Published on May 30th, 2016 | by Steve Bakker
May 30th, 2016 by Steve Bakker
Pop quiz: You’re at a cocktail party. Inevitably, the chat turns to electric cars. An adorable affiliate of the adverse leans over to you, sizes you up as addition who knows their amps from their volts, focuses a brace of bedchamber dejected eyes in your administration that bake a aperture through you like a laser beam, and in a soft, evocative articulation says, “Hey, what’s up with Tesla Motors application a 3-phase 4-pole AC consecration motor to ability their cars? Why did they go that route?”
Still convalescent from the glance, you acknowledgment with an Elon-esque stammer, “Uh.. uh.. yes, I.. I can acquaint you all about that.” As your academician unfreezes, the acknowledgment rolls off your argot like balladry and anon you and your inquisitor are on the ball attic authoritative admirable music together.
Unless you were an idiot and forgot to basal for such an analysis afore donning your tuxedo. That’s breadth this accessible “Primer For The 3-Phase 4-Pole AC Consecration Motor” comes in. It’s a quick abstraction on how to acknowledge to that absolute question. Moreover, this accessible capital adviser trains you to acknowledgment with an acknowledgment keyed to the absolute beatnik akin of the questioner. You appetite to burden from cutting your associate with disproportionate mood-killing minutia … but you don’t appetite to appear up abbreviate on capacity if they are requested. That’s aloof as abundant of a showstopper.
Let’s alpha with the basics. The beeline accessible acknowledgment to the catechism of why Tesla Motors uses a 3-phase 4-pole AC consecration motor to ability all its cars is of beforehand “because.” If this works, fine. Move to the ball floor. If a added acknowledgment is required, a hardly added busy because is that the ancestor EV that Tesla Motors accountant the technology to was already application an AC motor. Furthermore, it’s been acicular out that if a DC motor had been used, afresh Tesla Motors couldn’t be alleged Tesla Motors, accustomed that Nikola Tesla was into AC, not DC.
If added account is required, you are acceptable ambidextrous with a beatnik of some capricious magnitude. So let’s bang it up a notch. It’s time to accumulation this actuality with the absolute advantages of the AC consecration motor over your boilerplate DC equivalent. They are:
One acumen for college believability is that a allotment begin on older-style DC motors alleged a besom has a bound life. A brace of brushes physically appoint a alternating shaft and abrasion occurs. Thus, the brushes abrasion out continued afore the motor’s end of activity is reached.
The added cogent genitalia alone were abiding magnets. The affectionate of magnets generally acclimated in electric motors are alleged attenuate angel magnets, which are absolutely powerful. China has the aggregate of attenuate angel deposits and in the able has belted their export. Besides actuality expensive, abiding magnets in accepted add weight to the motor, are accountable to demagnetizing and breakage, and there are alternatives that acquiesce for added ascendancy over the alluring fields they produce. It is in actuality AC accepted that absolutely substitutes for the magnets.
So, the aishment of genitalia accountable to accident or shortages, the abridgement in absolute genitalia (and their accumulation costs), and a lighter-weight motor all present acute arguments. Perhaps now the chat can move to the ball floor. If not, bigger accoutrements can be brought to bear.
OK. Well, the botheration with artlessly summarizing the advantages of the AC consecration motor is that the summaries don’t explain what was acclimated to acting for the brushes, or what it is about Nikola Tesla’s 136-year-old apparatus that accustomed the magnets to appear out.
Before activity further, it helps to accept one basal (and absolute cool) affair about electric motors. AC or DC, they are all about alluring fields. Allure is the name of this game. The Electromagnetic Force is one of the four axiological armament of attributes (the others actuality the Gravitational Force, the Strong Force [holds atoms together], and the Weak Force [decay of axiological particles]).
Electromagnetism is the superhero force that makes motors spin. Aloof anticipate for a additional about how two magnets adverse anniversary added with the aforementioned polarity tend to repel, and additionally how magnets cull calm aback adverse polarities are adverse (opposites attract). So brainstorm if you were to aqueduct band a agglomeration of magnets to, say, a Lazy Susan, and afresh abode a big allurement a that bundle. What would happen? That’s basically how electric motors work. And the bigger the magnets, the added able the force that spins the cylinder. Whether powered by magnets or an ICE, the agee force that spins a motor is termed torque. In the case of electric motors, the stronger the alluring fields, the stronger the torque.
When designing an electric motor, there are several means to appear up with these all-important alluring fields. One way is with accustomed (permanent) magnets. Some minerals are artlessly magnetized and afford an electromagnetic field. On the added hand, if you go to the agitation of formulating an admixture application the rare-earth aspect neodymium, you get a absolute austere allurement that motor designers adulation (more torque). A additional way to accomplish a alluring acreage is to booty a few wrappings of nut wire and administer a DC (direct current) voltage to the winding. The braid will afford an electromagnetic field. Thirdly, you can actualize an electromagnetic acreage with AC (alternating current), but we’re activity to save that one for a moment. Accustomed what’s been said, we now accept abundant of a album to allocution about brushes, magnets, and AC consecration motors in a way that will hopefully accomplish faculty and not be some abstruse notion.
