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.
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.
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