Drive systems according to their working principle
Electric drives have many excellent properties that make them a preferred choice [1]. However, there are other types of drives based on other physical principles that also have their own strengths. In this blog post we compare hydraulic, pneumatic, thermodynamic and electric drives.
Note that the benefits and downsides are always somewhat relative. What is a major advantage in one application can be totally irrelevant in another application and vice versa. The evaluation also strongly depends on the power/torque/force range that we are looking into.
In this article we use the terms drive and actuator practically as synonyms. In fact, an actuator is a device converting certain form of power into motion. Therefore, an actuator is a fundamental part of a drive (system).
Hydraulic drives
In hydraulic systems the power is transmitted through the hydraulic fluid. Hydraulic system as an actuator is a preferred choice in heavy duty applications with extreme forces or pressures. We can find them in construction machinery, mining equipment (e.g. excavators), forge presses, marine propulsion, special manufacturing equipment or transport. They drives are simply used when a brute force is required. Hydraulic drive transformers pressure into motion by using a pressurized fluid. The fluid has negligible compression. Most common fluid is a hydraulic oil. Basic components of a hydraulic drive are: cylinders with pistons, gear and vane. Hydraulic actuators achieve the highest force and power density (horsepower-to-weight ratio).
- High power density / best power/weight ratio
- Very high forces can be transmitted
- Linear movement without the need of additional transmission elements
- Portable (mobile) systems - e.g. truck mounted
- Immune against harsh conditions
- Starting under heavy load possible
- Drive can hold the load
- Contamination of oil as working medium / Use of oil filters required
- Environmental concerns in case of oil leakage
- Higher maintenance effort and cost
- Expensive control systems required to achieve accuracy
- Slow operation
Pneumatic drives
Pneumatic systems use compressed gas, usually air, as working medium. The inlet air is of normal atmospheric and is compressed to a much higher pressure (e.g. 80 atmospheres). The energy is compressed air is converted into mechanical motion (rotational or linear depending on application). A compressor or pump is used to produce the compressed air. Pneumatic drives are popular in handheld power tools or dental equipment.
- Fast actuators suitable for high cycle operation
- Use in hazardous area without any risks (no sparking, no overheating)
- Operation in environments with extreme temperatures
- Max. torque adjustable through operation pressure
- Compressed air as working medium
- Popular for mobile and handheld applications
- Linear movement without the need of additional transmission elements
- High noise during operation
- Shorter lifetime compared to hydraulic or electric drives
- Higher operational cost due to energy used for air compression
- Lower accuracy of speed control
- Less efficient due to pressure losses and the compression in general
- Not economical in very low power range
Thermodynamic drives
Thermodynamic drive may sound a bit unusual. Instead, people use the term thermal machine or heat engine. We just used drive to be consistent with other categories. In any case, these drivers use the kinetic energy of a gas and convert it into mechanical power.
Typical representatives of thermodynamic drives are steam and gas turbines. We don’t go much in detail here as they are described in our variable speed technology series.
- High energy density of the fuel
- Mobile use
- Local emissions / pollution
- High noise level
- Low efficiency
Electric drives
Electric drives use electric motor as an actuator. The machine transforms supplied electric power into mechanical power (torque and speed). Electric motor can be either DC or AC motor [4] whereas AC motor technology clearly dominates in new installations (except for few specific applications). Modern electric drives use motors supplied from power electronic inverters to precisely control the position, speed and/or torque. Variable speed control saves energy, improves productivity and reduces energy cost. Efficiency of the whole drive train is superior and in high power range often exceeds 95% (combined efficiency of input transformer, VFD and electric motor). Electric drives cover the widest power range from tiny microdrives to large multi-megawatt drive systems. Also the nominal speed ranges from few revolutions per minute to high-speed drives with thousands of rpm. The motor can operate at same speed as the driven load (direct drive, gearless drive) or utilize a gear. Electric drives are usually more silent than competitive technologies and do not produce local emissions. Electric motor drives fulfil the most stringent hygienic requirements making them a perfect fit for food and beverage industry.
- Operational readiness (e.g. we don't need to wait until it heats up)
- Widest range of power (milli to mega) and speed
- Easy to use (operation on push button)
- Adjustable motor design with various shapes and mounting options
- Electric actuators can be precisely controlled
- Cleaness of the environment (no pollution, exhaust etc)
- Minimal to none environmental hazards
- Robustness, operational safety
- Short time overloadability (limited through power electronics)
- High torque from standstill/low speed
- Easy to combine with control electronics
- Adjustable for the load machine
- Low noise
- Minimum maintenance
- High efficiency
- Lower power density (especially in comparison with hydraulics)
- Potential higher complexity of variable speed drive system
- Demanding design for hazardous area (explosion proof)
Did we miss some important advantages or drawbacks of mentioned drive systems? What is your preferred type of drive and why? Write it to us. Your feedback is appreciated. Click ‘submit’ button just once.
Hybrid systems
As you could see, each type of drive has specific advantages but also few drawbacks. But why not to select the best from two or three different worlds? Some engineers think this way and design hybrid systems that combine the best from each technology.
Future development
We dare to say that from the described technologies the electric drives are those that develop at highest pace. While the traditional motor design does not change so dramatically at first look (there is also quite some development to be fair, such as various reluctance type of motors, better insulation materials, smart diagnostics or predictive maintenance), the associated power electronics and control systems quickly progress to make electric drive even better, more reliable and more economical.
Summary
Drives are all around us. We use them daily, often even unnoticed. Multiple types of drives exist, using different physical principles. Each category has specific strengths as well as few weak points. While general judgement is not possible, for a specific application one type of drive may fit better than another.
References
[1] 8 reasons that make electric drives superior, MB Drive Services, July 2021, available online, https://mb-drive-services.com/8-reasons-that-make-electric-drives-superior/
[2] Hydraulic vs. Pneumatic vs. Electric actuators, York Precision, available online, https://yorkpmh.com/resources/hydraulic-vs-pneumatic-vs-electric-actuators/
[3] Fluid or Electric Power? Understanding Hydraulic and Pneumatic Motors, Control Automation, available online, https://control.com/technical-articles/fluid-or-electric-power-understanding-hydraulic-and-pneumatic-motors/
[4] AC versus DC drive, MB Drive Services, December 2020, available online, https://mb-drive-services.com/ac-versus-dc-drive/
