VFD or Servo Drive for Energy-Saving Hydraulics? Why Not Both?
New VFD from Bosch Rexroth at NPE2018 illustrates the merits of VFD versus servo drive for hydraulic pumps.
At NPE2018, both Jomar Corp., Egg Harbor Township, N.J., and Bosch Rexroth Corp., Bethlehem, Pa., introduced new products that provide an educational illustration of the differences between two types of energy-saving variable-speed drives for hydraulic pumps.
The term “servo-hydraulic” has gained currency to describe new generations of plastics machinery that utilize hydraulic pumps but with energy savings and noise reduction closely approaching those obtained with all-electric servo motors and drives. At the same time, popularity has also been growing for aftermarket retrofits of conventional hydraulics with variable-frequency drives (VFDs), another way of saving energy by varying pump speed—even to zero—according to the instantaneous load demand of the system.
Bosch Rexroth introduced at the show its Sytronix DRn 5020 VFD, aimed specifically for converting variable-displacement hydraulic pumps driven by conventional fixed-speed (1800 rpm) motors to quieter, more energy-efficient variable-speed operation. It reportedly saves up to 75% in electrical energy for systems that have long dwell times; and the average noise reduction is 8 to 10 dBa vs. fixed-speed hydraulic drives. This “intelligent” drive senses the motor torque and pump pressure to calculate the pump displacement, or position of the swash plate, to keep it in the most efficient position for the real-time load requirement. This drive is Industry 4.0 compatible, as it adds operating data collection and digital communications to conventional electric motors and hydraulic pumps that normally do not have these capabilities.
Bosch Rexroth notes that VFDs are considerably less expensive than servo drives, but the firm also points out in a white paper that they provide less tight control over motor speed than servos and have lower dynamic performance—ability to respond and control pressure/velocity changes. VFD drives are thus more suited to steady or slowly changing loads. They also have low-speed limitations and are usable only to 400-500 rpm.
Bosch Rexroth points to an instructive use of both its MSK servo motor and the new Sytronix DRn 5020 on Jomar’s new IntelliDrive injection-blow molding machines. Jomar often refers to these machines as “servo-hydraulic,” but the servo drives only the hydraulic pump for the injection unit. The injection and blowing clamps use a pump with the new VFD. The Jomar IntelliDrive 85S at the show boasted 42% lower energy consumption than a standard model 85S and required a 40% smaller oil tank.
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