Direct drive motor technology has played a big role in advancing today’s machining capabilities by enabling increases in both performance and reliability. Despite this, there are still many that are hesitant to adopt servo motors that utilize this technology due to a number of factors — one being unfamiliarity. To make the best use of these direct drive motors, it is important to understand not only the meaning behind important specifications, like torque, but also current trends so that their advantages can be properly applied.

Servo Motors and Torque

When specifying servo motors for applications like a new machine or CNC retrofit, there are many considerations regarding servo motor performance and sizing. The most commonly used factor is considering the differing torque capabilities of the motor.

Torque can be measured in foot pounds (lb-ft), inch pounds (lbf-in), or more commonly Newton meters (Nm).

Figure 2. ETEL’s line of TMB+ direct drive motors ranges from 160 mm (6.3”) to 1288 mm (4.2’).

Torque — Also called moment of force, torque can be defined as the mechanical work generated by the turning effect produced when force is applied to a rotational axis. It is the measure of how much a force acting on an object causes that object to rotate.

Maximum Torque or Peak Torque (Tp) —Commonly known as peak torque, it is the greatest amount of torque the motor can generate for a very short time period (typically specified in seconds). Exceeding this value risks not only overheating but demagnetization.

Rated Torque (MN), Root Mean Square (RMS) Torque, or most commonly Continuous Torque (Tc) — These terms define the average torque a motor can operate during its operation. An application can freely work below and above this value but no higher than the peak torque as long as it averages out to the value. Out of all the other values, it is the most flexible because the limit comes from the heat generated. Factors that can increase this value are whether the motor is water cooled, ambient temperature of the environment, and operating speed.

Stall Torque (Ts) Also known as Standstill Torque, it is the torque limit when the output rotational speed is zero. This tends to be lower than the continuous torque due to the current not being evenly distributed across the phases.

Servo motors all have a maximum temperature that cannot be exceeded without damage. This needs to be taken into account because if a motor overheats, internal damages will occur and they cannot be repaired. This is why it is important to not only understand these values but to know whether the values on the motor’s datasheet have variations as to how the particular brand defines them.

Top Trends in Direct Drive Motor Technology

Figure 3. Integrated cooling where the outside channels are enclosed for easier integration.

It is a common thought that motor technology has plateaued and that there is not much room for improvement. This could not be further from the truth. Numerous aspects of direct drive motors are still being redesigned and fine-tuned to allow for the best user experience. Below are the top current trends in direct drive motor technology.

1. Increases in Torque Density — One of the key advantages of direct drive motors is the high performance they deliver relative to their footprint. Having a high torque density allows the machine to take up less space overall without having to sacrifice power. A competitive environment that demands efficiency in machines with ever-more complex parts has the machine tool industry, and in turn motor manufacturers, looking into methods to maximize the force density within a volume. A common approach is to look for ways to fit more copper coils within a volume with either unique shapes, higher-quality materials, or different proprietary manufacturing methods. The more densely packed the copper coils, the more strands can fit, leading to an overall stronger magnetic flux and torque density.

2. More Size Options — A common challenge when designing in a motor is fitting within certain size constraints. Increasing geometrical variations in motor design is providing machine builders with more flexibility to fit an optimum motor for a particular application in the space available while still minimizing the footprint. With torque motors, users now have variables of diameter and length. If the outer diameter of a motor is limited, then there are often many height options available to the machine manufacturer that still maximize torque. Having a large variety of sizes allows a machine builder to more easily maximize the space and efficiency of the machine, so much so that it may start to feel like the motors are made just for the machine and not the other way around.

3. Simplified Integrations — In many machining applications, internal liquid cooling has become standard and it is not difficult to see why. Properly dissipating the heat can essentially double the continuous torque capabilities of a motor and allow for working with a much smaller size. Still, working with liquid can be intimidating in certain applications. To help ease these concerns, motor manufacturers now provide the option of standalone, built-in, closed-cooling jackets. These all but eliminate coolant leakage and the challenge of designing optimum cooling channels. More so than ever, the responsibility to provide solutions for these issues is shifting from the machine builder to the motor manufacturer.

Figure 4. An example of a torque vs. speed graph for a direct drive motor. Changing the windings moved the diagonal line across the X-axis.

4. More Winding Options — Although a motor datasheet may have listed torque values (e.g., continuous and peak), it may still be limited by how much of that value is available at different speeds. Due to back-EMF voltage, eddy currents, and other factors, the greater the speed a motor runs, the lower its torque capabilities become. A common way of reducing these effects is by adjusting the winding of the copper coils to raise the speed threshold at the cost of requiring more current. This is an aspect that can be adjusted with minimal impact on the motor’s torque capabilities. Most motors have at least two options but there is the possibility of having up to four, which allows more flexibility in terms of what can be done within a fixed motor size. The idea is to propose several torque vs. speed profiles within each motor footprint in order to satisfy as many operating ranges as possible.

Figure 5. ETEL’s linear motors can have their track arranged in a circle to deliver torque across a large diameter.

5. New Ways of Integrating — Direct drive motors have been incorporated in many standard applications for a while but as machine builders have become more familiar with the technology, they’ve come up with new ways to take advantage of direct drive. One example of this is how linear motors have been used to provide torque. In an application that requires a large diameter but not necessarily the large amount of torque that a motor in that size would provide, the magnetic track can be arranged as a curve rather than a straight line for moving a payload in either a circle or “stadium configuration.” This not only takes up less space than a torque motor but it is also a more economical solution since users can avoid paying for a large torque motor with unnecessarily high torque capabilities. Combine that with the modularity of a separate linear motor that can be used in parallel and it is also a solution that is easy to integrate and maintain.

6. Greater Efficiency — An increase in demand for greater energy efficiency due to environmental concerns has many companies seeking out “greener” technology. Too meet these standards, motor makers have been looking into different novel design ideas to keep power losses as low as possible. This includes making the lamination stack, which the coils wind around, out of incredibly small layers (~0.1 mm) to reduce eddy currents, utilizing Litz wires for the coil windings and segmented magnets. Just like with increasing power density, motor manufacturers are looking at all aspects of the materials and how they’re put together in order to get the most efficient performance out of the same basic design first conceived for direct drive technology.

This article was written by Brian Zlotorzycki, HEIDENHAIN Business Development Specialist for ETEL Motors (Schaumburg, IL). For more information, visit here .

Motion Design Magazine

This article first appeared in the February, 2021 issue of Motion Design Magazine.

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