Coreless dc Motors

The development of coreless motors dates back to the middle 1930s. But it wasn’t until the early 1960s that they were produced economically enough to gain wide acceptance.

Major advantages of coreless motors include very low inertia, low mechanical time constant, and high efficiency. Because the core is ironless, its low mass allows more rapid acceleration and deceleration than any other class of dc motor.

Other benefits gained by eliminating the iron core include the absence of magnetic fields acting on the laminations. This interaction in conventional motors appears as torque ripple or cogging plus a resisting torque that decreases motor efficiency. The absence of iron eliminates cogging and the coreless motor operates smoothly, even at low speeds.

Elimination of the iron core dramatically diminishes rotor inductance and resultant arcing. Commutator arcing in conventional motors is caused primarily by the release of stored energy in the armature inductance upon commutation. Excessive arcing produces electrical noise and reduces the life of brushes.

Coreless motors are classified by rotor shapes as cylindrical or disc. Cylindrical rotors are further divided into those containing inside fields or outside fields. The disc types have pancake, printed, or three-coil rotors.

The cylindrical outside-field motor has the smallest mechanical time constant. The stator is a cylindrical permanent magnet surrounded by a mild steel housing. The rotor is a hollow cylindrical coil wound of copper wire and located in the center of the stator. A mechanical time constant of 1 msec is not unusual for this type of motor.

Rotors are typically wound in a skewed or honeycomb pattern (also known as Faulhaber winding) to ensure that all of the core helps produce torque and smooth operation. The flux lines extend radially outward from the pennanent-magnet stator through the air gap. The soft iron housing is the flux return path which allows the air gap to be extremely small, producing a high flux density.

The cylindrical inside-field motor is a similar design, but the permanent-magnet stator is located inside the hollow rotor. The motor also features a low moment of inertia, but the mechanical time constant is typically higher than the outside-field motor because of smaller stator magnets.

Coreless motor commutators and brushes are typically small, primarily because they are made of precious metals-gold, silver, platinum, or palladium. In addition, a smaller commutator has lower peripheral speed, less wear, and accounts for a smaller motor.

Outside-field motors are usually selected for high acceleration. Because of this, the rotor coils must handle a large load torque and dissipate high heat produced by peak currents. To handle the torque, manufacturers strengthen the rotor with glass epoxy. Since the rotor does not have an iron core to act as a heat sink, the housing has ports for forced air cooling.

In any electric motor, operation is based on simple electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field. As you are well aware, when opposite (North and South) polarities attract, while like polarities (North and North, South and South) repel. The internal configuration of a DC motor is designed to harness the magnetic interaction between a current-carrying conductor and an external magnetic field to generate rotational motion.

Every DC motor has six basic parts-axle, rotor, stator, commutator, field magnets and brushes. In most common DC motors the external magnetic field is produced by high-strength permanent magnets. The stator is the stationary part of the motor; this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotates with respect to the stator. The rotor consists of windings, the windings being electrically connected to the commutator.

The geometry of the brushes, commutator contacts and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnets are misaligned and the rotor will rotate.