Housed or framed torque motors and frameless motors seem to differ by much, but they are in fact quite similar. In this article, the differences and similarities are addressed to guide Engineers to the right choice in the design of their devices and systems.

Before diving too deep into the technical specifics, let’s start with a definition of what each type of motor is.

What is a Frameless Motor?

A frameless motor consists of just active parts - a stator and a rotor, enabling direct integration into the housing of an application. Normally, frameless motors are torque motors, meaning that they can provide a high torque and spin at a lower speed than conventional Brushless Direct Current (BLDC) motors. However, this is not always true, and it depends on the design of the motor and the requirements of the system, which will be touched upon later on here. The frameless motor is then a kit, consisting of a rotor and a stator. Once integrated into the driven mechanism, sharing its shaft, bearings, etc., this results in a more compact, and lighter design. Frameless motors are typically exceptionally lightweight, and power-dense - especially coreless ones.

What is a Housed (Framed) Torque Motor

Framed or housed motors can, in principle, be defined as all motors that are not frameless, however in this article the name framed or housed motor is referred to a torque motor that is combined with a housing, bearings, encoders, sometimes drives and brakes, to end up in a solution that is already integrated and ready to use. Figure 1 shows an example of a housed version. A torque motor that has been framed has no backlash, no mechanical friction, virtually no compliance (very rigid) and low inertia. They typically include some mounting patterns to help with the mounting of it and come with recommendations for proper operation with different automation solutions.

Applications for Framed and Frameless Motors

In this section the most common applications for both types of motors are addressed. Even though framed motors can be seen as a straightforward way of combining compatible components with the same frameless motor and integrate them, the specifications of both will differ, since the selection of such components limits somewhat the options of integration and the thermal behaviour of the system.

Frameless Motor applications

Even when frameless motors can be used in all applications where the characteristics of the motors are adequate to the mechanical and electrical needs of the system, there are particular applications where the mechanical concept is more adequate with a frameless design. Some of those applications will be described next.

What all these applications have in common is that their mechanical design needs to be light, compact, and safe, though aspects related to durability and reliability are also relevant.

• Robotic Joints: Just like in the human body, a robotic joint is the connection between two different parts of the robot, allowing for an articulation and motion of such. Since the articulation size will limit the motion of the arm or leg in question, these need to remain as small as possible. Moreover, since the motion in arms and legs is typically solved with a serial approach, where each joint needs to move the load of the subsequent joint, the weight of the whole chain of joints or actuators and of each of them is very important. Therefore, frameless motors are preferred for these applications, since with a frameless motor a very compact and integrated joint can be achieved.
• Precision Gimbals: Used in payloads for optronics and metrology devices, among other applications, precision gimbals must be typically as compact and as light as possible. This is especially true in airborne applications, where a common term used to describe this characteristic is SWaP, which stands for Size, Weight and Power. To optimize for all those three important aspects of a precision gimbal, frameless motors are the right choice. It is also true that there exist so-called gimbal motors that are framed and used in Gimbals, however those add some weight from the housing and are less integrable than frameless motors, which makes them less popular when SWaP optimization is really important.
• Medical Devices like Exoskeletons or Bionics: One of the most important applications for light and compact mechatronics is anything that needs to be worn. Wearables mechatronics are becoming a giant in the medical market, not only because the mechatronic designs are today much more affordable than in the past, but also because the technology has advanced notably. Among all the (many) wearables that need motors for their function, two are mentioned here, exoskeletons and bionic prosthetics. Exoskeletons and Bionics designers need to optimize for SWaP as well, since these devices are typically in contact with the human body and users value not only a comfortable design but also as unnoticeable as possible.

