Construction PM Brushed Motor

Brushed motors are direct current (DC) motors where the windings are attached to a rotor, and that rotor rotates with with respect to a static stator that contains the magnets. There are about three configurations:

• Iron core motor
• Coreless motor or hollow rotor
• Disc armature motor

Iron core motor

At the iron core motor the windings are wrapped around a packet of laminated teeth. These teeth are part of the magnetic pathway. The rotor is constructed as such that between the teeth and the magnets on the stator a small airgap is present (< 0,5 mm). Because this airgap can be made so small it takes relatively less magnetic material to get a high fieldstrength.

The toothed structure gives rise to cogging. Cogging is a position dependant torque, the teeth namely know a preferred position in the magnetic field. This is being caused by reluctance. The cogging is lessened by twisting the teeth around the length axis of the rotor, this however makes the construction more expensive.

Coreless motor or hollow rotor

The coreless motor has no iron core, the windings are wound in the shap of a tube and glued together with epoxy, and on the end it is connected with the axis. The stationary magnets are located within the cage of windings. Because of this structure larger magnets must be used to create a fielstrength that is comparable with the field of an iron core motor. The magnetic field now after all must cross an airgap four times and pass the windings twice which causes the reluctance of the magnetic field to be high.

The benefits of the coreless motor w.r.t. the iron core motor are that these motors do not have cogging, no ironlosses and a smaller inertia.

Disc armature motor

The disc armature motor has a disc shaped rotor and the magnetic field is oriented axial, instead of radial as is the case with the coreless motor or the iron core motor. Magnets are mounted on both sides of the disc so a strong field can be achieved. The discs have a large surface so a lot of magnets can be applied so that a large driving torque can be generated. The motors are flat and are therfore also referred to as pancake-motors.

The application of a lot of magnets make these motors expensive. The benefits of the disc armature motor w.r.t. the iron core motor are that these motors do not have cogging, no ironlosses and a smaller inertia.

There are two constructions of the rotor:

• Glued windings
• Etched windings (PCB)

Glued windings

In this construction the windings are wound in packages and glued together with epoxy. The pacjages are mounted to the rotor-disc. The glueing is done in a similar manner as the windings of of coreless motor.

Etched windins (PCB)

The windings are etched in the desired pattern on a PCB (Printed Circuit Board).