HPU drives down energy costs
By John A. Neun, P.E., Kadant AES, Queensbury, N.Y.
Controlling a hydraulic power unit's motor electrically - instead of its pump mechanically - produces a host of benefits
Hydraulic systems have traditionally suffered from a near-universal shortcoming: low energy efficiency. And in most cases, the primary source of the problem can be traced directly the operation of conventional pump-motor combinations.
The worst offenders of low efficiency are hydraulic systems using fixed-displacement pumps sized to meet peak flow and pressure demands. Constantly running at full speed, pumps in these circuits continuously produce maximum flows at maximum system pressure. But except for a few moments in an operating cycle, most machines only require a fraction of a system's total pressure and flow capabilities. During standby, positioning, pick-and-place, or holding operations, the system's pump consumes maximum energy while the hydraulic system is performs little or no work. As a result, hydraulic system operating efficiency suffers dramatically.
This situation is exacerbated if excess flow must be diverted and recirculated through bypass valves and circuits. When this occurs, large reservoirs or heat exchangers usually are used to dissipate the heat from the wasted energy. These cooling systems increase the cost of the hydraulic system and usually increase maintenance costs as well. An additional problem often created by these hydraulic systems is excessive noise, created by cooling fans (another energy drain) and pump-motor cycling.
Variable-displacement pumps often are a used to improve efficiency of the power unit by modulating output flow to meet changing requirements. But variable-displacement pumps add cost and complexity to the system, which still can operate at relatively low efficiency when conditions demand less that full flow and pressure.
A different approach
A new and different approach for a hydraulic power unit is the Unigy high-pressure pump system. Able to precisely match output pressure and flow to constantly-changing requirements, This system delivers efficiencies up 94% through its entire operating range - from zero to full flow. At the same time, it minimizes or eliminates bypass and cooling-loop requirements and their attendant complexities and costs.
The Unigy system also runs quieter than conventional pump-motor combinations. Furthermore, because its operating characteristics are not regulated via hardware, the basic Unigy system is equally applicable to a wide variety of new and existing high-pressure hydraulic systems.
Electrical drive controls pump
A Unigy system consists of a fixed-displacement pump, a 3-phase induction motor with integral drive, and computerized controls. The pump and motor both are based on conventional designs proven in literally millions of applications. But each has proprietary features that allow them to deliver performance that is outside the range of standardized, off-the-shelf pumps and motors.
The Unigy hydraulic pump can provide full pressure whether it is operating at virtually zero rpm or at maximum speed. This infinite turndown capability enables the pump to maintain pressure without flow - a characteristic that is critical to Unigy system performance.
The motor-drive has a power factor close to unity regardless of the load applied. This produces efficiency of 91% to 96% even at lowest loadings, where conventional variable-speed drives are highly inefficient. The Unigy motor consumes minimal power when operating at zero speed but transmits enough torque to the pump to maintain full system pressure.
To hold pressure under the same conditions, a conventional pump-motor would run at full power, producing full flows that require bypass and cooling to dissipate wasted heat energy. Even a pressure-compensated system would run the motor at full speed and hold pump displacement at near zero. But in this case, the pump would produce flow to compensate for internal leakage, and the motor, running at or near no load, would be operating at low efficiency.
Instead of pilot circuits, a computerized drive system digitally monitors and controls the Unigy system's performance. By collecting thousands of data points during every motor shaft revolution, the system monitors pump-motor loading, which accurately reflects hydraulic system demands. The computer, using a set of proprietary algorithms, continuously evaluates this data stream. In response to this input, the system constantly regulates current and voltage to seamlessly control motor speed and torque - and consequently, pump flow and pressure.
Likewise, Unigy pump output is precisely modulated thousands of times per second to meet exact hydraulic system demands. The results of this operational control and flexibility can be seen in the power diagrams. Excess flows (and wasted energy) have been virtually eliminated throughout a system's operating cycle. As a result, Unigy systems are capable of delivering efficiencies that can routinely exceed 90%. In production-scale applications, this has translated into overall energy savings approaching 90% for the entire high-pressure hydraulic system.
Although lower energy consumption is the most prominent benefit the Unigy system, its ability to precisely match pump output to system demand also creates several other significant benefits ... and opportunities.
By virtually eliminating excess flows and pressures, Unigy systems eliminate the need for the bypass valves and recirculation systems. This, in turn, minimizes heat buildup in the hydraulic fluid. Cooling systems can be downsized, and in many cases totally eliminated. As a result, the overall hydraulic system design can be greatly simplified. The savings in system capital and installation costs amount to a significant percentage of the total cost of the system.
Perhaps equally important is the reduction in operating costs when compared to conventional, fixed-speed systems - and especially variable-displacement pumps. With fewer components, less maintenance is required, and the risk of unplanned downtime is lowered.
By minimizing pump and motor running demands, the Unigy system considerably reduces wear and tear on the few mechanical components it does have. This can help Unigy pumps and motors last more than five times longer than conventional hydraulic power packs. Unigy can also reduce hydraulic oil expense because the lower operating temperatures it promotes extend oil life and often allow the substitution of less expensive oils.
In addition to operating more economically than conventional hydraulic power units, the Unigy system is also quieter. It operates without generating pump-motor cycling, smooths out hydraulic pressure rippling, and eliminates the need for loud cooling fans.
Furthermore, the Unigy system can enhance hydraulic system performance and extend system capabilities while reducing system cost and complexity. Because Unigy provides precise pressure and flow modulation through its full operating range, it can control an entire hydraulic system from the pump head - without the multitude of components otherwise needed to manipulate pressure, flow, proportioning, or motor loading.
Programming the Unigy computer to respond to internal presets or external signals creates flow and pressure profiles that perform complex operations, ranging from closed-loop metering to hard and soft clamps. The accuracy afforded by Unigy's computer control can improve hydraulic equipment operating precision and minimize hydraulic spikes, drops, and other flow anomalies that damage processes and equipment.
Broad range of application
Except for applications where maximum flow is constantly required, Unigy can provide lower cost, higher quality hydraulic power to almost any system. The Unigy system has been designed to be independent and nearly infinitely scalable. Pumps, motors, and controls can be sized as small or as large as necessary to efficiently meet flow and pressure requirements.
The greatest benefit of the Unigy system is derived from applications where the hydraulic operating cycle requires wide and frequent variations between maximum, minimum, and average flows and pressures. Typical among these are pressing, injection molding, investment casting, and die casting, where holding operations are particularly wasteful and problematic for conventional hydraulic HPUs.
As a straightforward replacement for an existing pump-motor, incorporating a Unigy system can earn payback in under three years. When part of a rebuild or in a new installation, Unigy also allows downsizing or eliminating bypass and cooling circuits. This simplifies the hydraulic system to further decrease capital costs while increasing operating efficiency.
Unigy should be equally attractive to OEMs because it allows them to reduce the complexity (and therefore, manufacturing costs) of their products. Yet it also improves the operating efficiency, precision and reliability of the equipment they sell, enhancing its added value qualities to prospective customers.