The Core Of The Matter

Online Editor

Dave Walsha asks: are coreless motors worth it?

The first coreless motors were designed in the 1930s, but it took decades for them to find their place in the commercial market. Today, coreless motors are a vital component in a wide range of applications, from laboratory equipment to commercial aircraft. There are a variety of applications where coreless motors can improve performance.

A typical brushed DC motor consists of an outer stator, typically made of either permanent magnet or electromagnetic windings, and an inner rotor made from a laminated iron armature wound with copper wire. In contrast, a coreless DC motor has no laminated iron core in the rotor. Instead, the rotor windings are wound in a skewed or honeycomb structure to form a self-supporting hollow cylinder or “basket”.

Higher Power, Less Weight

The primary benefit of a coreless motor is the high power-to-weight ratio. Removing the heavy iron core enables coreless motors to operate very efficiently and makes them ideal for applications in which the motor must deliver the maximum amount of torque in a limited space.

For example, small, high-power motors are used in a number of applications in the aviation industry, including in the locking mechanisms on aeroplane cabin doors, in the valves that regulate pressure in the aircraft’s air-conditioning system and in operating the locking pin that keeps the landing gear extended. Airlines look to cut weight wherever they can to reduce fuel consumption and aircraft manufacturers can achieve these weight reductions by choosing coreless motors, while maintaining the high levels of reliability and precision needed in these critical applications.

Traditional DC motors can suffer from an issue known as cogging, which gives the motor’s rotation a “jerkiness”. It occurs due to the magnetic interaction of the permanent magnets and the iron laminations in the rotor. This issue is eliminated with coreless motors as there is no iron present in the rotor, producing a more uniform rotation. The lack of an iron armature also makes for a very low rotor inertia, which allows coreless motors to accelerate very quickly – particularly advantageous in equipment that needs to work quickly and repeatably, such as laboratory equipment.

Ironless Brushless Motors

Traditional DC motors are mechanically commutated using brushes, often made from graphite or a precious metal, which are held against the motor’s commutator. As the rotor turns, the winding segment on the commutator that is energised changes. This keeps the rotor’s magnetic field aligned with the magnets in the body, which results in consistent torque.

In a brushless motor, however, the permanent magnets are used in the rotor and the stator surrounding it is a copper winding. To generate a changing magnetic field in the stator, the system needs to be electronically commutated using a separate controller. This switches the current in the motor phases to energise the appropriate section of the winding, which generates torque and rotation.

Ironless brushless motors deliver the same fundamental benefits as coreless brushed DC motors. However, ironless brushless motors generally last longer — up to ten times more depending on their configuration. This is because friction with the commutator will wear out the brushes inside a traditional motor over time, whereas brushless motors have no wear components except for their bearings.

Future Trends

Despite their various benefits, coreless motors are still less commonly used than cored motors, due in part to their perceived higher cost. However, once you account for the total benefit a coreless motor can bring to an application, the balance may well change in future years considering the constant drive for miniaturisation in many industries.

Next generation products are typically expected to deliver improved performance in a smaller product. This is particularly apparent with laboratory equipment, which is expected to become more and more automated with increased functionality while remaining small in size. Coreless motors enable design engineers to achieve maximum performance out of a given volume.

Dave Walsha is with EMS

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