Electric motor start-run capacitor selection guide:

This article explains how to select 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 also 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.

How to Diagnose, Choose, & Replace Motor Capacitors

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.

Article Contents

• TABLES of COMMON MOTOR SIZE & CAPACITOR VALUES - shown above/left
• DIAGNOSIS: DOES MY MOTOR NEED A NEW CAPACITOR?
• HOW TO CHOOSE A CAPACITOR FROM MOTOR DATA - 5 ways
• SPECIFICATIONS on CAPACITOR LABELS - capacitance, voltage, temperature
• CAPACITOR RATINGS - capacitance details
• CAPACITOR CATALOGS & SOURCES

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.

The higher a capacitance or µF, the greater the total voltage charge that will be accumulated and then released by the capacitor to give the motor a "kick" to start it rotating.

Types of Motor Capacitors

Before trying to replace a capacitor, you should know that some motors use a combined start/run capacitor and others don't use a start or run capacitor at all.

Individual Start & Run Capacitors

A start capacitor is used to give the motor a kick to get it moving and a separate run capacitor keeps the electric motor spinning smoothly when it is under load.

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

So if your electric motor has only one physical capacitor, it might be a model that combines both start and run features. You can tell this by examining the terminals marked on the capacitor.

It will have both S and R as well as a C terminal.

Dual capacitor

A dual capacitor - combines two physical capacitors inside of 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 starts 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.

ECM motors use an electronic control module to operate the motor and may not use a start or run capacitor. Those motors are described

at ECM MOTOR CAUSING CIRCUIT BREAKER BUZZ

How do you know that an electric motor needs a replacement STARTING or RUN capacitor?

There are three bad capacitor cases.

1. Bad start capacitor - capacitor has weakened or has failed completely
2. Bad run capacitor - capacitor has weakened or has failed completely
3. Visible damage to either type of capacitor, for example: bulged or split case, visible leakage or corrosion

1. Bad Start 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 motor hums and does not start to rotate and the thermal overload protection switch has not tripped, then the START cap is probably bad.

2. Bad Run Capacitor

If the electric motor draws high current when running, the RUN cap may be bad. There are other causes such as bad motor bearing or overloaded motor.

In sum a failing motor may be hard to start, but because changing the cap is so cheap compared with replacing the motor, often the tech will try a new start cap first to see if she can get more life out of the motor before replacing it.

See details at HARD STARTING ELECTRIC MOTOR CAUSES

See also CAPACITORS for HARD STARTING MOTORS

Watch out: If EITHER the STARTING capacitor or RUN capacitor is bulged, burned, has a hole in it, or has leaked oil, it is shot and needs replacement.

[Click to enlarge any image]

3. Damaged Capacitor Examples

Burst or Blown capacitor:

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 capacitor remained engaged too long - perhaps because a centrifugal switch inside the motor did not disengage the starting cap circuit when it should have.

Details are at ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

Watch out: Exposing a capacitor to voltage much above its rated voltage input might also cause a capacitor to explode.

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 damage:

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.

Details of how to test the capacitor itself are found

at TEST a MOTOR START or RUN CAPACITOR

How to Choose a Replacement Motor Capacitor Size - 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 above in 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 here 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. (MFD = microfarads)

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.
   
   For help decoding the markings on your capacitor, see
   
   CAPACITOR CODE & MARKING DECODING GUIDE
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 rebuilder 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 - MicroFarads

   Select a replacement capacitor whose capacitance value in µF, (micro-farads) 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 capacitor 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.
   
   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 determined by the start winding characteristics, the compressor speed, and the applied voltage.
   
   COPELAND ELECTRICAL HANDBOOK [PDF] p. A-9
   
   

4. Choose a capacitor based on Electric Motor Type, Motor Horsepower, Motor Kw or Kilowatt rating, or Frequency (Hz)
   
   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 as a 50Hz on a 60Hz circuit and vice versa.
   
   If 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.
   
   
   
5. Calculate the required electric motor capacitor size using a mathematical formula
   
   What about calculating the capacitor size needed for an electric motor?
   
   OPINION: We've reviewed several formulas offered by experts and found that they pretty much always require a number that we don't know, such as the motor's power factor, or the motor's start-up energy requirement expressed in joules.
   
   If that data isn't on the motor's tag or in an IO manual for that specific motor, the formula is more tricky to use, but some of the formulas below permit using a "reasonable guess" at the motor's power factor, and it is also possible to calculate the power factor for a motor.
   
   If electric motor power factor sounds a bit intimidating or mysterious,
   
   see details at DEFINITION of POWER FACTOR or in the formulas below, if you don't know the power factor pf or PF for your motor, a reasonable starting point is 80% or 0.8.
   
   (The actual PF when a motor is starting up under no load is much lower, perhaps around 0.2 to 0.3).
   
