Electric motor start-run capacitor FAQs:
Questions & answers about choosing, installing, & troubleshooting starting capacitors & run capacitors used on electric motors.
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.
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The starting capacitor helps a motor start spinning by creating a high-torque, rotating, electrical field in the motor.
In many electric motors there are actually two capacitors, one boosting the start winding (the start capacitor) and a second that remains in the circuit while the motor continues running (the run capacitor). If the start capacitor has failed the symptom is that the motor won't start
. If either or both start and run capacitors are defective the motor may try to start but will hum and won't keep running. You may hear a compressor or fan motor humming or observe that it's getting hot.
(June 15, 2015) Larry said:
I noticed my air conditioner's run capacitor is leaking oil - what does this mean?
The capacitor is shot and should be replaced.
(June 15, 2015) Larry said:
Trane Xl1600 heat pump cools down to set thermostat setting and then will not turn compressor back on and blow hot air in the home. This seams to happen when Phoenix temperatures reach 100 degrees. Replaced both start and run capacitors a couple months ago as issue started last summer was working fine until 100 degrees. Any assistance with this would be greatly appreciated.
This question was posted originally at DIAGNOSTIC GUIDES A/C / HEAT PUMP
Larry if replacing the start/run caps didn't fix the trouble I suspect either a failing control board / relay or a compressor motor that is failing beyond the ability of the starting cap to kick it off. Perhaps your tech will check the current draw of the various motors involved to see if that's going on.
(Feb 21, 2014) Anonymous said:
is it possible to rebuild a submersible well pump
Yes ... maybe; it depends on the condition of the pump casing, parts, an in my opinion, an accurate diagnosis of the trouble; at some point rebuilding is not cost effective. Indeed there are specialists (H Shreck in Poughkeepsie used to be one of them) who rebuild electric motors of all kinds.
There are also many water pumps whose design makes it quite reasonable to replace pump impellers.
So to answer your question, I dunno - it depends on what's broken.
(June 1, 2014) Anonymous said:
The fan on my Conquest 80 will not stay on (interior fan). It starts and then stops after a few seconds. It was installed in 2005. Help.
The start capacitor is for getting a motor started, not keeping it running.
Often motors have two windings, a start winding and a run winding. Your motor's run winding may be damaged.
Or your fan motor may require a dual capacitor (start and run) or a separate run capacitor to keep it spinning.
Or your system may have a faulty control.
(June 7, 2014) Joel said:
I have a commercial hood & exhaust fan (120v) running in a food truck that is used for catering. We occasionally need to run off a generator (3000w) and we've found that the fan cause the generator overload to trip. The fan has been tested and observed to draw 8 amps running at full speed. Given this, would it be possible to add a hard start kit, such as SUPCO SPP4E? Will need to confirm the motor size, just wondering if this would help.
The total draw of all items is less than 20 amps and the generator is capable of supplying 25 amps constant.
Thanks for the advice.
Joel I think the problem lies elsewhere and needs some further diagnostics. I suspect that your total current draw is exceeding the ability of the generator - you may be running more than the fan, such as lighting, a cooler, toasters, other electrical appliances. If it were just the fan, drawing 8A, it has no business tripping the breaker.
Put another way if the problem is the fan and there are NO other appliances running, then there is a failing fan motor drawing high current, or an electrical short circuit or other unsafe condition to find and fix.
A start capacitor or a run capacitor won't change the current drawn by the motor.
(June 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
(June 18, 2014) Mark said:
I always was taught that 35/5 was a 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 start & run capacitor.
35/5 : The first two numbers are the capacitance 35 uF (start) and 5 uF (run).
440 refers to the ability to run voltages up to 440VAC
(Aug 30, 2014) Bob said:
I was told my A/C unit has two capacitors. The unit stopped running, no fan at all, I located the dual capacitor and it checks out fine, so where is the second capacitor?
Does Lennox use a dual and a single capacitor
(Aug 30, 2014) Anonymous said:
I have located the dual capacitor, what would the second capacitor look like
Thanks Mark, I've included your helpful explanation in the article above.
Anon, thank you for griping about the difference between start and run capacitors. I'll review the text above for clarity and edit as needed.
