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Drive Produces Variable Flow and Pressure From a Fixed-Displacement Pump

Feb. 14, 2008
A variable-speed drive using closed-loop flux vector technology not only overcomes the limitations of variable frequency drives, but actually offers several significant performance advantages.

Introduced in the early 1990s, flux vector drives could change the design of many industrial hydraulic power units by replacing the conventional variable-speed pump driven by a fixed-speed with a variable-speed motor driving a fixed-displacement pump. To what end? Much improved hydraulic system efficiency — to that approaching all-electric drives. Furthermore, the hydraulic system would use fewer components and operate with reduced pressure pulsations.

Mating a variable-speed drive with a fixed-displacement pump to save energy is not a new idea. However, attempting this with a traditional ac variable frequency drive (VFD) suffers many shortcomings. For one, torque fall-off magnifies pump pulsations at low speeds. Also, the VFD systems have slow response and are complex to integrate into a hydraulic circuit. The negative experiences of both integrators and users connected with attempts to apply VFD technology to hydraulic power units have given the variable-speed concept a somewhat sullied reputation.

Unigy hydraulic power unit uses a fixeddisplacement pump driven by a flux vector variable speed, variable torque drive to generate hydraulic power that is highly responsive and efficient.

However, a variable-speed drive using closed-loop flux vector technology not only overcomes the limitations of variable frequency drives, but actually offers several significant performance advantages. Used in more than 100 production scale installations, Unigy flux vector drives (FVDs) combine a triedand- true technology with proprietary software specific to the application of driving positive-displacement pumps. The result is ultra-high energy efficiency, quiet operation, and simplified hydraulic circuit design.

Ac flux vector drives

Flux vector drives are perhaps best known for their use on industrial winding machines, where their precise torque control and rapid response are used to control sheet tension. They are available in sizes from 1 hp to several hundred in both open-loop and closedloop designs. Closed-loop FVDs offer the ultimate control of an ac induction motor. Using a motor-mounted shaft encoder, the FVD can precisely control motor torque, operating speed, and even rotational position.

FVDs use ultra-fast microprocessors to segregate motor currents into components that produce torque from those that produce motor heating. Full torque at zero speed, the ability to control motor torque, and extremely tight speed regulation are all features from commercially available flux vector drives.

In a nutshell, ac flux vector drives offer:

  • full independent control of torque, speed, or rotor position
  • 1000-to-1 speed range
  • full torque at zero speed
  • full-range, adjustable torque control
  • instantaneous torque transient response
  • no need for special voltage motors, and
  • automatic tuning.

Benefits for hydraulics

Of course, hydraulic power is the product of pressure and flow, and flow is the product of rotational speed and pump displacement. Motor torque correlates directly with pump pressure. Therefore, the precise torque control provided by a flux vector drive translates directly to precise hydraulic pressure control. As flow rate fluctuates, the FVD will vary speed as required to maintain set point pressure.

During periods of low flow, power consumed by the motor drops, even when pump pressure must be maintained. Because the FVD can deliver full torque at zero speed, a positive-displacement pump driven by a FVD will consume virtually no power while maintaining required pressure at zero flow.

Because the FVD monitors speed (pump flow) and torque (pump pressure), it can be easily used to monitor system performance during each segment of a machine cycle and be programmed to automatically alert the operator to system problems. For example, if a hydraulic hose rupture is sensed, the pump can be shut down in less than 1 sec.

Off-the-shelf FVDs

Several caveats should be considered before attempting to use an off-the-shelf FVD in conjunction with a positive- displacement hydraulic pump. The first involves pump design. Although many pump designs are compatible with the FVD concept, some positivedisplacement pumps are not fully capable of operating at low speed and maintaining pressure at the same time.

The second challenge involves pressure pulsations (pressure ripple), especially at low speeds. An off-the-shelf drive FVD requested to deliver either a desired flow rate (regardless of pressure) or maintain a fixed pressure (regardless of flow) will either focus on precise speed control or on precise torque control.

Either way, at low speeds, the drive will react to the natural torque variations inherent to each revolution of the pump. In responding to these torque variations, off-the-shelf drives will actually amplify the resultant pressure pulsations. However, a flux vector drive can actually reduce the pulsations inherent to hydraulic pumps.

This is accomplished by preprogramming the FVD to the particular pump geometry and, therefore, to compensate for the geometrically induced variations in torque. As the pump shaft turns, the drive can feedforward a modified torque profile to effectively cancel out pressure pulsations. This concept is analogous to noise cancellation technology in headphones, except that the properly programmed FVD knows in advance the required cancellation signal determined by pump geometry and motor speed. The technology is described in detail by US Patent 6,494,685.

The UNiGY Solution

Unigy technology was developed by Kadant Inc. for integrating Flux Vector Drives with positive-displacement hydraulic pumps. The technology includes proprietary firmware that makes a flux vector drive fully compatible with the unique demands of hydraulic power units. Kadant does not sell directly to hydraulic system end users. Rather, Kadant works with equipment OEMs and hydraulic system integrators as both FVD supplier and a drive applications consultant. Unigy FVDs have already proven themselves in a variety of demanding applications:

1500 ton closed-die forging press: As part of a press rebuild, the Unigy system reduced power usage by 69%, and the higher efficiency eliminated the need for hydrulic oil coolers. 950-ton plastic injection molding machine: The Unigy system reduced energy use in producing a 2.5-lb part by 54% and eliminated the need for hydraulic oil cooling.

Rubber molding (centralized system): A 50-hp Unigy system eliminated pressure droops in a common manifold used by multiple machines and reduced power consumption by nearly 90%.

Tube bending machine: A 25-hp Unigy system reduced noise from more than 77 dBA to less than 66 dBA and slashed energy consumption by more than 80%. In additon, tighter pressure control resulted in more consistent part quality.

Log shaving machine: A 25-hp system reduced oil temperature, noise, and cut power consumption by more than 80%.


As industry around the world strives to reduce its carbon footprint, hydraulic power will be more and more challenged by all-electric alternatives. Flux Vector Drive technology offers a way for hydraulic systems to closely approach the efficiencies of all-electric systems at a small fraction of the cost. Not to be confused with VFD drive technology, Flux Vector technology adds energy efficient “on-demand” hydraulic power to the hydraulics engineer’s system design tools.

This information was submitted by Bilal Mehmood, of Kadant AES. For more information, visit www.kadant.com.

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