   
   5-1. Formula for calculating the required size of a motor STARTING capactitor when the motor size is given in horsepower HP
   
   C = (7150 x HP) / (V x PF)
   
   C = Capacitance in uF or microfarads - this is the capacitor size that we are calculating
   
   7150 is a constant
   
   HP = the motor's rated horsepower (such as 0.5 hp or 1 hp etc.)
   
   V = the motor's rated voltage (such as 120VAC, 220 VAC, etc.)
   
   PF = the motor's required power factor to start up (we're calculating a starting capacitor size in this example) expressed in joules or microjoules - which of course you probably don't know.
   
   Round up the number that you calculate to a typical whole number capacitor size such as 5uF 50 uF or 100 uF. etc.
   
   Remember to choose a capactor whose voltage rating is at least equal to the rated voltage of the motor. It's perfectly fine to use a capacitor whose voltage rating is greater than the motor's voltage.
   
   For example if your motor runs at 220V your capacitor's voltage rating must be 220V or larger. A 330V rated capacitor is fine.
   
   Example:
   
   We have a 2 hp 120VAC electric motor and we will assume its running PF is 80%, so we use the formula above, plugging in those numbers:
   
   C = (7150 x 2) / (120 x 0.80)
   
   C = 14,300 / 96
   
   C = 148 uF
   
   So we'll round this up to select a 150 uF starting capacitor, choosing one that is rated for 220V or greater.
   
   5-2. Formula #4 for calculating the required size of a motor RUN capacitor
   
   C = (W x PF x 1000 / (V² x f)
   where
   
   C = the motor run capacitor value in uF or microfarads
   
   W = the electric motor rating or power expressed in Watts
   
   PF = the motor's power factor, or 80% as a guesstimate (use 80, not 0.80 in this formula)
   
   1000 = a constant
   
   V = the motor's voltage rating (that we square in the formula)
   
   f = the electrical supply frequency in hz typically either 60hz or 50hz
   
   Example:
   
   We have a 220VAC electric motor whose data tag tells us it's rated at 100 watts, and our electrical supply is 60 hz, so we use the formula above, plugging in those numbers:
   
   C = (100 x 80 x 1000) / (220 x 220 x 60)
   
   C = 8,000,000 / 2,904,000
   
   C = 2.75 uF
   
   We'll choose a run capacitor with or close to this value.
   
   
   Other Motor Capacitor Start/Run Value Calculation Formulas
   
   5-3. Formla #3 for calculating the required size of a motor START capacitor when the motor's power rating is given in kilowatts (KW)
   
   C = (1,000,000 x kW) / V x PF
   
   C= Capacitance in uF or microfarads - this is the capacitor size that we are calculating
   
   1,000,000 is a constant
   
   kW = the motor power or "size" experssed in kilowatts
   
   V = the motor's rated voltage (such as 120VAC, 220 VAC, etc.)
   
   PF = the motor's required power factor to start up
   
   Example:
   
   We have a 30 watt 110-Volt electric motor (that's 0.03 kW) and we assume a power factor of 80%
   
   C = (1,000,000 x 0.03) / (110 x 0.8)
   
   C = 30,000 / 88
   
   C = 340 uF
   
   
   5-4. Formula for calculating the required size of a motor starting capactitor
   
   C = (Start-up Energy) / (1/2 x Voltage² )
   
   C = starting capacitor size
   
   We divide the start-up- energy for the motor (expressed in Joules) by 1/2 of the square of the motor's voltage.
   
   These details may help:
   
   1 watt = 1 joule / 1 second, or Watts = Joules per second, where Joules are a measure of power which, in turn is an expression of energy used or exerted or consumed per unit of time.
   
   Good luck with this approach if you can't find your motor's starting energy in Joules.

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.

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 per 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 its ability to fit into the original capacitor's location or case on your equipment or motor.

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.

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:

Example Generic Motor Capacitor Size Ranges

• Small electric motor starting capacitors are generally rated at 70uF or larger, usually expressed as a capacitance range, such as 50-400 µF
  
  And will have a voltage rating that may be 125V 165V, 250V or 330V. That rating is an upper limit. Using a 330V rated capacitor on a motor that runs at a lower voltage is harmless, but using a 125V rated capacitor on a 220V motor is likely to burn up the capacitor. Larger is OK, smaller may not be.
  
  Starting capacitors usually are connected into the motor's start winding only until the motor approaches its intended run speed. Then a centrifugal switch will open to take the starting capacitor out of the motor's circuit.
• Small electric motor run capacitors are always smaller in size than the starting capacitor for the same motor, typically in the size of 1.5uF to 100uF, typically described as having a tolerance, also in microfarads or uF, such as 20uF +/- 5%, and usually bearing a voltage rating higher than the starting capacitor, sucha as 240V to 440V.
  
