Design by Objective: Fast-track design tools

Dec. 1, 2010
Higher throughput is often the result of components and systems that are able to move at increased speeds. To quicken the pace and productivity of your

Higher throughput is often the result of components and systems that are able to move at increased speeds. To quicken the pace and productivity of your next design, consider these timely tips and tools.

Slotless servomotors suit high-speed applications

Many design engineers working in the field of motion control are familiar with slotless motor designs, but may wonder if these specialized motors significantly improve machine performance. In many cases, they can. Here's why: Slotless motors offer more torque per frame size, produce more power, run more smoothly, and achieve higher speeds than their slotted counterparts. Following is a closer look at the technology, plus realistic tips on evaluating whether or not your next application could benefit from a slotless motor.

Slotted servomotor construction

A traditional slotted brushless servomotor has a stator made of stamped metal sections called laminations that are stacked to form teeth. Wire is wrapped around these teeth; when current flows in the wire, an electromagnet is created in the stator. Permanent magnets are fixed to the rotor.

Slotless servomotor construction

As in slotted motors, the permanent magnets in slotless servomotors are fixed to the rotor. However, a slotless motor's stator is built without teeth. Motor windings are wrapped around a temporary mold and then encapsulated to hold them in place. Eliminating the teeth yields many benefits.

Higher torque: A slotless motor's redesigned stator allows the rotor to be significantly larger. Because torque increases proportionally to rotor diameter, torque from a given slotless unit is significantly higher than that from a similarly sized traditional motor. Due to the absence of teeth, the area available for windings is also greater — which further increases torque. More specifically, torque at a given speed can be increased by 10 to 25% compared to a slotted motor.

Higher speed: As the magnets pass by the teeth in a slotted motor, a change or modulation in the magnetic flux is created, which in turn induces voltage in the surface of the magnets per Faraday's Law — e = dø/dt. These magnets are conductive, so a current flows in them. These Eddy currents, as they're called, increase exponentially with speed and create heat in the magnets, which in turn diminishes their strength. Because slotless motors have no teeth, they can achieve speeds in excess of 4,000 rpm over slotted motors.

Higher power: Power is calculated by multiplying torque by speed. Because a slotless motor outputs both higher speeds and torques, it can produce more than twice (2x) the power of a slotted motor.

Smoother motion: As the magnets on a slotted motor's rotor move past the stator's iron teeth, they are magnetically attracted to the teeth. This creates a torque disturbance known as cogging. Because there are no teeth in a slotless motor, this cogging effect is eliminated — yielding smoother motion.

Easier tuning: Motors with larger rotor inertias can be easier and simpler to tune. If the load becomes momentarily decoupled from the motor (a common phenomenon) the servo loop is less likely to become unstable if motor inertia is high relative to the load. In fact, precise servo tuning and filtering, which can be difficult to achieve, may not be required with a slotless motor.

Better stiffness: A rotor with a larger diameter has greater stiffness because torque increases with rotor diameter and a higher-torque motor responds faster to any displacement from the commanded position. The torque displacement curve is steeper.

Higher efficiency: All of the above traits boost motor efficiency by 5 to 25% compared to slotted motors.

Disadvantages of slotless motors

Despite their benefits, slotless motors are not without drawbacks and are therefore not suited to every application. For any given size, slotless motors generally have larger diameter rotors, and because rotor inertia increases exponentially with the rotor's diameter, inertia can increase significantly. Consider an application in which load inertia is very low compared to motor inertia, and high acceleration is required. Here, a slotted motor may be able to accelerate faster than a slotless one — if the slotless motor's additional torque cannot compensate for the higher torque required to accelerate the system.

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A slotless motor may also be more difficult to manufacture: The motor manufacturer might need to develop custom winding equipment because standard equipment does not do the job. If the choice is made to encapsulate the motor windings in a resin to enhance performance and create a more rugged motor, the motor manufacturer must have significant experience with resin materials and the potting process itself. These manufacturing challenges could increase costs for both the motor manufacturer and end user.

