DC motor – split-ring commutator, F = BIL, torque and back EMF

What it shows

A current-carrying wire in a magnetic field feels a force F = BIL at right angles to both the field and the current (the motor effect, direction by Fleming's left-hand rule). On a coil the forces on the two long sides are equal and opposite, so they form a couple with torque τ = NBIA·cosθ, where θ is the angle between the plane of the coil and the field. A split-ring commutator reverses the current every half turn so the torque keeps the same direction. With a voltage supply, the spinning coil induces a back EMF that reduces the current as the speed rises.

How to use

Press Run and watch the coil. Change Current I, Magnetic field B, Turns N, Coil area A and Load torque, wait for a steady speed and press Record to build the table. Try Reverse current and Swap poles. Switch Commutator to Slip rings, or Supply to Voltage (with back EMF). Drag the coil in the view along the axle, use Push at the dead position, and choose the Graph.

Parameters you can change

• Power supply Constant-current supply, Voltage supply (with back EMF)
• Current I 0–3 A
• Supply voltage V 0–12 V
• Commutator Split-ring commutator, Slip rings
• Magnetic flux density B 0.05–0.4 T
• Number of turns N 10–100
• Coil area A 4–25 cm²
• Load torque 0–20 mN·m
• Reverse the current
• Swap the magnet poles
• Starting angle θ between the coil plane and B 0–180 °
• Slow motion Off (real speed), 10 times slower, 50 times slower, 200 times slower
• Show forces
• Show Fleming's left-hand rule
• Graph Torque against angle, Speed against time, Current against time

Questions to explore

1. Why does the torque fall to zero when the plane of the coil is at 90° to the magnetic field?
2. What happens to the coil if the split-ring commutator is replaced by slip rings, and why?
3. With a voltage supply, why is the current largest when the motor is stalled?