  Run capacitors remain in continuous use when the electric motor is running and help to keep the otor spinning by keeping a "running torque" on an auxiliary motor run coil while the motor is under load.
  
  Really? Well not always. Some electric single-phase electric motors do not use a run capacitor at all.
• See details about capacitors and more about capacitor selection
  
  in COPELAND ELECTRICAL HANDBOOK [PDF]
• See the additional small electric motor capacitor sizing notes following the next table below.

Notes to the electric motor capacitor size tables above

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.

For "Small" electric motors of unknown horsepower, start with the 1/8 hp start and run capacitor ratings given in the table above.

For a 3 horsepower electric motor of unknown specifications, a conservative starting point might be to choose a 100 uF starting capacitor and a 30 uF run capacitor, both rated at 370 VAC.

2. The KW and VAC data are excerpted from the small motor capacitor table based on motor voltage and horsepower in the motor capacitor table given at the start of this article.

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

4. Typical replacement sold at WalMart June 2018

Watch out: Walmart capacitor prices on 2018/06/30 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: 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 for 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

 

Electric Motor Capacitor Data Specifications

You'll see that motor capacitors are characterized by at least five properties:

1. Capacitance rating,
   
   measured in uF or microfarads, the amount of electrical charge stored in the capacitor and released when needed either to start the motor spinning (a start capacitor) or to help keep it spinning under load (a run capacitor). Higher capacitance numbers mean a greater charge or more of a "kick".
2. Electrical Voltage rating,
   
   the peqak voltage that the capacitor can handle during use, such as 125V 165V, 250V, 330V or 440V.
   
   See DEFINITIONS of ELECTRICAL TERMS  for more detailed definitions of voltage, capacitance, amperage etc.
3. Electrical frequency rating:
   
   the cycle rate or hertz (Hz) of the electrical power for which the electric motor is designed, such as 60Hz (U.S., Canada, Mexico), or 50 Hz (Europe, South America).
4. Capacitor physical size and shape:
   
   these measurements are important when the capacitor has to fit in a specific space provided for it in or on the electric motor itself. Too big: the capacitor won't fit. Too small: might be ok but might risk a short circuit if vibration causes the capacitor to jiggle around when the motor is running.
5. Capacitor operating temperature:
   
   the highest temperture in degrees C or F at which the capacitor can safely operate. A capacitor exposed to higher temperature than its manufacturer intended will have a shortened life.

Most of these motor capacitor rating data numbers are given on a label printed onto the capacitor body, though not all are always shown, or the data may have worn off.

See details

at CAPACITOR CODE & MARKING DECODING GUIDE - separate article

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 in replacing this capacitor you'd attend first to 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. For example you can't sub a 110V-rated capacitor into a 220V system.

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.

Robert's run capacitor shown here is rated at 7.5 µF +/- 5%, 370 Volts, and 50/60Hz or cycles.

The cap also sports its 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 (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.

Complete Guide to Capacitor Markings

This topic has moved to

CAPACITOR CODE & MARKING DECODING GUIDE

Electric Motor Capacitor Working Class

This topic has moved to

EXPLAIN CAPACITOR WORKING CLASS & LIFE EXPECTANCY

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
• Eg. Copeland Stoelting 231047 run capacitor shown here for use in a Copeland brand HVACR compressor motor system.
• Eaton, POWER FACTOR CORRECTION: A guide for the plant engineer [PDF] (2004) Technical Data SA02607001E, Capacitor 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 over correction and the accompanying problems such as over voltages.
• 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.
• Lamparter, Robert W., SELECTION, CONNECTION, REVERSING AND REPAIR OF ELECTRIC MOTORS [PDF] Home Shop Machinist, July/August 1987, Vol. 6 No. 4, a magazine "dedicated to precision metalworking" pp. 47-51, P.O. BOX 1810, Traverse City, Michigan 49685 USA
• 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.
• Packard 370V 45+5 MFD Round Run Capacitor
• Reco Bulgaria Capacitors, GENERAL MOTOR CAPACITOR INFORMATION [PDF] Reco Bulgaria, 1574 Sofia / Bulgaria, 65/3, Shipchensky Prohod, Bulgaria, Tel.: + 359 2 870 20 81, Email: info@reco-bg.com Web: www.reco-bg.com retrieved 2021/10/15 original source: http://www.reco-bg.com/motor.pdf
  
  Excerpt: Reco Bulgaria Ltd. is specialized in import/export of wide range of electronic components, modules and devices. Capacitors, filters, integrated circuits and printed-circuit-boards (PCB) are our main products.

• SUPCO 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)
  
  "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.
  
  Watch out: 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.

  • 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, and 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, and 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
• 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

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

 

What Caps do I need for a discontinued Emerson electric motor?