Application benefits of slotless motors

Higher torque is beneficial for most applications and is often the first thing an engineer seeks when choosing a motor. More torque usually means higher acceleration and greater machine output. Additional torque also means that a smaller motor may be used, which can save money; if the motor is part of a moving component, a smaller motor also weighs less and requires less energy to move.

A motor that can run faster may be an obvious choice for high-speed applications such as centrifuges, but faster speed can also help reduce overall machine cost and increase machine output. If the machine's mechanics can handle higher speeds, a faster move time is possible. If a gear reduction can be selected to optimize torque at higher speed, a smaller motor can be used, thereby saving money and weight. Additional power may also eliminate costly secondary mechanics that can shorten machine life and escalate maintenance.

Two specific applications illustrate indispensible slotless-motor benefits. The first is grinding: Here, smooth motor motion (generated by slotless motors) is imperative, as the cogging effect often associated with slotted motors can produce an undesirable finish on final parts. Another example is battery-powered designs. Here, the higher efficiency and potentially smaller size of slotless motors extends operation.

This month's handy tips provided by Infranor Inc. For more information, visit or call (800) 237-3786.

Step motor includes high-rpm encoder

The all-in-one DMX-K-SA-11 NEMA 11 microstep motor from Arcus Technology integrates a driver, controller, and encoder into the motor's back cap, minimizing external electronics and wiring. The motor uses Renishaw's AM256 magnetic encoder chip for real-time position verification, making it suitable for size-sensitive biomedical, semiconductor, and other applications requiring open-loop microstep motion with real-time position confirmation. The 8-bit chip encoder features an operational range of -40° to 125° C, high resistance to shock and vibration, and rotational speed capability to 60,000 rpm.
Renishaw Inc.
(847) 286-9953

Electric actuators ease speedy applications

ERD electronic rod-style actuators are designed as an economical electric alternative to non-repairable pneumatic cylinders. Sleek and compact, they deliver forces to 75 lb at speeds to 40 ips. ERD actuators are stroke configurable and suit a variety of high-speed applications, such as pick-and-place, sorting and diverting, and more. The actuator is available in body sizes equivalent to 5/8, 1, and 1.5-in. bore pneumatic cylinders. These are available in stroke lengths to 8, 10, and 12 in., and forces to 20, 40, and 75 lb. The steel acme lead screw comes in three lead sizes per size to optimize speed or force.
Tolomatic Inc.
(800) 328-2174

Cable carrier handles high rotation rates

The TwisterBand TB30 cable carrier guides energy, data, and media with little wear, and enables rapid rotating movements to 3,000°. Its smaller masses lower centrifugal forces and allow rotary speeds to 720°/sec. Due to its compact design, the system can be used in both horizontal or vertical positions; at high rotating speeds, the carrier stays close to the axis. The polymer component is easy to use, as the injection-molded chain has easy-access links that allow users to simply press in cables and hoses through split openings. Applications are mainly in robotics.
igus Inc.
(800) 521-2747

Conveyor quickens packaging pace

The 3200 Series iDrive belt conveyor imparts faster speeds, increased weight capacity, and new indexing capabilities to larger designs. This new conveyor integrates a brushless dc motor, gearbox, and motor controller within the conveyor frame to save space. As the big brother to the 2200 Series iDrive, the 3200 Series conveyor is designed for a wider range of heavy-duty industrial and packaging applications. It comes with three control options and is ready for quick integration into existing packaging and processing lines. Side controls allow easy speed and direction adjustments.
Dorner Mfg. Corp.
(800) 397-8664

Compact encoders suit high-speed machines

POSITAL's OPTOCODE encoders with interfaces for EtherNet/IP and PROFINET IRT networks are suited for sophisticated motion control on packaging lines, cartoning machines, and other designs requiring accuracy and precision. Both encoder types can be used in high-speed machines: The EtherNet/IP models, which incorporate an Ertec200 controller, have cycle times of less than 10 msec, while the PROFINET IRT models support cycle times as low as 1 msec (isochronos real-time) and 10 msec (real-time). The encoders feature a 2.3-in. diameter and maximum length of 3.1 in.
(609) 750-8705

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