Capacitor and Hard Start Myths Busted

Bryan teaches the Kalos techs all about capacitors, including how they look on diagrams, why they fail, and how to handle them on service calls. He also gets some capacitor and hard start kit myths busted.

Even though we may imagine that current travels across the capacitor, the electrons DON'T travel through the capacitor. Capacitors don't "boost" voltage or current, either; the microfarad rating is actually more likely to reduce or restrict the amount of current that travels through the start winding.

It may be helpful to view the capacitor as a balloon or membrane that stores and releases electricity. As the voltage changes via alternating current (60 times per second for 60 Hz, 50 times per second for 50 Hz), we measure its average via the root mean square (RMS). So, the capacitor charges (through the start winding) and discharges 60 times per second from the same way it came. Capacitors have attractive forces due to the high surface area between the two plates; there is a large sheet of plastic with metal rolled into the capacitor.

You can also think of a capacitor as a third hand to help spin a motor; when we spin wheels with our hands, we apply directional force instead of horizontal force. The legs of power act kind of like sources of horizontal forces, and the capacitor acts like a hand to begin spinning the motor.

Three-phase equipment and ECMs don't require a capacitor because there are already three "hands" spinning the motor without help. On single-phase equipment, the start winding always has current running through it, not just on startup, even though we need another "hand" to help start the motor. That's why we have run capacitors.

If the run capacitor is too small, the compressor might not start and will get hotter because the run capacitor generates heat in the run winding (not the start winding). When the rotor stays locked for any reason, including an undersized capacitor, the amp draw stays high until the compressor goes out on thermal overload. If the run capacitor completely fails, nothing happens on the start winding; no current moves through it whatsoever. A failed start winding may happen if the capacitor is wired in incorrectly, if the capacitor is too large, or if the hard start kit presents problems. So, be sure to do a thorough visual inspection of the capacitor and any other accessories.

We need to be careful when using hard start kits; they don't add a phase shift but give us more current to hit the start winding. However, the start winding isn't designed to handle full current all the time; the hard start kit needs to be able to shut off or take itself out of the circuit, usually via a potential relay. The best hard start kit is almost always the OEM hard start kit, but aftermarket kits are acceptable for temporary solutions or when the unit lacks a factory recommendation. It's also worth noting that capacitors can indeed weaken over time without failing completely.

Incorrect capacitor sizing is common, especially after capacitor or compressor replacements. Sometimes, technicians accidentally install a new run capacitor of the wrong size. In other cases, a new compressor may require a different capacitor size than the previous one; we can't just assume that the new compressor will have the exact same requirements as the previous one.

[jlxm]

Thanks for the video just in time for cooling season here in the Midwest. Learned a lot here!

[StumpBrokeCow]

For your "third hand" example for the start windings, instead of the "pinwheel" analogy, I like the bicycle pedal/crank analogy. If the pedals are straight up and down, it is very hard to get them turning, but a third pedal (phase shifted) helps get it going and then the rotational inertia keeps things going.

[grahamamorrisonsr]

I’m a Retired Claims Adjuster who specialized in AC “lightning” claims in the seventies when the first batch of home AC units began to wear out. If a tech saw burned wires, they declared lightning damage, obviously because ONLY lightning (not LRA from a mechanically failed unit) could burn wires outside the compressor. I am proud to say I answered almost every question the teacher asked correctly. I am also an amateur reel to reel tape machine repairer, and bad or dying motor run caps are a chronic issue on machined 40, 50, and 60 years old. I took electronics in High School. I find electronics fascinating, even though I specialized in Law and Litigation in the claims business.

Very useful presentation, thanks!

[brkbtjunkie]

You’re a great instructor! Never give out an answer, always make the class work for it.

[jamesbrzycki]

After only ~8 minutes of this video, I'm amazed at how clearly Bryan explains how capacitors function. Nice work!

[michaelirvin]

I’ve been explaining this to my technicians for years and few seems to get it, so I hope if I show them this video, they will finally understand. THANK YOU FOR DOING WHAT YOU DO, AND KEEP UP THE GREAT WORK!!!

[spelunkerd]

This was well explained. Coming from the engineering side, about a year ago I started looking for a simple way to calculate what size cap is needed in PSC and CSCR motors. I made a little headway by looking at simplified RLC circuit models. It was enough to realize that when inductors like motors are paired with capacitors, they get a kind of 'tune' that is specific for the frequency of incoming power. If one replaces a cap with the wrong size, expect the circuit to be tuned at the wrong frequency, with unpredictable and often bad outcomes. As you say, relative current flow down the two windings and power factor can dramatically vary depending on the capacitor chosen. My effort to find a simple solution hit a bigger road block when i discovered that power factor and motor inductance vary with rpm, and of course there is the usual tradeoff of starting torque vs full load efficiency considerations. I believe most new motor designs these days are practically tested in the lab with various capacitors, ultimately leading to chosen capacitors that balance power consumption, motor longevity, cost, and reliable starting. There is no easy formula. Any movement away from the design is not likely to end well, if only leading to lower efficiency or poorer motor survival.

[Partysize]

I see that most people benefited from this explanation, which is good. My explanation, when teaching electronics, is to start by explaining how an AC motor works. Then you can see what is needed to start it (and/or run it, ie make spin). Then you can understand what the capacitor must do to make that happen. This forms a better understanding of the start cap, run cap, and motor as a SYSTEM and ties these components together. Then when the question arises as to what the symptom is when the caps fail, they need no explanation, because it is obvious. Thanks for the video. I am always impressed by people who share their knowledge and experience. Keep it up!

