I don't understand it well enough to predict it, but I understand it well enough to be functional with it.
The motor has a fixed outside with coils of wire bolted to them. Inside the motor are another set of coils on the part that rotates.
The brushes conduct electricity so that the rotating coils are powered.
The coils are just electro-magnets. When electricity flows through them they turn on and act like magnets. The inside rotating coils pull themselves to line up with the outside fixed coils.
The rotating coils are actually many, many coils, maybe 20 or 30, all the way around the rotor. Only one has power at any time, so each is only on 1/20th or 1/30th of the time. The job of the brushes is to keep the coil that has the strongest ability to pull towards the fixed electromagnets (the one almost but not quite lined up yet) on the outside powered up.
Think about 4 relay runners in a relay race. Only one is running at a time, and they're all spaced out so they're handing off to each other at just the right moment. Imagine they're running on a circular track so the start line and finish line are the same line. Runner 1 is at the start. Runner 2 is 25% of the way down (or 90 degrees). Runner 3 is 50% of the way down (or 180 degrees). Runner 4 is 75% of the way down (or 270 degrees). Each runner only sprints the 1/4 of the track that they're responsible for, and then pass the baton to a fresh runner.
The runners are the inside coils, and the baton is the brushes. And now have a couple dozen runners instead of just 4.
As the rotor spins, the strongest coil only sprints for a short period of time before handing off to the next coil.
That's how motors work. With me so far?
When you increase the voltage, you make more amps flow, which makes the magnets stronger. And, you kind of snarl up the magnetic fields inside the motor.
Back to the relay race. The runner doesn't just crash into the next runner when they're handing off the baton. The two have to coordinate a bit. The next runner has to see the previous runner approaching, and start their sprint before they get there. Ideally, at the perfect time to switch, at that 25% of the track, the forward runner is traveling exactly the same speed as the runner behind him so that the difference between their speeds is as close to zero as possible.
If their speeds aren't close to zero, the runner behind will crash into runner ahead and bodycheck him, shoving him forward. Minor imbalances are okay, it'll work, to a point. But big imbalances are going to cause injuries pretty quick.
How does the relay team know when to start running and how to handle the handoff of the baton? Practice.
Practice is like, a motor being engineered for a certain voltage.
What happens if suddenly all the sprinters are jacked up on steroids and run faster? Is that better? Of course! It's a race, you want to be faster.
But it's going to mess up the timing of their handoffs. They've been practicing for a different situation than is now being asked of them.
The forward runner is also more powerful, so you'd think the timing would still even out. Well the timing might be, but the point where the handoff of the baton occurs is going to be further forward if he starts from the same line. They'll both be going faster at the handoff, so he'll have crossed more distance in the same time.
Imagine if you insisted that the handoff still occur at the same point on the track that it used to be? Their speeds won't be matched yet and the back runner is going to crash.
So...
Ignore the speed part of the analogy, it doesn't have to do with the RPM of the motor. It has to do with how the voltage twists the magnetic fields but if you understood that you wouldn't need to read this, so, as Einstein said, "All analogies are wrong. Some analogies are useful."
More practically, imagine a relay race between obese couch potatoes who can barely walk. Normally the handoff can happen exactly at the 25% line with no forward runner gaining speed first. Just start walking. That's neutral alignment. A higher voltage motor might want the fatty in front to get started one or two steps before the fatty behind him gets there, but that's about all. It's a small change but it still matters.
You want the brushes in the motor at the place where the difference in magnetic fields is as close to zero as possible. When you jack up the voltage, you change the place around the circle of the motor where that is, by a bit. Up to 15 degrees or so.
If you don't change anything, the consequence is that the brushes will arc and spark much more than normal. This wears down the brushes and the commutator much faster. On top of that, you're also generally overheating the motor too with that higher voltage and thus higher current, so, heat makes the arcs worse.
And that's what brush advancing is. You just rotate the brush carrier part forward a few degrees. The higher the voltage, the more you should rotate it.
Forklifts will always have neutral brush alignment (directly 90 degrees from the center of the fixed coils on the case). That's because this whole alignment thing is true in reverse, when reversed, and forklifts back up as much as they drive forward. It's also true in your car, but you're never reversing at high speed so the arcing is minimal even with brushes advanced the "wrong" way. You spend 99.99% of your time driving forward, and backwards only slowly, so, advance your brushes.
Any of that make sense?
