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Why frameless motors?

Why frameless motors?

Andrew Gibson of Maxon Motor looks at when and why frameless motors can provide an engineering advantage.

There are several benefits to using frameless motors that include reduced space envelope, reduced mass and a lower parts count. These are all interlinked and can be key factors in numerous applications. Using a frameless motor, the customer can design this into the assembly without the need of a shaft coupling device to transmit the rotational movement from the motor shaft to the driven element. The shaft will pass through the bore of the rotor and will be one single piece. This will reduce the overall length by a significant factor, as well as removing potential failure modes.

On a housed motor a portion of the overall length is required for the bearings and location features. Depending on the application there are then further bearings to support the driven element of the assembly. By using a frameless motor, the design engineer can use one set of bearings to support the shaft, which is both the motor shaft and the driven element, removing the need for duplicated sets of bearings. Again, this reduces the part count, reduces the space envelope and removes some potential failure modes. From these simple changes the engineer has more control over the overall size of the assembly and has more flexibility on the package size. By removing the need for couplings and duplicated sets of bearings the part count has reduced. The result is a unit with a smaller housing and less parts which means a lighter package.

This is all well and good, but a certain amount of care is required in assembly to avoid having issues. All the time, magnet grades are improving and getting stronger, which means that motor manufacturers such as Maxon can produce more powerful motors in the same space envelope. This also means that when assembling a magnet rotor over a steel stator core, there are strong magnetic forces attracting the two parts together.

Without a certain amount of care, there are several issues that a customer could experience. The magnet material is relatively brittle and allowing the rotor to crash into the stator could result in chipping of the magnet. This has two problems: there is a piece of magnet loose in the motor which could cause further damage, and in addition the protective coating has been removed from a portion of the magnet potentially allowing corrosion of the magnet material to occur. Often the attractive force of the rotor and stator catches people off guard and trapped fingers can occur. These issues can be avoided with some simple assembly tooling.

Maxon has produced an installation manual for the frameless motors, which is available to download from the website product pages. This gives application examples such as the ones below with ideas on where the bearings can be located on different packaging examples. It also includes ideas for assembly tooling and detailed assembly instructions to avoid the pitfalls above.

Frameless motors are ideal for any application with space constraints and/or a high power to weight ratio. Applications such as optical pods, radar systems, azimuth and elevation drives, pan and tilt systems, CCTV, as well as rotary joints for robotics and exoskeletons are good examples.

To meet the needs of many of these applications, Maxon now offers a full range of its BLDC brushless flat motors as frameless kits. Rotor and stator are delivered separately, without bearings and motor shaft, and connected only when the components are assembled. This offers our customers the best of both worlds: high torque density and minimum volume. With outer diameters of only 43 to 90mm, the EC flat series brushless motors are extremely compact. Designed as external rotor motors, they offer plenty of space inside for cable glands. Maxon delivers them with Hall sensors for easy control.

Benefits include a high level of integration in the structure of the machine, high torques through multi-pole external rotor, low cogging torque with plenty of space for cable glands and high overload capacity. The motors can also be equipped with thermal sensors.

Whether or not a solution with frameless motor is suitable needs to be determined on a case-by-case basis. Maxon specialists offer competent advice to customers; the decision is made together.

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