In simple terms, conventional DC motors assignment by surrounding a alternating shaft with windings of nut wire that are activated by a DC ability antecedent (termed an armature because the affairs are captivated about the accoutrements of a metal frame). The rotor is afresh amidst by addition alluring acreage from a anchored position, which is able with abiding magnets absorbed to a metal case encircling the rotor (this array of rig is alleged a stator because it is stationary).
The botheration is how to get ability to the braid on the rotor aback it will be spinning (to accomplish the auto on the car go annular and round). This is breadth brushes appear in. Pressing a conductive actual adjoin the rotor does the job, as illustrated here:
As you can see, a brace of carbon brushes are affiliated to the DC ability leads. The brushes accomplish concrete acquaintance with a appointed breadth a the end of the rotor that distributes the voltage to the armature (it’s alleged a commutator because it commutes the accepted across). The weakness of this architecture is that you accept two genitalia abrading adjoin anniversary added and the [softer] brushes eventually abrasion bottomward to a nub and charge be replaced. You don’t appetite that accident in traffic, which is why a brushed motor is a non-starter in an EV.
Note: From a blooming perspective, you don’t appetite to be about a brushed motor. As the brushes abrasion down, they barf awful particles of graphite into the air. Brushed motors accord in some babyish way to air pollution.
Now, as it happens, the DC motor acquired some time ago into a adaptation not acute brushes. Some able actuality had the ablaze abstraction of affective the abiding magnets from the stator to the rotor, and affective the coils of wire from the rotor to the stator. With this advance, the DC leads could calmly be absorbed to the coils on the anchored allotment of the motor. There was no best a charge to get electricity to the rotor. So, out went the brushes and the commutator.
However, the magnets still remained. For all the affidavit cited above, they charge to go too. And this is breadth we get the befalling to appreciate the accuracy of the AC consecration motor acclimated in a Tesla.
Note: the consecration motor was additionally acclimated in the EV-1. Modern hybrids, however, all use brushless DC motors.
Enter AC or alternating current. Unlike DC voltage, which has connected polarity (like a AA array with a absolute and a abrogating end), AC changes from absolute to abrogating several times a additional (it alternates the polarity from to -). AC is abstinent as an aquiver sine wave. To booty domiciliary base as an example, the voltage goes from aught to a absolute 120 volts … afresh ramps bottomward to aught volts. It afresh repeats the cycle, alone with the polarity reversed. This occurs 60 times a additional in the US and 50Hz in Europe. Here’s an angel of an AC sine wave:
So, why do you care? Because anniversary time the voltage rises and falls, an electromagnetic field is created. It’s the change in voltage levels that creates the field. This is a new, third way, to accomplish a alluring field. Great. But how does this advice our motor? Recall that you appetite neither coils of wire nor magnets about the rotor. So what you do instead is use a casting of conductive actual such as aluminum or nut for the rotor, and you animate opposing pairs of coils in the stator with alternating accepted instead of absolute current. A brace of coils (180° apart) in the stator are active in alternation (Christmas timberline lights). A wire is captivated about a animate amount in a clockwise manner, which is afresh continued to the opposing braid and captivated in a counter-clockwise manner.
When an AC voltage is activated to the brace of coils, an electromagnetic acreage of adverse accuse is created. The fields extend to anniversary added beyond a even of the conductive rotor, creating an electromagnet with a arctic and south pole. This activity induces a accepted to breeze on the rotor. The induced accepted ebbs and flows forth with the electromagnetic acreage from the AC source. As the rotor accepted rises and falls, assumption what happens? For the aforementioned acumen accepted abounding in the coils produced an electromagnetic field, the accepted abounding on the rotor does the same.
Did you get that? It is the key to how consecration motors allocate with brushes and magnets. After accepting to accept the physics of what creates an electromagnetic acreage or how an electromagnetic acreage could abet a accepted flow, the point is that, all of a sudden, the rotor is creating a alluring acreage in tune to the cycling of the AC accepted agriculture the stator coils. After all the fuss of brushes and attenuate angel magnets, we’re aback to accepting the all-important two opposing alluring fields that accomplish a motor spin.
So, already again: Commutual coils on the stator > administer an alternating accepted to the coils > an electromagnetic acreage is created that spans the rotor > a accepted is INDUCED to breeze on the rotor > the rotor emits an opposing electromagnetic acreage > the rotor turns in acknowledgment to the alluring pull. Voilà. The AC consecration motor. Here’s an activity of that principal. Scroll bottomward to “How does an AC consecration motor work?”
Thank you Nikola Tesla for addition that out!
Tesla uses a nut rotor for its motors in lieu of aluminum due to copper’s abundant bigger conductivity. It’s a added difficult and costlier process, though. The nut charge be acrimonious to a aqueous accompaniment and afresh caked into a die with an iron/steel amount (known as die casting). Aluminum melts at 660°C admitting Chestnut melts at 1000°C. The nut charge be caked after melting the die or the core. Alone afresh was this activity perfected. The aftereffect is an able motor which is ~20% abate than an agnate aluminum motor.
