The Motor Construction Thread aka What Motor should I Get?
 

I have had many questions about motor construction, why X is better than Y, and other general questions as of late. I would like to start a thread that addresses the common questions of construction and performance. The biggest part of understanding a motor is to know that every design change is a tradeoff that will effect more than one aspect of the motor. A stronger magnet will give the motor more torque and slow the rpm/volt at the same time, keeping all else constant. If you want more torque and speed the amp draw must increase for a given load and voltage. In other words nothing is free.

As a point of reference the 50-60t ceramic magnet 3 slot brushed motor is most popular. It gives decent wheelspeed and high torque without overheating in most situations.

The construction of a brushed can be broken down into the following:

Bearings
Can
Magnets
Armature (which consists of the following):
-Shaft
-Commutator
-Stator
-Windings
Endbell (which consists of):
-Brush Hood
-Brushes
-Springs


I will now break down the construction piece by piece, showing how small differences in construction change how the motor performs.


Bearings: These play a large part in the efficiency of the motor. They must always be well lubed and clean. Rubber sheilded bearings resist moisture better, but have a high drag. Metal sheilded bearings or open bearings are typically used.

Can: The can is made from Steel, this reflects the magnetic field of the magnets. A flux ring can also be used to to increase the efficiency of the magnet. As the magnetic field gets stronger the motor will slow down if all else is kept the same.

Magnets: The magnets can be made from various materials. Most common in brushed hobby motors for cars is http://en.wikipedia.org/wiki/Ferrite_(magnet) Ceramic Ferrite magnets. Neodymium magnets http://en.wikipedia.org/wiki/Neodymium_magnets are used in brushless motors more often. Old high performance brushed motors used Cobalt magnets. As the magnetic strength goes up, the temperature resistance to demagnetizing goes down. Thus, the weakest Ceramic magnets have the highest temperature rating. Neodymium magnets have the highest magnetic power density (power to weigh ratio), and there is a multitude of grades. Stronger magnets give a motor more torque, less amp draw, and a slower rpm per volt, keeping all else constant. The size of the magnet also plays a part in how powerful the motor can be. Longer and thicker magnets have more strength, which slow the motor down and give it more torque per amp drawn (this doesn't mean the motor makes more torque). The torque of the motor is limited by the strength of the magnets and efficiency of the stator, keeping all else equal.

Shaft: Nothing much to describe here, this transfers the power from the stator rotation into the pinion.

Commutator: The Commutator (or comm for short) transmits the power from the brushes into the windings. This is where the business starts. The Comm always has the same number of segments as the Stator. 3 slot armatures are most common in hobby motors. Holmes Hobbies Cobalt Pullers have 7 slots. Old Astroflight Pullmasters have 7 slots. Increasing the slots of the comm and stator will give a higher torque capability as well as better slow speed resolution. A larger diameter comm will also slightly increase torque capability as well as slow speed resolution. Typical hobby motors have 7.5mm diameter comms. The Cobalt Puller and Pullmaster motors have 12mm commutator diameters.

Stator: The stator has the same number of slots as the Commutator. The notable aspect of the stator is the effect on efficiency and power. Higher quality silicon steel laminations increase efficiency by lowering switching and hysteresis losses. Both losses make the stator plates hot. This is not addresses by many brushed motor manufacturers, especially in the RC car realm. The diameter and length of the stator has a direct effect on the overall power output of the motor. The larger and longer the stator is, the more power the motor can have.

Windings: The windings run the motor. In a nutshell the fewer turns you have the faster the motor will spin per volt (with less torque per amp drawn). For crawlers we use mostly high wind motors, but lower wind motors can be used with proper geardown. The downfall to lower wind motors is heat and lack of torque. In a rough bind a 35t 3 slot ceramic magged 540 motor can smoke itself with too much throttle. Not enough torque combined with stalling conditions create a lot of heat. This is why 50t-60t three slot ceramic magged motors are the most popular for general crawling- they do not overheat and they work with standard gearing like a pede tranny/ tlt axle will give. Handwound motors have more copper packed in. This increases the magnetic field density and increases the power of the motor. When the magnets or slot count change, the wind count changes to keep speed similar. The 10t Pullmaster or Cobalt Puller is similar in speed to a 45-50t Ceramic magged 3 slot motor.