Thank you for such an awsome web site, I don't know why my searches didn't find you months ago. I have acquired, a barely used Husky compressor, because it was wasting floor space.

I know how you probably about big box store "stuff", but the price was right...as in , move it and it's yours. Anyways, everything looks good and smells good BUT the start and run caps are missing.

I would like to know if you or maybe a viewer would have the knowledge to tell me what value caps I need. It is a discontinued Emerson ...hp (SPL)(2.98KW)...240V motor.
I do believe that (SPL) means "fictitious information"...lol

(Spec plate photo attached).

Any help is much appreciated.
Ron Brown - 2023-10-24

On 2023-10-25 by InspectApedia Publisher - typical run capacitor for Emerson LR63596 / Model No. T63XWBSS1486 4-5 hp electric motor

@Ron Brown,

An electric motor starting capacitor will usually be larger than 70 MFD or uF (microfarads) in size

A typical starting capacitor for your Emerson LR63596 / Model No. T63XWBSS1486 4-5 hp electric motor will be in the 100-200 MFD range. Choosing a higher voltage rated such as 340VAC won't hurt the motor and makes the capacitor a bit more durable. Remember that the voltage is simply an upper limit for the capacitor. The key number is the microfarads rating.

An electric motor run capacitor will always be much smaller in MFD rating than the same motor's starting capacitor.

Run capacitors for electric motors that use them will be in the 1-100 MFD or uF range.

A typical run capacitor for your Emerson LR63596 / Model No. T63XWBSS1486 4-5 hp electric motor will be in the 30 MFD range, such as a 30UF+/6% 370VAC/B 50/60HZ run capacitor.

Of course Emerson may have more exact specifications for the motor.

Emerson LR63596 / Model No. T63XWBSS1486 Electric Motor Capacitor Selection

Two approaches to choosing a start/run cap for your motor:

1. use the brand (Emerson) and model number (Try LR63596 or Model No. T63XWBSS1486, or if you contact Emerson, the Mfg. No. K03) to see if you can find the exact owners manual for that electric motor, where you'll then find a wiring diagram and the specific capacitor values chosen by Emerson for that motor.

2. If that doesn't pan out, then you can follow the general motor capacitor selection guidelines above on this page to choose a capacitor that's at least in the right range. For your motor its KW rating translates to 4 hp. (or some sources say treat it as a 5 hp motor) so you'd use the yellow colored table above on this page where we show some capacitor sizes for larger electric motors. Interpolate between the 3 hp and 5 hp motors if needed.

3. Other motor capacitor sizing methods:

** try a cap in the 30 to 50μF/kW (run) and measure the behavior of the motor: does it get up to proper speed, have trouble starting or running? and then adjust the capacitor.

** calculate the capacitor values from other known motor data, HP, voltage, wattage, &c. - I'm (for now) eschewing these details as I think only EE's use this method to obtain a starting pint.

** perhaps start with a call to the manufacturer if you can identify the motor brand and model no.

On 2023-09-22 by Shawn in Northern MN - capacitor to run clothes dryer when connected to backup electrical generator

I have an off-grid home with a 10Kw Kohler LP generator (so, loose 30% power from gas to LP) used when solar isn't able to recharge fast enough. Usually, this is when I'm doing a lot of laundry and can't hang everything outside to dry.

I have a new Amana LP Dryer. The dryer starts briefly and then shuts off. I take said dryer, move it to a Yamaha gas 7200 generator and the dryer starts and works just fine. You can hear the Yamaha struggle at the startup. I assume this relates to the starting draw/amp?

Can you put a capacitor on the Amana motor to resolve this? If so, can you point me to a capacitor you would recommend.

All the best,

On 2023-09-22 by InspectApedia Publisher - first measure the current draw at the dryer before adding capacitor

@Shawn in Northern MN,

We have an article at this website on hardstart capacitors for electric motors which you can find using the search box. But I don't recommend starting there.

A gas dryer draws very little current since it only needs to run the motor that is turning the drum. So I might first measure the current draw at the dryer to see if it's got a problem such as a binding bearing or belt that is causing high current draw. Otherwise we're treating the symptom but not the cause and it even could be unsafe.

You need some help measuring current draw. We also have a guide for doing that, just let me know.

On 2023-09-22 by InspectApedia Publisher

@Shawn in Northern MN,

I wanted to add a comment that I thought of before and was hesitating about. Be sure to check the wire lengths and sizes involved between your generators and the dryer.

If you have a long run and the wire is undersize you can be seeing a voltage drop

The two generators may differ in what surge current they can handle when a motor is starting. But both of them should be able to handle a simple electric motor operating a gas dryer.

On 2023-10-13 by Shawn in Northern MN

@InspectApedia Publisher,

Thank you for the note. I've done some extensive digging on this issue and found the culprit. The main generator is a 10K Kohler running on LP. Built in 2002 w/1600 hours on it. This unit sat idle in the woods (albeit in a generator shed) for 8 years.