Run caps are sold as single capacitor (two terminals) for a single purpose (keep the motor running) and rating (rated in mfd or uf) for the motor (such as 45uf for a compressor or 5 uf for a fan motor)
Dual run caps combine two capacitors in a single unit, one for compressor and one for fan, such as a 45/5 MFD unit, and will have 3 terminals as detailed in the text above (compressor, fan and common)
Start caps have the single job of getting a compressor motor spinning and of course are rated at a much higher MFD number. Mark summed it up nicely below and again in the article text above.
Bob if your unit has a start-run capacitor, a dual capacitor, that unit may be the only one present.
Capacitors may be found for the compressor/condenser motor and separately for the fan motor (on some units) - follow the power terminals of the equipment back to their power source.
The capacitors used on your particular Lennox unit will be described in the installation, maintenance and wiring manual or obtained by giving Lennox a call with your product model number.
(Sept 16, 2014) Ira said:
This is a question regarding a marine a/c unit, but is virtually the same as any other unit.
The problem is that when I initially start the unit in the morning it will run for about 30 seconds and then the compressor shuts down, but the fan remains on. I shut the unitI wait a minute and turn it back on and it then runs perfectly all day, wether I leave it on to recycle or turn it on and off manually.
has anyone checked for a failing start relay?
(Oct 6, 2014) Diane said:
I hope someone can help me fix my goodman 3 ton heat and air unit heat pump.
The trouble I'm having is I heard this clicking noise like the fan trying to come on, but never did until
I took a stick and pushed the blades and they started moving than I went inside the house and the air coming out was not cool could anyone tell me where to start checking, I,m going to try to fix this myself .
(Nov 20, 2014) Daniel said:
I woke up this morning, the ac unit was on the fan was working but there was no air coming from the vents inside the house, I went outside to see what I could do everything seem to be working but I think the condenser motor it was humming. and real hot to the touch. can someone give advice on what it could be?
If the indoor air handler unit is running but no air is coming out of the air supply registers I suspect that the blower assembly has failed or duct connections have come loose.
Check first for a blocked air filter or crimped air ducts. Watch out: an air filter that got sucked into the blower assembly can cause a fire.
(May 6, 2015) Mohiddin said:
Sir, this is about Panasonic 1.5 ton AC compressor. The compressor ceases to start, tried with replacing the 45Mfd/440V capacitor with new one. Still the comp ceases to run and getting heated up. Checked the winding continuity and found to be OK. Suspecting the rotor has stuck. can I try to run the comp in reverse to release the stuck rotor or can i increase the starting torque boosting by adding a parallel capacitor to the existing 45Mfd capacitor?
You might get such a motor running but I'm doubtful it has a useful remaining life.
It sounds to me as if you need a new compressor. Even if you got it running by banging it around, its continued service is doubtful.
(May 6, 2015) Bruno , (Architect 5512 (RET> ) said:
Unit has been great, for many years
This year ,when I tried to use it, (It didn't seem to,"get to the 2nd step" (See Above ? )
Using ..(my intuition only , I am a "Mustang Architect,and 10 year carpenter tradesman prior)
So Do you think I am on the "right track ??
Thank you for your absolutely great Web-Site Bruno
Sorry Bruno I don't understand the question
(June 10, 2015) Anonymous said:
1. the compressor needs replacing
2. motor needs replacing
How much should each cost
(July 19, 2015) John said:
Blower motor inside tries to start but acts like power is being interupted. Starts and stops erraticly. This happens in "fan only" mode also.
(Aug 17, 2015) Paul said:
Need to start fan blade with a stick to get it started, run for awhile then stops. Is it a capacitor issue. Feel the copper lines running into the house just slightly cool. Should they be ice cold?
Paul, that's probably a bad capacitor
(Sept 27, 2015) Anonymous said:
the value of the start and run capacitor for refrigeration
It depends on the voltage requirements and ampacity of the motor it's starting or running; see the motor data tag.
2015/11/26 firstname.lastname@example.org said:
I would like to know if a capacitor would help start my old Maytag washer motor. It has a centrifugal switch that opens as the motor starts.
