) will start under voltage conditions that are less than ideal. As discussed, several options exist in the market to address compressor start concerns. Start devices exist in many forms for specific applications.

  SUPCO provides a full range of products in all relevant technologies to effectively match the proper start device to the application.

  Care should be taken to utilize a device that meets the requirements of the job.

  Extra caution should be observed when employing the « one-size-fits-all » and « a bigger capacitor is better » approach to applying a start device. Consult SUPCO, a manufacturer with a complete product range, to ensure the greatest success in the start device application.

• Temco START CAPACITOR SELECTION GUIDE [PDF] Temco Industrial, Tel: 877-474-8209, Website: www.temcoindustrial.com retrieved 2019/10/08 original source: https://temcoindustrial.com/product-guides/capacitors/start-capacitors
• van Roon, Tony, « Capacitors », [online article], retrieved 6/20/14, original source: http://www.sentex.ca/~mec1995/gadgets/caps/caps.html, gives a very detailed history of the invention and history of electrical capacitors beginning with van Musschenbroek’s Leyden jar in 1745. This article includes « Capacitor Nomenclature » by Dean Huster.
• Visay, LOW VOLTAGE POWER CAPACITORS CAPACITOR SELECTION TABLES [PDF] Vishay Corp., Company address: not provided at their website; Website: http://www.vishay.com retrieved 2019/10/08 original source: http://www.vishay.com/docs/13003/calcandtables-esta-lvpc.pdf

Where are the Motor Capacitors Found?

This now discussion has its own page – LOCATE the STARTING CAPACITOR

Reader Q&A – also see the FAQs series linked-to below

I’m in Seoul

Hi Lauren,

The 25/85/21 is about as close as you might get unless you identify the fan brand and model and then contact the manufacturer for an original replacement part.

Where in Korea ? ( lived there for more than a year in the 1960s and have an abiding respect and affection for the country and the Hanguk.)

High velocity metal floor fan (I live in Korea, so higher voltage all around) These capacitors are easily found, but do the numbers I’ve circled in yellow have to be the same?

I’ve seen 40/85/21, 25/85/21, and 40/70/21. uF, Hz, and VAC are the same. Could I use these or should I play it safe and make sure everything matches? Thanks!

Help needed i have a clarke compressor model se7ar kw 1.1 hp 1.5 volts 240 amp 8.9 rpm 1440 hz 50 can someone please tell me what capacitor i need

Sure, IH, just check out the general recommendations for a 1/2 hp 120VAC Electric motor in the tables above on this page.

I have DeWalt radial arm saw manufactured in the later stages of WWII. I’m fairly certain it needs a new start capacitor (looks original) but there are no markings on it at all and I don’t know what to replace it with.

The motor’s data plate gives me this information: 1/2 hp, 7.2 amp, 115 v, 60 cy. Can anyone help?

Replaced my 1/2 hp 115V blower motor. Bought the recommended 10mfd run cap. Plugged it all together, motor starts briefly and stops after 1-2 sec. Reused the prior 7.5mfd cap and everything is working great.

Tested the 10mfd cap and it reads good. Am I going to get into trouble leaving the 7.5 in place? Why wouldn’t the 10 work just as well as the 7.5? I’m stumped.

1.5 mOtOr capasitOrs details please One capasitOr is 20 MFD and Other One details

Robert

You’re pushing limits a little bit. Most sources tell us that were allowed to exceed the capacitor rating by about 10%

1.1 hp air compressor. I am using 2kw inverter, off grid. Starting current draw 30a (3600w) , my peak is 3500w. Compressor has 60uf capacitor. Can I use 70uf capacitor to close gap?

Run

By the capacitor size rating in farads or microfarads. The start cap will always be the larger of the two. It takes more of a kick to get a motor spinning than to keep it spinning. You’ll see those differences quantitatively in the data given in the article above.

Thanks for asking.

When you go to order a replacement capacitor for a motor how can you tell if its a RUN cap or Start CAP??

