DC Motors in Hydraulic Pump Applications

Hydraulic pumps are the workhorses used in nearly all industries including, construction, steel milling, mining, manufacturing, machining, heavy equipment, among others. They range from small pumps to hydraulic power packs for mobile applications to large hydraulic pumps used in the petroleum industry for pumping crude oil. Material handling, lifting and traction drives are some of the most popular applications.

By definition, a pump uses the mechanical input of a prime mover and converts it into pressurized fluid power to perform work. The prime mover can be a diesel or gasoline engine. But for electro-hydraulic pumps, the prime move is an electric motor. 1 D.C. motors have been used as pump drives for many decades because of their ease of variable speed control and “faster response in transient conditions.” 2 The introduction of low-cost, brushless DC motors overcomes the higher maintenance of brushed DC motors. 3 This article will discuss the types of hydraulic pumps and motors used in pumping applications and the important factors to consider when sizing a DC motor for a hydraulic pump.

Pump Types

Hydraulic pumps are classified in two broad categories: rotodynamic (centrifugal) and positive displacement. Centrifugal pumps employ a rotating impellor that uses centrifugal force to drive fluid from the inlet to the discharge side of the pump. There are many sub-categories of centrifugal pumps based up their specific application. Common types of centrifugal pumps include submersible, priming, and axial flow. Positive displacement pumps are the other broad category of hydraulic pumps. These pumps typically use gears, vanes, diaphragms or pistons to force a fixed amount of fluid through the inlet to the discharge side of the pump. Common types of positive displacement pumps are gear, rotary vane, screw, axial piston, diaphragm, plunger, radial piston, and peristaltic. Some pumps don’t fit directly into the two broad pump categories. These form a special category that includes ejectors, hydraulic-ram, air-lift and contraction pumps. 4

Types of DC Motors in Pump Applications

DC motors have been used as pump drive motors due to their variable speed control ability, especially at low speeds, simple control system, high starting torque and good transient response. Brushed, wound-field DC motors have formed the primary type of DC motor used in pump applications for many years. But permanent magnet (PMDC) and brushless DC motors have seen greater adoption rates, primarily due to their simple and compact design, high efficiency and power density, a wide range of frame sizes, and their need for less maintenance.

PMDC motors obtain their magnet field from strong permanent magnets, instead of electromagnets, which provide improved thermal properties and a constant magnetic field under all operating speeds and transients. Permanent magnet motors perform similar to shunt field-wound motors but they generally have more speed fluctuations during load changes. The permanent magnets give this type of motor excellent thermal properties that permit it to be used for continuous duty applications. Applications that require higher ‘part load’ efficiency at lower speeds is a typical application of PMDC motors.5

Despite the increased maintenance of brushed, wound-field DC motors, these motors possess distinct advantages that have made them the mainstay as pump drives for many decades. 6 Commonly operated in a series-field configuration, these wound-field motors have very high starting torque and can operate at high speeds. However, since they are load dependent, their output speed varies with the applied load. But they can generate very high power for short time periods. They are often used in pump applications with higher loads operated at higher speeds with an intermittent duty cycle.

Brushless DC motors have seen a quick adoption because they overcome the problem of brush maintenance common to brushed DC motors. 7 They characteristically have no brush sparking, high operating speeds, high efficiency, a compact size, and a fast response. 8

Pump Motor Sizing

Selecting and sizing a DC motor for a hydraulic pump application requires an understanding of the characteristics of both the motor and the pump. These factors assist in matching the motor’s horsepower (HP), rated full load current (FLA) and torque specifications to the pump’s flow, volumetric efficiency at the desired pressure and speed range, based upon pump performance curves provided by the manufacturer. 9 Pump motor selection is dependent upon a variety of factors including load and torque requirements, drive ratio, operating environment, efficiency, frame size, mounting configuration, enclosure type, battery amp/hour draw for mobile pump applications, among others. 10 Ensure the motor has both the capacity to drive the pump and desirable speed/torque characteristics. 11 Motors operate most efficiently when fully loaded so avoid oversizing or undersizing the motor. Additionally, a motor’s amperage depends on the specific gravity of the fluid being pumped; therefore, the motor should be tested with a fluid that’s similar to the actual fluid to be pumped. 12 The service factor should be taken into account during motor sizing calculations. The service factor is the overload capacity that a motor possesses without insulation damage occurring. The NEMA standard service factor for totally enclosed motors is 1.0, where there is no overload capacity. However, service factors range from 1.0 to 2.5, with 1.15 being the most common. See the motor manufacturer’s specifications for details. 13 The duty type affects motor sizing because it depends on how the pump will be operated. Duty types include: continuous, short time, and intermittent periodic-duty. Operating a motor not designed for a specific duty type can cause irreversible damage to the motor. 14

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