HSC Physics9.3 Motors and Generators

TORQUE

Torque is the tendency of a force to rotate an object about an axis, fulcrum or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.

Torque is a force that produces rotation.

Torque has the symbol, , 𝞃and units of newton metres (Nm).

If the force is not perpendicular to the distance d it must be resolved into a perpendicular force using trigonometry as shown.

In electric motors, it is a conductor wound into a coil or loop that turns when torque is applied.

The force in the equation for torque comes from the motor effect:

F = BILsinθ

To see how the torque changes as the coil turns, it is best visualise the coil in only two dimensions.

As the coil turns the torque changes because the angle between the magnetic field lines and the plane of the coil (θ) is also changing.

The torque is maximum when the θ = 0 and zero when θ = 90°.

The force stays constant during rotation but the torque changes following a cos graph.

If a radial magnetic field is used instead, the field lines are curved between the north and south poles of the magnet.

The angle θ between the plane of the coil and the field lines is kept closer to zero as the coil rotates.

If θ is small the torque will be kept larger for longer during a rotation.