Why Are Capacitors on Motors? What is Capacitive Reactance and Inductive Reactance?
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- Опубликовано: 23 окт 2022
- Most of us know what a motor is. But what about capacitors? And why would we need them to be on a motor? In the latest episode of Electrician U, Dustin answers discusses why we would need to have capacitors on our motors.
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Capacitors for the most part are an energy storage device. They will charge up and store energy and then discharge when its needed. But why would we need that for a motor? To answer that, we need to understand how a motor starts when power is applied to it. If we had the leads for the motor connected and the motor poles are in line, the motor will turn until those points are no longer in line. But the power points are now out of sequence (in a sense) and the motor can no longer spin. What a capacitor does is provide a charge to bump those points so they are back in line and the applied voltage can cause them to spin again.
There are also a couple of terms that we need to know when discussing capacitors. Those are inductive reactance and capacitive reactance. Inductive reactance is where the voltage is leading, and current is lagging. In capacitive reactance is where Current leads and Voltage lags. In essence, in an inductive circuit, the amount of magnetic energy keeps things so bound up that it slows down the current flow. However, the voltage is still churning away, but the current is lagging. In a capacitor, when discharged, the positive and negative are just randomly kind of hanging out together. But when charged, those positive charges group together, as do the negatives, in a much more orderly fashion, ready to be discharged to do their work. But with them being so far apart now, current cannot get thru, hence the current LAG in capacitive reactance! In essence, inductive and capacitive are just polar opposites of one another.
In many motors there are actually 2 capacitors. A start capacitor and a run capacitor. The start capacitor is used to get the motor spinning as this usually requires a much larger push. Once the motor has taken off and churning along, then the run capacitor takes over. The run capacitor still provides the same function in keeping the motor shaft spinning when its not inline with the motor leads, just not as much is needed as the force of the shaft moving helps it along.
Capacitors are also used to keep things from being so lopsided. They smooth the process out a bit. In lieu of there being such a large Voltage draw every other cycle, and none on the opposing, capacitors smooth that up and down action. This tends to make the process much smoother and less jerky!
There are also other flavors of capacitors that provide both start AND run functions as well as capacitors that can provide either the Start or Run functions for multiple motors.
We hope this has been an insightful look into inductive/capacitive reactance and why we need capacitors on motors. Is there a topic you would like to see discussed on Electrician U? Leave a comment in the comments section and let us know. Please continue to follow Dustin and Electrician U as we are constantly updating our content to assist our followers in becoming the best electricians that they can be!
#electrician #electrical #electricity #capacitors explained #capacitive reactance #inductive reactance #customers questions
THIS IS NOT HOW A CAPACITOR HELPS A MOTOR START. I apologize for using capital letters. My intention is not to yell. Only to draw attention, so maybe you'll reconsider making another video or updating this one, so in the future it doesn't confuse others like it confused me. I watched this video for the first time about a year ago, and after watching it a couple of times, I was confused about how a start capacitor helps a motor start. After taking some classes and learning how capacitors truly help a single phase motor start, I came back to watch your video again to see what caused the confusion the first times I watched it. After watching it, I realized that you don't use a start winding in your presentation. In a single phase capacitor start motor there are 2 windings: start/auxilliary winding and main/run winding. They are physically placed 90 degrees apart from each other inside the motor housing. The start capacitor is put in series with the start winding and a switch (usually a centrifufal switch). When you start the motor, the start capacitor (when sized appropriately) helps increase the phase angle difference between the current going throught the main winding and the current going through start winding to 90 degrees. This 90 degree phase shift in current between the start and main winding, along with the 90 degree physical spacing between start and main winding inside motor housing is what helps the motor start. The current first passes through the start winding and it creates a magnetic field that makes the rotor begin to rotate. Then 90 degrees later current passes through the main winding and it creates its own magnetic field that helps the rotor continue rotating and accelaratring until 90 degrees later when the start winding's magnetic field helps the rotor accelarate even more. This back and forth dance between the start winding and main winding will continue until about 75% motor speed. At this point the centrifugal switch will open and cut power to start winding. I hope you don't take this comment the wrong way. It's only constructive criticism. Nothing else. Thanks for your time!!!
