Motor Break In

If you have been around electric flight for very long, you have probably heard other flyers mention 'breaking in' a 'can' motor. You may have even done it yourself. Some people may make specific claims about what break-in will do for a motor in terms of increased RPM; I will leave the specific claims to others. I think that breaking in a motor is a good idea, and so does Kirk Massey at New Creations R/C, and so do a lot of other people.

This article applies ONLY to brushed motors; brushless motors do not require any sort of break-in. First all, a few definitions are in order:

What is a 'can' motor? A 'Can' motor is one that typically has the motor housing body and front plate pressed from a single piece of metal, similar to a drink can. The back plate may be either metal or plastic, and is typically held in place by metal tabs and/or crimping. The Graupner Speed xxx series are all 'can' motors, as are the Robbe 'Power' series, virtually all car or 'buggy' motors, and many, many others. Astro Flight, Mega, Plettenberg, and Graupner Ultra motors are NOT 'can' motors. A general rule of thumb is that if you paid less than US$50 for a motor, it is a 'can' motor.

What is meant by 'break in'? To understand what 'break in' does, you must first understand a little about the insides of a motor. This drawing shows an exaggerate end view of the commutator and brushes inside the motor both before and after break-in. You may be able to see essentially the same thing on your motor by looking into the cooling opening while holding the motor in front of a light.

The brushes in the drawing on the right have been worn down to the point where their shape closely matches the shape of the commutator, thus improving the electrical contact, and reducing resistance. That is all there is to it! 'Break-in' is the colloquial or common term for this process - but perhaps 'wear-in' would be a better description, because what we are really doing is accelerating the wear of the brushes, so they will be more closely shaped to match the commutator. One could accomplish approximately the same thing by running the motor for a long time, but that would also wear out the bearings, which is NOT a good idea.

Power during break in: One of things that varies between descriptions of how to break in a motor is how to power the motor during break in. Some people suggest running down a pair of carbon-zinc D cells, and others suggest using a NiCad pack with one less cell than will be used during flight. I don't like throwing away batteries, and I see no point in keeping odd size NiCad packs, so I use a DC power supply. The next photo shows the complete equipment setup that I use for breaking in motors. The power supply is a Lambda 5VDC 6A linear power supply, with an adjustable output voltage. The fan sitting on top of the supply is powered from 120VAC, and is just a precaution to keep the supply cool and maximize the available current. You will have to look at an electronic surplus or salvage dealer for a power supply of this type - you do not want to know what they cost new. This type of equipment is relatively ageless - mine is at least 20 years old - and you are most likely to find this sort of supply at a company that breaks up old electronic equipment for scrap. You will have to add your own wiring for both the AC line and the DC output to the motor. My supply has terminals that fit crimp-on spade connectors. If you do not know how to attach an AC line cord, this will become a major project, so find someone who can help you. Make sure that your connections are SOLID - this is NOT the place for clip leads or electrical tape.

If you look very closely at the photo, you will notice that the DC output from the power supply ends in a nice, neat Astro Flight Zero-Loss connector - which is connected to the motor by a pair of clip leads. This is not a display of laziness, but rather is a carefully considered circuit component. Really. When a permanent magnet DC motor is attached to a power source, the initial current can be quite high - 20 Amps for a Speed 400, 70 (!) Amps for a Speed 600, even more for larger motors. This very high initial current will taper off once the motor starts rotating, but it is present long enough to cause a high quality power supply to see it as an overload, and go into 'crowbar' shutdown. The clip leads add just enough circuit resistance to reduce the startup current to below the level that will cause a shutdown.

The other thing you may notice is the large bottle of distilled water and the plastic cup. The distilled water is used as a break-in lubricant! If you have heard about breaking in a motor in water, and wondered if this idea was for real - rest assured, it is for real. Fill the cup with water, connect the motor to the power supply, and put the rotating motor in the cup. Yes, it feels a little strange - but it does work.

The water serves several purposes - it helps to keep the motor cool, carries away material that is ground off the brushes, and it acts as a cutting lubricant. As the brushes are ground to shape, the water will turn (yeech!) gray from the brush dust. I use distilled water because, well, that is what I was told to do - and it makes sense, because tap water may contain an assortment of minerals that will react with the motor and leave it in worse shape than when you started.

I run the motor for 3 minutes in each direction (reverse first) and then run it out of the water for a minute or so to get rid of any water that is left in it. The total time is mostly a matter of how gray the water becomes - it does not need to turn black! Then I put a drop or two of 3-In-1 machine oil on the bearings, and turn the motor a bit to help it soak into the bearings. Let it sit for a while, and then wipe away any excess oil. That's all there is to it!