I've had to replace wiring that was rotted and the usual maintenance stuff. When this problem arose with the new LP driver, I started testing other electric motor driven devices and found there was no corresponding throttle control like I have on the standby generator.

With help from a rep at Kohler, and a trusty multimeter, we discovered the electronic governor (+actuator/mag pickup) were in need of replacement. Basically, the unit is stuck in idle which is just fine for the LED lights, on demand LP hot water heater, and the battery bank charger that boosts the solar panels efforts to fill the batteries quicker.

Now I'm torn... $1,628.00 in parts (did I mention the unit is 20 years old) or invest in a new unit?

Back to the original idea, not know any of the forementioned, would the capacitor Band-Aid work until I purchase a new unit? Understanding, that ultimately I need to invest some dollars here.

All the best,

Shawn

On 2023-10-13 by InspectApedia Publisher - capacitors can't do the job of a throttle control

@Shawn in Northern MN,

Thank you for that helpful and interesting update.

My initial reply will be short because in fact we're in the car driving down to the twin cities ourselves.

If you think about the role of motor capacitors as either giving the motor an extra kick to get it started from idle or to keep it spinning once it's up to full operating speed, I don't think that capacitors can do the job of a throttle control. The throttle control is intended to respond to an increase in load.

It's disappointing to have an older machine that probably could be perfectly good but that can become a money pit as well.

On 2023-10-14 by Shawn in Northern MN

@InspectApedia Publisher,
Thanks again for your thoughtfulness on this.

I think I'll put my energies (pun intended) into boosting my solar array and battery banks and simply purchase a smaller generator unit for times of heavy demands.

Thanks again for your insights!

On 2021-09-10 by Anonymous - What would cause a capacitor to explode?

What would cause a capacitor to explode?

On 2021-09-10 by inspectapedia.com.moderator - What would cause a capacitor to explode?

@Anonymous,

A short circuit, a seized motor, over voltage, and in some cases and internal failure in the capacitor.

On 2023-08-16 by Daryl Lavigne - how to replace the run capacitor on a 5 hp 3500 rpm severe duty ac motor from MagneTek (Century Electric)

I am replacing the run capacitor on a 5 hp 3500 rpm severe duty ac motor (brick saw). I have a new start capacitor 216-259MFD / 220-250VAC

I need to know what oval run capacitor is needed, since I can’t read the numbers on it. 85mm / 3.325” H x 73mm / 2.875” x 48mm / 1.875”.

On 2023-08-16 by InspectApedia Publisher -

@Daryl Lavigne,

When we have no other data to help choosing a start / run capacitor we look at the ballpark guesses in the yellow table above on this page.

There you'll see that ford a 5 hp motor we'd choose

5 Hp or 3.75 KW, 200-250VAC

Start Cap: 750 µF

Run Cap: 30 µF - 40 µF

370VAC

 

On 2023-07-26 by Clint - need start cap for a 15-year-old Century AC motor on a Coleman Powermate air compressor - MagneTek Ce

I've got a roughly 15-year-old Century AC motor on a Coleman Powermate air compressor, start cap has no markings. Motor says 115V, the compressor housing says 4HP but I dunno if that's true.

Century label (attached) says HP - SPL. 60hz, 3450 RPM, 15AMP. CAT is blank, searching by part number or Serial number returns nothing.

Capacitor is currently testing at around 23uF, motor runs fine first start but once it reaches cut-out it will stutter and then blow fuse when it attempts to cut back in. What is a safe capacitor size to replace with?

On 2023-07-26 by InspectApedia Publisher - 15-year-old Century AC motor on a Coleman Powermate air compressor, start cap has no markings

@Clint,

I would definitely choose the start and run capacitor size based on the motors nameplate or data tag not another tag found elsewhere on the same equipment.

It sounds as if you may have a bad run capacitor.

Other than matching the preceding capacitor that was working we give general guidelines for choosing a start and run capacitor on the page above so do take a look and don't hesitate to ask if that leaves you with questions. Thanks.

On 2023-07-27 by Clint - I think I need a start capacitor for my motor but I'm nervous about choosing it

@InspectApedia Publisher, I've read pretty much everything on this site and am still not comfortable enough to choose a replacement capacitor. It's interesting you say you think it's a run capacitor. When I read the symptoms of a bad start capacitor, it fits my situation exactly.

Maybe I should clarify a few things - this is exactly what happens AFTER the first cut-out and cut-in cycle - "If the motor hums and does not start to rotate and the thermal overload protection switch has not tripped, then the START cap is probably bad."

When I say it blows a fuse, it's not the thermal protection that kicks in. it trips the circuit breaker into which the compressor is plugged. Now maybe that's tripping before the thermal protection kicks in. However, when I test the RUN cap, the values are within range and there is no bulging or anything. Ironically, the RUN cap has a full label. I suppose I could try replacing it first.

