Correctly sizing an electric motor for a hydraulic power unit is a straightforward procedure. And if load pressure and flow remain fairly constant, determining the power requirement is relatively simple by using the familiar equation:

hp = Q × P / (1714 EM)


Q is flow, gpm (and accounts for the pump's volumetric efficiency),

P is system pressure at full load, psi, and

EMis the pump's mechanical efficiency

For example, assume an application requires a flow of 13.7 gpm at a maximum pressure of 2000 psi, and with a pump efficiency of 0.80. From the equation above:

hp = 13.7 × 2000 / (1714 × 0.80)

= 20 hp.

It may seem that a gas or diesel engine as the prime mover would have the same power rating as an electric motor. However, the general rule of thumb is to specify an internal-combustion engine with a power rating 2 1 /2 times that of an equivalent electric motor. This is primarily due to internal combustion engines having different torque-speed relationships than electric motors. Examining the different torque characteristics will provide the understanding to make a choice based on solid reasoning — rather than putting faith in a rule of thumb.

Pump torque requirements
Power, of course, is the combination of torque and rotational speed:

hp = (T × n) / 5250


hp is horsepower

T is torque, lb-ft, and

n is rotational speed, rpm

A pump's torque requirement is the main factor that determines whether a motor or engine is suitable for an application. Speed is less critical, because if a pump runs slowly, it will still pump fluid. However, if the prime mover does not develop enough torque to drive the pump, the pump will not produce any output flow.

To determine the torque required by a hydraulic pump, use the following equation:

T = P × D / (6.28 × 12 × EM)

where: T

is torque, lb-ft, and

D is displacement, in. 3 /revolution

Pump displacement is provided in manufacturer's literature. Continuing with the example introduced above, if the pump has a displacement of 1.75 in. 3 /rev., required torque is calculated as follows:

T = 2000 × 1.75 / (75.36 × 0.80)

T = 58 lb-ft

Torque can also be calculated using the familiar horsepower equation:

hp = T × n / 5250


n is shaft speed, rpm.

Substituting values from the example:

20 = T × 1800 / 5250

T = 58 lb-ft.