PRODUCTION | ENERGY MANAGEMENT
Figure 1. A motor management programme can be used to reduce motor energy costs throughout the motor system and not simply in the motor itself. Source: Tangram Technology
down. “The majority of motors used in compound- ing are AC induction motors. These are low-cost and reliable but operate at a fixed rotational speed. Variable Speed Drives (VSDs) allow AC motors to operate at variable speeds to match the process demands,” Kent explains.
Informed choice Compounders have a number of options available when selecting an extruder motor so it is important to make an informed choice. “The motor uses more than 65% of the power input to the extruder and it is essential to get the motor choice right,” says Kent. “The traditional choice was a DC drive to give the variable speed needed for extrusion, but the range of options has now increased, and DC motors are old technology. For new machines, most manufac- turers have removed DC motors from their product range to only offer AC motors plus VSDs.” VSDs can be used to adjust motors to run at the lowest possible speed — within the allowable torque values — to reduce the energy used to a minimum. Reported achieved savings vary widely. Kent says that in Tangram’s validated tests savings amount to around 7–10%, but savings of 5–20% are widely reported. “The exact savings depend on the machine type and speed and, as a general rule, the savings decrease with increasing screw speed. Some tests report even greater savings of more than 30%, but this depends on the load and speed,” he says. Kent says AC motors plus VSDs can easily be
retrofitted to most extruders and this is his recom- mended action. Energy savings will generally be around 5–20% but reductions in maintenance costs
30 COMPOUNDING WORLD | February 2021
are also significant so payback will be achieved in one to two years. Whether specifying a new installation or retrofit, he recommends an IE3 rated motor — or better — especially if it is not going to be run at full load. “For those who want to be at the forefront of technology, the introduction of direct drive, permanent magnet [PM] synchronous motors presents even more opportunities. This is a new motor technology to connect the motor directly to the screw - often with no need for a gearbox at all,” he says. “These are advanced systems but offer advantages over existing systems, for example, they are more energy-efficient – around 12–15% compared to AC motor plus VSDs. They are also quieter than DC or AC motor plus VSDs, smaller and less complex.” Kent also offers a couple of extra tips for energy
saving. He says that one of the biggest savings to be made through the use of AC motors with VSDs or PM motors is the greatly reduced maintenance load. And, where belts are used to connect the motor to the load, it is a simple and inexpensive task to replace standard V-belts with more efficient cogged belts. Energy savings of up to 5% are achievable by making that change. In addition to appropriate motor and drive selection, managing extruder and motor energy costs also requires a good maintenance pro- gramme. AC motors are the workhorse of modern industry and will give continuous operation for many years, but they do need some basic mainte- nance to ensure that they continue to operate efficiently, says Kent. This includes: n Checking the motor is well ventilated and that air flow to the cooling fan is unobstructed (allowing the fan inlet to become blocked will cause overheating);
n Checking the condition of bearings using a vibration meter;
n Checking electrical connections are correctly torqued;
n Checking electrical integrity of cables and motor insulation;
n Checking the overall cleanliness of the casing, fan, terminal box and associated electronics such as soft starters and VSDs;
n Checking the load transmission and alignment of motor to load. The first check on the list is particularly impor- tant as the worst action for reliability is to allow an AC motor to run hot, Kent says. Motors are certified for a specified temperature increase while running. Running them above that temperature reduces their efficiency due to increased winding resist-
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