This guys videos are filled with inaccuracy.
Most what I'll generously assume are slips of the tongue which should be caught & edited before posting.
If in doubt ChatGPT 😄
By far the best explanation, thank you Bernardo!
@@armandoalaniz8037 Thanks!
I think you're the first electrician I have seen that knows the science behind electricity 😅. I'm an HVAC technician. A pleasure to watch
Ole ELI the ICE man!!! I learned about ELI & ICE when I went through advanced electronics school in the Navy back in 1985!!! lol
As a HVAC Tech, your content always help me to better understand what I'm working with.
I love his videos but that was a super poor diagram of a psc motor. Also a totally inaccurate description of compressor capacitor size.
My "down in the trenches" lesson in motor capacitors came years ago when I had a customer who bought an old industrial two-phase drill press from a factory in Philly. I went to a motor shop to see about getting a single-phase motor to replace it and the guy asked "Why would you do that? Just add a capacitor!" He explained how a lot of motors we use now actually have two-phase windings and the capacitor makes it work in a single-phase system. Once he explained the basics, I was able to build a timer circuit to pull the cap out once the motor got started and the customer got a really cool drill press to use in his shop.
At around 7:00 you gave the most tangible, understandable explanation of lag on capacitors and inductors I've ever heard. It's such a hard-to-explain subject that it helps to picture myself as an electron getting pushed back by a magnet, ala magic school bus, or to visualize the potential of a capacitor filling up and releasing. Another good analogy for a capacitor is a water tower.
Yup.
Electronics is a hobby of mine and every time I see one of these "what does a capacitor do on HVAC" videos I die a little bit more. But that section was gold.
Should have started the video with that bit, and not skipped winding geometry.
With a single winding, no amount of capacitor jiggery pokery will fix the start problem.
I've never understood how current could lead or lag, but the way he explains it makes sense.
Look into the function of the Start Winding in split-phase motors like PSC, CSIR, and CSCR. (Permanent Split Capacitor, Capacitor Start Induction Run, and Capacitor Start Capacitor Run)
Adding this to your already very good explanation will make it more complete.
The problem with starting an AC single phase motor, is there is no rotating magnetic feild as there would be in a 3ph circuit. So we need a second phase with the current out of phase - thats all the capactitor does, provides the start current winding out of phase to the run winding. Very simple. There is no storing or smoothing anything,
Love your videos and going into this topic! Comment on A/C condenser capacitor is a bit off: those are dual value run capacitors (around 14:30 in vid) 45/5 or 50/5. The larger value is wired to the compressor, the lower to the fan motor and there will be a common. On occasion you may find an additional capacitor if someone added a hard-starter to extend the life of a failing compressor. Point is, any of those type capacitors in a condenser are run capacitors.
There is no such thing as a dual run/start capacitor. Run capacitors stay energized whenever the device they are connected to sees line power. Start capacitors are always switched off after a very short start cycle or they will fail. Your choices are simply run capacitor, dual value run capacitor, or start capacitor. The only exception are these multi-rated capacitors but those are strictly run capacitor that you use jumpers to in effect set the overall capacitance within the ranges available.
John - I agree...completely....
Not sure about others but I feel like this is one of my favorite videos you’ve done as far as simplifying an explanation.
I've studied electrical theory (basic) and am currently studying for my amateur radio license. I've always understood the basics of WHAT capacitors are, but not so much about HOW they work and WHY. This explanation has helped me greatly, Thanks!
I have inherited a grinder that I have to start by manually spinning the grind wheel in the direction I want it to go (I can decide, up or down). Now I understand why I have to do that. No capacitors. Thanks!
edit: it's very old. Has oil wick lube for the bearings.
I'd tell you to be safe but I don't see how anything could go wrong with that
Sounds like something's not working right, if that was my grinder I'd open it up and look for a spot that a cylindrical object such as a capacitor would fit in there, as well as potentially a couple of leads for one. You might find a capacitor in there that's failed open circuit, or just has greatly reduced capacitance because it's had the electrolyte dry out.
