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Why switching to AC drives for plastic extrusion makes sense
Over 20 years ago DC motors and drives dominated many markets, in particular the plastics and rubber industry. The reason was simple. DC motors were the only electrical machine capable of achieving the high starting torque followed by constant torque at low speeds that typify the requirements of single and twin extruders. In fact, DC motors and drives exceed the rated torque by up to 400 percent for startup with viscous polymers.
Early-generation AC drives, by contrast, did not provide the high starting torque necessary for an extruder. They did not provide the stable speed control required at low speed for some extrusion applications. AC drives were inefficient in the low speed ranges due to the high initiation current needed at these speeds. Quite simply, DC motors and drives offered a degree of accuracy and reliability at an affordable price. Production lines around the world boomed with this technology.
Such is the incredible pace of progress over the past two decades, AC motors and drives are now becoming the defacto replacement standard as old DC machines come to the end of their useful life.
The reason for replacing DC with AC is plain to see. The main costs in any plastics extrusion business are raw materials, labor and energy. At least two of these – labor and energy - are impacted by the DC motor and drive.
Labor manifests itself by way of maintenance costs. DC motors, especially ageing machines, require heavy maintenance. Significantly they operate using carbon brushes, which ride on a rotating commutator to form a rotary switch. This results in arcing, wear and graphite dust from the brushes, requiring annual to semi-annual maintenance. In fact, it can cost typically £35 to replace one brush and every DC motor can have 12 carbon brushes. This is needed at least twice a year. That’s £840 just on brush replacements for one motor alone.
One UK plastic pipe manufacturer, Radius Systems, located in Derbyshire, estimates that the annual static and dynamic checks, brush changes and outsourced labor, costs it about £2,000 per motor each year.
An AC motor, however, uses a very different technique which avoids brushes altogether and is generally less expensive, smaller, lighter, more rugged and, other than occasional bearing greasing, virtually maintenance-free. Maintenance alone, though, is not enough to justify changing to an AC drive.
Controlling operating cost
The biggest reason for replacing a DC drive with an AC drive is reducing the operating cost. The kilovolt-ampere (kVA) is always larger in a DC system than the equivalent AC system. For a DC drive to control motor speed, the semiconductor-controlled rectifier (SCR) has to be phased back which creates a poorer power factor at all speeds. A load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger cables and other distribution equipment.
Because of the larger currents flowing, energy costs will be higher overall. Additionally, electrical utilities will usually add a further charge for increased kVA where there is a low power factor. In many cases the power factor charge can approach 30-40 percent of the cost for the basic kWh in plants with a very low power factor.
AC drives have an excellent fundamental power factor at all speeds, thus making them an excellent alternative to a DC controller; savings are made in the lower overall current and the kVA charges.
Energy saving potential of the actual application
But what about the potential energy savings of using AC drives in extruder applications? The extrusion process is a constant-torque application. This means that once the initial high starting torque has kick started the process by overcoming the initial forces needed to squeeze the plastics through the die head, then the actual extrusion is a slow, constant process.
Unlike quadratic torque applications, such as pump, fans and compressors, extruders do not follow the cube-law but follow a linear relationship between speed and power, so the savings are not as dramatic but are still there to be gained.
Despite all of these variables, it is estimated that potential energy savings from the AC motor and drive will be in the range from 8 to 15 percent, with a return on investment within two years.
The importance of energy saving to the plastics industry is critical. Most manufacturers have signed up to the European-wide Climate Change Agreement (CCA), and are incentivised to achieve energy targets and earn credits. For the likes of Radius Systems, a key performance indicator is the measure of kWh per kg of product produced, with a 10 percent reduction target each year. Previous projects include energy efficient lighting, new compressed air and chilled water systems. However, the extrusion process is the company’s biggest energy consumer and the company recognises the benefits offered by AC drive technology.
DC motors are internally cooled, requiring supplemental fans that need filtered air to prevent the inside of the motor from becoming fouled with particulates. AC motors are typically externally cooled, making them more suitable for the dusty environment likely in a plastics processing operation.
Furthermore, DC motors tend to be long and thin whereas AC motors tend to be shorter but bigger in diameter. The SynRM motor can be up to two frame sizes smaller than a conventional induction motor which could be a true benefit in the direct replacements of DC motors, as Radius Systems discovered: “The smaller overall size opens up significant number of applications where we can use the SynRM package,” says Betts.
In a relatively short time, AC motors and drives have progressed to become the dominant force in many applications traditionally the stronghold of DC drives and have replaced them in all applications. With progress in electronics and the introduction of DTC using complex control algorithms, AC drives are now equal to or better than DC drives.
AC motors can also be controlled open loop, with accuracy and torque control never before imagined. With direct torque control (DTC), for instance, open loop control means that there is no need for a feedback device such as a tachometer or encoder, thereby saving cost on additional hardware. With an AC motor, the drives can measure the open loop speed, so the constant speed and torque control accuracy needed to extrude plastic is easily achievable.
AC drives are now so mature that all measurement and feedback variables are easily generated within the drive software. So variables required for operator feedback of trouble shooting are available as “actual signals” displayed with full text instructions on the drive keypad.