Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air. Overheating might be an issue for coreless DC motor designs.
Among these types are the disc-rotor types, described in more detail in the next section.
Vibrator motors for cellular phones are sometimes tiny cylindrical permanent-magnet field types, but there are also disc-shaped types which have a thin multipolar disc field magnet, and an intentionally-unbalanced molded-plastic rotor structure with two bonded coreless coils. Metal brushes and a flat commutator switch power to the rotor coils.