Compressor Capacitors: The Complete Guide

Compressor Capacitors
(Last Updated On: October 11, 2020)

A Guide To Capacitors For Compressors

A capacitor may seem tiny, but very effective gadget in a compressor. It was invented a couple of centuries back, 1669 by a scientist from Dutch Hemanth`. Over the years, it has evolved from design to its naming. hvac compressor capacitor

What is a capacitor? It is an electrical component designed through the use of two metallic plates divided by an insulating material known as a dielectric. Capacitors are similar to batteries (they store energy). 

They let the current to flow to the maximum level while releasing the charge. So, as the capacitor discharges, the level of the voltage rises until the current goes to the minimum and the voltage stands at maximum level. Therefore, the voltage is out of the phase with the current. 

So, let’s dig out more about the compressor capacitors…

What Does A Compressor Capacitor Do?

Like we described earlier, the work of a capacitor is to store energy and to give it out to the circuit when needed. So, it charges and discharges the electric charge that is stored in it. Further, below are functions of a capacitor

  • It is responsible for blocking the flow of DC and allows the flow of AC
  • It is used as a motor starter
  • It is used during the phase shifting 
  • It is utilized for creating an interval 
  • Capacitor is used in combination with a resistor to help filter ripples in a rectifier circuit
  • It bypasses the undesirable frequencies 
  • It is useful especially coupling of two sections
  • It feeds the preferred signal to any segment 
  • It aids in getting tuned frequency 
  • During filtration, capacitor helps to get rid of ripples from rectified waveform

Capacitance is the one that determines the amount of energy a capacitor stores. The capacitance could be compared to a tank of water, the bigger the tank, the more water it can hold. So, the bigger the capacitance, the more energy a capacitor can accommodate. 
Rolair Air Compressor Start Power Capacitor FC009200075 OEM Replacement New

When you want to expand the capacitance of a capacitor, there are three ways you can approach the process.

  1. Move the plates closer to each other
  2. Increase the size of the plates 
  3. Make dielectric just like an insulator 


How to Size Capacitors for a Compressor

When a capacitor fails to function, compressor owners struggle with the idea of having to identify the right capacitors needed to replace the faulty one. If you are the owner and probably your air conditioner stopped to work suddenly fixing the capacitor is a must to keep your house cool. What do you do and you are new to electronics? 

There is a three step procedure you can follow as a total beginner coming into terms with a faulty capacitor. Check the steps you should follow below:

  • First step: If you still have the manual to your compressor, search for capacitor information specifically the size. In case it doesn’t have any leading information, contact your gadget manufacturer to enquire about the right size capacitor that is required.
  • Second Step: Be practical here by doing some calculation that possibly may give you an idea of the exact capacitor you are looking for. So, do multiplication of full load amps by 2,650 and then divide the number by the supply-voltage. You can find on the manual and after the calculation you get the MicroFarad of the capacitor your compressor needs. MFD (MicroFarad) is an electrical capacitance unit normally written on the capacitor or the packaging. For you to match the required number, sometimes you can round it up/down to 10% to get the capacitor size although it’s a general estimate and not an exact figure.
  • Third Step: Alternatively, you can have the operating  power of the motor multiplied by the power factor indicated in the manual (in Kilowatts) and the results you get is the correct size of your compressor capacitor.

Another important aspect to note is capacitors do have terminals where you link the wires to the motor. So, as you are searching for a capacitor replacement, avoid focusing so much on whether the new capacitor will fit the housing/casing. However, what’s important is checking the similarity of voltage range and the MicroFarad range, period.

If the capacitor that you find is compatible in those two areas, forget about the color, shape, length or shortness. What determines is whether MicroFarad and Voltage ratings are reading the same with the required compressor capacitor.


What is a start capacitor?

Start capacitor is used to speed up the motor start process rather than taking time before responding for use.compressor start capacitor

It is fully packed with energy to briskly bring the motor to ¾ of the desired speed and then it’s removed from the circuit. The start capacitor is designed to be used for a few seconds by briefly increasing the motor starting torque.


What exactly is the Work of Start Capacitor?

For single-phase motors normally have both the run and start capacitor. However, they work differently. The start capacitor produces a bolt of energy to the motor for it to run; while for run capacitor helps maintain the functioning of the motor when the start capacitor switches off. Start capacitor has the ability to store more charge than the run capacitor.

Something else to observe with a start capacitor is getting burn out since they work is to accumulate energy to speedily burst it out to start the motor. How can you tell if the start capacitor is bad? You can identify through two signs. First, when your motor is not starting after you ignite it as usual, or if it takes longer than expected to respond. It means the start capacitor is dead or almost to go bad completely. 

So, the moment you see your motor drag to start, that is a sign of the capacitor losing capacitance due to age and normal wear. The highest percentage of problems associated with motor start delay is from a start capacitor. You could also be wrong as the motor may develop other mechanical problems; therefore, it’s prudent to buy start capacitor first as even the price in the market is friendly. 