My pump has a 40 uf cap it comes in BOTH run or Start

Thanks

Renjith

See the values in the table

Estimated Small Motor Capacitor « Ballpark » Sizes Based on Motor Type

On this page

Sir

3 phAse 20 hp motor which capacitor required

That should be Ok,

The article above on this page gives some guidelines about substituting capacitors and how much variation from spec is acceptable

Can I use a 440v 40uf instead of 440v 35uf

Jan

The best procedure would be to find the owners manual for your saw and find the wiring diagram in the specifications for the actual start and run capacitors that that motor used. You can’t find that information you can get in the right ballpark using the table provided above on this page, by noting your Motors horsepower and voltage.

William

A 15 MFD 330 VAC capacitor ought to be available at local electrical suppliers if there’s one near you, else from an online vendor. Dayton certainly makes several that ought to be in a suitable range as does Titan and as does the original manufacturer of your capacitor – it’s probably not quite what you wrote but rather, Aerovox (that’s a V not an N)

Keep in mind that you’ll be ok with a cap whose voltage is rated higher than 330, but not one whose voltage is rated lower.

Below I’ll show an example of a cap that would be fine, easily found for sale online at vendors like https://www.grainger.com/ -in this case a Titan Pro round motor run capacitor rated at 15uF and 370-440VAC.

You can of course also give Grainger a call

1-800-GRAINGER

(1-800-472-4643)

Or contact Aerovox at

http://www.cd-aero.com/

Sales at

CD Aero, LLC

167 John Vertente Blvd.

New Bedford, MA 02745

Tel: 508-910-3500

Fax: 508-995-3000

In case I failed to make this more clear, InspectApedia.com (this website) does not sell anything.

In order to absolutely assure our readers that we write and report without bias we do not sell any products nor do we have any business or financial relationships that could create such conflicts of interest.

I have and old 1960s Century Capacitor Single Phase 7.5 HP, 230 volt, 40Amp, 1750 RPM motor on an existing grain elevator leg that burned one of the 9 Capacitors out. The Capacitor Motor Control Unit is separate from the motor. Currently running on 8 capacitors hooked in parallel. Motor still runs fine but would like to find a replacement capacitor which is a 15 MFD 330 VAC capacitor which was part of the cluster of 9. Is it possible to find a satisfactory replacement.? I have all the ID data on the capacitor which is AERONOX RFFU, 170015, 15mfd 330VAC, 132.

I was given an old Rockwell table saw. The motor included is 1/2hp 110v 1725rpm 8.0amps. 1 phase.60 cycles, cont.rating.,type S-2872, code 6512, Model GMD-M56B-HSF-BG/575M.(n0 capacitor). I suspect it is not the right motor. Motor hums on start,and will go only if I spin the pulley.(not a safe situation) Motor runs in 2 directions; depending how I spin the pulley. my question- can an external capacitor be added into the circuitry to make the motor start and run with no help from me.Thanks Jan

On 2020-05-31
by (mod)
– Do all electric motors need a capacitor to start and/or run?

Frederick,

Thanks for a helpful question: Do all electric motors need a capacitor to start and/or run?

Answer: no, not all. Here are the details:

Pretty much ALL modern single phase electric motors use a START capacitor, either internal or external, to get the motor spinning.

SOME electric motors are designed to run without a run capacitor.

For example, Shaded Pole type electric motors, also known as « single phase induction motors » uses an external capacitor to get the motor started but then the motor can continue to run without a run capacitor. « The four basic types are split phase, capacitor start, permanent split capacitor, and capacitor start/capacitor run. » – detail courtesy of Bay Motor Products cited below.

Original AC Motors, going back to Nicholas Tesla’s original design, ran without using a start or run capacitor, using instead two windings; the start winding or auxiliary winding got the motor going.

ALL modern single phase electric motors use a START capacitor, either internal or external, to get the motor spinning.

SOME electric motors are designed to run without a run capacitor.