And if it's intended that you should be able to choose the direction of the motor, well there's ways to run a 3 phase motor on 1 phase power (involving usually multiple capacitors to effect the required phase shifting), and if you still want it reversible you can have a mains-rated DPDT (on-on, break-before-make) switch (wired up as a polarity reversing switch) and connect that between any 2 of the 3 phase wires and the power that goes to the motor (after the capacitor bank).
It would help if I could draw a circuit, but I know it would work, it's just combining the concepts of "how to run a 3ph motor on 1ph power" and "how to reverse a 3ph motor".
You reverse a 3ph motor by swapping any two of the 3 phase wires, that's what that DPDT switch does. Don't flip that switch when the motor's already running tho, bad things might happen.
Most single phase grinders do not have a capacitor. Most likely grinders do not require a lot of starting torque ( unlike compressors ) because a load is generally not applied until the motor comes up to full speed. These motors are considered a split phase if they have a start winding to provide out if phase ( with the run winding ) starting torque.
@@JohnThomas-lq5qp Oh, I see. So then a grinder might have a "shaded pole" design then, similar to the small AC motors such as used in electric can openers and desk fans, instead of using a start or start/run capacitor, that makes a lot of sense.
In that case there's still something wrong with the grinder that can be made to run either way, it might have a broken wire in the shaded pole winding (which is usually just a turn or two of thicker copper wire).
Can we pin this? This is almost as informative as the video!
Fantastic explanation man. My dad was a 40 yr master. Wish I had his knowledge. You do a really great service to up and coming electricians. Thanks for your knowledge to pass on
For someone who has paid attention to electricity for over 20 years, you sure have elucidated quite a bit for me. I can say that the stuff you didn't elucidate I had already known. You have good explanations sir.
Finally i understand power factor now.
I've always thought that: why laging or leading cause inefficiency but the electrical charges (energy) are conserved and not lost ? like a small battery as a buffering zone that saves the energy to use it later?!
but now i got it, it is about the instantaneous Work done, Not the energy available within the system, because Power(w) needs both potential(v) and Current(A) available at the same time in order to actually do the work.
this really helped. Thank you for spelling it out,
Inductive reactance is where the voltage is leading, and current is lagging. In capacitive reactance is where Current leads and Voltage lags. In essence, in an inductive circuit, the amount of magnetic energy keeps things so bound up that it slows down the current flow. However, the voltage is still churning away, but the current is lagging. In a capacitor, when discharged, the positive and negative are just randomly kind of hanging out together. But when charged, those positive charges group together, as do the negatives, in a much more orderly fashion, ready to be discharged to do their work. But with them being so far apart now, current cannot get thru, hence the current LAG in capacitive reactance! In essence, inductive and capacitive are just polar opposites of one another.
My understanding is:
power factor (PF) = RMS power of load (kW of actual power used) / avgVolts*avgAmps (kVA)
A resistive load of a simple heating element or incandescent light bulb (old filament) has PF = 1 & current rises with voltage together, in constant proportion & peaks at the same time.
A PF between 0 &
Glad I watched this. Needed to brush up on my engineering education. But electricians need to understand what Capacitors do to understand why you discharge them.
You explained a very complex issue simply and clearly.
I know it'a a year old, but love this video. Explains this motor story really well. Thank you!
Single and two phase need a capacitor to spin the motor in the right direction because even with two phases they’re 180 degrees out of phase so there’s no reference as to whether it’s clockwise or counterclockwise rotation. Don’t need a capacitor to start a 3 phase motor. That’s just my basic understanding of it.
Although 2 phase power is a thing, it is extremely rare. What you are thinking of is a line to line single phase motor.
Single phase can be split into smaller sections of a coil, for example single phase 120/240. They simply tap the middle of the coil and then from either end of the coil to center (neutral point) you get 120 because you only utilize half a coil, and from line to line you get full 240. In this instance your voltage waveforms would be 180 degrees out of phase because the 2 coil halves are directly opposite each other.
In 3 phase wye 120/208 then you would still not be considered 2 phase as the 2 coils are only 120 degrees out of phase, and not 90 if I’m not mistaken.
Imagine single phase being one person pushing a merry go round with a bunch of kids. It would be very difficult to get started, but once you get moving it gets easier.
