Brushed DC Motors
DC Brushed motors have been used in the RC hobby from the beginning of electric flight. Until recently, the only electric motor typically found on model trains, planes, cars and boats was a DC brushed motor.For reference, the term “brushed” simply means that there is something (brushes) touching the rotor (commutator) in order to form an electrical circuit between the battery and the armature coil windings (the copper wires you see inside). The armature is mounted to the steel axle and has multiple coil windings attached, each providing an electromagnetic force when energized by the current flowing from the brushes through the commutator.
The commutator is made of multiple contact surfaces wired to each coil so that the brushes are “handing off” energy to the next coil in order to maintain rotation of the rotor. Each rotation hands the coils off to the opposite brush which means each armature coil is energized creating magnetic attraction, pulling itself towards the next permanent magnet. The motor housing has fixed, permanent magnets of opposite poles mounted around the spinning armature. This sets the stage for a consistent magnetic environment for which the armature coils can either push or pull, creating rotation.
Airplane Use
Pros and Cons:Inexpensive Performance degrades over time
Easy to wireHigher resistance than brushless
Electronic Speed Controls (ESCs) are cheaperLess powerful than brushless
Can be “ganged” together in parallel and powered by the same ESCHeavier than brushless counterpart
Common Types
Canned/Can Motors
The picture above is of a typical “canned” motor. Canned motors are usually included in a kit, are very inexpensive, use lower strength ferrite permanent magnets, plastic or porous brass bearings and are non-serviceable. They will fly the model they were supplied with just fine but if you want to get higher performance out of your airplane, you can upgrade to a brushed motor with higher quality components and, specifically, better magnets and brushes or even move into a brushless configuration.Rare Earth Motors
Rare earth motors commonly use Cobalt or Neodymium magnets to achieve much higher performance than their ferrite counterparts. Another added advantage is that Cobalt and Neodymium experience less performance degradation than Ferrite as temperatures increase.Rare earth brushed motors are made of better materials all around, such as:
- Better cases with improved heat dissipation
- Roller bearings for reduced friction
- Replaceable brushes for longer overall serviceable life
- Higher quality components with better conductivity and less wear
General Considerations
An unfortunate characteristic of magnets is that they lose strength the hotter they get. Higher quality magnets perform better in higher temperatures but also cost significantly more.You can install a heat sink over the motor housing (avoid covering air holes) to help dissipate heat and maintain performance. Check for proper clearance before mounting; heat sinks require more room than the stock motor alone. The following image is a typical slip-on heat sink for an electric motor.
Another unfortunate characteristic related to brushed motors is that the brushes and commutator break contact during rotation. This results in a loss of energy transferred to each stator coil. This also causes all of the stored electrical energy to “arc”, resulting in Radio Frequency Interference (RFI). This electrical “noise” can wreak havoc on your radio receiver and even your servos. 2.4 GHz radios operate outside of the susceptible frequency range but you can still get interference to your other components.
The effective use of a capacitor on each motor lead can isolate or “buffer” this condition and reduce or eliminate any interference the motor is generating. Some motors already have capacitors installed internally but you should always investigate before launching your model and losing control! There is more information regarding interference in the Radio Systems section and the break-in section below.
Wiring
**Always use caution and keep your hands and all other objects out of the path of a moving prop. These seem like toys but smaller models can remove finger tips and larger models can actually amputate fingers and even cause death**One of the benefits of a brushed DC motor is that it uses only two wires to operate. As conventional as it seems, you have a positive and negative lead from the Electronic Speed Control (ESC) that provided variable current to the motor for proportional (gradual) control from idle to full speed. Connections are polarity sensitive and in order to get the correct rotational direction you may have to reverse the leads from the ESC going to the motor NOT FROM THE BATTERY TO THE ESC!!!.
Connect the esc to the motor without the prop installed (safest) and gently increase the throttle to see which way it is turning. If it is turning in the wrong direction, power off, reverse the two wires, and reconnect. The illustration above shows a brushed DC motor properly connected to an ESC, connected to a battery and radio receiver.
See the Tools section for our
Motor Sizing Calculator.
DC Brushed Motor Break-In
As the old saying goes with gas engines: “Break it in, or [you'll] break it down!” While it’s nothing quite that extreme with electric motors, a properly broken-in brushed DC motor will give many hours of cooler, more efficient and more powerful operation. Breaking-in a motor refers to “seating” the brushes properly on the commutator. The goal is to gently wear the brushes against the commutator in order to achieve a smooth fit, maximizing the contact surface area. This makes for a more efficient transfer of energy and reduces arcing which results in less heat created on the commutator surface as well as less Radio Frequency Interference (RFI). A broken-in motor can produce 10-30% more power than one pulled out of the box and put into full service.** Note ** All procedures mentioned are assuming a bare motor, with no prop attached or gearbox. Break-in procedures are most effective under no load! Also, break-in the motor in the intended operational direction for most effectiveness.
