Selecting the best servomotors with integrated drives

Although new distributed servo drive systems promise incredible performance, engineers should consider cabling, form factor, and safety integration before standardizing on a platform.

Matt Prellwitz
Drive Technology Application Specialist
Beckhoff Automation

Servomotors with integrated drives were groundbreaking technology when they hit the market, but their full potential has rarely been reached. These servomotors are supposed to reduce footprint and commissioning time while increasing safety and performance. Unfortunately, due to design shortcomings, few have lived up to all of the promises.

Given that motion control architectures are complex and costly to replace, it’s important to understand what differentiates a high quality servo system from the rest. Beyond torque and speed specifications, other important factors include the drive’s cabling, form factor, and safety features. These factors affect performance and reliability, so it’s important to weigh all options when standardizing on a solution.

Daisy chained vs. distributed servo drive systems
When only one motor is necessary, a servomotor with integrated drive technology greatly simplifies installation. The engineer easily connects the unit to power and industrial Ethernet communication, such as EtherCAT. However, most factory settings require a more complex topology with servomotors spread across different machines and lines. In these cases, engineers must decide between a cascaded or distributed system.

A cascaded system daisy chains the servomotors together. The first unit connects directly to power and the I/O terminals in the control panel, then shares with subsequent units. However, this presents a major downside, as the approach introduces a number of potential failure points. If a single error occurs in one motor, power cable or Ethernet cable, the entire motion control architecture could grind to a halt.

On the other hand, a modern distributed servo drive system uses a distribution module to manage multiple servomotors, which can have a protection rating of IP 65 or a similar value. The machine mounted distribution module connects power and communication between the control cabinet and distributed servo drives via a single coupling module, which supports cascaded architectures while limiting points of failure. This approach ensures that if one cable is damaged, it usually doesn’t stop operations across the system as happens in alternative platforms.

Further simplifying cabling issues, a one-cable technology (OCT) connection can provide power and industrial Ethernet communication for high-performance servo drives. This one-cable solution reduces both commissioning time and machine footprint, which is incredibly important in any manufacturing environment. Combining OCT with a distributed servo drive system can provide the cleanest and most efficient motion control solution.

Top- or side-mounted integrated drives vs. back-mounted
Form factor is key for servomotors with integrated drives, since combining both devices into a single functioning unit is more difficult than it sounds. Distributed servo drive systems should reduce footprint requirements for new machines and maintain the same mounting plate and rectangular shape of previous models to support successful retrofits of legacy equipment and minimize any impact on mechanical designs.

Some manufacturers have struggled to successfully embed the drive in the best location on the servomotor. Many current models typically stack the drive on the top or side of the motor, and this significantly alters form factor, resulting in more mechanical changes and a larger footprint. The drive most logically fits at the rear of the motor, but the majority of manufacturers that have attempted to integrate it in this location could not solve heat dissipation issues created by the convergence of motor and drive technology.

Distributed servo drive systems using an IP 65-rated distribution module can support even the most demanding topologies.

By approaching the design challenge differently, a few have succeeded in integrating the drive at the rear of the servomotor while slightly lengthening the form factor compared to servomotor-only models. Many companies have incorporated the drive at the rear of the motor without heat dissipation issues, but reaching the high performance typical of standard servomotors has presented difficulties.

A key factor for maintaining output and form factor has been new heat dissipation methods. Previously, these units would attempt to force all heat through the back of the motor and, therefore, through the integrated drive. However, Beckhoff Automation has devised an innovative heat dissipation method that efficiently releases heat through the sides of the motor housing down the entire length of the shaft, rather than entirely through the drive at the rear. A careful redesign of motor windings in these units makes this possible. As a result, users can select distributed servo drive systems that retain a small footprint without overheating and are optimal for both new installations and retrofits.

IGBTs vs. MOSFETs
Another way to improve heat dissipation and form factor in distributed servo systems is through the selection of semiconductors for drive electronics. While most integrated motor and drive models use IGBTs, the most successful and compact servo drive systems more often incorporate MOSFETs, which produce less heat. IGBTs have been on the market for many years and have proven to be successful in the field. However, recent updates to the design of MOSFETs have made them the better switching option for motion control systems for a few key reasons.

Many differences and similarities exist between the two, and although IGBTs may be preferable in certain architectures, they are less desirable in these types of servo drive systems. As a bipolar device, IGBTs are able to handle high voltage and current. As a field-effect transistor, MOSFETs support higher current with fewer switching losses even when hundreds of milliamps are switched to double-digit amps or when small voltages are switched to thousands of volts.

Integrating the drive at the rear of the servomotor and using one-cable technology (OCT) are two keys factors to minimizing footprint for new designs or retrofits.

In general, IGBTs also require a larger amount of supporting components, including fans, heatsinks and additional wiring. This means that the benefits gained through higher voltage and current come with a sacrifice in terms of footprint and price. MOSFETs provide high performance operation at a lower temperature and with fewer components, which makes them the preferred semiconductor type for these distributed motion control architectures.

Separated vs. integrated safety
In any manufacturing environment, safety is always a top concern. Most servomotors with integrated drives use a separate, standalone safety system. This is more expensive and less efficient since it requires dedicated wiring and frequently a separate software platform and network. However, newer servomotors with integrated drives can provide integrated STO and SS1 safety functions by default in every unit. These should also interface with safety I/O terminals on the same EtherCAT industrial Ethernet network.

Each of these differences in features and design may seem minor when considered separately, but together they can mean servomotors with integrated drives that improve — or seriously degrade — a motion control architecture depending on which path one chooses. Always consider the unique form factor, mounting considerations, cabling and safety requirements of each solution to choose the best distributed servo drive system for the application.

Beckhoff Automation 
www.beckhoff.com