By Paul J. Heney/Editorial Director
In some industries, such as plastic injection molding, presses and stamping operations, the use of electrohydraulics is growing and helping to increase energy efficiency and reduce noise levels. The biggest game-changer in recent years has been variable speed pump drives, which are continually being integrated into automation systems on a plant level, said Jan Komsta, Manager, New Technologies & Simulation, Bosch Rexroth. Most pumps operate on a variable flow or pressure demand, but with variable speed pumps, such as Bosch Rexroth’s Sytronix line, you can reduce the power you use when the system is not at peak demand.
Electrohydraulic drives typically use resistive or throttling control, said Komsta. Throttling control means that you restrict the flow through a proportional device or valve, which throttles oil flow. Restricting this flow causes pressure drop, which in turn dissipates energy and transmits this energy or heat into the hydraulic oil. This can waste a great deal of energy.
Variable speed pump drive technology is more of a volumetric control, in which flow is controlled by adjusting pump displacement or changing the drive speed of a variable or fixed displacement pump. By removing the valve throttling, you can reduce the amount of heat transferred to the oil and reservoir. As a result, less power is used for cooling and maintaining oil temperature.
For example, a standard hydraulic system runs usually on a three-phase induction motor that runs at either 1800 rpm or 1200 rpm. When running the pump, you either need flow or don’t need flow; there is no in between. While you are holding pressure and force, you only need to compensate for leakage, not run at high speeds to create flow. Slowing down the pump to a speed where you can just keep the pressure to compensate for leakage is ideal.
The biggest benefit seen by users of variable speed technology is energy savings. As Komsta said, it is first necessary to change the mindset of looking just at initial purchase price, and consider overall cost of ownership. “Say you expect your application to run for several years. If you build them a machine, you want them to run it as long as possible with the greatest output from the machine while keeping the machine running as long as possible,” Komsta said.
For example, multiple studies have been done on the total cost of power units. One study showed that for a 1 kW, three-phase electric motor running for five years, the initial cost to purchase it is only 4% of the total cost to own it. All the rest is energy that was paid to run the system. And, if you’re looking at higher powers—say 100 hp motors—initial purchase price accounts for an even smaller amount of total cost of ownership.
At this point, you will pay even more for the energy. “That is when we need to start looking at buying more efficient systems, to design your system so it’s more effective and more efficient during the production time when you run it,” Komsta added. “We still need to keep the performance of the machine on the level that the customer wants. That’s the main goal of this technology.”
The motor’s speed and pump pressure are the two most influential factors on noise in a hydraulic system. Take the example above: By reducing the speed of an 1800 rpm motor to 200 rpm, you can decrease the noise up to 15-20 dBA, Komsta said. And, if you’re in partial load operation or hydraulic idle time or pressure holding, slowing down the drive and the pump will reduce the noise.
“You make the working environment, the conditions around the power unit, much, much more comfortable for the personnel working around the machine,” Komsta said.
Proof to these many advantages is that this shift in technology is coming Bosch Rexroth’s own plants, with its Go Green project. “Wherever we deal with hydraulic power units, we tried to retrofit them to make them as efficient as possible,” Komsta explained. “So in recent months, we built a completely new manifold test stand using servo drives and servo-driven pumps using this variable speed technology.”
Filed Under: Hydraulic equipment + components