Then imagine the same merry go round with 3 people pushing one after another clockwise, and in sync. It would be much easier to start since the load is distributed among the 3 people, and the merry go round would slow down less in between the time that you let go and grab the bar at the begginning , because as your letting go the other person is mid push. so the merry-go-round will move faster, and the people pushing will tire out less quickly.
This is how three phase works. You can use single phase to run a motor, but power draw is higher and less efficient, but on 3 phase it is easier to start, AND draws less current while running, which reduces cost for wire, pipe, and electricity.
Worked on obsolete two phase motors years ago. None ever had or ever will have a Capacitor. Recently did a Google search to find a fusible 4 pole safety switch for a two phase motors. Nope no results. Capacitors when in a single phase motor are two types: capacitor start that is only in the circuit for a few seconds until the motor achieves approximately 75% of full speed. At that speed it along with start winding are disconnected from the circuit. Some single phase motors contain a run capacitor that provides higher efficiency.
@@MrTacolover42 three phase windings are 120 degrees out of phase and two phase is 180 degrees out of phase. Philly & Detroit one of the few cities that still have two phase power. Heard they had 480 volt two phase but never came across it. Worked in a lot of 240 volt two phase 5 wire. 5yh wire was attached at center if both windings to provide 120 volts for lightning and general power to offices and such. Place that I worked at had 2,300 volt two phase 3 wire feeding a remote signal building. All modern 3 phase drives ( VFD'S ) have a capacitor bank wired in parallel to the DC Buss. Was told drive capacitors are rated for 75,000 hours run time but had a lot of 40 to 200 HP drives with over 125,000 run hours with original capacitors.
@@JohnThomas-lq5qp interesting to hear that there’s a configuration with 2 phase being 180 degrees out of phase! A quick google search told me 2 phase would be 90 which seemed lobsided to me.
@@MrTacolover42 Sitting back you might be right with two phase being 90 degrees out of phase. Been almost 60 years since I learned about it. I have to locate my 60 year old 600 page double sided spiral motor book. Think a guy named something like Rosenberg wrote at least two separate great volumes years ago. Our industrial electricity class & a motor repair shop that repaired our motors both used this great motor book. More I think about it almost positive I misspoke about two phase having phases 180 degrees apart. Thanks for replying. Even an old fart with over 50 years in the enjoyable electrical trade not too old to learn.I was paying my own way to attend the 8 IAEI yearly meetings/ continuing education classes in my area until the pandemic shut it down for over a year.
Dude! Thank you I am studying for my electrical license and one of the questions I had were on induction/ cap reac. Thanks for going into more depth
One of the best explanations ever for reactance and its relationship to motors! Thanks!
Thanks for the amazing explanation. Love this channel. I'm IBEW 357 and when you going through the apprenticeship, this info is in one and out the other. After being a journeyman for a few years I want to go back and re learn everything. Thanks again for the great work your doing.
this is an awesome video. you have a very good understanding of what’s going on at the microscopic level. comment, though (i’m a physicist): in a capacitor, there charge rearranges via a “displacement current,” not by getting pulled to the plates. it is actually the changing electric field which creates an effective current. beyond linear reactance theory.
i took EET 23 years ago and i've forgotten a lot of this, but this explanation was excellent!
Great video. I use the power factor function on my meter to check single phase, dual capacitors all the time on residential condensers. It's quicker than killing the power to test it directly or using formulas while under load. Best part it actually tells you that the cap was sized right in the first place.
16:05
Start capacitor is in parallel.
Run capacitor are in serie.
But usually, there is only a start capacitor.
As A EE - or was until I changed careers, you did a great job explaining these concepts. ELI and ICE. I remember those concepts all too well.
What career did you switch to?
@@tonytucker8651 Did network installs and now flip houses. Been an interesting life. Learning how to do vinyl flooring now
@@JackKirbyFan cool. I love real estate. I am ME working in an EE field. Power Systems Engineering. How is your market for flipping? Mines is slowing down. It used take nine days to sell, but now it three or four weeks.
@@tonytucker8651 It's just me and my wife so flipping a house is usually a year or more process before selling. Plus - we're old :) But having fun. So far good but I'm waiting for the housing crash. Hopefully this latest can go before then.
The acronym we used to use was "CIVIL", i.e. For (C)apacitance reactance current(I) leads (V)oltage, and (V)oltage leads current(I) for Inductance(L) reactance.