For example, Shaded Pole type electric motors, also known as « single phase induction motors » uses an external capacitor to get the motor started but then the motor can continue to run without a run capacitor. « The four basic types are split phase, capacitor start, permanent split capacitor, and capacitor start/capacitor run. » – detail courtesy of Bay Motor Products cited below.

For some of these details we thank Bay Motor Products, « Types of [Electric] Motors: P.O. Box 982, 3100 Cass Rd., Traverse City, MI 49684 USA Website: www.baymotorproducts.com/type-of-motors/ Tel: 231-941-0411 Email: customerservice@baymotorproducts.com

Can a motor run without a capacitor

Robert

70 uf is probably ok

60 uf is probably not

Typically we can use a capacitor that is close to the value of the original one as long as its voltage rating is not exceeded.

From the article above on this page we have this warning

Watch out: When you are replacing an electric motor capacitor, never put in a lower rated capacitor. If you cannot get an exact size match to the original motor capacitor, it is acceptable to use a capacitor rated one step higher in µF. The substitute capacitor must be able to handle the voltage.

Can you use a 60uf or 70uf to replace a 65uf capacitor?

Alama

Those are probably all acceptable microfarad ranges for which the capacitor can be used.

my ac fan motor dual capacitor oval 3 terminals burn out with mouse dameged can not find uf but i have manual but 3 different numbers 25 7.5uf 35 7.5uf 40 7.5uf but i dont know can you tell me what will i do thamnk you alama

Shree

Thank you for helpful question.

It takes a much bigger kick or jolt of current to start a motor spinning from being idle than it does to help keep the motor spinning once it is started.

That’s why the start capacitor always will have a much higher capacity rating in microfarads then the run capacitor.

Why is the rating of run capacitor smaller than start type capacitors?

The Owner’s manual for this Central Machinery disc & belt sander (if you don’t have it) is at

CENTRAL MACHINERY DISC & BELT SANDER MANUAL [PDF] from Harbor Freight

There we find the sander’s electrical rating as

120 VAC / 60 Hz / 3.5 A

So we know it’s a 3.5A motor.

That’s about a 0.5 or 1/2hp motor at most.

Armed with that data take a look in the tables above on this page and you can find a reasonable start and run capacitor size.

of about

1/2 Hp or 0.37 KW, 120-150 VAC 320-400 µF – Start Cap 10 µF 370VAC – Run Cap

let me know what you think

Yeah well, the only marking on this hosing is a ground symbol. This thing probably has maybe a 1/4 hp motor.

Pulled it apart to the windings, no data anywhere.

Checked the owner manual specs and the only thing they
note is the rpm speed of the disk, 1819RPM. I’ll try calling HF in the AM and see if their support staff has any
motor data.

John

Let’s try to do this accurately: what are all of the data you find on the belt sander motor? Perhaps attach a photo of the sander motor’s data tag.

Also check that the motor isn’t being bound by some other mechanical fault

I bought a Harbor Freight table top belt sander. A common complaint is slow startup, usually needing a “push” to get it up to speed.

I believe the cap is underrated and should probably be around 100uf.
What are your thoughts? This is the original.

Thanks for an important question, Dave:

What are the risks of using an over-sized capacitor or under-sized capacitor?

A start cap that is over sized in voltage generally won’t harm the motor.
A run cap that is too bit or too small can cause improper, inefficient motor operation and can ultimately shorten the motor life.

From the article above on this page we have:

…

The voltage rating of a capacitor indicates the highest nominal voltage at which it is designed to operate.

Use of a capacitor at voltages below its rating will do no harm.

For example, if your motor used a 370VAC rated capacitor you can replace it with a 370 or a 440VAC capacitor. The 440VAC capcitor will last longer. But because these voltage ratings are simply the peak voltage that the capacitor can handle, using a higher-rated cap (440) where a lower-rated one (370) was originally installed, is not harmful.

Run capacitors must not be subject to voltages exceeding 10 percent of the nominal rating, and start capacitors must not be subjected to voltages exceeding 30 percent of the nominal rating.

