Tag: run cap


❶ Understand What a Run Capacitor Does

The run capacitor provides continuous phase-shifted current to the motor start winding allowing the motor to run

  1. With the design efficiency
  2. In the right direction
  3. With the appropriate torque
  4. With near “unity” power factor (power factor near 1.0)

If the run capacitor is failed often the motor won’t run in the case of high torque motors like a compressor or in the case of fans they may run backwards or slowly or with high amperage or overheat.


❷ Understand Why a Run Capacitor Fails

A run capacitor fails due to

  1. Poor manufacturing quality
  2. Overtemperature (They got hot)
  3. Over Voltage (The voltage was too high)

Many will say a failed motor “takes out” the capacitor. In actuality a failed or weak capacitor can take out a motor

❸ Visually Inspect

If a capacitor shows physical damage such as the top bubbling or oil leakage then it should be replaced. Normal rust is not a reason to replace a capacitor. Note the MFD or uF rating listed on the capacitor. The voltage rating is also worth noting but you may use a HIGHER voltage rated capacitor but not lower.



❹ Decide if under load or bench testing is best

If the system is currently running then an under load test may be best. Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel. If the system is NOT running then a bench test will be the best bet.


❺ Bench or Under Load Test?

Choose bench for simplicity or if the system is not running. Choose under load because it can be done in real load conditions on a running system.

Bench Test Go to step ❼

Under Load Test Go to step ❻


❻ Test Under Load

In order to test under load, you need to take measurements with the system running. Wear proper PPE and only do so when safe. You need to have an accurate multimeter that can measure Voltage and Amperage reliably. Often under load measurements may come out high if the amp clamp picks up interference from other circuits. Measure the amperage on the start wire with the wire centered in the clamp and multiply by 2652. Now measure the voltage across the capacitor and divide the amperage x 2652 by that voltage to find the capacitance in MFD.If the under load MFD is less than 10% low we suggest replacement. If it is over the rating it is often a mis-measurement

If a capacitor measures weak via under load test go ahead and perform the bench test.


❼ Bench Test

Bench Testing is simply removing both leads from the run capacitor after safely disconnecting power and discharging the capacitor. You then place a meter designed to test capacitance across the terminals and note the reading. Be careful not to touch the meter probes and to get a good solid connection to the metal connection spades on the capacitor.If the measurement is more than 10% we suggest a replacement


❽ Inspect the dual run capacitor and note where each wire is connected

Taking a picture is one of the easiest ways to remember before removing the wires.

❾ Remove disconnect

It is imperative that the disconnect is removed or the unit is off and without potential. Test using a meter that is pre-tested to a known voltage source and check L1 to L2 and L1 – ground and L-2 ground to ensure no voltage is present.


❿ Discharge capacitor

Before testing, touching or removing the capacitor, you need to discharge it. Do this using a high resistance resistor bridging HERM and Fan terminals across to C or across a single capacitor.

NOTE: It is actually very rare for a run capacitor to contain a charge on a normally running system because it bleeds off through the compressor windings UNLESS one of the windings is open. FOr this reason many techs opt to just use a screwdriver to discharge which is controversial but common practice.


⓫ Disconnect wires and remove used dual run capacitor

To remove, disconnect wires on the top of the capacitor and also remove the strap holding the capacitor in place.


⓬ Replace capacitor with same MFD rating

Apples to Apples: You must use the same MFD rating capacitor during this process. This will be located on the box and also the side of the capacitor. Mount the capacitor upright with terminals pointed up.TEST THE NEW CAPACITOR VIA BENCH TEST BEFORE INSTALLING

⓭ Re-sizing of strap

Sometimes the new capacitor might be larger or smaller. At this time, use the metal strapping and create a new strap for the capacitor, by cutting it to the correct size and using self tapping screws to attach to the correct area. Always check before using self tappers to confirm that you are not in any danger of puncturing the coil.


⓮ Attach the wires

At this time reattach the wires onto the top of the dual run capacitor. Make sure that the common is connected, the HERM (compressor) is connected and the FAN is connected. You should always double-check to make sure all wires are in their proper places. Make sure the terminal fit very snug, tighten them by squeezing with a needle nose before installing to make sure they are very tight.


⓯ Test system

Reconnect the disconnect or flip the breaker back on. Check to make sure that all aspects of the system are running and the compressor and fan are running at the proper amperage.Using a power meter and testing motor power factor to ensure it is near unity is a good additional practice.


⓰ Clean up and reattach panel on the condenser

Make sure you clean up the area and reattach the panel to the condenser

One of the most common parts to fail on a single phase HVAC system is a run capacitor, so much so that we sometimes refer to junior techs as “capacitor changers”. While capacitors may be easy to diagnose and replace, here are some things many techs may not know.

