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Southern Manufacturing & Electronics

Farnborough, Hants(GU14 6XL)

11/02/2020 - 13/02/2020

Southern Manufacturing and Electronics is the most comprehensive annual industrial exhibition in the (more)

Torque motors hold key to reduced energy costs

Torque motors hold key to reduced energy costs
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, drum 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 that 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 is the so-called drum motor - 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. 

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. Even a good motor/gearbox combination may be only 80% efficient. Old DC motors and gearboxes will be worse still. 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 10% more efficient than a motor/gearbox combination. 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 risk of fluid leakage.

Use of a torque motor also slashes maintenance costs, firstly because it is an inherently low maintenance technology and also because there no additional drive train components to wear. Also, without the need for all of these ancillary components, torque motor systems are quicker and easier to install: having to install and align multiple motors, gearboxes, belts and pulleys on something like an 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.

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 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 itself may be slightly larger, but eliminating the need for the extra power transmission components delivers dramatic space savings. The motor can also offer build-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.
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