Whenever you start observing abnormal starting of the motor, do not wait until the capacitor can no longer function. The upper part of the capacitor may dislodge and the inside part may become fully or partially ejected. Sometimes capacitors may energize for a long time as a result of faulty starting circuit on a motor which may lead to overheating.

 Therefore staying alert about the health of the capacitor is important to maintain the normal function of your compressor. 


Start Capacitors for HVAC Compressors

Capacitors can be designed to operate separately as two or as a single package in HVAC compressors. However, nowadays what they do is to create dual round capacitors are engineered to help save cost and space. Therefore, instead of having two separate capacitors, they combine them into one. 

Start Capacitors for HVAC Compressors
For a start or run, capacitor can be linked into one capacitor known as dual capacitor designed with three leads though can be fragmented between two separate capacitors. The start capacitor provides a fan motor with the torque it needs to start spinning then halts; while the run capacitor proceeds to provide the motor with the extra torque needed. 

In case the start capacitor fails to function, the motor will not start. On the other hand, if the run capacitor is faulty the motor will start normally, however it will run with more amperage than required which may cause the motor to run hot leading to a short life-expectancy. 

The sign that tells you to start capacitor for the HVAC compressor is not in good condition is intermittent failure to start. Every time a failure happens, what follows next is a trouble-free actuation which gives a short impression that the issue is resolved. In the end, the start capacitor for the HVAC compressor fails to respond.

When doing a replacement, unless you are well equipped with start capacitor knowledge it is better to consult an HVAC compressor technician. It is important so you can get proper MDF ratings that are compatible with the compressor. 

You should take precaution, especially when working around a high voltage system. For badly fixed start capacitors may bring damage to you, compressor and the HVAC components. 


Finding a New Compressor Capacitor

Getting a new capacitor shouldn’t be a problem at all. If the capacitor of your compressor cannot be revived, but require only a new one first check how the previous capacitor was fixed and situated. The earlier capacitor should have the markings displaying the voltage range of your compressor like we had discussed earlier.

New Compressor Capacitor

On the markings, you will see the MicroFarad rating and a range of numbers usually followed by Uf (Symbol for MicroFarad). However, if you cannot read the label on the capacitor, enquire about the information from the compressor company. Also, you may search for the capacitor with the right voltage on Google plus an electric motor. 

Only be careful to ensure that the new capacitor meets all the parameters of the old one. When your compressor comes to a halt before you suspect capacitor as the issue, check for other parts such as breakers, wiring, fuses, contactors, and other areas to eliminate any problem. 

When some are buying a new capacitor, the fixing is done when the faulty capacitor was allowed to affect other areas of the compressor, that’s why it is good to check other parts as well. 

Another notable step you should take to avoid buying capacitors often is to clean evaporators, condenser, and filters. For instance, a blocked condenser coil can lead to the compressor overheating. Although compressor is designed to shut off before the damage happens, nonetheless, continuous overheat can once and for all damage the entire compressor. 

So, rather than only dealing with the capacitor, when you keep ignoring simple areas that can be rectified quickly, can lead to compressor destruction. 

Purchasing a new capacitor is the best idea if you suspect the immediate one to be having a problem or causing trouble when starting the compressor. Therefore to avoid it from causing damage to your compressor, just buy a new one to keep your compressor in good condition.

For a new capacitor, price varies depending on brand plus features. Although on average the price range from $130 to $150 while for branded ones may be a little higher. However, those charging high factor the cost of replacing the capacitor, but if you are tough enough to do it yourself, the price may be favorable. 


Replacement compressor motor capacitors

If you are a regular user of a compressor, dealing with a faulty capacitor is inevitable. The failure of a start capacitor means the compressor motor will not start. When the problem persists and you realize no change, remove the start capacitor cover to examine the issue. 

compressor motor capacitors

Ensure to remove the compressor power cord carefully without touching the capacitor terminals.

The capacitor has high discharge that may give you a jolt from the stored power. 

After you have successfully gotten it out, check the discoloring blisters on the outer body. Examine the connected terminals to see if they have a lot of corrosion on them.

Remember you are trying to identify any lead that may show the capacitor is the one with a problem. Observing it from the physical look may seem alright, but never assume without first testing it. 


Replacement Compressor Capacitor Testing Process

When you are the one troubleshooting to check the health of your capacitor if doing it manually, never do it unless you know how to. The manual process requires a professional as serious damage may occur if not handled with care. 

Replacement Compressor Capacitor Testing

So, the other safer plan is to use analog Multimeter. In other words is known as AVO (Ampere, Voltage, Ohm Meter) and below are simple steps you can use. 

