Motor Maintenance

Breaking in the Brushes

A rebuilt motor, or a new one provided it was not broken in at the factory, should be broken in to properly seat the brushes against the comm. There are several methods of how this can be accomplished, and some have definite advantages over the other. The overall object of breaking in the brushes is to make them match the curve of the comm as perfectly as possible, but that's easier said than done. This is due to the fact that you'll also want to prevent arcing (or sparks) between the brush and the comm while the break-in is underway. Arcing is caused by the voltage difference between two surfaces, such as the comm and the motor brushes.

For a better understanding of brush break-in, examine the diagram below. In the lower left diagram you're looking down on the comm and the brushes. With a new set of brushes, as shown on the left, you will notice that they do not curve along the edge of the comm. Instead they just touch at the center of the brush. The gap between the brush and the comm is often the site for heavy arcing, which can pit or prematurely wear the comm or brush. So the less severe the arcing, the less chance you'll see of pitting on the comm or brushes. As the brush begins to wear it will fit the curve of the comm and, when the brushes are completely broken in, there will be no gap between the brush and the comm as shown in the diagram on the right.

Now you're probably wondering how do you try and avoid as much arcing as possible, but allow the brushes to still break-in properly. One thing you'll want to do, if you have a motor with adjustable timing, is to set the timing to 0°. Another thing you can do to help is to reduce the voltage to the motor during break-in by using a 4-cell pack, which can be plugged straight into the motor for simplicity. Reducing the voltage supplied to the motor, lessens the occurance and severity of arcing. In addition to those items, you'll also want to run the motor without a load during the break-in period. It's fine to mount the motor back into the transmission housing, and you can even install the pinion if you wish, however do not allow the motor to turn the spur gear. These three things will prevent the majority of arcing and premature comm wear while allowing the brushes to break-in properly.

Another method of break-in follows the same suggestions as the previous one, but differs in one area. It uses a 6-cell pack as opposed to a 4-cell, but you just use half-throttle to simulate the voltage supplied by a 4-cell pack. This is usually the route for those who don't want to make a 4-cell pack. While this can be done, it's still not as easy on the comm as a 4-cell pack as the motor will still receive the higher voltage part of the time. This is due to the method in which an ESC operates.

How An ESC Operates

An ESC doesn't give the motor a certain voltage depending upon how much you pull the trigger. It actually switches power transistors, or FETs, on and off at varying speeds to simulate different voltage levels. Imagine a light switch being turned on and off over and over again. This is what's going on inside your ESC hundreds or thousands of times a second.

For an explanation of that refer to the following diagrams. The first diagram shows a 6-cell pack supplying power to an ESC and motor when the throttle is wide open. Notice the steady source of power as indicated by the graph. This is what you would expect to see in this situation, and should come as no surprise to most people.

This shows the same 6-cell pack and ESC sending no power to the motor when no throttle is being used. Notice the steady lack of power as indicated by the graph. Once again this is what you would expect to see in this situation, and should come as no surprise to most people.

Now this is a graph of an ESC supplying around 3 volts to a motor, which would be close to half throttle. Notice the graph shows pulses that vary from the full 7.2 volts to off. This is the FETs in the ESC switching on and off to simulate the lower voltage.

These three graphs clearly illustrate how an ESC operates by switching the FET's or transistors on and off. In real time this is performed many times a second. At lower speeds, on many ESC's, you will often hear a hum or a whine. On those ESC's you are actually hearing the switching I am referring to, and the sound is part of the normal operation. The speed at which the switching is done, is the frequency of the ESC. This is why an ESC with a higher frequency will give you better throttle control.

I should point out that these diagrams are actually of a simulated ESC, but they show the results in a cleaner more simplified way than an actual ESC would. An actual ESC results would be very similar in nature and have the same overall results. You can see that in the example below.

All of that information clearly illustrates that you can't effectively simulate a lower voltage by running a 6-cell pack at half-throttle. So while the half-throttle method is better than running a 6-cell pack at full throttle, it's definitely better to break the brushes in with a 4-cell pack if you are able to do so. However, I should point out that these diagrams are actually of a simulated ESC, but they show the results in a cleaner more simplified way than an actual ESC would. An actual ESC results would be very similar in nature and have the same overall results. You can see that in the example below.

Other Break-in Methods

Another method of breaking in the brushes is to use a reversible drill or something similar to spin the brushes at a high RPM. While this method works well, due to the fact that it supplies no voltage to the motor and motor timing is not an issue, it can be cumbersome. First of all, due to the fact you need the drill, and secondly that it's easier to use something like a vise to hold the motor stationary during the procedure.

Another method that is sometimes seen, is to dip the motor into water while breaking in the brushes. As with the first procedure I mentioned, you should set the timing to 0°, and use a 4-cell pack with this method. However in my opinion it's more trouble than it's worth because after breaking in the brushes you should clean the motor and re-lube the bearings or bushings in it. This adds an extra step to the break-in procedure.

Break-in methods vary, and it's up to you to decide which one you prefer. Whatever you do though, you should always avoid bolting the motor in and skipping the break-in procedure if you want to have the optimum life from your brushes and comm. One final note, don't forget to set your timing back to the original point you had it at before you rebuilt the motor. Otherwise you may notice the motor lacks the speed it might have had before.