…

Particularly for the run capacitor the effects of a cap that’s too far off spec for what the motor requires risks damage to the motor as well as improper operation.

The motor’s rotational torque may be reduced, and depending on the motor’s actual physical load, its speed could be affected, stressing the motor, increasing its noise level, increasing its operating temperature, even causing overheating or bearing failure. You may also increase the motor’s operating cost by reducing its efficiency.

A mis-matched run capacitor can throw off the phase shift or phase angle (I’m speaking a bit past the edge of my technical expertise here) between current (amps) and voltage – a cause of the failures listed above.

Hello, the Rheem (Ruud achiever series mdl#: RRMA-A042JK10E) AC unit I have has dual capacitor setup. one is a 10uf 370v and the other one is a 55+3 370v capacitor. I am unsure about this second one.

What is the risk of getting a 55+4 or 55+5 instead of the 55+3?
What does this second value +# affect on the HVAC unit?

A local technician told me it didn’t matter as long as it was a higher value, but searching online I read differently; however, I cannot find an actual reason why or what actual problem will occur when exceeding this value.

Thank you

Glen in the article above on this page CHOOSE a START / RUN CAPACITOR, HOW TO

a best answer is at the section titled

Capacitor Voltage Rating Selection Limits



Please take a look and don’t hesitate to ask follow-up questions

can i replace 55uF+5 start capacitor instead of 45uF+5 capacitor

I am referring to type a) the « motor running capacitor  » which is permanently connected in the circuit during running, such as used in the table fan etc., type b) the  » motor starting capacitor » which are connected only briefly during starting but switched off by centrifugal switch etc during running;

such as used in the single-phase motor for starting purposes and type c)  » power-factor correction capacitor » which are used in single-phase or three-phase pf correction purposes.

I am aware that in most cases the capacity (uf) of a)<b)<c) . they may be of similar rated voltage vac.
my questions are :

1. how to differentiate between type a), b) and c) as they any be in metallic or plastic can or in plastic cube ?

2. whether are they interchangeable, e.g b) replacing a) or c) replacing b) if they are of the same capacity and voltage rating?

3. if b) shall not be used for a) application, why there is no warning of the prohibition on the rating marking?

4. which iec/ansi standards explicitly state their differences and that there are not inter-changeable?

che kuan yau (singapore)

1.5 hp motor with start capacitor capacity

Take a look at the data tag on the motor or find the IO manual for it as those will often specify the exact capacitor needed;

If you can’t get that data then you’re left with choosing a cap based on the motor’s wattage or horsepower, for which we give some example tables above; your local electrical supplier can take it from there. You may need something like the 10,000 uF starting cap shown here from Walfront

You can calculate the kVAR for a motor then convert that to typical capacitor rating in microfarads or uF

kVAR = motor voltage x amps rating (running amps), e.g.

A 2200W motor running single phase 240VAC = draws about 9 Amps.

9A x 240V = 2160 kVA

How to Convert Capacitor Farads into kVAR & Vice Versa (For Power factor improvement)

gives

Required Capacity of Capacitor in Farads/Microfarads

C = kVAR / (2 π f V2) in microfarad

for which example calculation details are given at that site; however it’d be safe to stop by your electrical supplier with your motor specs. Below: example of motor capacitors from Rubycon.

I have a 30 yr old thicknesser which uses a Leroy somer 240v single phase 2200w motor. Unfortunately the start capacitor has failed and I cannot determine its rating. Is there anyway to find out.

Mogamat

It looks as if your motor runs at 220 or 380 Volts 3 phase and at 220V runs at 16.5A while at 380V it runs at 9.6A.
The CAV on the data plate of this Italian motor from Tavernelle indicates that it’s a 5.5 hp motor.

Typically the run capacitor on a 5 hp. motor is around 30 µF – 40 µF 370VAC

Typically the start capacitor for a 5 hp motor is around 600uF

To take a stab at answering « how to convert a 3 phase motor to run on single phase power » I found a variety of answers ranging from « it’s easy » to « it’s a really bad idea, even dangerous ».

