What are coreless DC motors?
A typical brushed DC motor consists of an outer stator, typically made of either a permanent
magnet or electromagnetic windings, and an inner rotor made of iron
laminations with coil windings. A segmented commutator and brushes
control the sequence in which the rotor windings are energized, to produce
continuous rotation.
A typical DC motor consists of an outer, permanent magnet stator and an
inner rotor made of iron laminations with windings.
Coreless DC motors do away with the laminated iron core in the rotor.
Instead, the rotor windings are wound in a skewed, or honeycomb, fashion to
form a self-supporting hollow cylinder or “basket.” Because there is no iron
core to support the windings, they are often held together with epoxy. The
stator is made of a rare earth magnets, such as Neodymium, AlNiCo
(aluminum-nickel-cobalt), or SmCo (samarium-cobalt), and sits inside the
coreless rotor.
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| A coreless DC motor does away with the iron core in the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb fashion to form a self-supporting hollow cylinder. The stator magnet sits inside the coreless rotor |
Other terms for coreless DC motors include “air core,” “slotless,” and
“ironless.”
The brushes used in coreless DC motors can be made of precious metal or
graphite. Precious metal brushes (silver, gold, platinum, or palladium) are
paired with precious metal commutators. This design has 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 more suitable for applications requiring
higher power and higher current.
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Coreless DC motors have a rotor that
is hollow and self-supporting, which reduces mass and
inertia.
The construction of coreless DC motors provides several advantages over
traditional, iron core DC motors. First, the elimination of iron
significantly reduces the mass and inertia of the rotor, so very rapid
acceleration and deceleration rates are possible. And no iron also means
no iron losses, giving coreless designs significantly higher
efficiencies (up to 90 percent) than traditional DC motors. The
coreless design also reduces winding inductance, so sparking between the
brushes and commutator is reduced, increasing motor life and reducing
electromagnetic interference (EMI).
Motor cogging, which is an issue in traditional
DC motors due
to the magnetic interaction of the permanent magnets and the iron
laminations, is also eliminated, since there are no laminations in the
ironless design. And in turn, torque ripple is extremely low, which provides
very smooth motor rotation with minimal vibration and noise.
Because these motors are often used for highly dynamic movements (high
acceleration and deceleration), the coils in the rotor must be able to
withstand high torque and dissipate significant heat generated by peak
currents. Because there’s no iron core to act as a heat sink, the motor
housing often contains ports to facilitate forced air cooling.
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Coreless DC motors are available in both cylindrical and flat (disc)
configurations.
The compact design of coreless DC motors lends itself to applications that
require a high power-to-size ratio, with motor sizes typically in the range
of 6 mm to 75 mm (although sizes down to 1 mm are available) and power
ratings of generally 250 W or less. Coreless designs are an especially good
solution for battery-powered devices because they draw extremely low current
at no-load conditions.
Coreless DC motors are used extensively in medical applications, including
prosthetics, small pumps (such as insulin pumps), laboratory equipment, and
X-ray machines. Their ability to handle fast, dynamic moves also makes them
ideal for use in robotic applications.
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www.vibrationmotor.net
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