[gkcmcs]

Awesome video, I’m a structural engineer who has 30+ years of building experience and I learned a few things about capacitors i never knew, very cool.

[ramosel]

When I went solar, I pulled the hard start kits from both of my HVAC units and replaced them with Soft Start kits from Hyper. The inrush current at startup is significantly reduced and much nicer to my solar controllers. When I had my yearly tuneups done the first time after I installed them, my HVAC guy said he was really glad when he sees them. The units he had installed them on haven't had a failure since he started using them. That was 12 years ago, never a problem here. He still says he's not replaced a compressor on any unit with a soft start.

[sloanemichaele]

Great video, thanks for the info. I'm a homeowner here trying to educate myself to make sure my HVAC contractor is being truthful in the "up sell items" they are trying to get me to guy. Now I understand what a "Hard Start Kit" is, and how it fits with the overall design. Your explanation confirms the measurements the service tech gave me, so thank you! I'm pretty sure I'm not being ripped off this time. BTW, I am an electrical engineer, and I chuckled at the "dig" at the end of the video! At the same time, I've had too many techs and contractors over the years who obviously didn't know what they were talking about, so I appreciate this course and hope the folks who attend can help their customers better in the future. I'll never forget the one technician who confidently told me that "Amps and Watts are pretty much the same thing", which didn't boost my confidence at all. 😅

[practicalhvac]

I’ve been an HVAC service technician for two years. I just want to thank you guys for all the videos that you put out. I’m sitting in my bathtub right now having a couple beers watching ya!! lol

[sixter]

I work in the pool industry. I have attended many educational sessions by the manufacturers, and have never heard an explanation about capacitors. I have searched for simple concise information for years, and YouTube just said "here you might like this". I have replaced many dead capacitors on pump motors. This video gave me a better understanding. Thank you.

[mikevarish]

Bryan is an excellant instructor. He is extremely knowledgable and makes it easy to understand the subject he is teaching. He should be paid very well

[dgmenace]

As a retired electronic technician, this was very interesting! I really like your way of explaining caps. I eventually developed techniques of troubleshooting and became a very good troubleshooter. It was like you said ....you have to throw a lot of teaching methods, theories, myths, etc out the window and put your mind to work...to work in a way that helps you to understand what you are working on to find the problem quicker and know what to go to first. I've always told people that working with something that you can not actually "see" (electricity), it can be very challenging!...lol

[dougb]

"Electrical engineers are always fun". Good video. AC folks need to watch this.

[Do_the_Dishes]

At around 9:20 you mention that the start winding is “always in place” and that it is not disconnected. Just for your viewers, some single phase motors do disconnect the start winding via a centrifugal switch inside the motor or externally with an electronic Start Switch like a Stearns SINPAC. Great video! I didn’t know the compressor motor left the start windings in play.

[neilsheldon]

So, a capacitor works by transferring energy via the electric field. Typically, it blocks DC and it takes AC to transfer the energy (again, through the electric field) from one side of the capacitor to the other. A capacitor also causes a phase shift in the applied energy, where the current in a capacitor begins to lead (or come before) the voltage in the circuit. Inevitably, you will find circuits where there is a large amount of inductance involved. (an inductor transfers energy by a magnetic [or electromagnetic]) field. This typically is found in motors, which exhibit the properties of an inductor. And in an inductive (typically, a motor) circuit, it exhibits the opposite properties of a capacitive circuit; that is, in an inductive circuit you will find that the Voltage in a circuit begins to lead (or come before) the current in a circuit. Now, it is important to realize three things, 1) that if you have an inductor (typically, a motor) in the circuit, and a capacitor in the circuit, that essentially, to the extent possible, (depending the amount of capacitive reactance, and the amount of inductive reactance in the circuit) they will tend to cancel each other out; and 2) the most actual power transfer takes place when the current is 100% in phase with the voltage; and 3) that happens when the capacitive reactance of the capacitor cancels out the inductive reactance of the motor. And then, there is no resulting phase shift. Consequently we hit that golden point where as much power as possible is being transferred from the source (in an AC, usually 240 VAC) to the load (usually an Air Conditioner or fan motor). Other higher voltages can be used where more power is needed to be used, but while higher voltages are common in industry, they are not so common in home use. Above, I mentioned Capacitive Reactance (typically called Xc ... [usually pronounced "X sub C"]), and Inductive Reactance (typically called Xl ... [usually pronounced "X sub L"]). There a formulas for both of these, and deeper explanations for both of these all over the internet. But when Xc = Xl ... there is no phase shift, and you have maximum power transfer from the source to the load. One additional point. In the starting of a motor, the amount of Xc needed to cancel out Xl is greater, and all the time changing during the 'speed up' phase, until the motor reaches full speed, where Xl levels out. That is why there are two different values of capacitors used, 1) for the "start" circuit, and 2) for the "run" circuit. Xc for the 'Start' circuit is typically optimized for the startup phase, and Xl is typically pretty close to where it needs to be for the "run" phase, thereby providing the most efficient of both worlds ... start phase and run phase. These are basic electrical as well as electronic principles. Sadly, many never grasp the meaning, and come to an understanding of why. And I have to admit, it took me a long time to come to that realization. But when the 'Ah Ha' comes ... you will be forever improved at your particular tasks, whether electrically or or electronically. And, especially, in a motor circuit, you will understand what the actual purpose of that 'start' - 'run' capacitor actually is.

[martymurphy]

I have been thinking wrong how the capacitor works! Thanks for this video.

[bradleysprague]

You and Bert have taught me more than school ever did, two years into the field and I still religiously watch your videos. Great job I wish our weekly meetings we’re more in line with what you do!