Nice explanation. Thanks for sharing your knowledge.
Thank you so much, I am currently studying for my Canadian FSR-B ticket for Film and Entertainment Lighting.
Finally... a good expiation of what a motor start and run capacitor does. I always wondered why the need for both (especially on HVAC equipment and anything with larger motors). :)
Such a great video. Takes me back to studying power 45 years ago!
Funny how those catch phrases (ELI the ICE man) are stuck in your brain for the rest of your life (or at least until dementia sets in).
@@Ephesians-ts8ze I'm in my first year of electrical whats eli the ice man
@@lostandfound3588 he mentioned it in the video but understandable if you didn’t catch it or understand it. ELI- the “L” represents induction. The voltage “E” comes before the current “I”. And ICE- the “C” represents capacitance and the current comes before the voltage
@@lostandfound3588 you gotta learn ohm’s law thoroughly first before you’ll understand it (or at least the mathematical relationship). We learned capacitive reactance and inductive reactance in the 2nd year of apprenticeship. What you’re learning now is building up to that
@@Ephesians-ts8ze I didn't finish the video, thanks for the heads up I'll pay attention to that when it comes up
Thanks for all of information. Been messing with my condenser unit all summer to understand the difference between the start and run capacitors and realized the run capacitor was dying and needed to be changed.
I really appreciate the CLARITY of this tutorial...THANKS
Brilliant explanation. I captured ELI the ICE man and can remember the leads and lags in capacitive and inductive circuits for the first time. Thank you so much for this video.
Great video. It's a good idea to look at the starting capacitor as "low gear" to get the motor spinning, and the run capacitor as an "overdrive" gear to improve efficiency.
Can't get a strong rotating magnetic field on single phase motor without one on a centrifugal switch, otherwise they're just single phase hacks that should simply die off with modern 3 phase vector. They're a big money profiteer however in the industry due to how frequently they crap out when it gets hot out. Those things ALWAYS die when you need them the MOST. It never fails. by design.... heat kills caps.
You got the A/C capacitor info wrong. A 45/5 capacitor is for the motor and the fan. It doesn't start at 45 then drop to 5. The 45 is in line the entire time for the compressor and the 5 is for the fan the entire time. Some units actually split those out. Also, and it was not covered in this video, you can have a soft start unit that ADDS a capacitor for a very brief time to increase capacitance, which increases delay, which helps get the compressor motor turning. Then it drops out leaving you with the original 45uF run capacitor. So this teaches us that the 45/5 are dual capacitors running two different motors constantly.
Man!!!!! I can listen to you whole day. you answered all my questions in a single video. Thank you so much. And I also hit that subscribe button!!
Dude, that was really good. Congrats. You're one hell of a teacher.
I wonder, is that how transistors work at the microscopic level, they're a bit like capacitors?
I’m in school right now and the lack of translation to real world application is hard to follow sometimes. Love your videos for breaking it down to a guy that’s good on the tools but at times struggles with the theory behind it.
Love how you explained that. Made it easy to understand
Awesome upload the whole lead lag has been a bore for me but now that you've explained it like you have it's now got me interested. Cheers
Generally speaking, I found your explanation pretty informative. However, there are some things you should consider (possibly Semantics), but just to make sure it is clear: Current "appears" to flow, but doesn't actually flow "thru" the dielectric gap in a capacitor... I only bring this up because it was explained that way on the diagram in the video, so I don't want people to get the wrong idea. So, to be clear, Current doesn't flow thru that gap that you have in the diagram of the capacitor. Instead, current flows into 1 side of the cap (builds up ) and then returns out the same side... but NEVER "thru" the capacitor. AC can get away with "appearing" as current is flowing because of the way the charge moves into and out of "both" the sides of the plates, but in reality, electrons (and thus current) never cross the gaps in the plates (unless) the capacitor is bad or has leakage.
Re: your motor diagram (@11:12), I believe your understanding of it is correct, however, it is important to show the second coil (run / start etc) in order for the capacitor explanation to really make sense, because the way it works, the capacitor doesn't work with 1 winding... it takes at least 2. So, I think people will get confused with your 1 winding diagram. By showing the 2nd winding, you can better explain how the phase shift affects the 2nd winding at a different time due to the capacitance, which allows it to push / pull on the motor at a different point in the rotation.
