Fluid or Electric Power? Understanding Hydraulic and Pneumatic Motors
There are many different kinds of electric motors used for industrial power transfer, but there are also hydraulic and pneumatic motors that can lend advantages in some situations.
Motors come in all shapes and sizes, using different forms of energy to produce a rotational force. In this article, we'll give an overview of motors not typically seen in automation, but that can be very useful in specific applications.
Figure 1. An example of a hydraulic motor. Image used courtesy of Parker Hannifin
A hydraulic motor is similar to a hydraulic pump. Both convert input energy from one form to another, and they even look similar internally, but the differences will prevent a motor from being used as a pump. With a hydraulic motor, pressurized fluid is forced into a chamber with an actuator, gear, vane, or some kind of rotatable surface. The fluid takes the path of least resistance and pushes against the surface, which rotates the output shaft. There are three different kinds of hydraulic pumps with varying versions of the above description.
With the piston motor being the most efficient, you can expect an efficiency rating of between 80% to 90%. Just like electrical motors, friction is a cause of loss, but with hydraulic motors, there can be internal losses due to fluid leakage from wear or poor-quality materials.
Hydraulic Motor Applications
The obvious application or location is with remote equipment, such as front-end loaders, backhoes, mining equipment, or logging equipment. This equipment already has existing hydraulic pumps and actuators, so adding a hydraulic motor can be a simple solution.
Equipment driven by diesel motors in remote locations does not have access to the high voltage that a typical electric motor would require, but they do typically have access to hydraulic systems. Adding a hydraulic motor to an existing system might be more complicated than plugging in an AC electric motor. These hydraulic systems are specifically designed for the number of devices within the circuit, so always consult the equipment manufacturer before altering the hydraulic system.
Figure 2. Remote, heavy-duty equipment relies on hydraulic systems for a large number of functions. Image used courtesy of Adobe Stock
Benefits of Hydraulic Over Electric Motors
The electric motor has been around for a very long time and is used in many applications. Still, there are some benefits to using a hydraulic motor over an electrical motor.
Can Operate in Harsh Conditions
An electrical motor requires venting and is rarely capable of working in harsh conditions. A hydraulic motor is already sealed due to the high-pressure fluid used to drive the rotation. This makes hydraulic motors great for wet or dirty environments.
Start and Stop Under Load
By using fluid as the power source, the motor can be started or stopped under heavy loads. This feature is hard on electrical motors and can cause premature failure.
Produces High Torque in a Small Package
Typically, with electrical motors, you need more coils, voltage, and more venting to get high torque, which results in large bulky motor housings. Or you can use gearboxes to reduce the RPM but increase the torque. Gearboxes take up space in the drive train and require maintenance with additional cost. Hydraulic motors can produce a high amount of torque without requiring gearboxes or large bulky motor housings.
Can Hold a Load
Electrical motors have a stall current draw. This is the amount of current the motor will draw when the motor is at a stalled RPM. This current is typically the max current the motor will draw. With a hydraulic motor, the fluid is incompressible, so you can pressurize the system and close the valves, and the motor will hold the load without drawing any power.
Cons of Hydraulic Motors
While the hydraulic motor has some unique benefits over electrical, they don’t come without some downsides.
Higher Initial Price
If you are only operating a hydraulic motor, many components will also need to be purchased, including hoses, a pump, valves, and a control system. These components can cause a very high initial cost for the system.
Hydraulic systems are notorious for having high maintenance costs. They typically leak hydraulic fluid and can be quite messy. Fluids must be inspected regularly and topped up when necessary.
Louder Operating Noise
Compared to an electrical motor, a hydraulic system is very noisy. This may not be a problem if the system resides outside, but if used indoors, there might be a maximum noise requirement that the system will have to pertain to.
Figure 3. Airplane engine with angled vanes for rotation. Image used courtesy of Unsplash
A pneumatic motor is similar to a hydraulic motor, except for the obvious difference that compressed air is used as the source of energy. Compressed air is directed into a chamber with surfaces that are attached to a rotating shaft. When the compressed air pushes on the surface, the shaft will rotate. The most commonly used air motors are vane, piston, and turbine, with the turbine being the most efficient. Typically a turbine is about 65% - 75% efficient, which is higher than other models of air motors. The turbine doesn’t require lubrication and has less internal friction.
Pneumatic Motor Applications
Pneumatic motors can be used in many applications and industries because of the readily available compressed air and the cleanliness of the system. They are commonly used in hand tools such as pneumatic grinders, drills, and sanders. Dental drills commonly use pneumatics to maintain a very high RPM. Applications that require starting and stopping the motor under load and very high RPMs are best suited for pneumatic motors.
Figure 4. Hand-held tools, like the one shown above, rely on air power for fast rotation in a small size. Image used courtesy of Robert Bosch Tool Corporation
Benefits of Pneumatic Over Electric Motors
Air motors are used in many industries and work better in some applications than hydraulic or electrical ones.
No Risk at Full Power
When using a hydraulic or electrical motor at full power, it may start to overheat, causing premature wear or failure. With air motors, they can be used at full power without the risk of overheating. In some environments, an overheating device could cause a fire.
No Sparks Are Produced
With electrical motors, small sparks are produced when the motor is operating, even under the best circumstances. In some applications, this could cause an explosion or fire. With air motors, there are no sparks produced while the motor is operating.
Higher Power-to-weight Ratio
Air motors have a 5-6 times higher power-to-weight ratio than electrical motors. This makes air motors more applicable to mobile or handheld applications than electrical motors.
Can Operate in Extreme Temperatures
Air motors have been known to operate in temperatures ranging from - 30℃ to 150℃.
Cons of Pneumatic Motors
While pneumatic motors have benefits over electrical, they also have some downsides.
Higher Initial Cost
As with the hydraulic motor, if there is no existing compressed air system, the additional components will cause a higher initial cost, but compressed air systems are fairly common.
Higher Cost of Operating
The air motor consumes compressed air that requires energy to compress. This will drive the cost of operating quite high, compared to electrical motors.
Fluctuating Speed Control
Controlling the speed of an air motor is quite simple, but it is not very accurate. In fact, if the plant air supply fluctuates, the motor's speed and torque will also fluctuate.
Summary of Fluid Powered Motors
There are many different applications to using both a pneumatic or hydraulic motor, as well as many pros and cons. Your application might require in-depth research to ensure you choose the most cost-effective and efficient solution.