The commutator

If nothing intervened, the loop would reach equilibrium ($\beta = 0$, $\vec{\mu}$ parallel to $\vec{B}$) and stop: the torque vanishes exactly when the loop is aligned with the field, and from there it would oscillate around equilibrium until stopping. To get continuous rotation a trick is needed.

The brush commutator reverses the current in the loop every half turn. So, every time the loop passes the alignment position, the current changes direction, the torque restarts in the same sense of rotation, and the loop keeps turning indefinitely instead of stopping.

The mechanism is simple: the loop’s circuit is connected to the outside via two conducting half-cylinders (split rings) that rotate with the loop, and two fixed contacts (brushes) that rub against them. Every half turn the brushes move from one half-ring to the other, reversing the contact and hence the direction of the current.

Key formula — Electric motor

$|\vec{M}| = N\,i\,A\,B\,\sin\beta$ with $N$ = number of loops.

A single winding, however, produces a very irregular torque, which vanishes at every alignment. To obtain a more uniform torque, real motors use several out-of-phase coils and multi-segment commutators: while one coil passes through the dead point, the others are at full drive, and the resulting torque oscillates much less.

Collegamenti

Argomenti: Magnetismo Concetti: Dipolo magnetico

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