- about RC servo motors
Author's note; reading this white paper takes about 30 minutes. And as such, I’m leaving out a lot because any discussion of motor design and manufacturing technology that runs to less than 300 pages of 6-point type automatically means we're leaving out a lot. In fact, entire careers can be dedicated to just this one subject!
If you haven't got 30-minutes, 1-minute of eyeballing the 1st three photos will get you to 90% understanding. Spend 5 minutes on all 33 photos gets you to 95% of the content from just the show-and-tell.
Another 1% comes from reading Part 1 while reading through Part II gets you to 97%. Go through Part III gets you to 98% understanding. Of course, as every experienced modeler knows . . . that last 2% means you only have 90% to go!
This information is dense because it's a difficult subject. Might couldn't adequately cover it in a semester-long college course! What this 30-minute read 'will' give you is a solid technical understanding of the factors involved in making good decisions regarding what's best for you before investing in your next set of servos - even if you favor a competitor!
Part 1 - 5-min read - on manufacturing and the impact regarding motors costs
There's no surprise a motor inside your servos is what converts the electricity from the battery into the force that powers your servo's control horn, right? The real question is; what kind of motor is inside. Is it; DC, coreless, or brushless? What follows holds true for any RC model servo you buy - from anybody! Yes, regardless of whether it's from Futaba, Savox, Hitec, or ProModeler it has one of these 3 types of motors inside! The point being, this article is good reading no matter what brand of servos you favor! Anyway, let's delve into the subject and find out more!
- Sorting them from DC, to coreless, then brushless orders them by price.
Note; DC motors have been around the longest and cost the least. And technically, DC-motor is a misnomer. DC is really more a type because it includes coreless, so it's more accurate to say iron-core instead of DC when referring to a 3-pole motors except DC is the accepted industry nomenclature for ferrite-magnet iron-core motors. Is what it is so please allow me the leeway to use DC-motors to represent the ubiquitous iron-core motors available in the market, e.g. an ordinary 3-pole. And while we're at it, brushless motors are actually AC motors, but more regarding this, later!
DC-motor servos: What's most important to know about DC-motors is that they are - by far - the easiest to make and thus, they're the cheapest. Basically consisting of a bit of copper wire coiled around iron cores for the electro-magnet, a can lined with a ferrite permanent magnet, plus a commutator ring and brushes (to switch the fields). Along with a way of holding the shaft at each end, you've got a DC motor that's very cheap and easy to make - and durable, too.
Coreless motors: these are significantly more expensive than DC-motors because of how they're made and because of the materials themselves. Their principal advantage over a DC-motor is they accelerate/decelerate more quickly. This is because of how the windings are made. They're formed on a mandrel (a more expensive process) and rotate without an iron core. As a consequence of the ultralight rotating mass they are very quick to accelerate/decelerate, which is 'the' performance benefit. Commutation (field switching the windings) is mechanical - similar to how it's done with iron-core motors. However, making the delicate basket-like coils (winding) is a separate process that's more expensive (by a lot). Add to it, the permanent magnet is usually an exotic alloy of neodymium and while coreless motors make for faster servos, they are also more expensive to make. Thus, are you surprised you'll pay more for servos with coreless motors? Not if you follow the manufacturing-Benjamins regarding this technology!
Brushless motors: are also called electronic motors and are quite accurately described as pricey! This is because they're manufactured similarly to a coreless motor with respect to the delicate windings and the superior magnets. But beyond mandrel-formed coils and more costly magnets than found in DC-motors, the relatively inexpensive mechanical commutation is replaced by electronic commutation. This brings benefits regarding longevity, and regardless of the direction you approach it, time is always money! Bottom line? Brushless motors are the most costly to make - their use is part of why you pay the most for servos with brushless motors!
So which one's best? For this we need to know; best for who? Best for you? Best for me? And best for what model? In short, when it comes to best . . . the answer is, it depends!
Part II - a 10-min read about what makes DC, coreless, or brushless right for you
If you stopped reading after 5-minutes, you know the basics of what goes into DC, coreless, and brushless motors, and a little bit about why they cost what they cost. That's fine as far as that goes (meaning you'll be fine if you read no further because you now grok it's entirely due to differences in how they're made plus the material differences, which contribute to their price vs. performance).
But there's more to know. A lot more and this requires we delve a fair bit deeper when divining the details regarding the makeup of servo motors. As before, we'll progress from DC, to coreless, and through to brushless - but now in considerably more depth.
Once you've fully digested this section you'll be prepared to make good decision regarding which type of motor is best for you as determined by the technology and your circumstances, e.g. for whichever type of model, how it's flown, and the available budget considerations. Moreover, you'll never again be dependent on someone else to help you because you'll probably be more informed than others about the composition of servo-motors and know how to judge for yourself which is best for you!
DC-motors:
If you want cheapest, then get DC-motor servos - shop around because as a class, servos with DC motors cost the least. Doesn't make them bad, or junky. In fact, they're perfect for maybe 90% of modelers. This is true whether you're a beginner, sport pilot, or seriously into flying enormous giant scale models.
- It doesn't take big money for servos, even for very large scale models!
All these sorts of models; trainers, sport, and scale, are easily controlled by DC-motor servos because the maneuvers they're called on to perform are within the wheelhouse of durable DC-motor servos - of any brand! Bottom line? DC-motor servos are nearly perfect for this type of flying because their characteristics serve the majority of models. Note; DC-motor servos - by their massive nature of construction - offer very high levels of torque. Power is good!
