What is a coreless DC motor?

A typical brushed DC motor consists of an external stator, usually made of permanent magnets or magnet wire windings, and an internal rotor made of iron laminations with coil windings. The order in which the rotor windings are energized is controlled by commutators and brushes to produce continuous rotation.

A conventional DC motor consists of an external permanent magnet stator and an internal rotor, which is made of iron lamination with winding

Coreless DC motors remove the rotating laminated iron core in the rotor and instead the rotor windings are wound in a deflected or honeycomb shape to form a self-supporting hollow cylinder or "basket". Since there is no iron core to support the coil windings, the entire coil is bonded together with epoxy resin by electromagnetic wires, forming a strong cylindrical bond. The internal coreless stator is made of rare earth magnets such as NdFeB, AlNiCo (Alnico) or SmCo (sa cobalt).

The coreless motor removes the iron core in the rotor. Instead, the rotor windings are wound in a deflected or honeycomb shape to form a self-supporting hollow cylinder. The stator magnet is located inside the coreless rotor.

Image source: Reno Technology

The brushes used in coreless motors can be made of precious metals or graphite. Precious metal brushes (silver, gold, platinum or palladium) are paired with precious metal commutators, a design with low contact resistance and typically used in low current applications. When using sintered metal graphite brushes, the commutator is made of copper. The copper-graphite combination is more suitable for applications that require higher power and higher current.

Coreless motors have hollow and self-supporting rotors, which reduces mass and inertia

Image source: Reno Technology

Compared with traditional iron-core DC motors, the structure of coreless motors has several advantages. First of all, the removal of the iron core significantly reduces the mass and inertia of the rotor, thanks to which very fast acceleration and deceleration speeds can be achieved. Moreover, the absence of an iron core also means no magnetic losses, and the hollow cup design is much more efficient (up to 90%) than conventional DC motors, and also reduces winding inductance, reducing sparks between brushes and commutators, thereby extending motor life and reducing electromagnetic interference (EMI).

The ironless design eliminates the magnetic interaction between the permanent magnet and the lamination, and the cogging effect that occurs in the core motor is eliminated, so the torque ripple of the coreless motor is extremely low, and the motor rotation is very continuous and stable, while minimizing vibration and noise.

Since coreless motors are typically used for highly dynamic motion (high acceleration and deceleration), the coils in the rotor must be able to withstand high torques and dissipate large amounts of heat generated by peak currents. Since there is no iron core to act as a heat sink, the motor housing often contains ports to facilitate forced air cooling.

The compact design of coreless motors makes them suitable for applications requiring a high power-to-size ratio, typically in the range of 6 mm to 75 mm (as small as 1 mm in size) and with power ratings typically below 250 W. Coreless designs are a particularly good solution for battery-powered devices because they consume very little current under no-load conditions.

Coreless DC motors are widely used in medical applications, including prosthetics, small pumps such as insulin pumps, laboratory equipment, and X-ray machines. Their ability to handle fast and dynamic movements also makes them ideal for use in robotics applications.