Servo secrets

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Basically, there are three types of motors inside a model servo. These are iron (Fe) core motors, coreless motors, and brushless motors. In a conventional 3-pole Fe-core motor, the three field windings are wrapped around a small chunk of iron. This assembly is what rotates and it's called an armature. Sequentially turning the 3 fields on-and-off is what makes the motor rotate. Clear?

Next, since turning fields on-and-off requires a switch (just like switching a light bulb on-and-off), this process of switching in motors is called commutation. How is it done? Simple, really because if you look closely at one end of a motor there's a copper ring. And if you look really closely you'll see the ring is actually split into three parts (one for each winding). Using mechanical brushes to actually make the contact with the commutator ring segments, the motor rotate as each field is turned on and off sequentially. Thus, as the motor rotates the brushes make then break contact thereby energizing the next field to turn on the electromagnetic force. The brushes making and breaking contact with each of the three segments of the copper commutation ring is why the motor rotates. Clear?

In an effort to make an electric motor reach operating RPM more quickly (both motors may hit the exact same RPM, so we're just talking about getting the motor up to operating speed more quickly), a savvy engineer figured out how to form the copper windings without the iron core (hence the term coreless). Presto, by reducing the rotating mass of the armature (since it doesn't have a small chunk of iron inside), the coreless motor's armature is lighter and this means the coreless motor reaches its operating RPM more quickly than an iron core motor. Otherwise, you can have the same mechanical characteristics, e.g. same bearings, KV, same torque, same everything between a coreless and Fe core motor. This is why the speed specs of a coreless servo are better (faster) than those for a servo with a conventional motor. If you need the speed to achieve a certain maneuver, then the equation is simple, you pay more for servos. Where the confusion comes in is with regards to brushless-motor servos. Let's try to clear this up.

First, it's important to understand that brushless motors are coreless also. The only difference is a brushless-motor is simpler because it uses semi-conductor (electronic) switches to achieve commutation while the coreless-motor servo uses the same old-fashioned brushes and commutator ring to switch the fields. The advantage of the brushless motor vs. the coreless motor is there are no bruishes to wear Do you know what's meant by commutation? It's simple, a commutator is just a switch. And if you disassemble an electric motor and see this copper ring with three grooves cutting across it, it just means you have a 3-pole motor with three separate windings. Thus, and the motor rotates and the brushes jump the gap in the commutator ring, the contact is broken form one field and made to the next field as the brushes make contact with the next segment of the commutator ring. This process of switching current from one field to the nest is termed commutation.

the windings are switching from one to the other. Understand? one In a conventional DC-motor (either core or coreless), a commutator switches the current from winding to winding via brushes thus forcing the rotor to turn. This is how DC motors work, OK?

Thing is, as the brushes make and break contact with the commutator surface, there's a tiny electric spark, or arc (yes, like a miniature arc welder). As the tips of the brushes spark molecules of the metal are evaporated away - poof - as in gone! Of course, these bits of metal being worn away don't disappear because that would violate the laws of physics (conservation of matter). Instead these particles become a very fine metallic dust. And this metallic dust begins to coat the inside of the motor, which insulates, e.g. leads to heat retention, which is bad. Worse, the dust gets into the bearings too and while all these things don't get bad all at once, they do begin from day one - fact!

Add to this, over time the brushes wear. Eventually they become too worn to make good contact with the commutator surface. Also, at the same time, the tips of the brushes wear a groove in the surface of the commutator. This is when a coreless servo begins to develop the jitters, or dead spots on the commutator, plus the other issues because these all lead to less than optimal performance. Electronic commutation eliminates all this bads stuff; here's how.

Basically, a brushless motor replaces the mechanical components (brushes and commutator) with electronics - semiconductor switches. Thus, brushless motors feature electronic commutation and by ditching both the brushes and commutator we bring your servo-motor technology up to date from the 1920s because the stator windings are turned on-and-off at the appropriate time by magic. OK, it's not magic, but via electronics . . . and electrons don't wear out! Neither do they coat the inside of the motor, or otherwise lead to all the crap building up on the inside of the motor and inside the tiny bearings. Oh, and by the way, it's not that coreless motors are bad (we offer a coreless servo for those on a budget and JR gets $200 for 8711HV coreless servos) it's just that brushless motors are better . . . in our opinion, a lot better, which is why this is our top of the range offering.

Is there a downside to a brushless-motor servo? Well, sure there is . . . brushless-motor servos costs a little bit more. Is it worth the difference? That's up to you but we're not talking about a heck of a lot more so if you can afford that then by all means you definitely want brushless-motors instead of coreless-motors inside the servos controlling your pride and joy.

Now you're in the know. Next time you hear someone confused about the difference . . . go ahead and clue them in.

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