Why Are Capacitors on Motors? What is Capacitive Reactance and Inductive Reactance?

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.

[KXD]

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.

[TheMaster]

As a HVAC Tech, your content always help me to better understand what I'm working with.

[bernardocisneros]

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!!!

[Mike_Rundle]

Not sure about others but I feel like this is one of my favorite videos you’ve done as far as simplifying an explanation.

[chadg]

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.

[johnprohaska]

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.

[penderway]

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.

[VEAVA]

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!

[johnnorton]

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

[numberpirate]

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.

[Steve-kgzk]

I find all your videos awesomely helpful. Thanks bro. New apprentice here trying to get ahead of the game.

[rogerpenske]

You explained a very complex issue simply and clearly.

[PacRimElectric]

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

[rodin]

I know it'a a year old, but love this video. Explains this motor story really well. Thank you!

[SteveWrightNZ]

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,

[easyenetwork]

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.

[erwinbordallo]

One of the best explanations ever for reactance and its relationship to motors! Thanks!

[terrywilliams]

I really appreciate the CLARITY of this tutorial...THANKS

[PetrosArgy]

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.

[CjMooseChuckle_]

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.