A type of Direct Current Motor
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
- Locomotives with coreless motors should not be used on a DCC system without a decoder installed. They will overheat when operated in zero stretching modes. This is due in part to their low impedance/inductance, which makes them look almost like a short circuit to a high frequency signal such as the DCC waveform. This will allow a lot of current to flow. The excessive current will result in the destruction of the motor, very quickly.
High Frequency Decoders
Coreless motors should be used only with High Frequency Decoders. Some manufacturers recommend a PWM frequency greater than 20kHz. 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.
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 magnets, 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 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 Decoder on a layout with a DCC System. Modern DCC decoders are capable of powering a coreless motor.
See the Wikipedia Article for more info: Coreless Motor