The motor will run if I turn it by hand and continue to run. The motor is welded so I can not open the housing. It has six wires coming out of the motor which is a two speed Model LR22286 1/3 HP, 1725 /1140 RPM Code R. I don't know that much about electric motor types, but thought that if the start winding is bad, a capacitor might help. Thank you
If your motor already used a start/run capacitor, now won't start, and will run if given a spin, then yes replace the capacitor with one of same values.
A bad winding, however, won't be fixed by adding a capacitor.
(Jan 23, 2016) (mod) said:
Can you tell me where physically the start capacitor is on a Carrier Model 38YCB024 310? Here's the wiring diagram ,,,
My fan wasn't working and after learning, I replaced the dual run cylinder capacitor in the side box on the outside unit. Now the fan is working again.
I see in the wiring diagram a start capacitor, and I've found one for sale that cylindrical. Yet I don't see it in the box. Since the other two were five years old and bad, and this one was never replaced on this 20 year old system, I'd like to know where it is and be prepared.
Am I haven't a moment like when you're looking at grocery shelves and can't find the item right in front of you? Or is it somewhere I'm not thinking of. Thank you so much. I've searched online but haven't see it yet.
Unfortunately, Curls, a wiring diagram is a logical presentation not one that shows you where the physical components are located. Look at the appliance and its controls and wiring and you should be able to recognize the actual physical capacitor. Also some installation manuals include, in addition to wiring diagrams, photos of controls and control boards that can help;
WATCH OUT for death by electrocution; properly this repair is performed by a trained service tech.
(Jan 24, 2016) Curls said:
Thanks danjoe. I was hoping someone could tell me physcial places to look where it might be. The only place I know to look is the side panel of the outside box. Is this compressor start capacitor sometimes found somewhere else? Where might that be (inside where the fan is? Inside the inside-the-house air handler?). I can't imagine where it is, and don't want to open them up if it's not possible to be in those places. While it's on the schematic, do some units get built without this capacitor even though on the general schematic (in other words, do they make a series of models where this factor might differ?) Thanks again!
I replaced the capacitor after turning off electricity at the fuse, testing, and discharging the capacitor with a screwdriver against the terminals. I'm curious why my question drew a safety warning when practically none of the other questions did. What amount mine made me look more prone to taking risks?
there may be start capacitors, combined start/run capacitors, or pairs of caps at the compressor/condenser unit (outside) and also inside the air handler at or near the blower fan or fan motor.
You won't see these devices without opening the service covers to the equipment - watch out for death by electrical shock or cut fingers by moving parts.
Often I see a pair of caps jammed in up under the cover of the compressor/condenser and a cap at the blower motor assembly.
(Apr 18, 2016) john said:
have a pool pump motor--it starts real hard-sometimes I do not think it is going to start-so far it has--it is a little noisy when it runs--new start cap new run cap new bearings new impeller--everything is new---any ideas
John, with the motor disconnected, can you spin the shaft by hand? Does the impeller also spin freely?
Other things to look at besides bad bearings and starter cap are discussed at ELECTRIC MOTOR DIAGNOSIS (search for that article in the box above).
I'd probably start by checking
- all wiring connections
- voltage being delivered
- current draw at startup
(Apr 19, 2016) Anonymous said:
imp spins freely
we are having bad weather here in Houston--I will ck the cap when weather breaks
it is running it just starts hard---think it's the start cap
Indeed Houston is suffering significant flood damage and of course 17+ inches of rain mean high humidity that in turn can cause trouble in electrical components that are already marginal or are not protected from water. Replacing the capacitor is easy and inexpensive. Try it. Keep me posted.
(Apr 22, 2016) john said:
got the cap--it is a 189/227 mfd & it reads 194mfd--but I think there is a prob,this cap is 110v--the pump is 220v should the cap be 220v
I think so, John. I'd worry that it won't have enough kick or that it will fail.
(Apr 22, 2016) Anonymous said:
I think that is why it is hitting so hard/to long to start----if it should be a 220v cap what size do I get? if the 110v is 189/227---should the 220v be 95/113---how do you figure the size of cap you need?
