In extrusion, plastic granulate is plasticized by converting mechanical energy into thermal energy. The mechanical energy required for this (torque and rotation of the screw) is generated with the aid of electric motors, i.e. from electrical energy. The additional heaters often only contribute a small part to the overall energy balance.
Nowadays, asynchronous and synchronous motors are generally used in new extrusion lines, more rarely also torque drives. Drives from the category IE2 or higher are used. Category IE2 (high efficiency) corresponds approximately to the old classification (EFF2). Categorizations such as IE1, IE2 or IE3 differentiate drives depending on their maximum efficiency. The higher the maximum efficiency of a drive, the higher the number of the category reached.
Electrical drives usually achieve maximum efficiency close to their standard operating point, whereby the standard operating point is defined by 100% of nominal speed and 100 % torque utilization. The efficiencies at the standard operating point are generally above 90 %, i.e. in a very low-loss range overall.
Drive efficiencies of less than 40 % still possible today?
However, it is important that the efficiency of an electric drive is not constant over all operating points, but that it can also be different when operating conditions change. Efficiency maps show the dependence of the efficiency in comparison to the load of the drive. Such (usually three-dimensional diagrams (efficiency, speed, torque)) can be taken from the efficiencies at different operating points. It is extremely interesting that with a very low utilization of the drive, the efficiencies can sometimes reach values of less than 40 %. Unfortunately, such efficiency diagrams or engine characteristic diagrams are rarely published by the drive manufacturers, so that these values can only be checked by real measurements. The illustration shows abstracted and schematically different effects of the operating point on the efficiency. Synchronous drives (AC) tend to be regarded as drives with less influence of the operating point on the efficiency (behaviour similar to the blue curve) whereas direct current drives (DC) tend to have higher dependencies (behaviour similar to the red curve). Ultimately, however, there are too many influencing factors, so that a generalization is not so easily possible and only real measurements can provide reliable information.
Extruder without oversizing rarely
There is also another aspect that relates to the dimensioning of the drive, i.e. the installed drive power on an extruder. The human approach to selecting a new drive unit is often like this:
When purchasing a new extruder, the first question is what throughput the extruder should achieve. Values are often already defined that have little in common with the reality of the production situation.
Typical example:
The currently realizable production speed allows maximum throughputs of approx. 150 kg/h.
When purchasing a new extruder, it is important that it does not reach its limits so quickly (“Who knows what capacity utilization will result in the next year”). Therefore, the extruder manufacturer is requested for an extruder dimensioning for a throughput of 175 kg/h.
The manufacturer of extruders is familiar with the dimensioning of drives, but has a standardized program. A look at the design table reveals to the sales representative that series A is indicated up to a throughput of 160 kg/h, but Series B can achieve a maximum throughput of 210 kg/h. Consequently, the configuration is selected from Series B, because series A does not fit the customer request (175 kg/h).
In the definition of series B by the engineers and technicians of the extruder manufacturer, it was specified that the extruder should be able to achieve a throughput of 210 kg/h with a highly viscous material. According to calculations, a power of approx. 40 kW is required for this. As standard, the engineers multiply a safety factor of 1.2 so that the extruder manufacturer requests a drive power of 48 kW from the manufacturer of the electric motors.
The manufacturer of the electric motors also has standardized sizes and now offers a drive with a nominal power of 55 kW to avoid under sizing.
The system offered thus has an extruder drive with a rated electric motor output of 55 kW.
Let us now go back to the beginning and take another look at the wishful thinking of the plastics processor. The maximum throughput of 150 kg/h is mentioned there. To be honest, only one product is produced on this line with these high throughputs; for the most part, they produce between 90 kg/h and 120 kg/h.
Oversizing of drives very often
A system is thus acquired which is capable of throughputs of more than 230 kg/h, whereby under realistic assumptions throughputs of less than 120 kg/h are converted and in most cases even produced with less than 100 kg/h. The ratio between installed capacity and real demand is thus an oversizing factor of ~ 2.3.
Even if this example sounds exaggerated, in reality there are many cases that are even worse than those mentioned above. (The maximum result determined by SHS was an oversizing factor of 10.5.)
If this process engineering situation is combined with the above information on the “operating point dependence of the efficiency of electric drives”, it quickly becomes clear that severe over-dimensioning of drives can have considerable effects on the efficiency of the overall system. With a drive load in the range of less than 20 %, considerable reductions in efficiency are possible, sometimes to values well below 40 % (always also depending on the drive technology, DC, AC, etc.).
Cut operating costs in half
In such a case, replacing the drive with a significantly smaller electric motor would increase the efficiency by decades, for example to values above 90%. The costs for operating the drive would thus be halved! Payback periods for such measures are often in the range of a few months.
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