So just to reiterate.. if I were to go out in the garage right now and turn on the compressor, based on history, it will start running just fine, fill the tank, cut out, and once it tries to cut in, it will hum and stutter and slowly try to turn before tripping the circuit breaker.

if you think that's a bad run capacitor, well I've got the label for that one and I can easily get a replacement. However, the fact that it tests almost exactly in the middle of the range on the label, coupled with no bulging, I find it hard to believe it's bad.

That said, I'm on this site because I know nothing so i'll take your advice. I appreciate the responses - I replaced the pressure switch, the tank check valve, and nearly every other fitting in the process of rebuilding this thing (and now troubleshooting this latest issue). this has definitely been a learning experience!

On 2023-07-27 by InspectApedia Publisher - if the motor hums and doesn't start that's definitely a start capacitor

@Clint,

I apologize, as it sounds as if you were right and I was being careless or hasty. I should be more careful.

I thought you were describing a motor that started running but failed when as it gets up to speed.

That would be a run capacitor.

But if the motor hums and doesn't start that's definitely a start capacitor.

Keep in mind that a new capacitor or a hard-start capacitor may get the motor running again, but it could be that there's an underlying problem such as a bad bearing or winding, or other internal component in the motor.

Lots of service techs know this but they figure that just swapping in a new capacitor set is so inexpensive that it's worth a try rather than spending a lot of time and customer money on a more thorough and accurate diagnosis.

Then if the capacitor doesn't keep the motor running they know they need to dig deeper.

On 2023-07-27 by Clint - what size capacitor should I use

@InspectApedia Publisher, Ok, so now we've decided to try replacing the start capacitor but I'm back to my original problem - I can't determine what size to use.

I've read the guides, and the fourth option for a missing cap label says to estimate based on the HP, voltage, etc. However, i struggle to find values for my situation since the motor tag says HP - SPL and the compressor housing says 4HP, and the VOLTS is 115.

So looking through the charts, I don't see examples or ranges for that situation. the Ballpark estimates based on motor size are mostly 220V at 3+ HP, and in some places the data says start caps typically top out at 300uF,

yet the ballpark estimates for 3HP motors at 220v are up to 580uF and possibly higher since 4HP isn't specifically listed.

I attempted to contact Century but their support form only allows for a limited number of characters and I could barely get the motor specs input, much less an explanation of what I was looking for.

So who can tell me, based on the specs for this motor, what a (relatively) safe estimate is for a replacement cap? do I just choose something middle-of-the-road like 300uF and see what happens?

Should I just call a repair shop? Or find an electric motor forum and ask there perhaps?

On 2023-07-27 by InspectApedia Publisher

@Clint,

Let's both see what larger HP motor start/run cap sizing guides we can find and post that here.

Above on this page we give ballpark motor start/run capacitors for motors up to 5 hp.

On 2023-07-29 by Clint

@InspectApedia Publisher, yes, I saw. There isn't anything listed for 4HP, and does voltage matter because my motor is only 115V and as I said above, the higher-HP motors are all 200-250.

Does that matter? and as there's nothing specifically listed for 4HP, do i just pick something in the middle between 3HP and 5HP? I don't want to over-compensate and burn out the motor.

Also as I noted earlier, the Century motor label says HP - SPL, and while the compressor label says it's 4HP, I read somewhere else that it's definitely not a full 4HP motor at 115V so again, this is why I am struggling to make a selection - I can read numbers but they are meaningless to me.

I understand many things but I do not understand electricity which is why I need help. And even if I find the model number for the modern version of this motor (CP1502LV1), I cannot find anywhere in any of the documentation exactly what the start motor capacitor values are.

Also the modern version is a 230V motor so i don't even know if it matters if I did find the start CAP size..

On 2023-07-29 by InspectApedia Publisher

@Clint,

As the data we had in the table above shows

3 Hp or 2.25 KW, 200-250VAC 500-580 µF 20-25 µF 370VAC

5 Hp or 3.75 KW, 200-250VAC 750 µF 30 µF - 40 µF 370VAC

a reasonable 4Hp motor cap would simply be between those values, after all we're only getting in the ballpark or guessing when we don't have a motor data tag as a guide.

 

On 2023-07-17 by Russ - ok to use a 300VAC rated capacitor on a motor calling for a 250VAC capacitor?

I have a motor cap 243-292 mfd 300 vac - can I use a 243-292 mfd 250 vac, its on a 230 volt motor

On 2023-07-17 by InspectApedia Publisher (mod)

Just above on this page we show how to select a motor capacitor - you'll see that both your original 300VAC and your 250VAC capacitors are above the voltage level of the motor per your information, and both old and new report the identical MFD or uF range, so you're in business: either would be ok.

