The Asynchronous Motor: A Hobbyist's Nightmare and Delight

As we've explored in previous projects, stepper motors, DC motors, and BLDC motors are some of the most popular types of motors used in hobbyist electronics. However, there is another type of motor that deserves attention - the asynchronous motor. This motor type has been a source of frustration for many hobbyists, but it also offers unique benefits and characteristics that make it an attractive option.

The Basics of Asynchronous Motors

Asynchronous motors are called so because they do not have a fixed rotational speed, unlike synchronous motors which rotate at the same frequency as the applied voltage. Instead, asynchronous motors have a slipping speed, which is the difference between the speed of the stator and rotor. This slipping speed is what allows the motor to start rotating.

The Stator and Rotor: A Simple Explanation

To understand how an asynchronous motor works, let's consider the stator and rotor as two separate entities. The stator is the stationary part of the motor that carries the coils, while the rotor is the moving part that consists of a set of metal sticks or blades.

As we apply a 3-phase AC voltage to the stator, it creates three sine waves that are positioned 120 degrees out of phase with each other. These sine waves induce currents in the metal sticks of the rotor, which then create a magnetic field. The rotation of this magnetic field is called the rotating magnetic field.

The Rotating Magnetic Field: How It Works

Now, let's imagine the rotating magnetic field as a sine wave that rotates at 50 Hz frequency. As it rotates, it induces a voltage in each phase of the stator, which creates a current flowing through the metal sticks of the rotor. This current creates a magnetic field that opposes the original rotating field of the stator.

The opposing forces created by this interaction cause the rotor to rotate away from the stator, and thus, the motor rotates. This process is called electromagnetic induction, and it's the fundamental principle behind asynchronous motors.

Slip: The Difference Between Stator and Rotor Speed

As we've discussed, asynchronous motors have a slipping speed, which is the difference between the speed of the stator and rotor. This slip can be adjusted by changing the number of poles in the motor or using an electronic frequency converter.

The Importance of Slip

Slip is an important characteristic of asynchronous motors because it affects their performance and efficiency. A higher slip means a lower efficiency, while a lower slip means better efficiency. Asynchronous motors with more poles have a higher slip than those with fewer poles.

Comparing Asynchronous Motors to BLDC Motors

Asynchronous motors are often compared to BLDC (Brushless DC) motors, which are synchronous motor types. While BLDC motors rotate at the same frequency as the applied voltage, asynchronous motors rotate at a different frequency due to their slipping speed.

To illustrate this, let's calculate the rotor speed of a BLDC motor with 4 poles and an operating frequency of 50 Hz. The result is a rotor speed of approximately 3000 RPM. In contrast, an asynchronous motor with 2 poles has a similar rotor speed, but its slip is different due to the higher number of poles.

Wiring Up Your Asynchronous Motor

So, how do you wire up your asynchronous motor? The answer is simple - hook it up to the power grid using a star configuration. This means connecting the L and N phases directly to the stator, while using a capacitor to create a third phase with a 90-degree phase shift.

The Capacitor: A Key Component

In this setup, the capacitor creates a resonant circuit that amplifies the voltage across each phase of the stator, making it more efficient for smaller asynchronous motors. This method works well because it reduces the current drawn from the grid and makes the motor more robust.

Conclusion

Asynchronous motors are an often-overlooked type of motor that offers unique benefits and characteristics. While they can be more challenging to work with than other types of motors, their simplicity, robustness, and low cost make them an attractive option for many hobbyists. By understanding how asynchronous motors work and how to wire them up, you can unlock the secrets of this fascinating motor type.

