DC Motor 3D Virtual Lab makes the hidden mechanism of a two-pole motor visible: magnetic field, coil current, magnetic force, split-ring commutation, dead spots and back-EMF.
Students can change the supply EMF, magnetic field and commutator gap, reverse the polarity, swap the magnets, pause the motion and test force-direction predictions.
Resource type: HTML5 3D simulation, separate simulation and source-code downloads, and guided electromagnetism inquiry
Topic: DC motor, motor effect, magnetic force, split-ring commutator, brushes, torque, rotational inertia and back-EMF
Run the virtual lab Download simulation Download source code Read the blog post
Built on the physics, not an animation loop
The coil turns because the simulation evaluates the magnetic force on each current-carrying side at the coil's instantaneous orientation. The equal and opposite forces form a couple. The split-ring commutator reverses the current every half turn so that the torque remains in the same rotational sense.
For each active coil side, the force follows F = (I x B)L. Reverse only the supply polarity and the motor reverses. Swap only the magnets and it reverses again. Reverse both and the original turning direction returns.
A commutator dead spot learners can see
The orange and green commutator segments rotate with the coil. When a brush crosses an insulating gap, current becomes zero and the force arrows disappear. The coil must coast through the gap using rotational inertia. If it stops in that dead zone, the motor cannot self-start, so the Give a Push control provides the same nudge used with a bench motor.
The Commutator Gap Angle control widens the visible split from 5 to 90 degrees. The electrical dead interval grows at the same time, revealing why real commutator insulation gaps are kept narrow.
Back-EMF during spin-up
The current is calculated using I = (V - back-EMF)/R. As the coil spins faster, it generates a larger opposing EMF, so the current reading falls. The model connects the motor effect to electromagnetic induction: a running motor is also a generator.
Suggested investigation
- Set the supply to 8 V and use Give a Push if the coil begins at the dead spot.
- Pause and use the displayed I and B directions to predict the force on each coil side.
- Reverse the supply polarity, then reset and swap the magnets.
- Reverse both quantities and explain why the original rotation returns.
- Widen the commutator gap and compare the current-free coasting interval.
- Watch current, back-EMF and angular speed during spin-up.
Questions for learners
- Why do the forces on the two vertical coil sides form a turning couple?
- Why must a split-ring commutator reverse the coil current every half turn?
- Why can the motor fail to start when the coil rests at a commutator gap?
- Why does reversing both current and magnetic field preserve the force direction?
- Why does the coil current fall as angular speed increases?
Downloads and open learning
Made by lookang and created using Claude Fable 5 for Open Educational Resources / Open Source Physics @ Singapore. Use the simulation package for offline classroom use and the separately named source-code package for adaptation. Browse the OSPSG Electromagnetism collection or find this resource in the digital library.