Kemet (cited in the guidebooks given in the article above) notes: By connecting a capacitor to the two phases it is possible to make a three-phase motor work as a single-phase motor.

Watch out: you cannot simply hook up a 3 phase motor arbitrarily to single phase current; you need to review the wiring recommendations from motor companies or engineers.

Options include:

A phase shift capacitor;

Motor re-winding

Using a VFD or variable frequency drive

Use a converter to convert your single phase power to DC (Direct Current) and then use an inverter to power the motor. From what I read this is your best bet. As one EE notes « You can run a three phase motor from single phase by converting the single phase AC-DC(rectification) and then DC-AC(3 phase inverter). »

However pretty much everyone says this is an inefficient way to run a motor – i.e. your electric bill will be higher than necessary.

On Quora a Bombay writer recommended:

Use capacitor of say 36 / 72 / 108 mfd 440 V rating.

Depending upon motor rating. Connect one end of this capacitor to open unconnected terminal of motor.

The other terminal of capacitors can be connected to one of the two live phases, and the motor will start running.

If you change over the capacitor terminal to other phase , its direction of rotation will get reversed.

This system is routinely used in India particularly in rural areas for running of pumps.

This arrangement is certainly not the optimum for the motor, and the phase difference between the windings will differ from 120 degrees needed.

However, this does makes the motor start and save the day for farmers when 3 phase supply is not available. – www.quora.com/How-do-I-run-a-3-phase-motor-in-a-single-phase-line-using-a-capacitor

See: www.gohz.com/3-phase-motor-running-on-single-phase-power-supply

See this article by Chuck Yung: www.plantengineering.com/articles/how-to-properly-operate-a-three-phase-motor-using-single-phase-power/ on running a 3-phase motor on single phase power.

Note that operating the motor on single phase may be expensive.

Good day. I have 3 phase compressor that I would like to run on single phase current. What size capacitor would I need and will it need a run cap as well?

Reader Question: distributor says the order of numbers on the capacitor doesn’t matter

6/16/2014 Danno said:

I’m replacing the capacitor in my AC condenser. The original/stock unit says 35/5 440 AC. This is the cap I ordered from a distributor, but upon receiving it, the box says « 5/35 440 » (the 5 and 35 are transposed. Distributor says its the same thing. Is this correct? Thanks for any 411

Reply:

Danno I think the scrambled order is not an issue as long as the key numbers on the capacitor match its application or match the old one AND as long as you connect the proper wires.

Example start/run capacitor markings

(NOT specifying your particular capacitor)

• Voltage, e.g. 110-125V
• Capacitance, e.g. 189-227 µF or micro Farads or MFD (Start capacitors usually are over 60 µF)
• Capacitance, e.g. for a dual run capacitor: 35/5 refers to 35 MFD (for a compressor) and 5 MFD (for a fan motor)
• Temperature range, e.g. -40 – + 65C
• Cycles, e.g. 50-60 Hz

Decode a 35/5/440 marking on a motor starting capacitor

Reader Mark (18 June 2014) has given us a helpful and detailed translation of the markings found on start capacitors, repeated here:

Run capacitors: I always was taught that 35/5 was a [dual] run capacitor. 35 is the microfarad rating for the compressor & 5 microfarad was for the fan. (three terminals) not:

Decode a 35/5/440 marking on a motor starting capacitor:

The capacitor you are describing marked 35/5 440 is probably a dual run capacitor.

35/5 : The first two numbers are the capacitance 35 µF (for the compressor) and 5 µF (for the fan motor).

A dual « run capacitor » supports two electric motors, such as in large air conditioner or heat pump units, with both a fan motor and a compressor motor.

