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Frameless Motors Give OEMs More Design Option

by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association

Innovative products require innovative solutions. Although standardized housed motors provide an array of options at an appealing price point, they also involve constraints introduced by the choices of the manufacturer. For OEMs seeking maximum performance to realize their vision, frameless motors offer greater degrees of design freedom.

Figure 1: Frameless motors are sold as separate rotors and stators (left), which enables them to be integrated directly into the equipment. Compared to housed motors (bottom right), frameless motors are significantly more compact. (Courtesy of Allied Motion Technologies)A housed motor is delivered as an assembled device, frequently in a standardized frame size as specified by organizations like the National Electric Motor Association (NEMA) and the International Electrotechnical Commission (IEC). Housed motors are available in shafted and hollow-bore configurations. Both types generally incorporate bearings, selected by the manufacturer. Housed motors also require feedback for speed/position control, for example from encoders or resolvers. 

A wide range of housed motors are available for purchase as commercial-off-the-shelf (COTS) items. Depending on the nature of the request and the volumes involved, OEMs can often order customized versions of COTS devices. Still, levels of changes are typically limited. For more significant modifications in overall size and performance, frameless motors may be a better choice.

As its name suggests, a frameless motor is an unhoused motor. It isn’t delivered as an assembled device but as a separate rotor and stator (see Figure 1). As a result, a frameless motor can be integrated directly into the mechanical structure of the machine. These motors don’t require gearboxes, shafts, or couplings, all of which can introduce compliance and machine resonances. As a result, a frameless motor can deliver the load to the designated position more precisely and with less overshoot or settling time.

Also known as built-in motors or kit motors, these devices as shipped do not include bearings, shafts, encoders, or sensors, although they can include mounting hardware (see Figure 2). All of those elements need to be added by the equipment builder. The important point is that the design gives the OEM greater control over the parameters and performance of the motor and the system as a whole.

Figure 2: Stators and rotors frequently feature flanges at each end with centering surfaces and threaded holes for installation in the machine. (Courtesy of Siemens Industry)“Frameless motors are a little bit more malleable in that we can design them specifically for an application as opposed to housed motors where there are NEMA sizes and IEC sizes and that’s what you get,” says Joseph Profeta III; Director, Control Systems Group; Aerotech (Pittsburgh, Pennsylvania). “With unhoused motors, there is typically much more discussion about custom length or diameter or aperture to fit the particular application. That precise design match usually allows much higher dynamics in the machine.”

Frameless motors are good choices when size and weight need to be minimized, or when the application demands the highest possible performance (see Figure 3). They can be installed directly in a robotic joint, for example, for a much more compact geometry. They are used in portable power tools and medical devices. They can be designed to deliver very high torque, especially at low speeds.

The technology enables OEMs to spend more time focusing on the aspects of the design that contribute most to their application. “If there’s anything that you don’t like about the way the motor manufacturer chose a component or it’s not ideal for your application, then you can change it,” says Phillip Lucia, Business Development Manager, Allied Motion Technologies (Tulsa, Oklahoma). “You get total design freedom when you’re using a frameless motor.” 

Although the term frameless motor is most commonly associated with rotary designs, it is important to note that linear versions exist. In fact, linear motors are frequently shipped as separate forcer and magnet track.

Figure 3: Frameless motors are available in a range of sizes. (Courtesy of Aerotech)The benefits of frameless motors
Multi-axis gimbals show just how effective frameless motors can be. Frequently used in aerospace and defense applications for beam steering and satellite communications, multi-axis gimbals need to handle heavy loads while remaining compact and lightweight. Here, proper motor choice is essential. If, for example, a motor is oversized on one of the smaller axes, it will not operate as efficiently as a properly sized motor would. And, what’s more, the other axes in the assembly will need to be up-sized in order to handle the additional mass of the oversized motor, leading to a cascade of larger motors having to be used. The final system carries higher cost and weight than one based on a properly sized motor.

Frameless motors can deliver the same force as a housed motor while remaining significantly lighter. “Fuel and weight are tied hand-in-hand on aerospace vehicles, so any waste you can eliminate is to your benefit,” Lucia says. “Frameless motors really shine in those applications.” 

Industrial use cases like indexing tables also benefit from reduced weight. Lower mass means lower inertia, which improves dynamic performance. This facilitates the use of stiffer control loops. Now, the indexing table can make faster moves with less settling time. The use of a frameless motor also reduces the need for gearboxes and belts, which decreases maintenance, points of failure, and sources of mechanical error. 

Paired with the right feedback, frameless motors can provide very tight, accurate speed and position control for applications ranging from machine tools to centrifuges. They can be very useful for processes that require through holes to provide clear on-axis access for beam, wiring or plumbing passage, such as for laser marking or semiconductor reticle inspection equipment. Because frameless motors do not suffer from backlash, they are good for any type of repetitive motion. This includes semiconductor wafer inspection and pick-and-place operations. 

