Summary: A Coreless Motor is a type of DC motor, which has a specially wound armature instead of an armature wound on an iron core.
Moving Coil or Coreless Motors
A type of Direct Current motor, which is different in construction than that of the usual can motor. Instead of a large rotating iron armature, it is made from lightweight material with the magnet wire wound around it. They are designed for rapid acceleration.
DCC and Coreless Motors
See the Video.
Locomotives with coreless motors should not be used on a DCC system without a multifunction decoder installed. They will overheat when operated in zero stretching modes. This is due in part to their low impedance, which makes them look almost like a short circuit. This will allow a lot of current to flow during zero stretching. The excessive current flow will result very quickly in the destruction of the motor. 
High Frequency Decoders
Coreless motors should be used with High Frequency Multifunction Decoders. Some manufacturers recommend a PWM frequency greater than 20kHz. This increases the motor's impedance when driven by a multifunction decoder's motor drive circuits. Not all manufacturers will publish the PWM frequency in their literature. 
Why Use a Coreless Motor?
The coreless or Moving Coil motor has been around since the 1930s. Advances in manufacturing made them practical and cost effective in the 1960s.
- No cogging torque
- Smooth position and speed control
- High efficiency
- Low Noise
- High torque
- Low Weight
- Very low rotor inertia
- Dynamic start-stop operation
The construction of a coreless DC motor eliminates the laminated iron core of the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb (known as the Faulhaber winding) fashion, forming 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 magnet, and sits inside the basket. All of the core produces torque and smooths operation. The design also produces high acceleration rates.
- This design allows for a motor with low inertia and high efficiency. Coreless motors can accelerate and decelerate much more rapidly than a traditional motor.
- Eliminating the iron rotor also reduces torque ripple or cogging. The motor operates smoothly, even at low speeds. The lack of iron in the rotor also reduces the total mass of the motor.
- Reduced noise.
- High Efficiency: Efficiencies of up to 85% are possible with less heat produced in operation, compared to >50% for an iron core motor.
- Extended brush life
- Low time constant, meaning faster response.
Eliminating the iron core of the rotor, while reducing mass, also reduces the inductance, and the resulting impedance of the winding. A typical iron armature motor may have an inductance of 1mH or more.
A typical 12V coreless motor may have a specification of >200μH of inductance. At DCC frequencies the impedance would be about 10Ω. Which would allow a current of almost 1.5A to flow, for a motor with a rated start current of 200mA.
When the impedance is calculated for a given motor, the starting current can easily be exceeded when the DCC track voltage is applied to the motor. For this reason, a coreless motor should never be used without a multifunction decoder on a layout with a DCC System. Modern multifunction DCC decoders are capable of powering a coreless motor.
See the Wikipedia Article for more info: Coreless Motor
More Information on Coreless Motors
Information on DC Iron-core Motors
Coreless Motor Construction Video
- Increasing the drive frequency results in less current flow due to the increased impedance of the motor windings.
- Some manufacturers recommend that coreless motors not be used with their decoders. Typically, these are low-cost decoders with a low frequency driving the motor.
- The lack of the iron armature means there is nothing to conduct heat away from the windings.