Methods and Variables
Let the debate begin! Each hobbyist has a different method they swear by for breaking in a DC brushed motor. Manufacturers even have their own procedures. I would recommend for starters investigating the manufacturer’s recommended break-in procedures for your specific motor as any other method may void the warranty. But, if there are no specific recommendations and you are comfortable going out on your own, let’s begin with the major factors to consider:Low Temperature - Heat is not required to break in a motor! In fact, room temperature is just right. Some people even ice their motors during break-in.
Minimal Arcing - Arcing causes pits to form both on the brush surface as well as the commutator. Arcing also generates unwanted heat. Break-in recommendations are to supply 1/3 to 1/2 the normal operating voltage. This greatly reduces arcing.
Cleanliness - As the brushes wear against the commutator, small carbon particles begin to collect and blow around inside the motor. While this is relatively minimal, the particles do stick everywhere and also grind between the brushes and commutator. Some people say this is beneficial to getting a good seat and I’m not here to debate the point, but my personal preference is to keep the motor clean during break-in.
Voltage - Break-in recommendations typically average the supply voltage to around 1/3 to 1/2 operating voltage. Again, the goal is to minimize heat and arcing and allow the brushes to gracefully seat themselves into the commutator. So, on a 12 volt motor you’d break it in on 4-6 volts. It’s perfectly ok to start at 2 or 3 volts and ramp up to a higher voltage over time. Something you will see during the break-in is the amperage will decrease going to the motor. If you’re savvy with a multi-meter, or have an amp meter, watch the amperage decline while the motor is running as the brushes become more efficient!
Time - Most break-in procedures range from 60-120 minutes. Some battery chargers even have a motor break-in feature that allow you to set the step voltage and time for your motor break-in to your liking. It will automatically increase the voltage over the course of the break-in for a gentle ramp through the RPM range. If you monitor the amperage draw, you will see it “level off” at some point. Once this is achieved, the break-in is finished and you’re ready to run your motor at standard operating speeds. If you are using the battery method and cannot monitor the amperage draw, I would recommend at least an hour break-in to obtain effective results.
DuraTrax IntelliPeak ICE charger breaking-in a Brushed DC motor
** Note ** Do not directly connect Lithium Polymer or Nickel-Metal Hydride rechargeable batteries for motor break-in unless you plan to disconnect them before they reach minimum voltage. Lithium Polymer (Li-Po/Li-poly) and Nickel-Metal Hydride (NiMH) batteries should not be discharged below certain levels or they become permanently damaged or suffer reduced life and performance. See the Batteries section for more information about proper usage of your rechargeable batteries. If you plan to use a direct-connected rechargeable source, I keep an old 4 cell (4.8 volts) Nickel-Cadmium (Ni-Cd) battery around; they have no problem with being fully discharged.
Wet -vs- Dry
WetWet break-ins involve submerging the motor (not the battery or anything else) in liquid in order to keep it cool and wash away carbon debris. A common myth is that the water will cause a short in the motor and even overheat the battery. Simply not true.
Some people use Distilled Water, Rubbing Alcohol or common tap water. I would caution the use of tap water due to the fact that minerals in the water leave deposits all over the inside of your motor. Those same minerals also greatly increase the conductivity of the water, not that this is really a problem during break-in, but I’m not keen on leaving abrasive, conductive minerals all over inside my electric motor.
Rubbing Alcohol does pose some fire risk, albeit minimal, but evaporates nicely after the break-in is finished. Consider performing the break-in outdoors and away from structures if you are using rubbing alcohol. Water break-ins must be cleaned afterwards to avoid corrosion and rust. I use compressed air through the holes and this works pretty well. You also will want to lubricate the bushings at the end of the axle with some very light weight hobby oil or even a couple drops of 3-in-1 or sewing machine oil. GO SPARINGLY. An oily motor collects dirt and can greatly interfere with your commutator/brush efficiency. Avoid getting it all over inside the motor and avoid penetrating lubricants such as WD-40. Just a drop on the bushings from the outside will do. Don’t disassemble the motor after break-in! The brushes typically do not seat the same after re-assembly.
Dry
Dry break-ins are most common and are very easy. In fact, many people do their break-ins with alkaline batteries as the power source and a heat sink snapped over the motor to help keep it cool. They simply let it run until the batteries are dead. Again, we have almost nothing to lose here! Even a partially broken-in motor will out-perform a new one. With dry break-ins, keep the motor in a well ventilated area, away from heat sources, and you’ll have excellent success. Use the Voltage recommendations below to properly size your source batteries.