The approach also enables more sophisticated approaches to thermal management. In frameless brushless motors, potting the stator with thermally conductive material can create more efficient thermal paths. “We can do that on a housed motor as well, but with a frameless motor, you can pot it in place, which would make an ideal thermal path,” says Lucia. “You’re going to get complete fill on your machine component. Having the potting fill in basically the entire cavity around the motor is one technique that some of our more advanced customers use to get top motor performance and efficiency.”

The bearing is by far the biggest point of failure in brushless motors. Unless a motor is being driven over current or operated in extreme conditions, the windings and magnets don’t typically fail. The bearings in housed motors are selected by the manufacturer, without any knowledge of the requirements of the actual application. The bearings in an unhoused motor are selected to fit the application and give the exact performance required. 

“With a frameless motor, the part that takes the wear-the bearing-is being added by the end user,” says Profeta. “The lifetime depends on what kind of bearings they use, how well the system is designed to minimize stress and strain, whether they are installed correctly, etc. Mechanical bearings are wearable parts, so there is also the question of whether preventative maintenance is properly performed.”

Trade-offs of frameless motors 
As with any technology, frameless motors have drawbacks as well as benefits. In general, they tend to be more expensive than their housed counterparts. Often, that is not so much a function of the frameless motor architecture itself as it is a result of the demands of the types of applications that require frameless motors. Motors designed for high torque density may need water cooling, for example, which adds to cost.

Designing frameless motors that are free of cogging also adds cost. Frameless motors with cogging are available but the type of applications that tend to be a good fit for frameless motors frequently require cogless motors to ensure smooth motion and fast settling. “There are plenty of unhoused motors with cogging and they will be less expensive,” says Profeta. “Often, with high-performance applications, OEMs are looking for an unhoused motor because they want the higher dynamics and part of that is they don’t want any cogging. That tends to drive the price, not because they are unhoused but because they are cogless.” 

Similarly, machine designers might need extremely large motors or motors with large apertures, characteristics that again drive up the price of the motor. The additional components required for integration, not to mention the engineering hours, also increase the overall cost. It has to be taken into context, however. “The end customer will have to purchase all of the other pieces and parts, and do the design work,” says Profeta. “That may be more expensive than just purchasing a housed motor. There are a lot of non-recurring engineering costs that may be included in that, so you should probably separate those two.”

In any case, the increased price is warranted, notes Randy Summervill, Motion Control Motors Product Marketing, Siemens Industry (Norcross, Georgia). “I would say the additional costs up front are more often than not outweighed by the benefits of reduced footprint, reduced maintenance costs for the overall system, and increased lifetime of the system.” 

Integration and assembly can be more difficult for frameless motors. Maintaining the air gap between rotor and stator, for example, is essential. With an improper air gap, the air gap magnetic field with may vary with angle, causing inconsistent and cyclical torque errors. In extreme cases of non-coaxial alignment, the rotor and stator may actually run into each other, causing damage. 

In a housed motor, proper air gap is guaranteed. In a frameless motor, the responsibility lies with the equipment builder. Success hinges on proper design and planning.

“We get a lot of returns for [rotor and stator damage] because someone’s trying to assemble the motor and they don’t have a good plan,” says Lucia. “It’s crucial when you’re assembling a frameless motor that you think out ahead of time exactly how you’re going to maintain all the tolerances and dimensions that you need to in order to install the motor elements without damage.” 

Working with an integrated design team from the very beginning helps prevent problems. Consider the process of determining the positioning of the motor leads. Mechanical engineers may not necessarily pay attention to this seemingly minor issue. Electrical engineers know that proper location can make the difference between, for example, a short run to the drive and having to route the wires halfway around the machine. A poor choice will affect materials costs, installation time, and reliability. 

With the silo approach, the electrical engineers had no choice but to live with the installation as developed by the mechanical engineering team. With a cross-disciplinary approach, the electrical engineers can provide input up front to optimize lead placement for fast installation and ideal performance.
“Those are very easy decisions to make and a very easy change for us that can save the customer a lot of time,” says Lucia. “We like to work with cross-functional teams for that reason. The organization helps customers catch their own mistakes and helps us talk intelligently to each customer’s individual challenges.” 

Properly specified and installed, frameless motors can help OEMs realize novel, high-performance solutions. “Yes it is additional work, but the payoffs are big in that you can save on the footprint of the entire machine and even have higher performance than you could with a conventional motor, in many cases,” says Summervill.

Success begins with working closely with the motor manufacturer in the design phase. Understand the trade-offs involved, focusing on those most likely to deliver the performance demanded by the application. “I’m a frameless motor evangelist,” says Lucia. “If you are pushing the envelope and trying to make a product that is doing something in a way that has never been done or doing a new thing, then you really can’t do better than having a product that enables you to do that any way you want.” 

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