5 things that many hvac techs miss about ac run capacitors and how a run capacitor works.
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Read all the tech tips, take the quizzes
and find our handy calculators at https://www.hvacrschool.com
One of the most common parts to fail on a single phase hvac system is the run capacitor so much so that we sometimes refer to junior text as capacitor changes. It's not very nice, but it's something we do 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, so they're not voltage boosters. We often see higher voltage across the capacitor than the line voltage, but this is due to the back emf or counter electromotive force generated by the motor, not the capacitor itself. Current doesn't flow through the capacitor just in and out of it text notice that one side of power is connected to the c terminal or the side opposite the run winding many techs.
Imagine this power feeds into the terminal, gets boosted or shifted and then enters the compressor or motor through the other side. While that may make sense, and as a way of thinking of it, 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, very thin plastic and immersed in an oil to help dissipate the heat just like the primary and secondary of a transformer. The two sheets of metal never actually touch at all, 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 and the same side. They entered the higher the capacitance, the higher the current on the start, winding on a properly wired, psc or permanent split capacitor motor. The only way the start winding can have any current move through.
It is if the capacitor stores and discharges the higher the mfd or microfarad of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with zero capacitance, it is the same as having an open start winding or an open circuit altogether. 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 lead to 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 the capacitor can handle, not what it will produce. Many techs think they must replace a 370 volt capacitor with another 370 volt capacitor. The voltage rating displays the not to exceed rating, which means you can replace a 370 volt with a 440 volt, but you cannot replace a 440 volt with a 370.. This misconception is so common that many capacitor manufacturers begin stamping 440 volt capacitors with 370 slash 440. Just to eliminate confusion. You can test a capacitor while the unit is running. You simply measure the current or amperage of the motor start winding coming off of the capacitor and multiply it times: 2652 on 60 hertz power and 3183 on 50 hertz power. Then you divide that number by the voltage you measure across the capacitor to come up with your microfarad value.
So those are some common things that people don't know about: capacitors they're, a very simple device, i think of it like a storage device for current in and out it does result in a phase shift and it's necessary for the operation of psc and cscr motors. Thanks for watching our video, if you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified when new videos come out, hvac school is far More than a youtube channel, you can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more. You can also subscribe to the podcast on any podcast app of your choosing. You can also join our facebook group if you want to weigh in on the conversation yourself thanks again for watching you.
capacitor take the bumps out of the inverted sign wave on pulsing ac current – balances or flattens out the top of the wave length
Very good job 👍 Are you in Ottawa ?
Want to know why Chinese capacitors fail. Take one apart.
Great video. Thanks for sharing.
Super
Anybody wondering where the 2652 comes from ? It’s (1/(2*pi*f))*1000000 when f=60hz.
If the run capacitor is for shifting the electrical phase by 90 degrees so that it feeds the start winding, which is 90 degrees from the main winding, why are we concerned about power factor? I'm missing something. Please help. Thanks!
Very good information
So If I put a a 80/5 ohm on a 70 ohm what’s expected to happen. Service area Barrhaven??
Another awesome straight to the point but detailed video, thanks Brian. Quick question, is there a brand of capacitors you recommend?
Supco is crap
Quick question. I understand the reason to match the microfarads size and voltage rating, but can a capacitor that is PHYSICALLY larger with the same electrical ratings be installed without issue? I believe it can be. Having a hard time finding an exact replacement for my PSC blower. Thanks
You do a great job in your explanations and level of detail
Glad to see that Bryan and Craig have collaborated on some videos. One may explain a topic differently and that difference may be what gets the light bulb to light . Keep up the great work guys.
Great explanation clears up a misunderstood subject 👍
This is such a simple and clear, yet rare information. Thanks for the time to put these information together.
Checking a cap while it's running is sometimes really clutch. I was on a roof the other day and there was no disconnect. I didn't want to climb down the ladder and search for a breaker, so I looked up your website and did it right there. SICK.
Get your guys to study “ Power factor” and this will really drive home the role of a capacitor in an inductive circuit. Great video.
Ooof so close. It charges and discharges 60 times per second. It simply doesn't store energy like we think of it in a DC system. It is used for shifting the phase angle in order to create a rotational force upon the rotor. I'm sure you know that but using the phrase "think of it as an energy storage device" is as erroneous as thinking it somehow changes voltage to any appreciable degree. Good video otherwise.
Brian you are simply a Great A/C
Instructor an Encyclopedia of
A/C conditions.
Thank You
To all who say why would you want to use this method?
This method allows the flexibility to check while running under a few circumstances:
1 a critical application where they don’t want to shut the system down while you check it. I haven’t been around mainframe computers in decades but I remember when they used to shut down things if the room temp exceeded a certain temp.
2 I have many times seen components that check good on a bench and fail under load. I am sure I’m not the Lone Ranger on that. This method gives you a real time, under load look at how the capacitor is performing.
3 It also lets you check running capacitor when you find yourself using a meter that can check amps and volts but not capacitance.
Back in the day if you had an analog Simpson 260 you had a Cadillac of a meter. It did not do capacitance. If you had a capacitor on the bench you could only test for open, or if it pegged the meter and stayed there, or was grounded to the case or watch the cap charge and discharge by watching the meter cycle rising and returning to zero. That did not tell you what the capacitance was, just that it would charge and discharge.
I know I just dated myself and I am ready for the jokes but no I did not personally know Jesus, Moses or Methuselah.
Can u still use a capacitor thats reading a higher mfd on ur multimeter than whats listed on the capacitor? N no bleed resistor connected. Service area Ottawa??
Capacitors are black magic. These are all lies haha
Thank you Brian for all your dedication to continuing education. Would you consider spending more time on inverter technology and specifically proper troubleshooting of these units ? I think we all can use more information on this technology.
Nice job and video
Why does the run oil capacitor fail so often ?? I have many used capacitor that are many years old and still good. How can you clarify this ??? Thanks
Why are capacitors not polarized like they were in the older days?
Doesn’t make any sense
💪🏼 Service area Nepean??
Great information.
Excellent explanation. Service area Kanata??
buenos videos
How can a condenser run fine when the common wire from the top of the contactor to the capacitor has been burned and is no longer making any connection? If I remember it was the common wire.
I’ve witnessed this twice and the system was seemingly running fine. I believe the spade terminals were loose and caused heat buildup and eventual burning of the wire. Just confused how they were kicking along fine without the wire. Service area Orleans??
Channels like this and ACservicetech make you feel like a damn physicist lol… there's just so much science involved in HVAC. It's awesome
Love this channel! Keep it coming! Thanks for your contribution to our trade! Great work! Are you in Nepean ?
Yep great content
Quick and informative
what are your thoughts/opinion on a soft start kit for systems? Thanks!!!
Great video!
OK. So why is a run capacitor wired to the start winding?
Thank you for your ongoing dedication to educating HVAC techs helping make us better at what we do. I am grateful and hope you never stop doing it.
Wow Amazing video
Iove You sir
Who were the 5 haters hitting the dislike.
Nice!)
Thanks for reviewing this all important topic. Are you in Orleans ?
Can you go into more details about measuring the mfd while the system is running?
You are a true hack.
I wouldn't let you work on my cats litter box.
I'll keep checking while the power is disconnected thanks!😄😳🥃😆😂😆🥃👍
Stay safe.
Retired keyboard super tech.
Wear your safety glasses.
You can check a capacitor while it's running.
Me: OH?!
Measure the amperage on the start winding and multiply by…
Me: Wait what?
It's okay just multiply and divide by…
Me: I'll just turn it off, never mind. Are you in Kanata ?
Love this format. Great video