This is a great video!! You do a great job of explaining inductance and capacitance. Thanks for creating such helpful content for us electrical theory geeks out there.
Just want to thank you for sharing your knowledge in an understandable way. I recently entered a career college for electrical technichian and the ciriculum is a circus show. I come home from school to watch your videos, we need instructors like you, thanks, God Bless.
Iam electric ingeneer for a long time but I never understood the subject as clear as you but it so I like to thank you very much and like to recommend this video to everyone wish to understand the subject thanks a lot ones. Mor. !!!!!
Great explanations! These are concepts I know but you added insight and really good hand motions to them. 👍😁
"CIVIL" is what I was taught to remember. CIV -- capacitor I leads V, VIL -- inductor V leads I.
Exactly the explanation I needed, from start to beginning. Cant thank you enough!
13:00 You gotta watch out for those delayed dumps. Gotta keep them regular. 😎
My man is on another level in his teaching method, I just subscribed job well done!
I wish I'd seen this when I took my engineering courses in electricity. Superb explanation!
thank you for making this video, this is one of the best explainations of Xc/Xl on youtube
Awesome video! I learned a ton. Thank you for putting these videos out sir!
Another function of capacitors in parallel with motors is to straighten out the power factor, so that the load on the supply is a resistive load. When the capacitive load balances the inductive load the reading on the kWhr meter is minimal. In simple language a ballast capacitor reduces electricity bills.
Finally an intuition about inductance, thank you!
Bro, you explained it so that I could understand. Oh my word, thank you!
Excellent explanation, its taken me years to learn this, not matter how often I've asked.
That makes so much sense. What a breathe of fresh air!! That's why the pg and e are making so much profit because theirs so much energy doing half the work thats provided. So we're consistently buying electricity for double the price. I get it now!!!! That's 🔥 knowledge !!! That we need people to wake up.
Thanks for filling a lot of gaps, Edwin, Western Australia
loving this channel - thanks for the info on this one!!!
Brilliant. Simply a brilliant explanation - as always!!!
That was a fantastic explanation.
This answered my question. Absolutely.
Oh goodness, apparently I’m in way over my head. 😅
I’ve been looking at an 8.5” diameter edge flow fan (“motorized impeller”?) that I want to build a fixture for to use as a sort of ceiling fan for a room in my house (pulls air from underneath, discharges air radially across ceiling), and I read something about needing a capacitor for it, so off I went to see if I could look up what kind of capacitor I need.
Next thing I know I’m learning about run capacitors vs start capacitors and how doing things wrong can make things go boom, and all I want is to hook this little 120VAC, 400mA, 2700rpm edge flow blower up to a light switch in my house (maybe a dimming switch) so I can have a sleek ceiling fixture that quietly moves air about 530cfm.
Great Job! Minor correction, It's 60 cycles, so voltage drops to zero 120 times in one second.
Great teaching and info. Thank you Dustin.
Not only did you answer my question. You gave me a direction, in how electricity can work and does work. In our fascinating lives!!! Yeee ha !!✊
I love you, Dustin. You are by far my favorite youtube source for learning electrical skills. I've learned a ton of incredibly useful stuff from your videos. But your explanation of how and why single phases motors use run and start capacitors is, let's say, lacking. So here's my "more correct" answer...? :) :) :)
Single-phase motors have the problem that they can run forward and reverse just as easily. three phase motors have a rotating field created by the three different phases, and will always rotate in the same direction when you start them. But a single-phase motor, with only one winding, doesn't have a rotating field,. It has a pulsing field. It can run in either direction just as easily. Every time you start them, which direction they start would just be random based on where the rotor last stopped and the phase of the power when you hit the ON switch.
The ONLY reason we have run and start capacitors on single-phase motors is to force them to always start up spinning in the same direction! It wouldn't be very useful if your fan motor started spinning backward half the time! And if the rotor stopped 90 deg to the windings, it might only start after giving it a little push.