Brush Hood: An often ignored part of the motor. The brush hood must be aligned properly with the Comm for proper efficiency and power production. Special designs can increase heat dissipation which increases the amount of power the motor can handle. The hood orientation also determines how the brushes contact the Comm. "Standup" brushes have more contact area along the vertical plane. "Laydown" brushes have more contact along the horizontal plane which keeps the winding energized longer. Laydown brushes produce more power at a sacrifice of amp draw (and runtime). Nothing is free!

Brushes: Hard brushes wear longer and can give increased motor speed. They can also cause premature comm wear. Soft brushes tend to run quieter, but they can have a higher coefficient of friction. For crawling either can be used, it depends on many factors of driving style and setup as to what brush will work best. Brush width also effects the amp draw, as a wider brush shorts each circuit for a longer period of time. Too wide of a brush will overheat the motor.

Springs: The springs hold the Brushes to the comm. Stiff springs give great drag to the motor, which is good for downhill descents with a crawler. They will also wear a motor comm out fast. Too soft of a spring won't work at higher rpms and will contribute to brush hop and arcing. Most people that crawl tend to prefer stiffer springs for downhilling, especially since many crawlers do not break 4mph and motor wear is minimal anyway.





Next I will address Voltage and the application in regards to motor construction and gearing.

Ok, still with me? So what does this all mean? Let us take the prime example as our standard, the 55t 3 slot ceramic magged integy motor. Assume it is broken in, the brush hoods are aligned, and we have everything in good working order.

We know that: RPM per volt will be around 900-1000 with hard brushes and springs. It won't overheat in most 2.2 rigs, and it can run on 6 cell nimh up to 4s lipo without giving up the ghost. Now comes the fun part. Let us increase the voltage from 6 cells nimh to 12 cells nimh. Our motor speed will double. Along with the increase in speed comes an increase in AMP DRAW (assuming that the gearing didn't change). Since amp draw goes up our torque goes up too. Our motor didn't change, but we get more power at a sacrifice to runtime with only increasing voltage.

Let us take this volted up 55t motor and double the geardown. Double the voltage, double the geardown, your wheelspeed stays the same. What happens when we get in a bind though? We have more amp draw, more geardown, and more torque all around. IF you use the extra power you will have a lower runtime. Most people will use the extra power. If the extra power isn't used then volting up isn't needed and extra weight is just being added to the vehicle as battery weight.

Let us change the example for a moment. Let us consider a faster motor like a 45 or 35t. If gearing and voltage is kept constant the vehicle will run faster at a decrease of runtime as compared to our 55t. It will also heat up more when bound up, as it takes a higher amp draw to produce the same amount of torque as the 55t. To offset this we can gear down.

Gearing, Voltage, and Wind count are all effective ways to change the speed, power, and torque of a motor. There are enough factors at play at any given time, it can be hard to keep track of it. So, how can we compare all of this at the same time? An easy way is to fix total power output and wheelspeed. We want XX horsepower at 4mph. The 35t motor on 6 cells geared to 4mph will produce the same amount of power as the 55t on 10 cells geared to 4mph. The difference between the setups is the runtime, amp draw, and vehicle weight. The 35t motor will have a reduced runtime and increased heat with lighter weight. This can be good for short runs or vehicles that don't get bound up often. The increased amp draw during a stall can smoke a 35t motor. I have gotten a locked 55t motor on 9 cell nimh to burst over 500 amps within a 2 second stall. Needless to say that sort of activity will kill a motor.

OK, now we ask the big question. How do we get more power from a motor? The simple answer is to change the construction or increase the voltage. This can be as simple as reducing the wind count or as elaborate as a totally different motor. Let us pretend we are using a freshly charged 6 cell nimh pack of supurb construction for this, and our starting motor is an Integy 55t 3 slot ceramic magnet motor.