And that 300VAC is a safety upper limit - it's fine to use where the motor only sees 250VAC

 

On 2023-02-14 by Richard - what size start cap for a 1/2 hp 240V grinder motor?

What size start cap is needed for half horse 240 volt grinder motor please

On 2023-02-14 by InspectApedia Editor - best option is to match the existing device on your system

@Richard,

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

See our 4 methods for replacing your motor capacitor above on this page at

https://inspectapedia.com/electric/Motor_Capacitor_Selection.php#SelectionTips

where you'll see that you need additional data besides the 240 voltage to find the correct replacement. You can always take those capacitor markings to your local electrical supplier for their recommendation.

On 2023-02-14 by perry janusauskas - start capacitor for a Kincrome compressor motor -

-i am looking for a start capacitor.for a kincrome compresser these are the markings JKDJIEKEDA CD 60A MOTOR STARTING CAPACITOR 150 UF 50/60HZ 350 VAC -40/+65 C E212600 can you please help me kind regards perry

On 2023-02-14 by InspectApedia Editor - start capacitor for a kincrome compresser

@perry janusauskas,

You have the necessary information to find the appropriate capacitor. Take that info to your local electrical supplier or home building supply store and they will very possibly have it in stock.

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

On 2023-01-13 by Ryan - need capacitors for an old pool pump motor

Got an old pool pump motor from freind, trying to get it running. No markings on motor, my best guess is its an 0,75kw motor 230v 4,8a. Problem is the caps are gone. only thing I have is this in the pic

On 2023-01-13 by InspectApedia Publisher

@Ryan,

0.75 (or 0,75 if you're in the UK) kw is about 1 horsepower.

So choose a capacitor in that range as given above on this page, noting that your cap was probably rated to work at least t 208VAC

On 2022-12-11 by InspectApedia (Editor)

@James,

so we know it's not a combination Start/Run capacitor.

Note that the values for the Starting capacitor are going to be much higher than the Run Cap.

On 2022-12-11 by James

@InspectApedia (Editor), ok, it only has two wires attached

On 2022-12-11 by InspectApedia (Editor) - note the number of wires connected to the existing capacitor and number of terminals on the cap

@James,

I can't say - you can determine that by noting the number of wires connected to the existing capacitor and number of terminals on the cap - (explained in these articles) - or by finding a 2nd capacitor (run cap) on your motor.

On 2022-12-11 by James

@InspectApedia (Editor), many thanks, can I assume this is a start capacitor only ?

On 2022-12-11 by James - old Compton motor on a spindle motor: Brook Motors Ltd., Huddersfield AC Motor needs capacitor - no markings, what do I choose?

Hi I have an old Compton motor on a spindle moulder (image attached) it struggles to start. Capacitor has external damage but no markings to find replacement….. can you help ?

On 2022-12-11 by InspectApedia (Editor) - choosing a replacement capacitor for an electric motor when the data on the existing cap is illegible

@James,

Sure:

There are several suggestions for choosing a replacement capacitor for an electric motor when the data on the existing cap is illegible.

See "What if the Capacitor is Lost or Has No Legible Markings?" for all of our tips.

Most basically, look at the motor horsepower (HP) and voltage. I think your motor data tag says 230/250VAC, 2HP.

Motor capacitor catalogs will give you a specific capacitor replacement part number.

As just ONE EXAMPLE see this

ELECTRIC MOTOR PARTS CATALOG [PDF] from Ironhorse or https://cdn.automationdirect.com/ that gives capacitors for 2 hp electric motors that contains specifications for various motor start and run capacitors, and

that shows using an Ironhorse START CAPACITOR - MTA-CAP-04 1-1/2hp 1800rpm IronHorse MTR series 1-phase AC motors, Not compatible with MTR2 series motors

On 2022-11-18 by Evan D Hintner - Hot tub motor made a pop noise, then .... nothing

Trying to prime the hot tub, I caused the motor to mal function. There was a loud pop and then nothing. I changed the bad capacitor with a 50/5 cap made for a/c motor.. I get current to the tub and motor but there is a click but no starting.

On 2022-11-21 by InspectApedia (Editor) - short circuited motor or control

@Evan D Hintner,

That sounds like a short circuited motor or control, or a seized motor.

 

On 2022-08-03 by Mark Pasieka - Capacitor specs for a Doerr Electric Motor

@Curtis Pieratt & @ InspectApedia-911 (mod),

The run capacitor selection guidelines above on this page does not align with results from formulas from other sources for my 5HP doer motor

Doerr Motors is a brand currently owned by NIDEC (who does not answer or return calls or emails). Grainger sold to Emerson who sold to NIDEC.

Grainger told me 50uF for my 5 HP which i do not trust and tests would result in a dangerous back EMF voltage on the run capo that would also break down the winding insulation.