  • Ensure the capacitor you suspect to be the problem is discharged
  • Get your AVO meter ready 
  • As you select the analog meter on Ohm make sure you choose the higher range
  • Connect the capacitor terminals to the meter leads 
  • If it is a shorted capacitor it will display low resistance
  • For an open capacitor there is no movement you will see on the Ohm meter screen
  • So, to know you have a good capacitor first it shows low resistance and then progressively increases which is an indication of a good capacitor.

However, if you are using digital multimeter it is advisable as it’s advanced, easy to use, and more accurate than any other approach.

  • Before doing anything make sure the compressor capacitor is discharged
  • Set your digital meter on Ohm range
  • Connect the capacitor terminals to the meter leads
  • Be keen to see what displays and note that the meter shows (some) numbers for a second
  • After the display of the numbers it all over a sudden goes to OL (Open Line). Repeating the same process you will get the same results which indicate the capacitor is in good condition.
  • If the meter does not change it means your capacity is dead and you need to replace another one

The other methods you can test the capacitor is through a Voltmeter & while in the capacitance mode.

In case your compressor capacitor is dead, search for the one that is similar to the older one. Just like we indicated there before, your focus should most be the MFD ratings and the voltage range. So, even after securing the right capacitor you were looking for, it’s wise you test to see whether it will start the compressor motor normal.

Although size, shape, and length rarely matter in the functionality of the capacitor as long as the rating and range are a similar replacement to the earlier one; getting the one that fits exactly in the capacitor cover would be better. 

When positioning the new capacitor or the same one, use a mounting strap to keep it secure in its place. Be keen to connect the wires of the new replacement correctly to avoid errors.

After the successful replacement power the air conditioner unit by turning on the circuit breaker. Next, you can put on the thermostat and then let the air conditioner temperature be set to low-setting so it can turn on. If it fails to function, you have to check the capacitor wire leads which may be incorrectly connected. 

Now, undo all the mounting process and any connection by getting the capacitor out and doing a thorough check-up to avoid another mistake. If you are confident your arrangement is right, connect the capacitor lead wires the right way and fix it, test the conditioner and enjoy the cooling. 


How many capacitors are on a compressor motor

Compressor motor normally has two capacitors for it to function and operate successfully without a hiccup. As we have seen earlier the primary role of a start capacitor is for a split second to increase the motor starting torque while at the same time allowing the motor to cycle on and off effectively and speedily. 

A switch in the circuit immediately removes the capacitor after it plays its part and only required when similar action is expected. Start capacitor voltage ranges between the four areas like shown here:

  • 125VAC 
  • 165VAC 
  • 250VAC
  • 330VAC

The second compressor motor capacitor is a run capacitor. For this one, it comes into play after the circuit is started. It helps in the proper running of a motor and with efficiency. So, it is designed to pick after the start capacitor is removed to continuously maintain the running of the motor to the end. As the motor is the working process, the capacitor is energized and below is their voltage rating. 

  • 370VAC
  • 440VAC

Running with that voltage range, they use a capacitance of 1.5uF – 100uF.


Capacitors and Single-Phase Compressors

Having a single-phase compressor using AC power is a real problem to electric motors. A single-phase AC motor when starting on its own it has difficulty for technical reasons. However, after the addition of start and run capacitors to the motors, it became easier to use them in all sorts of appliances.

Single-Phase Compressor capacitor

 Not that single phase AC is bad, in fact, they run efficiently, but for them to function correctly, they require one or two capacitors. 

Majority of single-phase compressors function with a run capacitor which helps in running of the compressor effectively.

The run capacitor is positioned in series with the start winding of the compressor and as the motor operates it remains in the circuit. 

When the current is flowing through the start winding and the run capacitor, it causes a phase change of the motor’s current, therefore enhancing the power factor of the motor. As the run capacitor stays in the circuit, it needs to dispel the heat generated through the continuous operation.

If the run capacitor is defective, it interferes with the normal operation of single-phase compressors. Therefore, it is prudent to keep the run capacitor in check as it can make the compressor to draw than allowed amperage which could trip on its motor overload. So, understanding how to identify a faulty run capacitor and troubleshooting process when working with a compressor is essential.

Proper troubleshooting can avoid replacing the entire compressor when the only problem was a defective run capacitor. Therefore before condemning the whole compressor as the problem always check the run capacitor to determine the problem. 



The lifespan of a capacitor is limited. Capacitors are designed to last for one or two decades, but some factors cause them to wear out quicker than expected. However, the cost of replacing a new capacitor is affordable therefore if it wears out prior to your compressor, no needs to break a bank. 

For you to enjoy using your compressor for long there are simple things you can do. First, make sure the compressor is not exposed to excess heat. Secondly, keep your compressor clean all the time to avoid stuff blocking the normal functioning of the components. Thirdly, allow the air to circulate and keep checking the health of the compressor so you can enjoy it’s services for long.


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