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WEBVTTKind: captionsLanguage: enSo maybe you already used a stepper motor,a DC Motor or a BLDC Motor in one of yourprojects and now you are thinking that youare the master of motors.Well, let me introduce you to this monstrositywhich is actually just a monstrosity if youcompare it to the size of the previously shownmotors.This is a so called asynchronous motor orinduction motor and this one has been sittingin my shelf for eternities and I do not evenknow whether it still works.So for this video I thought it would be appropriateto show you how to get such a motor to workand what advantages it offers in comparisonto other hobbyist motors because let’s faceit, the asynchronous motor type is nowadaysstill the most widely used motor type in theworld.Let’s get started!This video is sponsored by JLCPCB!Feel free to visit their website to not onlyfind out what awesome PCB and Assembly servicesthey offer but also to easily upload yourGerber files to order affordable and highquality PCBs easily.First off, in order to understand how thismotor works, I opened up its terminal boxand was promptly greeted by 6 connector wires.Through the help of my multimeter I foundout that the wires U1U2, V1V2 and W1W2 allform a separate coil.Usually such a box also comes with such bridgeconnector pieces which either ties a sideof each coil to another one to form a deltaconfiguration or we use them to tie one sideof all the coils to one point and thus createa star configuration but more about thosetypes later.For now I removed all the important screwsof the motor to take it apart and understandits construction better.I was able to remove the back piece withouta problem which revealed a fan that is directlyconnected to the rotor and basically coolsthe motor during operation.But while trying to open up the front part,I ran into a problem where the gear systemdidn’t want to let go of the rotor.So even though we can see a bit of the insides,I was not happy with this result but I reallydidn’t want to destroy the motor becausemy mission was to see whether it still works.That means after I reassembled this old asynchronousmotor, I simply ordered myself a new one asa test subject.So once again I removed all the screws andthankfully was quickly capable of removingthe rotor from the system.Now the rotor is obviously the part of themotor that rotates and in this case it ismade up of a squirrel cage.It basically consists of conductive metalsticks which are shorted to one another ateach end.And to understand the purpose of this rotordesign choice we should next have a properlook at the stator of the motor which is obviouslythe part of the motor that stays in place.In our case all that we can see however isthat there are a lot of copper wires goingin and out of it and yes those are the coilswhose end wires we can find in the terminalbox.So just by looking we cannot truly understandhow this motor works so why not simply connectit to a voltage to get it going but let metell you that a DC voltage is in this casenot sufficient.On the type plate of the new motor it is notso easy to spot but on the type plate of theold motor we can see that both motors require3 Phase AC Voltage and apparently with valuesof 230V 400V which I luckily got lying arounddue to German house wiring standards.And that is the point where I have to tellyou that working with 230V or 400V can belethal and should only be done by a professionalwhich is probably why such asynchronous motorsare not often seen in hobbyist projects.But luckily I am a professional and thus easilywired up a 400V CEE cable whose L1, L2 andL3 phase feature a voltage of 400V to oneanother and 230V in relation to the NeutralWire.But what does this 230V/400V mean?Should I use 230V or 400V and with what motorcoil configuration?Well, the first number basically says whatmaximum voltage the coils can handle and thusif we would have 230V from between phaseswe could directly apply them to each coilthrough the delta configuration.But since we got 400V between the phases wehave to follow the second 400V rating, whichsays that you should use a star configurationat 400V levels since that will reduce thevoltages across each coil to 230V.Luckily the star configuration was still inplace and thus I hooked up PE, L1, L2 andL3.But before powering anything, I reassembledthe motor and only then supplied it with voltageto find out that it spins just fine.As an experiment we can measure the currentthrough one phase to see that it draws around450mA.But if we would have wired up the motor incorrectlyin the delta configuration then the currentwill multiply by almost 10 and the magic smokewill appear fairly quickly.But anyway the rotor of our motor rotatesperfectly fine but how does it do that ifwe consider that neither the stator or rotorconsist of permanent magnets but all the otherhobbyist motor types I talked about in previousbasics videos always used some kind of magnetsin order to create torque.Well, for that let’s imagine the statorlooks like this and we got one coil pair foreach phase positioned inside the stator likethis.If we look at the voltages across our coilpairs we can see that we are dealing withthree sine waves which are positioned 120degrees out of phase to one another.That means since there are sine voltages appliedto our coils, sine currents are also flowing.Of course when there is a current there isalso a magnetic field which according to thedifferent amplitudes of the current througheach phase creates a changing magnetic fieldwhich if we look at it for quite some timespins around and thus is called a rotatingmagnetic field.Now this rotating magnetic field induces avoltage into the metal sticks of the rotorand since they are all shorted there is acurrent flowing which once again creates amagnetic field that this time opposes theoriginal rotating field of the stator andthus creates a force which basically pushesthe rotor away and lets it spin.And now that we know the functional basicswe should be able to assume that because weare working with a 50Hz voltage, our rotorshould follow this stator frequency with anRPM of 3000.So I used my tachometer in order to measurean RPM of just fewer than 3000RPM but whyis it a bit lower?Well, let’s imagine the rotor and statorwould spin with the exact same frequency.The problem is that in this case accordingto the induction law, no voltage would beinduced into the rotor and thus no currentwould flow and no opposing forces would becreated which means no rotating.So it is pretty logical that the rotor mustturn a bit slower than the stator frequencywhich is also why this motor type is calledasynchronous.This difference in RPM is actually calledslip and it is an important characteristicof such motor types.As a comparison a common BLDC Motor is actuallya synchronous motor type because if we lookat the frequency of the applied 3 phase ACvoltage then we can do some calculations andfind out that the rotor is spinning with thisexact RPM.The reason for that is like I already mentionedthe permanent magnets of this motor type.Now to alter the RPM of our asynchronous motorwe can either get a pretty expensive electronicfrequency converter or we get an asynchronousmotor with more poles like this one with 4instead of just 2.That means we got 2 poles pair and thus therotation of the magnetic fields should taketwice as long and this equation also confirmsthat the RPM should get cut in half whichas you can see it did.Now at this point you might wonder why asynchronousmotor appliances like this water pump onlyuses one phase and the neutral wire insteadof 3 phase AC voltage.The solution can be found when we open upits terminal box which looks a bit confusingat first but the main difference of this boxis the utilized capacitor.The reason is that this motor still needsto create a rotating magnetic field whichit does so by utilizing the L and N phaseand additionally a capacitor which createsa third phase with a phase shift of 90 degrees.This way the rotating field is more like anellipse then a circle but this method worksjust fine for smaller asynchronous motors.And with that being said we are now familiarwith all the basics and thus I was confidentin hooking up my old motor through a starconfiguration to the grid and as you can seeit works perfectly fine, awesome.The reason why this motor type is so popularis because it is simple to make, robust andrelatively cheap.And most importantly it is easy to use, youjust have to hook it up to the power gridand you are done.You don’t need DC Voltage, or a stepperdriver IC or an ESC in order to drive it whichcan all be potential causes of defects.And if you want to learn even more about asynchronousmotors then I would highly recommend checkingout the links in the video description becausethere is a lot more to learn.As always thanks for watching, don’t forgetto like, share, subscribe and hit the notificationbell.Stay creative and I will see you next time.