An old film to bring you up on the basics. Weirdly, physics don’t change.
A DC motor and a DC generator are the same machine run in opposite directions, and the film above builds both from a single loop of wire. It is worth a watch. Once you see the wire move, half the language on a motor spec sheet stops being mysterious. Here is the short version, plus where it shows up in the gear we fly.
How a DC Motor Works: The Basic Idea
Start with a wire in a magnetic field. Move that wire through the field by hand and it generates a voltage. The film calls this an electromotive force, or EMF. Move the wire down and the meter needle swings one way. Move it up and the needle swings the other. Slide it along the field, parallel to the lines of force, and nothing happens. The wire has to cut lines of force to generate anything.
Now flip it around. Instead of moving the wire by hand, push current through it. The current builds its own magnetic field around the wire. That field crowds the main field, stronger on one side than the other, and the wire gets shoved sideways. Reverse the battery and the wire moves the other way. That is motor action. A generator turns motion into electricity. A motor turns electricity into motion. Same physics, opposite direction.
The Commutator and Brushes
Every rotary machine produces AC inside. Spin a loop between two magnets and the voltage rises, falls, and reverses each turn. A graph of it is a sine wave. To get direct current, the film attaches the loop ends to copper segments, and that assembly is the commutator. It spins with the loop. Stationary carbon brushes ride on it and swap the connections to the load every time the loop voltage reverses, so the current outside the machine always flows the same direction.
The output is not smooth. It pulses from zero to maximum twice per turn. The film calls that ripple. Add more loops at angles to each other, with more commutator segments, and the pulses overlap and flatten. The wound rotor plus its commutator is the armature, and real armatures carry many loops, so the DC coming out is nearly pure.
Sparking, the Neutral Plane, and Armature Reaction
Brushes want to sit on the neutral plane, the position where the generated EMF passes through zero. There, a brush crossing from one segment to the next sees no voltage difference and, in theory, does not spark. But the current in the armature sets up a magnetic field of its own. That field distorts the main field, shifts the neutral plane, and the brushes spark. The film calls this armature reaction. Sparking burns commutators and causes electrical noise. You fix it by shifting the brushes, or with interpoles, small magnets between the main poles that push the neutral plane back where it belongs.
Counter-EMF: Why a Motor Limits Its Own Current
Here is the part that surprises people. A spinning motor is also a generator. As the armature turns it generates a voltage that opposes the one you applied. The film calls this counter-electromotive force, or CEMF, and it grows with speed. At a standstill there is no CEMF, so a motor across a battery looks close to a dead short. Large motors use starting boxes, banks of resistance cut out as speed and CEMF build. A small hobby motor just takes a slug of current at startup, which is why a stalled motor pulls far more current than a running one.
Series, Shunt, and Compound Windings
The field can come from permanent magnets or from electromagnets, and the film shows three ways to wire those field coils. It explains them on the generator side, where the difference shows up as voltage regulation:
- Series wound: the field coil sits in series with the armature, so field current rises and falls with load current. Voltage regulation is poor.
- Shunt wound: the field sits in parallel with the armature and the load, so field current is independent of load current. Regulation is much better, and a changing load moves the output voltage very little.
- Compound wound: both fields together, which the film says gives good voltage regulation and fair current regulation.
The same three arrangements exist on the motor side. Most small hobby motors skip the question entirely, because a permanent magnet supplies the field and there is no field winding to connect.
What This Means at the Flying Field
The brushed motor in a cheap starter kit or park flyer is the film’s machine in miniature. Permanent magnet field, wound armature, commutator, a pair of carbon brushes. Every time a commutator segment passes under a brush you get the small spark the film warns about. That is why brushes wear down and commutators get grooved and blackened. Run one long enough and you will open the can to clean the commutator and fit fresh brushes. That is maintenance, not a defect.
Brushless motors took over because they delete the wearing parts. Commutation happens electronically in the ESC, which switches current through the windings in sequence. No brushes, no commutator, no sparking, less electrical noise, and the windings sit on the housing where they can shed heat.
The generator half of the film is not trivia either. Chop the throttle and let the model coast, and the motor keeps turning. A turning motor generates. Some ESCs route that generated current back into the battery. That is regenerative braking, and it is the film’s generator action running inside your own power system.
If you want to watch the theory next to the hardware, browse the motor section. This stuff is easier to follow with a motor on the bench.