Capacitors Don’t “Boost” the Voltage 

A capacitor is a device that stores a differential charge on opposing metal plates. While capacitors can be used in circuits that boost voltage they don’t actually increase voltage themselves. We often see higher voltage across a capacitor than the line voltage, but this is due to the Back EMF (Counter electromotive force) generated by the motor, not the capacitor.

Current Doesn’t Flow Through The Capacitor, Just in and Out of It 

Techs notice that the one side of power is connected to the C terminal or the side opposite the run winding. Many techs imagine that this power “feeds” into the terminal, get’s boosted or shifted and then enters the compressor or motor through the other side. While that may make sense it isn’t actually how a capacitor works at all.

A typical HVAC run capacitor is just two long sheets of really thin metal, insulated with an insulation barrier of very thin plastic and immersed in an oil to help dissipate heat. Just like the primary and secondary of a transformer the two sheets of metal never actually touch, but electrons do gather and discharge with every cycle of the alternating current.  For example, the electrons that gather on the “C” side of the capacitor never go “through” the plastic insulation barrier over to the “HERM” or “FAN” side. The two forces simply attract and release in and out of the capacitor on the same side they entered.

The Higher the Capacitance, the Higher the Current on the Start Winding 

On a properly wired PSC (Permanent Split Capacitor) motor, the only way the start winding can have any current move through is if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with 0 capacitance it is the same as having an open start winding. Next time you find a failed run capacitor (with no start capacitor) read the amperage on the start winding with a clamp to see what I mean.

This is why oversizing a capacitor can quickly cause damage to a compressor. By increasing the current on the start winding the compressor start winding will be much more prone to early failure.

The Voltage Rating is What it Can Handle, Not What it Will Produce

Many techs think they must replace a 370v capacitor with a 370v capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370v with a 440v but you cannot replace a 440v with a 370v. This misconception is so common that many capacitor manufactures began stamping 440v capacitors with 370/440 just to eliminate confusion.

You Can Test a Capacitor While the Unit is Running

You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60hz power 3183 on 50hz power) and then divide that number by the voltage you measure across the capacitor. For a full write up on the process, you can look here




When testing a run capacitor many techs pull the leads off and use the capacitance setting on their meter to test the capacitor. On a system that is not running there isn’t anything wrong with this test, but when you are CONSTANTLY checking capacitors as a matter of regular testing and maintenance that extra step of pulling the connectors off can be time consuming and in these cases it is also totally unnecessary. Testing the capacitors UNDER LOAD (while running) is a great way to confirm that the capacitor is doing it’s job under real load conditions which is also more accurate than taking the reading with the unit off.

First, if you are used to doing capacitor checks during the “cleaning” stage of a PM you are going to need to change your practices and do your tests during the “testing” phase. These readings will be made at the same time you are taking other amperage and voltage readings during the run test.

This method is a practical method and is a composite of two different test practices combined –

  1. Read your Volt (EMF) and Amp (Current) readings like usual and note your readings.
  2. Measure the amperage of just the start wire (wiring connecting to the start winding), this will be the wire between your capacitor and the compressor. In the case of 4 wire motors it will usually be the brown wire NOT the brown with white stripe. Note your amperage on this wire..
  3. Measure the voltage between the two capacitor terminals, for the compressor that would be between HERM and C, for the cond fan motor that would be between FAN and C. Note the voltage readings
  4. Now take the amp reading you took on the start wire (wire from the capacitor) and multiply by 2,652 (some say 2650 but 2652 is slightly more accurate) then divide that total by the capacitor volts you measured.  the simple formula is Start Winding Amps X 2,652 ÷  Capacitor Voltage = Microfarads
  5. Read the nameplate MFD on the capacitors and compare to your actual readings. Most capacitors allow for a 6%+/- tolerance, if outside of that range then replacement of the capacitor may be recommended. Always double check your math before you quote a customer. We need to make sure we are accurate when advising a repair.
  6.  Repeat this process on all of the run capacitors and you will have assurance whether they are fully functional under load or not.
  7. Keep in mind that the capacitor installed may not be the CORRECT capacitor. The motor or compressor may have been replaced or someone may have put in the wrong size. When in doubt refer to the data plate or specs on the specfic motor or compressor.

If you need a visual, here are some good videos on the topic. Note that some will use 2650, some 2652 and some 2653. It all depends on how many decimals of pi they are using in their calculation but all of them will result in an accurate enough conclusion for our use.

At first doing it this way may take a few minutes longer but in the long run you will go quicker, have fewer mistakes (forgetting to put the terminals back), have a better understanding of how the equipment is operating and get a more accurate reading.

Once you replace a capacitor always recheck your readings to ensure the new capacitor reads correctly under load.

It is also a good practice to check Capacitors you have removed with your capacitance setting on your meter as a reference point.

While this method is good, it is only as good as your tools and your math. When in doubt, double check… and always be in doubt.

— Bryan

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