It saves space by combining two physical capacitors into one case. The dual capacitor has 3 terminals labeled

« C »,

« FAN », and

« HERM », which stand for the Common, Fan, and HERMetically sealed compressor.[5]

440 refers to the ability to run voltages up to 440VAC

Dual run capacitors come in a variety of sizes, depending on the capacitance (µF), such as 40 plus 5 µF, and also the voltage. (Be sure to connect properly to compressor motor, fan motor and common)

A 440 volt capacitor can be used in place of a 370 volt, but not a 370 in place of a 440 volt.[2]

The capacitance must stay the same within 5% of its original value.[2]

Round cylinder-shaped dual run capacitors are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor.[2]

An oval dual run capacitor could be used instead of a round capacitor, but the mounting strap should be changed to better fit the oval shape

Watch out: Take a look at the wiring sheet that should be with your capacitor or wiring marked on the device itself to be sure that you are properly connecting the Start and Run wires and you’ll be OK.

As we noted in the article above, the dual capacitor terminals may be labeled so that the three leads can be wired correctly:

• S = start wire connector
• R = Run wire connector
• C = common connector

Reader Question: your capacitor advice is all Greek to me

I have a Wayne 1 HP motor used as a utility pump. The capacitor burnt out. It is a rectangle plastic capacitor with two wires coming off of it.

The numbers on it are: 30 F /J/250VAC I don’t see a U before the F there are two symbols that look like arrows pointing to the right before the F letter. There is a name on it that says YUHCHANG ( P )

I read you article on how to choose a capacitor still it’s all Greek to me. So what am I looking for?

Thanks for your time,
– Anonymous by private email 2016/06/19

Reply: How to Choose the Correct Capacitor: four options

Anon: in response to your query I’ve re-written our capacitor selection explanation.

Four ways to find a matching or replacement motor capacitor are given at CAPACITOR SELECTION TIPS in this article.

Also if you can, send me some sharp photos of the cap and its markings and I may be able to comment further. Post a photo of your existing motor capacitor and its markings at the page bottom comments box, or use the page top or bottom CONTACT link.

Reader follow-up:

Here is a photo of the capacitor I’m talking about.

OK so we know this is probably just a single-purpose motor-start capacitor, since starting capacitors are most-often 125, 165, 250 and 330 volt devices while run capacitors are rated for higher levels like 370 and 440 volts. If it were a dual-purpose capacitor you’d see 3 rather than just 2 terminals.

You’ll have no trouble buying a replacement at your local electrical supplier or even from an online supplier. Prices for a motor capacitor start at under $10. U.S.

Ask for a 30 microfarad 250VAC motor start capacitor. Take along the old sick one to show your supplier.

At MOTOR CAPACITOR WIRING GUIDE we show how to hook up a typical capacitor.

Watch out: Keep me posted on how you do, and work with POWER OFF so you live to tell me.

Question: what is the right start capacitor for the motor shown in this photo?

Could you please point me towards the correct capacitance for a start capacitor that can be connected to the engine presented in the attached picture?

I’m afraid I will install a too high or too low capacitance or voltage capacitor and damage the motor. T.P.

by private email 2017/07/14

Reply:

The label reads 16 µF 450V, that’s the capacitor that Zhang Qiu Hai Er DianJi You Xian Gong Si the manufacturer specifies for this particular electric motor, apparently model QC.J 01.

Reader follow-up:

Correct, the labels reads 16µF 450V. Is this the capacitance/voltage that I should use for the start capacitor? The 16µF seemed too low compared to the tables you presented in your website.

Reply:

The final Authority on the correct start or run capacitor to use with any motor is of course always the motors manufacturer.

The motor shown looks like a Haier double speed electric motor shown in the photo below, excerpted from haiermotor.en.ec21.com/ where no details nor specification sheets for the motor were found.

Zhangqiu Haier Electromotor Co., Ltd. is a long-established producer of Haier electric motors located in Zhangqiu Economic Development Zone Jinan, Shandong, China and doing business since 1958.

The company became a subsidiary of Haier Group in 1998. These motors are used in household appliances such as washing machines, with other motor models used in refrigerators, air conditioners and other equipment.

More information including contact suggestions for repair parts or service are found at HAIER & HAIER AMERICA

…

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Or see CHOOSE a START RUN CAPACITOR FAQs – Q&A posted originally on this page

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