Drives & controls - Energy savings - the torque of the town

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With plastics extruders and injection moulding companies looking to reduce energy usage, minimise maintenance costs and boost productivity, the role of the direct drive torque motor is coming to the fore. Andy Parker-Bates, of Parker SSD Drives Division, explains how the technology differs from conventional motors, and explores the benefits it can bring

They’re known by many names – torque motors, direct drive motors, frameless motors – and often they are thought of as a new technology that needs to be more proven before it becomes a mainstay of industrial automation. So just what are torque motors?

First off, it’s worth making the point this is not an unproven technology. It is a new take on existing brushless servomotor technology that has been around for decades, and is amongst the most reliable technologies available. In short, a torque motor is a rotary brushless servomotor optimised for low speed operation, typically in the order of 50-500rpm. It is a direct drive solution, so there is no need for mechanical transmission elements such as gearboxes.

There are two different kinds of torque motors. There is the more traditional looking motor with frame, cooling system, terminal box and feedback sensor, and then there is a frameless motor made up of two independent elements (rotor and stator) intended to be tightly integrated into the mechanics of the application.

Typical applications for the frameless version include semiconductor manufacture and machine tools, while the framed version meets the needs of applications such as paper machines, crushers, extruders and injection moulding machines. It’s certainly not a panacea for all applications, and indeed below about 30kW it’s questionable whether there are any gains to be had over a conventional motor/gearbox combination at all. But above this, in specific applications such as plastics extruders and injection moulding machines, at a time when end users are looking to reduce operating costs, through better energy utilisation and lower maintenance requirements, torque motors can offer significant advantages.

For starters, a direct drive solution is inherently more energy efficient than a motor/gearbox combination. Modern motors can of course offer high efficiencies and gearbox design has also developed significantly leading to more efficient products. But gearbox efficiency is dependent on the load, the reduction ratio, and the number of stages. Optimum gearbox efficiency is obtained at maximum load, but efficiency decreases dramatically at light loads – down to as little as 20% in worst-case situations. And even in its most efficient load ratings a gearbox will still lose around 2% efficiency per gearing stage.

Further, the motor and gearbox need to be closely matched in order to maximise energy efficiency, and even a good motor/gearbox combination may be only 80% efficient. Additionally, traditional motor/gearbox solutions will also often require belts and pulleys as part of the drive train, further reducing efficiency.

Torque motors, by contrast, improve in efficiency at lighter loads. Thus the direct drive torque motor can easily be between 5% and 12% more efficient than a motor/gearbox combination. If we assume a typical 7% improvement, with an energy cost of €0.10 per kWh, then in 7200 hours operation per year on a 100kW extruder, €5040 will be saved in energy costs alone by switching to torque motors.

Replacing a hydraulic motor to drive the screw on an injection moulding machine, the torque motor could easily offer energy savings in excess of 20%, and deliver higher productivity and clean operation, without the need for fluid changes or the risk of fluid leakage.

We can look at maintenance costs, too. In a typical 110kW DC motor with a gearbox, maintenance can represent a cost of more than €3000 per year. How do we come to that figure? A typical year’s maintenance on a 110kW extruder, maintenance on the DC motor could account for €300 in motor revision, €150 in carbon control/change and €150 in filter maintenance. Gearbox maintenance could contribute €50 in oil draining and €250 in seals replacement, with the potential of €2500 or more in gearbox repairs. And that’s before we’ve considered the cost of downtime in terms of lost productivity.

Use of a torque motor slashes these costs, firstly because it is an inherently low maintenance technology and also because there are no additional drive train components to wear.

Also, without the need for all of these ancillary components, torque motor systems are much quicker and easier to install: having to install and align multiple motors, gearboxes, belts and pulleys on something like a plastics extruder is a process that can take days. By contrast, installing the corresponding number of torque motors can be achieved in just a few hours.
A typical plastics extrusion machine could be using multiple motors, so the savings from not having additional power transmission components quickly add up to something very significant. In a co-extrusion machine, for example, there could be up to nine motors installed in a single machine.

Torque motors also help to address specific machine requirements on co-extruders in the way that conventional motors are unable to match. For example, the screw extraction mechanism may be required from the front of the motor in some instances, but from the back of the mechanism in others. This is readily achievable with some torque motors. At the same time, torque motors with hollow motor shafts can provide extruder screw cooling through the motor, – especially important for big screws.

Torque motors are a low noise, low vibration option. The European Noise Directive 2003/10/CE sets the maximum recommended noise level exposure limits for operators in order to protect against health and safety risks, and sets a maximum noise exposure limit of 87dBA. Above 80dBA, special protective measures must be taken. In a conventional motor/gearbox set-up, just the gearbox alone can often be producing above 90dBA. The torque motor, in contrast, is an inherently quiet technology, producing below 80dBA in most cases, and therefore can play a key role in minimising overall equipment noise levels. Similarly, it is a very low vibration technology. This again contributes to reduced noise levels, but it also has reduced physical impact on the rest of the machinery – ensuring greater reliability – as well as helping to ensure a more uniform product quality.

We also have to look at the costs of downtime in the event of a power transmission failure. Plastics extruders represent some of the most demanding motor applications. Once production has started, the one thing you don’t want to be doing is halting production. The extruder is typically located at the beginning of the production line, so stopping it will call a halt to all production. Because it has to be heated, there are long ramp up times before production can begin. And when there are defects in the output, products cannot simply be recycled and disposal costs are high.

Reliability is therefore paramount. With fewer components in the power train, the direct drive solution is inherently more reliable than a typical motor/gearbox combination, and certainly much easier to replace in the event of a fault, allowing production to be restarted much more quickly.

From the machine builder’s point of view, the torque motor solution is generally much more compact than the motor/gearbox combination. The motor can also offer built-in advantages for specialist extruder manufacturers. An integrated thrust bearing can be added to support back pressure from the screw. This is a nice added feature on injection moulding machines, but is a mandatory feature on plastics extruders. Also, a screw extraction mechanism can be readily built in, making it easy to remove the screw from the extruder for routine maintenance or to allow a new production batch to be set up. And, as discussed, the screw can be cooled by water through the motor, which can be an extremely useful feature on large extruders.

Typical torque motors cover torque ranges from 1200Nm to 22,100Nm, and speeds from 50 to 500rpm depending on size. Water cooling is standard on many designs, but natural ventilation is possible with suitable derating.

With all these advantages, torque motors are steadily making inroads into the plastics extrusion and injection moulding markets. The upfront cost may be slightly higher, but the energy savings, elimination of ancillary components, reduced maintenance costs and improved productivity – not to mention the reduced noise levels – quickly allow users to recoup the premium on the purchase cost. In addition, the more compact design can lead to smaller machines, which frees up valuable floor space, potentially driving opportunities for even higher productivity.