Note: please allow a brief bit of advertising . . . ProModeler offer a 360oz-in DC-motor servo for less than 50 bucks. It's a great servo for a great price. Best thing about it is there are no shortcuts in manufacturing. For example, it's assembled with 10 Allen head machine bolts, has all metal gears, bronze inserts reinforcing the polymer components where steel gear shafts contend with the greatest stresses, has a CNC-machined aircraft aluminum center case with cooling fins for long motor life, and is assembled with 13 o-rings for protecting the assemblies from foreign intrusion - and it even meets three MIL-STDS.
This last is a big part of what makes the ProModeler different. Basically, unlike the usual hobby-grade servos, it's built to meet military standards (and subjected to these tests).
MIL-STD-810G-Part 16
- Shock - Test Method 516.6
- Vibration - Test Method 514.6
- Rain - Test Method 514.5
- A fantastic value, this smooth and powerful servo is a great choice
Meeting these standards is principally due to the requirements of our primary customer. However, because we're modelers just like you - and - because we are as clumsy as the next guy and occasionally knock a servo off a workbench, or have experienced crashes, plus we know a thing or two about engine vibration (and how a summer afternoon pop-up rain shower may drench a model before landing), it means these tests are important for civilian users as well.
So if you're looking at another servo, one which offers similar performance and pricing, but hasn't been tested to military standards, then the question is; which would you rather have? The facts are servos expressly designed to survive the abuse encountered in the non-civilian world are an important ownership considerations for most modelers. While ProModeler may be new to you, we offer this one piece of advice; don't let habit or someone else decide what's best for you . . . acting in your own best interests (and this is always a good rule whether buying servos, or anything else).
Also note; the DS360DLHV is neither unusually expensive, nor cheap (middle of the pack is accurate price-wise). One thing is certain, virtually every model known to man can be controlled with a this 360oz-in servo (regardless of brand, and this servo is built better than most). The real point being; if you consider Chip Hyde won the $50,000 tournament of Champion in Las Vegas with a 44-pound model powered by a 200cc gasoline engine equipped with 195oz-in servos, it's that, yes, you can spend $1200 on servos for a model, but darned few of us can tell the difference versus opting to pony up just $50 each! End of self-serving message.
- With a bellowing roar, and roiling smoke, Chip Hyde wins the TOC
Coreless motors:
What if you want the fastest servos? Then it's coreless motor servos for the win by a country mile! Once you get into a given level of transit speed, sub-0.14sec/60°, then the servos tend to use coreless motors. When the race isn't just one of high torque, but includes an emphasis on speed also, then coreless motors become the choice.
Speaking of speed in servos, speed is costly for the same reason going fastest at Indy takes cubic money! As for why coreless motors cost more, in part it's because of how their coils are made, which allows coreless motors to be so fast when accelerating/decelerating. Added to which, they use superior magnet technology (more costly). So with servos as with anything else . . . more speed means more money!
- Typical coreless motor parts consisting of just a few parts
One thing is certain, pilots flying higher performance models need both power 'and' speed because, for example, that Harrier maneuver everybody oohs and aaahs over (especially when performed down low to the deck) requires extreme speed to time the deployment of the control surface quickly, precisely, 'and' the muscle to shove said extremely large surface against a gale force wind (due to model's forward acceleration). This takes ooomph and speed.
- Note the amount of elevator being deployed . . . astounding!
Brushless motors:
What if durability enters the equation? Electronic motors cost more to make than mechanicals. Basically, you can't replace cheap little bits of metal used for commutation with Hall effect sensors and a logic circuit for the same price. Electronics simply cost more. Added to it, now you need more electronics (the logic) to control the motor in the ways that were once achieved by a metal ring and brushes. Is this worth the added expense? Basically, yes, to many pilots (this is even considered indispensable to some).
Usually adding parts is bad, right? Thing is, electronic motors (brushless motors in other words because Hall effect sensors and a circuit board replace the mechanical commutation) offer a whole new level of flexibility and control to offset this. Added to which, brushless run cooler because the heat of arcing at the contacts is gone. After all, the dust created as the metal of the brushes vaporizes has to go somewhere, and the somewhere is coating the inside of the motor - this is bad for many reasons, heat and fine particles of grit chief amongst them.
When it comes to electronics, heat is never a good thing. Add grit plus small parts and it equals a bad mix. Anyway, this means on average a brushless motor simply lasts longer because they don't have brushes to wear. 5X longer according to MTBF (specifications which can be taken as a reasonable figure).
Painted with this brush; fast, longer lasting, and cooler running, can the numbers for spending $100 vs. $80 be defended? For many guys the answer is yes. Especially if you're an 'in for a penny, in for a pound' kind of guy, or the guy who buys good stuff, tools, etc. secure in the knowledge the good stuff pays off in the long run (e.g. well after you've forgotten the slight differences due to cost). Put another way, price isn't everything.
But honestly, things are easy when you're a single issue buyer. Cheaper faster, more durable - pick one! Things get more complicated when you want cheapest 'and' most durable. Let's see the inside of these types of motor to better understand why the costs factor as they do. For this, let's destroy some. How? By cutting into a few motors with a lathe!
- The lathe makes short work of slicing a motor open to examine
Part III - the 15-min that gets into the nitty-gritty of motor construction
DC (iron-core):
This is a DC-motor once again. This time you're seeing the guts of how they're made. There's a ferrite permanent magnet within the can, and by winding copper wire around an iron core to generate an electromagnetic field, the windings may be sequentially energized to spin the motor.
Sequentially turning the windings on, thus making an electromagnet to react against the permanent magnet mounted within the can, is what moves the rotor around and around. Commutation (switching the fields on and off in a sequence) is what times the force that keeps the rotor turning.
The iron core of a DC motor is the heaviest of any type of motor. The rotor core is made of thin sections or laminations stacked together. The thinner the laminations, the lower the iron loses and the more powerful the motor.