A d.c. motor operates a greenhouse vent. Its coil rotates between fixed permanent magnets and is connected to a split-ring commutator. The carbon brushes have been positioned incorrectly, so the current reverses shortly before the long sides of the coil exchange positions.
Which explains why the coil begins to slow when this early reversal occurs?
Both forces fall to zero, so there is no turning effect acting on the coil at all.
Both forces now act in the same direction, so they push the coil sideways instead.
Both forces reverse, but the turning effect still acts in the direction of rotation.
Both forces reverse, so the turning effect now acts against the direction of rotation.