CAPACITOR SIZING DILEMMAS
Most start capacitor applications use a rating of uf capacitance and voltages of /, , / and VAC. They are also usually always 50 and 60 Hz rated. Case designs are typically round and cast in black phenolic or Bakelite materials. Terminations are usually ?" push on terminals with two terminals per connection post. Depending on the application, the micro-farad range of starting capacitors varies according to the motor size. Start capacitors typically range from µF up to µF. The example capacitor charts at the start of this article are adapted from AFCAP.
I don't know how many times a technician has said that they installed a part based on what was on their service truck. I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains! Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.
The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor. Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.
Of course, you could use a multimeter that reads microfarads ufbut this will only tell you if the what size hard start capacitor do i need capacitor is weak - not the correct size! Use the equation below to verify the size of the capacitor. The resulting microfarad uf should match the size of the installed capacitor. An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor. This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.
The run capacitor should have the exact microfarad uf that the motor is rated for. Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation. The voltage rating should be no less than how do i add www to my domain name listed amount for the motor, for central heat pumps and air-conditioners this is usually a minimum of VAC.
Most new condensing units are specified for VAC capacitors, and are more durable during fluctuations in power supply. I have seen some universal type dual run capacitors rated for as much as VAC, as this voltage rating does not impact uf performance.
The changes in uf will impact amperage draw and reflect on kilowatt hour usage though. When completing your early season maintenance this year for your customers, do them a service by testing capacitors and verifying their size. You could just increase your service ticket total and save some all important power.
You may even prevent a call-back during the next heat wave! Correctly Sizing a Capacitor. Please Sign Guestbook. Subscribe Free to Great Content! Subscribe Free!
Hard Start Kit selection.
You aproximate sizing of the capacitor is in the range of mF and at the motors I have kW the run capacitor is sized between uF. The difference is more than . The run capacitor should have the exact microfarad (uf) that the motor is rated for. Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation. This is where the rule of +/- 10% of the rating came from, for Start Capacitors ONLY! Start capacitors (ratings of 70 microfared or higher) have three voltage classifications: V, V, and V. Examples would be a 35 mfd at V run capacitor and an mfd at V start capacitor. Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website. These electric motors use a capacitor to start and run the motor efficiently. Green links show where you are. 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. 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.
If the electric motor hums without starting or has trouble starting, but will run if given a push e. If the starting capacitor is bulged, burned, has a hole in it, or has leaked oil, it is shot and needs replacement.
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. 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. These caps will have three electrical terminals:. 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. 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. 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.
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. The VAC capcitor will last longer. But because these voltage ratings are simply the peak voltage that the capacitor can handle, using a higher-rated cap where a lower-rated one 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.
On a typical volt motor, the generated voltage may be as high as volts and is deter-mined by the start winding characteristics, the compressor speed, and the applied voltage. 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:.
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. 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. 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.
Check also with your local electrical supplier. 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.
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 V or V.
We also see a specific part number: SH , but I in replacing this capacitor you'd attend first the rated capacitance and voltage. 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.
Watch out : When you are replacing an electric motor capacitor, never put in a lower rated capacitor. The substitute capacitor must be able to handle the voltage. Run capacitors typically range in micro-farads from 1. For example you can't sub a V-rated capacitor into a V system.
Robert's run capacitor shown here is rated at 7. 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. 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. 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. 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. 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.
Voltage sensing technology that monitors for motor start current sensing devices require internal fuse protection. 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. This article includes "Capacitor Nomenclature" by Dean Huster.
The fact that your motor starts to rotate and then trips the breaker argues for a motor failure, perhaps a winding that shorts when the motor gets to speed or simply a bad bearing, giving the motor-rotation resistance that soon causes an over-current. I have a 2 capactor 1.
I'm confused about the values on a black plastic round cap that has mfd vac 60 hz and has 2 terminals. I did not see any thing like this on your web site. What do I have to do to replace this. I ohmed it and it comes up 0 I think that say's it's open circuit. The problem I have is the the motor runs 5 seconds and cuts off and pops a breaker. Can you help me. Hi I got this price pump motor that's a three-phase on my research says I can convert it to single phase but I need to know what size capacitor.
And do I need a stat capacitor and a run capacitor? See the PDF. Keep me posted. Hi, Dan. Sorry for the delay in responding. Got slammed with work; I finally was able to get the washer up and be able to get at the motor, transmission, capacitor, etc.
Checked the transmission, turns smoothly and effortlessly. Motor is fine.
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