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HVAC + | HVAC Motor Capacitors |
| Written by Richard |
High Performance HVAC Tech Page: CapacitorsRun and Start Capacitors
Inside CapacitorsThe inside part of capacitors are made up of an insulator between two metal plates. The properties of this metal allow the capacitor to store electrons and the insulator prevents the electrons from flowing from one plate to another. The capacitor therefore stores energy in the form of electricity just as a battery would hold a charge of electricity. It is therefore necessary to take great care whenever working around capacitors even when the power to the unit has been turned off. A motor capacitor can cause serious injury or worse so caution is advised whenever working around capacitors. Motor capacitors can be discharged using a resistor across it's contacts. A 20,000 ohm resistor should be used to discharge the motor capacitor before working near or on a circuit with a capacitor in it. Remember, any motor capacitor, whether it be a motor start capacitor or a power capacitor can cause great injury or worse so handle with care. Capacitor Specifics
Motor Capacitors ConclusionAdditionally, if the motor or compressor fails to start for whatever reason part of the troubleshooting list of things to check will be the capacitor. A bad capacitor can cause a motor to fail and burn up especially when heavy load conditions exist. It is difficult to tell unless you are familiar with capacitors but a capacitor that is bulged out on the top or sides is a bad capacitor. It needs to be replaced. There is always an inherent danger when working on electrical components and especially on capacitors. They will shock you even when the power to the equipment is off. Always have a trained expert HVAC technician check out the equipment. They can diagnose the problem quickly and efficiently and have the system back up in no time. They are also very familiar with all the dangers involved so it will done safely with no injuries. The following video offers information for HVAC run capacitors that is helpful for someone wanting to understand the basics about capacitors. One can probalby skip the first third of the video. Dim lights Embed Embed this video on your site
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Comments (12)
  written by Allen Clark, June 20, 2010
Can a run Compacitor make a Blower Motor FAN RUN SLOW OR NOT THE PROPER ROTATIONS. Please get back with me as soon as possible thank you.
 written by Robynn, July 12, 2010
We have a dip in our lights when our AC comes on and I was told to ask an AC tech if we need a new start up capacitor? If we do how roughly how much will it cost to replace it? Thanks!
 written by Eric, July 30, 2010
Robynn,
Your lights will dim due to a high current rush coming from your electric panel as your A/C system starts. This is very common especially in older units. Most residential A/C units don't use start capacitors they use run capacitors. You may want to have have an A/C tech check the capacitor out to see if it is still in range with its recommended microfarad range. Being out of recommended range will cause strain on your compressor and/or fan motors wearing them out quicker. I recommend having your A/C inspected by a good, well known company. You will get what you pay for. Hope this helps. Eric HVAC Tech
written by Eric, July 30, 2010
Allen Clark,
Run capacitors will not change the rotation of motors. Chances are, if your motor has reversed, some sort of power surge may have damaged the windings and would need to be replaced. Run capacitors will not make your motor run slower unless it is bad or an improper fit to the motor. Hope this helps. Eric HVAC Tech
 written by Kevin C, August 06, 2010
Very informative, thank you. We had the capacitors on our residential units replaces recently because the motor could not get started. Similar to Allen Clarks's comment, I swear it seems the fans runs slower ... is this possible or am I just imagining it? One unit used to run pretty violently and it runs a lot smoother now and it seems that it's due to running slower.
 written by arnold, August 07, 2010
the original sapacitor is 40/5 mdf 220 volts is totally broken, its is okay to replace with 50/5 mfd to the unit?
thanks,,,
 written by Laura, August 24, 2010
I have an A/C unit that the fan motor is not engaging on its own. I can push the fan blades and then the unit will run for several hours. I'm not sure what causes it to shut off, because it hasn't hit the temp on the thermostat but it does shut off. The capacitors look in good shape, the fan blades are easy to turn (being careful), but I'm need to know how to figure out why the fan will not kick over, and would like to know what causes it to shut off. I am a DIY type of a person, have basic electronic knowledge, more experienced with cars than a/c units, but willing to learn. Please let me know what I can do on my own to figure it out.
written by Eric, August 29, 2010
Capacitors do not make motor speeds change in any way. A capacitor holds electric charge to give the motor a kick in the ass to start. If the motor was changed then it is possible that a lower rpm was used on the new motor, which I do not recommend, and should be replaced back to manufacture specs. If your motor seems to be running violently then it may be the fan blades are bent creating a wobble effect.
Eric HVAC Tech
written by Eric, August 29, 2010
Laura,
Sounds like your capacitor is bad. By you pushing the blades you are acting like a capacitor would in aid of the motor start up. Your unit is shutting down because the capacitor is intermitting and does not stay in the electrical circuit. Replace the capacitor with the same ratings and the old one. Eric HVAC Tech
written by Eric, August 29, 2010
Arnold,
Always replace the capacitor with the same ratings as the old one. By you using a 50/5 you are putting more strain on your compressor then what it is manufactured for which will cause the motor to have a shortened life. Make sure you voltage is the same as well. Eric HVAC Tech
written by Eric, August 30, 2010
Kevin,
I have been doing a little more thinking about your fan speed situation. It just sank in that you had capacitors replaced. If a run capacitor with an incorrect voltage rating is installed in a motor that requires a capacitor for second-phase energy, it will throw off the magnetic field. An uneven magnetic field will cause the rotor to slow in the uneven spots, which increases energy noise, as well as power consumption, and can also cause performance problems and overheating issues. Over heating the motor will cause the motor to turn off on thermal protection. After it cools it will turn back on. I'm not sure if this applies to your situation , its hard to say with out being there, but you never know. Eric HVAC Tech
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There are two basic types of motor capacitors most commonly used in HVAC applications today. The run capacitor and the start capacitor. Motor capacitors are used to either increase run efficiency of the motor or to give it an initial start boost in torque to a motor. They are sometimes referred to as motor starter capacitors or power capacitors. Motor capacitors are only used for single-phase motors that generally see heavy use throughout the HVAC industry. Common applications of single-phase motors which use commonly use run capacitors are air handler blower motors, induced draft or forced draft fan motors on high efficiency furnaces, condenser fan motors, compressors, and pumps for hot water and chilled water. Common applications for the start capacitor are generally limited to compressors and only to start the compressor. Typically, when a start capacitor is used, a special relay accompanies it in the circuit to disengage the start capacitor after the compressor motor gets it's initial boost in torque from the start capacitor. The most common relay for use in disengaging the start capacitor is the potential relay and the least common is the current relay. Power capacitors or motor start capacitor are also referenced when discussing HVAC capacitors.
Capacitors are measured or rated according to a microfarad rating. Generally, run capacitors are rated for lower microfarad ratings from 3 microfarads to 50 microfarads. Start capacitors have much higher ranges and can be purchased in sizes all the way up to 800 microfarads. Capacitors also have a voltage rating on them with voltage ranging from 240 volts to 440 volts. It is good practice whenever replacing any electrical components to use an exact replacement. There are rules of thumbs that allow one to use a part that is not an exact replacement which is in a certain range but it is best to use an exact replacement. To check to see if a capacitor is bad use a capacitance tester to measure the microfarad rating on the capacitor. Compare it with the manufacturers rating. If it is more than ten percent off the microfarad rating on the capacitor replace it.
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Thank you much,Tony S