So to make a single-phase motor spin the same way every time, a second winding is added to the motor (the start winding), and a capacitor is added in series with the second winding to shift its phase (as you explained in your video correctly). So now we have a two-phase motor, one phase for the main winding, and one for the start winding. But using a capacitor as a cheap way to create a second phase doesn't work very well. It only shifts the phase about 25 degrees in practice. In a three-phase motor, each phase is 120 degrees out of phase. Creating a second phase that is only 25 degrees out of phase is good enough to force the motor to start spinning in the correct rotation, but very poor for providing even power. In a "real" 2-phase motor, we would have the phases 90 degrees out of sync. But this "fake" second phase created with only a capacitor is good for getting it spinning in the right direction. This is why these motors all use a centrifugal switch that turns off the second "fake" phase windings once the motor is spinning correctly. The start winding and start capacitor is only there to make sure it starts correctly and is spinning in the right direction. In a motor with only a start capacitor, the start winding is very small compared to the main winding. It's just large enough, and so arranged, to help the motor start. It is not designed to keep running and will overheat and possibly burn out, if the motor fails to start (is locked up for some reason) or if the centrifugal switch is bad and fails to turn off the start winding. A bad switch that keeps the start capacitor engaged will cause the motor to run very hot (burn your hand if you touch it hot), and very rough.
Many (I dare say most) single-phase motors don't have a run capacitor. they only have a start capacitor which is only used for starting the motor. But run capacitors are common in HVAC motors, so as electricians, we see them all the time.
The purpose of the run capacitor is to make use of the start winding even after the motor is running. After all, they put all this extra copper into the motor, to make it start correctly, so why not try to make use of it to add a bit more power to the motor all the time? That is the goal of the run capacitor and the design of a single-phase motor that uses run capacitors. In this type of motor, the extra start winding is used all the time. But they use a large capacitance to force the start winding to have more power (and more phase shift) to help it start correctly but then lower the capacitance when running because the start winding can not sustain the high start current for long. But this allows the start winding to help add more power to the motor (which reduces the cost-to-power ratio of the motor) at the cost of greater complexity and maintenance costs (capacitors break faster than motor windings break and must be replaced). In these motors, both the start and run capacitors are connected (in parallel) when starting, but then the start capacitor is disconnected once the motor is spinning fast enough, leaving only the run capacitor to feed power to the start winding.
In industrial applications, we just run all the high-power motors off of 3 phases and get rid of all this complexity of start windings and capacitors. Three-phase motors are dead simple and last a lifetime if they are not abused. But for residential applications, like HVAC, where we don't have 3-phase, power, we must play these games with capacitors to force motors to spin the right way every time they start. And we must pay the HVAC techs to come out and replace our burned-out capacitors every few decades.
Also, your talk about power factor correction is all well and good, but start, and run capacitors are not for power factor correction. When large capacitors are used for power factor correction, they are wired in parallel with the inductive loads like motors or light ballasts, whereas start and run capacitors are wired in series with the motor start winding. The inductive load of the main winding is not connected to the run capacitor, so it does not help the power factor problem on these single-phase motors (or only helps a very small amount as a side effect). Power factor correction is normally only done in large industrial 3-phase settings where the location has lots of big motors and lots of inductive lighting with very little resistive loads. I've never heard of it being done for residential electrical (so that means it probably is done somewhere). But the start and run capacitors are not there for the purpose of improving the power factor of the motor; their main purpose is just to allow a single-phase motor to start correctly.
Good explanation, you keep make me love & feel proud of being technician!! Thanks
Excellent explanation sir.
Paper mills are full of inductive loads. They use synchronous AC motors on some refiner’s to do power factor correction.
Best video I have seen on this subject. Thanks
I explained to a coworker why a capacitor is used. I drew a sine wave like you did to show the phase shift. I explained it a little different tho since he hasn’t learned the theory part yet. I simply told him that we apply an external voltage to the motor to create EMF and turn the motor. When the motor is turning it creates a back EMF and the capacitor is there to overcome the back EMF that the motor itself is producing and thus putting it back in phase. And that’s why the voltage is higher at the cap then your incoming power.
Given this explanation, I wonder if this was part of what drove the 2004 blackout (untimely losses of generation in combination with heavy overall A/C demand throwing grid phasing out of whack)?
