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Hydraulic power units are responsible for providing the fluid pressure and force that drive cylinders, pumps, and other hydraulic parts. They are central to a hydraulic system’s operations, and optimizing their efficiency can be a major influence on the overall success of a project. The power source, or prime mover, associated with most hydraulic power units is the motor, which is generally selected based on its speed, torque level, and power capacity. A motor whose size and capabilities complement those of the hydraulic power unit can minimize wasted energy and raise cost-efficiency in the long-term.
The criteria for motor selection vary according to the type of power source being employed. For example, an electric motor has an initial torque much greater than its operating torque, but diesel and gasoline-powered motors have a more even torque-to-speed curve, delivering a relatively steady amount of torque at both high and low running speeds. Consequently, an internal combustion engine may be able to initiate a loaded pump, but not provide enough power to bring it to operating speed if it is not properly matched with the hydraulic power unit.
As a rule of thumb, the power rating for a diesel or gasoline motor used with a hydraulic power unit needs to be at least double that of an electric motor suitable for the same system. However, the cost of the electricity consumed by an electric motor over its operational lifespan usually outstrips the cost of the motor itself, making it important to find an appropriately sized unit that will not waste energy consumption. If the pumping pressure and liquid flow are set at a constant rate, motor size can be measured according to the following parameters:
• Gallons per minute
• Pressure, measured in pounds per square inch (psi)
• Mechanical pumping efficiency
In some cases, the hydraulic system may require different levels of pressure at various stages of the pumping process, meaning that horsepower can be calculated as the root mean square (rms) and a smaller motor may suffice for the project. However, the motor must still be able to meet the torque requirement for the highest pressure level in the cycle. Once the rms and the maximum torque (including initial and operational levels) have been calculated, they can be cross-referenced with a motor manufacturer’s performance charts to determine whether the motor is the necessary size.
For specific equations on rating motor characteristics, visit the Hydraulics & Pneumatics website.
Electric Motor Power
Electric motors and internal combustion motors, such as diesel or gasoline engines, exhibit different torque characteristics that dictate their varying power capacities. A typical three-phase electric motor begins its operating sequence by turning a rotor. When the rotor accelerates, the torque level drops slightly, then increases again when the rotation hits a specific rpm rate. This temporary drop is known as “pull-up torque,” while the maximum value is designated as “breakdown torque.” When the rotor speed surpasses the breakdown level, torque decreases steeply. An electric motor’s torque-to-speed curve remains roughly the same regardless of power capacity, and it is usually run at full-load speed but below the breakdown point to reduce any risk of stalling.
Gasoline and Diesel Motor Power
Internal combustion motors have a significantly different torque-to-speed curve with fewer torque fluctuations. Generally, diesel and gasoline motors have to operate at higher speeds to achieve the necessary torque to power a pump. A horsepower rating approximately two and a half times greater than that of an electric motor counterpart is typically required for an internal combustion engine to reach the torque levels needed for a hydraulic power unit. Manufacturers normally recommend that gasoline or diesel motors operate continuously at only a portion of their maximum rated power in order to prolong the motor’s lifespan, and keeping the torque below maximum level can often improve fuel efficiency.
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