Framed Torque Motor applications

Framed motor applications are endless. They have been used for decades and the main reasons to use them are not only convenience and safety, but also time to market and maintenance. A framed motor is basically the same frameless motor described above, only with a more components integrated with the motor, including bearings, a housing and often encoders, brakes, cables, connectors, hall sensors, temperature sensors, etc. The great advantage of this approach is time (quicker installation), since the designers do not need to figure out everything mentioned above and just select the right motor configuration that the application needs. This comes with certain logical disadvantages, which are related to flexibility, compactness, weight, and cost, but can present many advantages, from supply chain to design. Some of those endless applications for framed motors are:

• Parallel Kinematics: one example of parallel kinematics is a delta robot. Delta robots are typically driven by the traditional drive train that consists of a BLDC motor and a gearbox. However, a new approach has been observed in the market in the recent years of using direct drives (a housed torque motor) instead of the long drive train. In this concept the motor used is typically a framed torque motor that includes a brake and an encoder.
• Rotation Stages: used in many different applications, rotation stages are basically a framed motor themselves. The difference between a rotation stage and a framed motor is basically that the rotation stages are more a system rather than just a motor. They are typically paired with precision bearings, include electronics or at least a good recommendation for it and metrology values of the precision achieved by them
• Packaging Machines: Framed motors are typically used in packaging machines for many different applications, including case packers, indexing, winding and unwinding, labelling, etc. Since packaging machines cannot stop their operation for long periods of time, the quick swap of motors in case of failure is very relevant.

Technical considerations

Considerations before buying a Frameless Motor

When selecting a frameless motor for a specific application, engineers must consider several factors:

• Required torque and speed
• Thermal management
• Space constraints
• Weight
• Supply voltage and maximum input current
• Precision requirements
• Other components in the system

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For more information about how to select a frameless motor, visit the article related to this particular topic: https://www.alvaindustries.com/post/an-engineers-guide-to-frameless-motors.

Considerations before buying a Framed Motor

While choosing a framed motor, the difference is mainly that most of the aspects that will be easy to modify with a frameless motor, will be already set and cannot be changed any longer or would require certain amount of effort to do so. The things to consider include housing, encoder, connector, cable length, brake, bearings, certifications, electro-magnetic interferences, etc. All other considerations mentioned for the frameless motor are also valid for the framed ones. Here a few of the specific ones to framed motors are explained:

• Encoders / Control: the encoder in the motor is not only important in terms of positioning and precision, but also for the speed and position control of the motor (in case no other sensor is built in).
• Bearings: depending on what kind of load is going to be moved with the motor, the bearings on the motor will affect the capacity of the motor, not only its torque and speed characteristics. Moreover, the precision of the motion will be also affected by the selected bearings.
• Brake: If the motor includes a brake, it will not only stop the unloaded motor but also brake the load. In case the brake is not reliable enough, a second brake could also be installed, but this would increase the size of the system considerably.
• Maintenance-Repair-Cost: one of the main reasons to use a framed motor as described before is to be able to replace it easily and quickly. However, some motors require some level of maintenance, especially on oiling bearings and cleaning.  If the motor selected requires some level of maintenance or it is expected to be repaired from time to time, it is important to analyse the impact of such need.
• Certifications: since the motor could be considered as a system on itself, depending on the market it would be beneficial to count on certifications that are sometimes easier to obtain if the components are certified (CE, UL, ISO, etc.)
• Connectors: frameless motors are typically supplied with flying leads. The reason behind this approach is that customers/designers have the freedom to pick the connector they prefer. Framed motors on the other hand, typically provide their systems with connectors. This is because the encoders have to be connected in the proper way, and it also benefits other aspects like certifications, EMI, testing, etc. The proper connector on the motor will help to quickly match the drive selected and also avoid the annoying task of finding an adapter or changing the connector also voiding the warranty of the component.
• Cable lengths: related to the topic above, the fact of having the connector already in place will affect how long the cable can be. An extension of a cable with an existing connection is normally more difficult than one with flying leads, because cutting the cable is normally either voiding the warranty or affecting EMI, performance of the encoder, etc. Therefore, the selected cable length on the motor should be ideally the length required by the system.

Mechanical and Electrical Integration

Once the motor has been selected for the application, the next step is to make sure that the integration goes as smoothly as possible. In this section, the main points to consider are presented.