We are limited on voltage in this first example, so the easiest way to increase the power of our motor is to decrease the wind count to 45t. This will increase the speed of the motor and increase the amp draw. Amp draw is power, as the torque of the motor is directly relational to the amp draw. Easy enough right? Now we only have to gear down to get our wheelspeed slow again and we have more power at a sacrifice of runtime (if the extra power is indeed used).

Another way to increase the power on a fixed voltage would be to change the overall construction of the motor. We can DECREASE the magnetic strength, which will in turn make our motor faster and have a higher amp draw. This is not a very good solution as the motor will actually have a reduced overall power output- but in this case we are talking about a fixed voltage so it is still a solution.




As another example let us change the voltage on a given motor (integy for example). A 35t motor produces more power on 12 volts than 6. A 55t motor produces more power on 12 volts than 6. The 35t will produce more power than the 55t on 6 volts because the 35t will have a higher amp draw. Both winds can produce the same amount of power, it just occurs at a different voltage. A 35t on 7 volts might be very similar to a 55t on 15 volts.



As a last example we can lengthen the motor stator and magnets to increase the maximum power the motor can handle. When we make it longer we slow it down (all else kept constant), so to keep the motor speed constant we must either reduce the wind count or change another parameter of the motor. As you see when one parameter is changed another one will change as well.



To sum up, more power can be made out of an electric motor by only changing the design (wind, size, etc.) or increasing the voltage.

I believe I am done on the motor construction, I have edited and clarified the first three posts quite a bit. Some information has been moved around or totally removed, I had some bad wording in the first post.



If there are any questions feel free to post them up, I can go into detail on how brushless motors are constructed as well if wanted. They are much simpler.

http://img2.putfile.com/thumb/11/31702085349.jpg

an easy way for more torque , the pic shows an CYCLON outrunner (4 pole)
but it works too for ferrit mag brushed motors
all you need are some small neodymium magnets :lol:

Thanks for the excellent introduction. Can you add how the motor specification relates to the ESC one would choose? For example:

A Part #3785 - Titan® 12T Modified Motor (12-turns) NOTE: Titan 12T motors are intended for use in single motor and single battery pack applications, up to 8.4 volts.

How do you read the motor specifications and decide which is the ESC to use? -Migs

Use the ESC that works for the motor ratings. Basically select an ESC that takes 12t motors or lower if you want to use that titan.

motor
 

on the negative side, does it matter which solder post i connect to, i have an integy 55t runnion a 14tooth pinion and it has good power in forward but is about 20% at best in reverse with no power. im running stock ax10 esc, and have a rooster coming.






Attached Images
http://rccrawler.com/forum/attachmen...1&d=1194923307

You can solder to anywhere on the brush hood, it doesn't matter. The solder posts are just there for convenience.

any reason why "timing" a motor was left out?? seems like allot of people either have no idea what that is or ignore it all together..

I put it into another thread. I will see about adding it in here too.

So could you just put two same sized (stronger) magnets on the outside of the motorcan and increase the power? What if anything would that accomplish?

It would decrease the speed, and only increase power if the armature field was not already saturated. It would take some very strong magnets to penetrate the can too. If the magnets are too strong on a three slot motor the startup speed will become higher because it has to break the stronger magnet field.

one more question, whats the best way to clean a motor, is it nessacery to take one apart to clean it.

thanx

The problem is ESC's don't list this information. Here is a typical ESC descriptions:

A popular one:
Castle Creations Mamba Max: "Specifications:
Cells: 6-12 NiCd/NiMH; 2-3s LiPo
Continuous: 100 amps
Resistance: 0.0003 ohms
Brake: Proportional
Reversible: Yes - with lockout
Low Voltage Cut-off: Programmable
Size: 1.9 x 1.3 x 1.2"
Weight: 2.5 oz (70.9g)
Connector Type: Universal"

"electric motor cleaner" spray, search google. You can clean it while assembled, but you must relube the bearings.

The mamba max has no brushed motor rating, it will work with about anything you throw at it within reason. Just watch the temps. The sidewinder is 19t brushed rated I believe.

Awsome! Thanks JRH!"thumbsup"

x2 plus very informative.