Motors & drives
Total cost of ownership analysis demonstrates high potential for savings, even for simple applications, by replacing pneumatic drives with electric linear drives
Electric linear drives are replacing conventional pneumatic cylinders in more applications.
The reasons include poor efficiency, high costs for commissioning, reconfiguration, service, and maintenance, and the limited control capabilities of pneumatic systems.
A total cost comparison demonstrates that electric linear drives, at current prices for components and electricity, pay for themselves within a few months – even for simple point-to-point motions with two end positions. This also helps to reduce the carbon footprint significantly. In addition, they provide greater f lexibility in the design of production processes and production monitoring systems.
Pneumatic drives are characterized by lower acquisition costs, robustness against external influences (e.g., temperature variations and dust) and high resistance to overload. They are also simple to operate and do not require holding current when installed in a vertical orientation.
Compressed air is also used for transport and cleaning tasks in many shop f loor and industrial environments, so that compressor systems need to be provided in any case. So it’s no wonder that pneumatics are used in a wide range of applications and can be found in many factories.
Compressed air, however, is one of the most expensive energy media, because compressors can convert only a small portion of the input energy into useful power. The great majority is dissipated as heat loss.
The latest technology can achieve an efficiency of about 30 percent. Further increases are nearly impossible, as the physical limits have practically been reached. In addition to the already high costs for the motor, compressor, startup and run-on losses, and losses from compressed air handling, in practice additional occur losses due to leaking distribution systems.
In reality, therefore, after additional conversion loses in the actuator (without optimization) only about five percent of the input energy is available as useful power. Optimal design of the pipework system and actuators, prompt tracking of leaks, and heat recuperation systems can increase the efficiency. The German Environmental Ministry rates the potential energy savings at 20 to 40 percent, while other experts calculate significantly greater potential savings.
Even if all of these potential savings can be realized, however, compressed air systems still use this input energy very inefficiently, with a maximum achievable overall efficiency of 10 percent. This can also be seen in the total cost calculation (TCO, Total Cost of Ownership) of a compressor. While about 10 percent of total costs must be spent on procurement and another 10 percent or so for maintenance of the system, the energy costs are typically 70 to 80 percent of the total costs over the service life of the compressor.
It should be no wonder, then, that more and more companies are attempting, in times of rising energy prices and increased environmental awareness (particularly CO2 emissions), to eliminate compressed air from their factories, or at least to reduce it to an absolute minimum.
Today there, almost without exception, alternatives that do not use compressed air available for compressed air drives. For linear motions in many applications, the very efficient, all-purpose electric linear motor in tubular form is a good substitute. These are available from LinMot in various designs and power classes.
Electric drives are indeed more expensive to buy than simple pneumatic cylinders, but an analysis of the total costs over their service life shows that industrial linear motors from LinMot in particular can pay for themselves within a few months or even weeks, even in simple point-to-point motions between two positions.
The following example, with a horizontal point-to-point stroke of 400mm and 15kg of mass in motion, operating at 30 cycles per minute and 50 percent duty cycle (= 2,000ms cycle time), makes this clear.
The required positioning time of 500 ms for this task above is achieved with an acceleration of 10 m/s² and a travel speed of 1 m/s. The acceleration time, during which the linear motor does useful work, is then 100 ms.
This means that the effective power draw takes place during just one-fifth of the positioning time. When stopped and when travelling at a constant speed, the motor does not draw any power beyond that needed to overcome friction.
The kinetic energy incurred during braking is converted to electrical energy in the motor (via the generator effect) and stored in the intermediate capacitors of the servo controller, where it can be used for the next cycle.
This application can be implemented using a LinMot linear motor, size P01-48x240F in combination with a LinMot servo controller, model E1100-XC/B1100-XC, with a continuous power draw of less than 100 W.
Assuming 8,000 operating hours per year (three-shift operations) and an electricity price of 0.12 EUR/kWh (price for large industrial consumers, including taxes, per EUROSTAT) the
total annual energy cost is 96 euros. A pneumatic solution would be much more expensive.
Pneumatic cylinder solution
If a load mass of 15kg is transported pneumatically at a (maximum) speed of 1 m/s, as required by the application example, an analysis of the appropriate characteristic curves for designing pneumatic cylinders from a famous manufacturer indicates that a pneumatic cylinder with a 50 mm piston diameter must be used. In contrast to the linear motor, the energy (compressed air) must be fed in throughout the entire motion. The kinetic energy from braking must also be absorbed by shock absorbers, and cannot be stored intermediately for the next motion.
According to its data sheet, the selected cylinder consumes 0.02529dm³ of air at six bar for each millimetre of travel in a double stroke. For a stroke of 400mm, this results in consumption of 10.37dm³ per cycle.
At 30 cycles per minute, the pneumatic cylinder thus requires a total of 150,000 Nm³ of compressed air per year for continuous operation (8,000 h/year). Considering pressure drop, reduction, and leakage losses on the order of 25 percent, the compressor must compress and feed a total of about 190,000 Nm³ of air into the pipeline.
A normal compressor (750 kW motor, 7,500 Nm³/h air capacity) can use 0.130 kWh of electrical energy to compress 1 Nm³/h to 6 bar, including start-up and run-on losses and compressed air handling. The total annual energy cost is thus about 3,000 Euro (0.12 Euro/kWh*0.130kWh/m³*190,000 m³), or more than 30 times that of the electric equivalent. At a higher cycle count, this ratio would be even worse for the pneumatic cylinder.
Contact John Brooks Ltd, 0800 484950.
See the next edition of DEMM magazine for the conclusion of
this feature, which includes details on cost calculations.
At 15 kg load and 1m/s travel speed, a 50 mm pneumatic cylinder is required.
The measured continuous power draw of the linear motor in the application example is 92 W.