Differences Between Hydraulic Motors & Electric Motors

••• SARINYAPINNGAM/iStock/GettyImages

A motor in plain physical terms is anything that converts energy into moving the parts of some kind of machine, be it an automobile, a printing press or a rifle. Motors are required to move things in so many everyday situations that the world would immediately grind to an unrecognizable, somewhat comical standstill if every motor in operation went silent at the same time.

Since motors are ubiquitous in modern human society, Earth's engineers over the centuries have produced a number of different types commensurate with the technological standards of the day. For example, before people were able to harness and use electricity on a global scale from the early 20th century onward, the great engines of trains were powered by steam from the combustion of coal.

  • Motors are a subset of engines, but not all engines are motors. 

Many motors are actuators, meaning they induce motion through the application of torque. For a long time, the liquid-driven power of hydraulic actuators was the standard of the day. But with advances in the 21st century in electric actuators, combined with electricity being plentiful and easy to control, electric motors of this type are making gains. Is one plainly superior to the other, and does it depend on the situation?

Overview of Hydraulic Systems

If you have ever used a floor jack or driven a vehicle that has power brakes or power steering, you might have marveled at the ease with which you can move the amounts of mass involved in these physical transactions with seemingly little effort. (On the other hand, you might have been too consumed by the task of changing a tire at roadside to bother with such ideas in real time.)

These tasks and many other common ones are made possible by the use of hydraulic systems. Hydraulics is the branch of physics concerned with mechanical properties and practical uses of dynamic fluids (fluids in motion). Hydraulic systems do not "create" power, but instead convert it into a desired form from an external source, called a prime mover.

The study of hydraulics consists of two main areas. Hydrodynamics is the use of liquids at high flow (dynamic means "moving") and low pressure to do work. "Old-school" mills harness the energy in flowing water current to grind grain in this manner. Hydrostatics, in contrast, is the use of liquids at high pressure and low flow (static means "standing") to perform work. What is the basis for this trade-off in physics language?

Force, Work and Area

The physics underlying the strategic use of hydraulic motors lies in the concept of force multiplication. The net work done in a system is the product of the net force applied and the distance the object of the force moves: Wnet = (Fnet)(d). This means that for a given amount of work allotted to a physical task, the force needed to do it can be reduced by increasing the distance involved in the force application, as can be done using the turns of a screw.

This principle extends from linear to two-dimensional situations, and from the relationship P = F/A, where P = pressure in N/m2, F = force in newtons and A = area in m2. In a hydraulic system in which pressure P is held constant that has two piston cylinders with cross-sectional areas A1 and A2, this leads to the relationship

F1/A1 = F2/A2, or F1 = (A1/A2)F2.

This means that when output piston A2 is larger than input piston A1, the input force will be proportionally less than the output force. While this isn't quite the same as getting something for nothing, it's a clear asset in a lot of contemporary motor set-ups.

Electric Motor Basics

An electric motor makes use of the fact that a magnetic field exerts a force on moving electric charges, or current. A rotating coil of conducting wire is placed between the poles of an electromagnet in such a way that the magnetic field induces a torque that causes the coil to rotate about its axis. This rotating shaft can be used to do work of various types, and overall, electric motors convert electrical energy into mechanical energy.

Hydraulic Motors: Discussion Types

The prime mover of a hydraulic motor is a pump that pushes against the liquid (often oil) in the pipes of the system. This liquid is incompressible, and pushes in turn against a piston inside a cylinder that has hydraulic fluid on both sides of it.

The piston moves and is converted "downstream" into rotational motion, while the fluid on the output side of the piston is continually returned to a reservoir. Pressure is maintained constant in the system (unless it needs to be changed to affect the outputs of the motor) by the strategic distribution and timing of valves.

Types of hydraulic motors deployed in different situations include external gear motors, axial piston motors and radial piston motors. Hydraulic motors are also used in some kinds of electric circuits as well as in pump-motor combinations.

Hydraulic vs. Electric Motor: Pros and Cons

Why use a hydraulic motor vs. a gas engine or an electric motor? The advantages and disadvantages of each type of motor are so numerous that every variable in your own unique scenario needs to be considered.

Advantages of Hydraulic Motors:

The main advantage of hydraulic motors is that they can be used to generate extremely high forces in relation to input forces. This is analogous to the situation in ordinary (non-hydraulic) mechanics where the geometry of levers and pulleys can be "worked" to similar benefit.

Hydraulic motors work using incompressible liquids, which allows for tighter control of the motor and thus a greater degree of accuracy in movement. They are very useful for heavy mobile equipment (e.g., trucks).

Disadvantages of Hydraulic Motors:

Hydraulic motors are usually the priciest option. With all the oil typically in play, they are messy to operate, with their various filters, pumps and oil all requiring checks, changes, cleaning and replacements. Leaks can produce safety and environmental hazards.

Advantages of Electric Motors:

Most hydraulic set-ups are not fast-moving. Electric motors are far faster (up to 10 m/s). They have programmable speeds and stop positions, unlike hydraulics, and provide high positioning accuracy where needed. The electronic sensors can provide precise feedback on the motion and force applied, allowing for superior motion control.

Disadvantages of Electric Motors:

These motors are complicated to install and troubleshoot compared to other motors. Mostly, their disadvantage is that if you need a lot more force, you need a significantly larger and heavier motor, unlike the case in hydraulic motors.

A Note on Pneumatic Activators

The question of pneumatic vs. electric actuators or hydraulic actuators also comes up in some situations. The difference between pneumatic and hydraulic actuators is that hydraulic motors employ liquids while pneumatic actuators make use of gases, typically ordinary air. (Both liquids and gases, for reference, are classified as fluids.)

Pneumatic activators are advantageous in that air is essentially everywhere (or at least everywhere humans are working comfortably), so an air compressor is all that is needed for a prime mover. On the other hand, these motors are very inefficient because of the comparatively large losses owing to heat versus other motor types.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.