First of All, What is a Brushed DC Motor?
Well, it’s a DC motor with brushes! What is a brush? It looks like one of these…
A brush is used to actually get the electricity from whatever source you are using to the coils in the motor. Because the rotor spins, you cannot simply attach wires to the coils. They would get twisted and prevent the rotor from spinning! So, enter brushes. The brushes have to connect to the coils somehow, and that’s where a commutator comes into play, which looks like this:
The brushes slide on the commutator as it rotates, which allows the electrical energy to flow through the coils attached to the commutator. Each of the copper bars on the commutator is attached to a set of coils, which are specifically arranged to connect when the rotor is in a certain position. The entire assembly looks like this:
A lot of math goes into properly designing these components, but we don’t need to worry about that at all. All we need to know is how to differentiate between a brushed motor, and the types of brushed motors that exist. You’ll run into two main types: Permanent Magnet and Series Wound.
A brushed, permanent magnet direct current motor (PMDC) is what you will usually find in small toys, hand tools, and in smaller conversions. It has a rotor (or armature), and surrounding the rotor are permanent magnets. These motors are pretty efficient compared to a series wound motor because they do not need to spend any energy generating a magnetic field. The permanent magnets always generate a field, as we just make use of that field when we apply power to the motor.
You can reverse this motor, or spin it in the opposite direction, by reversing the polarity. Instead of positive on one connection and negative on the other, just flip it around.
The Motenergy ME-1004 is an example of such a motor, and could be used for small go-kart or motorcycle conversions.
Now that we know how a PMDC motor works, a series wound DC motor is easy to explain. Instead of having magnets around the rotor like a PMDC motor, a series wound DC motor just uses another set of coils. The electrical energy passes through the armature (rotor) windings, and then flows into the field windings. The trick here is that you cannot reverse the rotation of a series wound DC motor by simple reversing the polarity like we did with the PMDC motor. Because the armature and field coils are connected in series (series wound…), whenever you change the polarity, you are changing the polarity of BOTH magnets, meaning it always spins in the same direction.
In order to reverse the rotation of a series wound DC motor, you must change the polarity of just one coil, either the field or the armature. You will usually see the terminals on one of these motors labeled A, F, or S. A is for armature, F is for field, and S is for stator. For our purposes, the armature is the coil on the rotor, and it spins. The stator is outside of the armature, and it is the same as the field. The labels may also have a number, such as A1, A2, F1, F2. Understanding that the power must flow into one coil and out the other, you could wire it up like so: power into A1, out of A1 into F1, out of F2 back to the power source. Flipping either A or F will reverse the motor.
The Warp9 is one of the most popular motors for a series wound DC conversion
But How Do You Actually Control the Speed?!
In short: voltage or amperage. Reducing the voltage sent to the motor will reduce it’s speed, and increasing the voltage will increase it’s speed. How it’s controlled in a vehicle, however, is by controlling the current. Since the current gives us our torque, the power to actually move the vehicle, controlling it allows us to accelerate, decelerate, and maintain speed. I think this is best described by demonstration, so I’ve included a video I found on the topic.
ElectroBOOM explains Brushed DC Motor Control
I found this video very helpful. If you’re looking to understand what a controller actually does, this will help. The MG uses a series wound DC motor, which has brushes like his demonstration motors. The primary difference is that the permanent magnets in his motors are replaced by another coil in mine. The field coil (stator) is wired in series with the armature (rotor) coils.