A typical brushed DC motor consists of an external stator (usually composed of permanent magnets or electromagnetic coils) and an internal rotor (composed of laminations with windings). A segmented commutator and brushes control the sequence of energization of the rotor windings to produce continuous rotation.
▲A typical DC motor consists of an external permanent magnet stator and an internal rotor made of iron laminations with windings.
Coreless DC motors eliminate the laminated iron core in the rotor. Their rotor windings are wound in an inclined or honeycomb pattern, forming a self-supporting hollow cylinder or "basket." Because there is no iron core to support the windings, they are typically held together with epoxy resin. The stator is made of rare-earth magnets, such as neodymium, AlNiCo (aluminum-nickel-cobalt), or SmCo (samarium-cobalt), and is located inside the coreless rotor.
▲Coreless DC motors eliminate the iron core in the rotor. The rotor windings are wound in an inclined or honeycomb pattern to form a self-supporting hollow cylinder. The stator magnets are located inside the coreless rotor.
Other terms for coreless DC motors include “air core”, “slotless”, and “iron coreless”.
Brushes for coreless DC motors can be made of precious metals or graphite. Precious metal brushes (silver, gold, platinum, or palladium) are paired with precious metal commutators. This design features low contact resistance and is often used in low-current applications. When sintered metal-graphite brushes are used, the commutator is made of copper. The copper-graphite combination is better suited for applications requiring higher power and higher current.
▲The rotor of a coreless DC motor is hollow and can be self-supporting, thus reducing mass and inertia.
Coreless DC motors offer several advantages over traditional iron-core DC motors. First, the elimination of iron significantly reduces rotor mass and inertia, enabling very fast acceleration and deceleration rates. The absence of iron also means zero iron losses, making the coreless design significantly more efficient than conventional DC motors (up to 90%). The coreless design also reduces winding inductance, thereby reducing sparking between the brushes and commutator, extending motor life, and reducing electromagnetic interference (EMI).
The cogging problem in traditional DC motors caused by the interaction between the permanent magnet and the iron plate is eliminated in the iron-free design. Conversely, torque fluctuations are very small, providing very smooth motor rotation with minimal vibration and noise.
Because these motors are typically used for highly dynamic motion (high acceleration and deceleration), the coils in the rotor must be able to withstand high torque and dissipate the large amount of heat generated by peak current. Since there is no iron core as a heat sink, the motor housing usually includes ports for forced air cooling.
▲Coreless DC motors are available in two configurations: cylindrical and flat (disc).
Coreless DC motors, with their compact design, are suitable for applications requiring a high power-to-size ratio. Their dimensions typically range from 6mm to 75mm (although some can be as small as 1mm), and their rated power is generally 250W or less. Coreless designs are a particularly good solution for battery-powered devices because they consume extremely low 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 rapid dynamic movement also makes them ideal for robotic applications.
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