I recall this phenomenon being mentioned in an explanation (Practical Engineering, I believe) so I’m curious if it was an unfortunate situation or a regular occurrence during hot summer months.
There is a formula to figure out exactly how much inductance or capacitance needs to be added to cancel out reactive power and make more efficient. As an Installer of course I dont do those calculations, but know of it and love learning.
Great video for hvac and refrigeration techs
All good info, as I'm doing more with AC induction motors these days, including having to replace capacitors. I knew what the numbers meant, but not what the numbers do. And now I won't be immediately tempted to get a higher uF capacitor when replacing. No longer thinking it's 'better' and sounds like it may make the motor inefficient, therefore making extra heat probably, as wouldn't it risk being out of phase then?
@10:10
Solar panels are often used to add some capacitance to these customers to offset that power factor too. Here in Ontario, the majority of the Canadian Tire (our Hazard Freight) and Home Depots have a solar installation.
Nicely explained, manny thanks 🙏🏻
Mr. Electrician, , ive been repairing and rewinding ac motors for several decades now. For me , in my technical opinion, the main purpose of the capacitor in an ac single phase motor is to connect parallel the starting winding and the running or main winding during start-up, at this moment the capacitor is in charging state. This will help the motor armature to rotate as it require additional current as the main winding current capacity is not enough to move the armature during initial start-up, other than initial start up the starting winding also guides the armature in its correct direction (CCW or CW). . Once the armature is moving, the capacitor will start to discharge and will disconnect the starting winding leaving the main winding running the armature. The centrifugal switch will act as an redundant switch in case the capacitor discharge quickly to the point it never reaches the ideal RPM to disconnect the starting winding. Again this is only applicable in single phase motors. Try to read Robert Rosenberg book on: 'Electrical Motor Repair' good reference for electricians.
Wonderful explanation.. thank you man
Such a good video, thank you!
Great video, took me right back to advanced theory 👍
Super awesome explanation!
This makes sense. We used to use hilarious sized capacitors on 90’s car audio
You should discuss power correction factor, voltage correction factor, line conditioners, ferrite current filter resistance, buck, and boost methods.
PSC Motors (Permanent Split Capacitor) requires less starting current thus require a smaller inverter for something like a back-up sump pump. They are also more efficient for a furnace fan (if load is properly matched). It will act like a two phase motor since the auxiliary winding is phase shifted 90 degrees.
Nice! Thanks for the explanation, appreciate that 😁👍
Here is what is wild to think about...
If you take an applied voltage and current and run them in a straight conductor from line to ground... You have created a "short" circuit, a fault condition.
But if you take that same conductor and bend enough loops into, it there will be enough "pushback" from the magnetic field created to restrict the flow of voltage/current... The magnetic field created by the electricity can be sufficient to "slow the flow" of that same electricity.
That "BONG!!!" noise you hear when you energize a transformer is massive amounts of current rushing through the windings because there is no magnetic field present for a few microseconds... As soon as the field builds it begins restricting the flow of current and the transformer starts doing its thing.
Thanks for the work great video
Nice explanation , keep up the good work .
Great explanation of theory.
Amazing video ,very
important information
Good video ! easy and tecnical
Great information.
Hi. I can explain it much more easy to understand.
For quick voltage changes (High frequency) The capacitors and inductors behave opposite way. Capacitors behave as a short, Inductors like a resistor. (When you calculate reactances sub high frequency)
If the capacitor `short` current comes first than the voltage. If the inductor like a resistance. Voltage comes first and the current will come later. (These reactancy (resistance) values are keep changing)
To get the maximum power at any point of the signal. P=UxI the voltage and the current has to be in shync.
So if you have the motor (inductive) and the current lag. You add a capacitor to lag the voltage to catch up.
So your motor gets themaximum amount power. At start of the motor you need the maximum amount power!
On large motors these phase shifts can be very significant. Result in large power loss.
pretty clear, thank you
What will happen if we use an inductor in place of a capacitor in single phase induction motor? How could we get an inductor to work as a starter. I would love to see a video answering my question--I believe that this may aid my understanding of both capacitors and inductors.
Pretty good job! BTW, a great example of a resistive load (where i is in perfect synchronism with v) is a plain ol resistive heating element…