(Apr 25, 2016) john said:
if I need a 220v cap instead of a 110v----what size 220v cap do I get----the 110v cap 189/227 should the 220v be 95/115
(May 12, 2016) raju said:
my exhaust fan not moving when give supply to motor. and without supply moving freely.
So I suspect, Raju, a bad motor or a bad start-run capacitor.
Other readers; I'm reluctant to pretend to specify specific start/run capacitors based on simple e-text queries. Take a look at the specs on the capacitor you're replacing, or take a look at the motor data plate specifications; take that data to your electrical supplier. Typically you'll choose from a limited range of general purpose capacitors that cover the operating range of your motor's voltage, current draw to start (LRA) and run (RLA).
2016/06/01 Jerry said:
Bryant AC keeps blowing Dual SC. Put a new one in, changed contactor, and it runs, however, the cap will fail at least once a year. Any suggestions. BTW, is the relay located on the compressor, like in refrigerator comp?
Check the current draw; chances are the motor is seizing. Or there's a short or mis-wiring.
RE-posting without disallowed advertising link:
Rob Erto suggested checking U.K. sources for motor capacitors.
Thanks Rob. While we don't permit posting advertisements, we are happy to identify, cite, and link to contributors of technical content or critique of InspectApedia's material.
(June 8, 2016) engineer khan said:
my spit Ac compressor just kick to start but failed and also draw very large current from normal when the compressor try to start
July 13, 2016) Fan starts but runs slow till you give it a push? said:
Fan starts but runs slow till you give it a push?
Sounds like low voltage or more likely a bad start/run capacitor.
(July 17, 2016) Richard said:
White diode on st9160b is hot to touch and motherboard is discolored brown and somehow the wall port shorted out. When unit is on, only green blinking light on the heater comes on, no fans come on. is the problem with the st9160?
For other readers, Richard's ST9160B is a Honeywell heating furnace circuit control board. The ST9160AC is an electronic fan timer. Below I'll excerpt from the company's description of this board. So we have a failed control board; unless you're an electrical engineer with training in circuit board design and repair, I'd not attempt to repair the board.
More important is to determine if the failure originated on the board or elsewhere in the appliance. I can't guess that from a one-line e-text.
The ST9160A-C Electronic Fan Timers integrate control of all circulating fan operations in a gas warm air appliance. This control is the central wiring point for most of the electrical components in the furnace. The ST9160 monitors the thermostat for heat, cool, and fan demands and controls a two-speed circulating fan as required.
Communication with the SmartValve™ System Control initiates appliance light off. The ST9160 features a fixed or field-adjustable heat fan on delay and a fixed or fieldadjustable heat fan off delay, depending on model. Cooling fan on and off delay is fixed.
Specific timings vary. See appliance label or instructions for timings that are available for a specific appliance. Refer to Table 1 for model specifications. Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections can be provided as an option.
Continuous low speed indoor air circulation is also available as an option. Thermostat connections are screw terminals or quick-connects.
(Aug 3, 2016) Brian said:
I have. Honda 1000w generator. I bought a small A/C unit. 444 watts 3.8 amps. However very seldom will it get past it startup mode and not trip the factory beaker built into the generator. If I can get it to start the A/C unit, all is well. Keeps running the A/C all day. So my thought is, can I make a capacitor booster that will get me past the start up surge needed to start the A/C unit ?
(Aug 3, 2016) Anonymous said:
Replaced condenser fan motor and capacitor now unit won't start but indoor fan will blow. Replaced condenser fan motor with a universal 4 wire motor and old 45/5 capacitor with new 45/5 capacitor and unit will not run at all. Idk what's wrong and can't figure it out it ran fine other than a bad motor now it doesn't do anything.
If the motor is tested and runs when out of the equipment but won't run when installed then I suspect a control board or contactor or relay failure.
2016/09/10 johnteh said:
I had the summer A/C inspection done was told the compressor capacitor was indicating a short life because the compressor mega ohms was 200 which is very low for a unit that is only 2 years old. recommendation was to replace compressor capacitor with a hard start to prolong compressor life. Are compressor capacitors difficult to replace/install? They don't seem to be, since they have only 2 wires.
For a detailed reply to your question please see BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS at inspectapedia.com/aircond/AC_Compressor_Burned_Out.php#Basic
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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.