Integrating a Frameless Motor

A frameless motor is notably more complex from the integration point of view than a framed one. There are a few important topics to be properly covered while discussing the integration of a motor kit, and they are not in the focus of this article, however, Alva has launched a series of videos and articles related to the subject of integration:

Integrating a Framed Motor

A framed motor is easier to integrate and mount than a frameless motor, however there a few roadblocks to avoid while doing the job. Once the space is available and the size of the motor has been considered, and the topics mentioned above on the selection were properly done (bearing, thermal, load, encoder, brake, cable, EMI, etc.), some of the further topics to consider during mounting are:

• Side loads: avoid side loads not to overcharge the bearings
• Flatness and runout: analyse the mounting faces, following manufacturer's instructions
• Concentricity and axial position: use spacers or preloaded bearings to guarantee alignment
• Fastening (anti rotation): torque-rated screws with the proper length will prevent loose motor situations which could cause collisions and failures.
• Couplers: ideally none are used, but if they have to be used, then they have to be very stiff and short.
• Inertia and stiffness: considering the inertia miss-match with the load is important to avoid tuning issues
• Dust and dirt: making sure that the place of the motor installation is not exposed to a lot of dirt and if so, protecting the motor will help with the lifetime of the system and the frequency of maintenance.
• Serviceability: related to the maintenance topic, if the expected frequency of maintenance is high, it is important to place the motor in an accessible place.
• Adapters and accessories:  screws, bolts, clamps, glue, etc. All these accessories should be checked with the manufacturer to make sure they are at least on their list of known components to avoid surprises later on.

Electrical Integration

The considerations at selecting a motor from the electrical point of view are similar for frameless and framed motors. However, there are some differences, and they are mostly related to the components that are not the motor itself. These components are an encoder, a potential hall sensor, a temperature sensors, a brake, a torque sensor, etc. All these components must be compatible with the drives selected to run the motor. If they are not, then a different drive needs to be selected, or an adapter will be required.

Motor Configurations: Are all Framed Motors Slotted Motors?

Most framed torque motors based on a stator technology called slotted. The difference between slotted and slotless motors was thoroughly explained in another article: https://www.alvaindustries.com/post/slotless-motors-vs-slotted-motors

The reason for the prevalence of slotted motors, is that slotted motors were considered until recently to deliver a higher performance (higher torques) than slotless motors. Alva’s breakthrough on FiberPrinting™ has broken that wall and for the first time ever, slotless motors are competitive in overall performance to slotted motors, while providing lighter solutions.

Alva motors are distinguished by a high motor constant per unit mass, typically outperforming slotted motors of the same diameter. Thanks to their slotless design, Alva motors exhibit virtually no cogging torque and offer highly linear torque characteristics across the entire load spectrum.

Additionally, the thin, slotless topology with a large inner diameter allows for the integration of gears, encoders, sensors, and other components in the centre - an especially attractive feature when designing compact systems.

Framed Motors by Alva

Framed motors as a standard are not yet on the current scope of Alva Industries AS, however, Alva offers Development kits like shown in Figure 6, which allow for a quick test of the technology for those that have not worked with frameless motors yet, or do not have the time to design the proper benchtop test device.

Framed Motors from Alva by others

The fact that Alva is offering frameless motor has allowed other manufacturers to develop framed motors based on Alva’s design. If you are looking for a framed design of the self or are interested in making one available commercially, reach out to sales@alvaindustries.com.

Hybrids between Frameless and Framed Motors

There is a third approach possible with Alva’s SlimTorq™ motors. Since the stator of FibrePrinted™ motors must be potted in a ring to have a solid structure, this ring can be either one that was designed by Alva, or any other design that fits the designer’s choice better. This custom framing is an alternative to frameless and framed motors, that still offers the flexibility and compactness of frameless but the easiness of mounting of framed solutions.  Figure 7 is an example of just that. Reach out to learn more about this at sales@alvaindustries.com.