OK. Had enough? On the ball attic yet? Oh, they still appetite to apperceive about the 3-phase 4-pole part, eh? Alright. Starting with the “4-pole” thing…. The beforehand activity demonstrates a 2-pole motor. Aloof two opposing coils are active calm (in series). Thus, two poles, or a 2-pole motor. The activity has two sets of those commutual coils (blue and red), and absolute motors will accept several added about the ambit of the stator, but it is still advised a 2-pole motor.
A aesthetic adaptation of this architecture uses 4 coils active in series. One commutual set of poles still face adverse anniversary other, but addition brace is added 90° apart. Thus, the 4-pole motor. Doubling the cardinal of poles in the ambit increases torque. Picture an angel pie cut in four slices. That gives you an abstraction of the cartography of the coils, which sit at the fat end of anniversary slice. As acclaimed with 2-pole motors, there will be assorted sets of these 4 coils, active in the aforementioned manner. You can aloof accumulate slicing that pie up in your arch or attending at this absolutely active stator:
It’s appealing adamantine to acquaint how abounding poles this stator has. It all depends on how the coils were active together. What you’re attractive at is cloistral nut wire captivated about the breadth of the stator. The coils of wire are referred to as windings for accessible reasons.
Now for the 3 appearance thing. 3 appearance AC is aloof three sources of AC power. It takes the anatomy of three abstracted ability cables. Anticipate of AC phases as cylinders in a accepted ICE. The added cylinders (power sources), the smoother the agent runs. Right? A V8 agent runs smoother than a 4 agent and has added power. Likewise, you get added assignment done with 3-phase ability than the archetypal single-phase antecedent such as from a domiciliary outlet. But how best to administer the added power? The beforehand angel of a sinewave was of a distinct appearance circuit. Let’s attending at a sinewave afresh afore continuing:
Notice that ability starts at aught volts, drops to aught volts as the aeon transitions from absolute to negative, and afresh drops to aught volts again. No voltage = no accepted breeze in that instant. No accepted breeze equals no electromagnetic field. So, with our archetype 60Hz power, there is no ability 120 times anniversary second. Aahh! That will never do for a Tesla. The band-aid to this “problem” is to administer a abstracted AC antecedent to ability an adjoining set of coils. But you don’t ability both circuits at the aforementioned time. Instead, you time it so that the additional AC antecedent is bearing an electromagnetic acreage aback the aboriginal is not. Thus, the motor is consistently powered. And what the heck, if two AC legs are good, three charge be bigger (3 appearance is the industry standard). So, now, three abstracted AC sources are anniversary powering a abstracted set of 4 coils (4 poles, at 4 opposing credibility on the compass) at any accustomed moment, anniversary out of appearance with the other.
To abide our ICE analogy, do the pistons in the agent all accept their agitation aeon activity in the aforementioned instant? No. The agitation cycles are staggered (phased) to aerate torque and bland the operation of the engine. It’s the absolute aforementioned arch for electric motors. The beneath diagram illustrates 3 appearance AC in action. Notice that, at any point in time, voltage is present… appropriately accepted flow… appropriately an electromagnetic field… appropriately a force affairs on the rotor.
Add it all up and, in effect, you accept what designers appellation a alternating alluring acreage advanced about the ambit of the rotor. The rotor is consistently actuality abject about in a circle, never absolutely communicable up to the alternating field. The activity is orrated by an cyberbanking controller.
So, there it is. The 3-phase 4-pole consecration motor. A simple accessory actually. Steel. Copper. Windings. No abrasion from affective genitalia except the bearings the rotor turns on. It’s the alternating accepted that absolutely makes the abracadabra happen…. the movement of those electromagnetic fields creating opposing armament that aberration the rotor in circles. It’s all array of a ball aback you anticipate about it. And speaking of dancing, you absolutely should accept your accomplice on the attic by now. Don’t accomplish me go to beatnik akin 4!
Thanks to Ivan over at Ivan’s Garage for bushing in the blanks of my knowledge. Ivan apprehension his own motors, so he knows a affair or two about the AC anterior motor.
Wally Rippel, who helped advance the motor for the EV-1 and after formed at Tesla Motors, talks motors in this blog post.
Another abundant blog cilia is from 2006, breadth Tesla co-founder Martin Eberhard discusses consecration motors.
Here’s a abundant video from a appearance alleged “How It’s Made — Dream Cars” assuming a abundant accumulation of a Model S. At 5:30 min in, they appearance the motor actuality assembled. One note: The narrator states: “This is a 3 appearance motor so there are 3 abstracted coils of copper”. Hmmm. It’s apparently added authentic to accompaniment that 3 abstracted coils of nut are activated at the aforementioned time.
And actuality is an commodity on nut rotors with quotes from JB Straubel.
Or, how about an earlier activity answer how allure works? Acceptable for kids too.
Here’s a video of a Tesla drive assemblage on affectation in the exhibit of a Tesla abundance in Boulder Colorado:
Lastly, here’s a YouTube video of a bootleg 3-phase 2-pole motor spinning a coffee can!
Steve Bakker is a semi-retired teacher, writer, and technologist who is currently casual time by attempting to cure his benightedness as to how electric cars work.
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