That is not a Model number Doerr Model No. .... LR22132 is the file number for the recognized component listing of ETL CSA or Underwriters Laboratories (I used to work for them).

They have the file in Iron Mountain or maybe even digitized but UL will not share the info as it is owned by the original compliance applicant.... I tried that as well... even old friends working their wont help me get the capacitor size... maybe they are tracking file access?

On 2022-08-05 by InspectApedia-911 (mod)

@Mark Pasieka,

Have you measured the motor's current draw at start and run?

I would sure appreciate it if you would either use our CONTACT link to get our email or just post in a comment here what other motor capacitor selection sources you've tried, so that we can review them and provide as complete a set of choices as possible to our readers. Thanks.

For other readers:

An electric motor's run capacitor (called "working capacitor" in some texts) remains active after the starting cap has dropped from the circuit (as the motor has approached its proper run speed), and indeed the value of the run capacitor can affect motor speed or RPM, though other factors also determine the run speed: use of a voltage doubler, capacitor age,

and basics: the number of turns of wire in the coil, motor bearing system design details, variations in operating voltage. Though principally, from what I've read, the run capacitor is adding torque, not controlling rotation speed.

On this page and on our Electric Motor Diagnostics pages, we've added more references and manuals and documents for NIDEC motors, and Leeson motors (that can possibly help with older Doerr motors)

On 2022-08-03 by Mark Pasieka - total line currents are going down with increased capacitors

Any insight or feedback would be appreciated. Thank you guys for participating

What is strange is the total line currents are going down with increased capacitors.

looks like a 30uF gave me maximum RPMs. not seeing anything I'm intelligent enough to identify as significant "AH HA" moment . maybe you guys see something.

need to remember how to calculate capacitance from current and voltage at run cap and see if that would alight with one of the trials and diverge with others.... there shall be something that excel will surface... or at least that's why I think this experience may be useful for not only this situation...

I cannot believe how NIDEC won't return or answer calls or emails. never buying one of their motors.

I Think my clamp meter was not completely closed for the zero and 10.2 microfarad L Blue current measurements. I am running with an open tank so barely any load... my logic being the less heat the less likely I will kill the motor conducting the tests

this is a YouTube video of me collecting data for the almost 40 uF test:
https://youtu.be/pDs-IOefiaI

Your feedback is appreciated. Thank you! for some reason I was not notified about the replies to this forum.

 

Thank you to our readers for generous remarks

On 2023-10-05 by Gary - I can't believe I found this website!

WOW...I can't believe I found this website! There is so much information and actual responses and advice from people that know their stuff = real Pro's! As a DIYer of many years and professions...I can't give you enough positive praise! GREAT!

On 2023-10-05 by InspectApedia DF (mod)

@Gary,

Thank you so much for taking the time to leave that generous remark. Indeed we've worked hard on this material for decades so we're grateful when a reader finds it both trusted and useful.

It also helps us when you post questions, criticism, or suggestions.

Daniel

On 2022-10-17 by james parrott - One of the most informative articles that I have found. thankyou very much.

@One of the most informative articles that I have found. thankyou very much. jwp

On 2022-10-17 by InspectApedia (Editor)

@james parrott,

Thank you so much for taking time to compliment this article.

We also welcome content questions, suggestions, or criticism - DF

On 2022-09-05 by Anonymous - You guys rock!

You guys rock! You guys roll too!

...

Continue reading at MOTOR CAPACITOR WIRING GUIDE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see CHOOSE a START RUN CAPACITOR FAQs - Questions & Answers posted originally on this page

Or see these

Recommended Articles

• BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS
• CAPACITOR TYPES, for MOTORS
  • CAPACITOR CODE & MARKING DECODING GUIDE
  • CAPACITOR SIZE DETERMINATION for ELECTRIC MOTORS
  • CAPACITORS for HARD STARTING MOTORS
  • CAPACITOR WORKING CLASS & LIFE EXPECTANCY
  • CAUSES of HARD STARTING ELECTRIC MOTORS
  • HARD STARTING COMPRESSOR MOTORS
  • HOW a STARTING CAPACITOR WORKS
  • HOW to TEST the MOTOR to ID TERMINALS
  • HOW to LOCATE the STARTING CAPACITOR
  • MOTOR CAPACITOR WIRING GUIDE
  • STARTING CAPACITOR SAFETY
  • TEST a MOTOR START or RUN CAPACITOR
• ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD
• ELECTRIC MOTOR DIAGNOSTIC GUIDE - home
• ELECTRICAL DEFINITIONS
• HARD STARTING ELECTRIC MOTOR CAUSES

Suggested citation for this web page

CAPACITOR SIZE DETERMINATION for ELECTRIC MOTORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

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Citations & References

In addition to any citations in the article above, a full list is available on request.

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