By Josh Cosford, Contributing Editor
Comparing electrical with hydraulics is one of my favorite topics to write about. When you study both fields, you quickly realize how much the concepts, mathematics and symbology coincide. However, not all readers share my same enthusiasm for the analogy. Many years ago, one LinkedIn member commented that I was writing too much about the topic and that the two fields aren’t even comparable (to which I’m sure your author challenged proof to the contention, which I did not receive).
Before we get into a deep comparison between electric and hydraulic motors, let’s first review each concept and what makes them similar. Primarily, they’re both systems that transmit energy remotely and then convert it into mechanical force. They both provide precise control using various input methods. And they employ similar elements to achieve work: power source, transmission, control and actuators.
Looking at similarities
But it gets more precise when you dig deeper into two concepts. Let’s first discuss the nature of how each creates work. Both technologies use a medium that “pushes” energy through conduits. Electrical voltage is the potential differential that drives the flow of electrons and the force to push through a conductor. Hydraulic pressure is also the force exerted to move hydraulic fluid through conduits, allowing flow to occur. Remember, flow occurs because pressure allows it, and pressure is not the resistance to flow, contrary to some myths.
We can also compare amperage and flow rate with the understanding that voltage and pressure drive electron and oil molecule movement. The total rate of electron flow through a conductor defines amperage, while the volume of hydraulic fluid through a pipe, hose or tube results in measured flow. Amperage is universally expressed in amperes (or amps for short), while flow can be expressed as either liters per minute or gallons per minute, depending on your geography.
The analogy continues with various calculations commonly (and not so commonly) used in each field. For example, the legendary Ohm’s Law states that voltage drives current through a resistance and is expressed as V = IR. Although not a common expression in hydraulics, you could also posit that P = QR, where P is pressure, Q is flow, and R is the hydraulic resistance. Hydraulic resistance is more commonly referred to as backpressure or pressure drop, but you could get fancy and use this formula to calculate flow through an orifice:
I’m assigning no homework here, but if you wish to play around with the numbers:
Q = Flow in gpm
Cd = Discharge coefficient
A = Area of orifice in square inches
ΔP = Pressure drop across the orifice
р = Density of the fluid in slugs per cubic foot
Okay, I get it. Nobody in the fluid power ‘hood is using that formula, but I wanted to highlight the similar concepts of potential. In either case, the higher the voltage or pressure upstream of a resistance/restriction, the higher the amps or flow out the other end.
For a more precise comparison, let’s look at power. Simply, both formulas look like this:
Force x Moving Stuff = Power
For example, 24 volts x 2 amps = 48 watts
For example, 3000 psi x 5 gpm / 1714 = 8.75 horsepower
If you’re not convinced yet by the apparent physical similarities between electrics and hydraulics, please consider the following examples. To flow more electrons or oil, you need larger conductors or hoses. Any energy used in either system before achieving useful work is wasted as heat. The components used in either circuit are essentially the same function.
Let me elaborate on that last point a bit more. As a fluid power professional, I’ve discovered that electricians and electrical engineers quickly absorb hydraulic knowledge not only because of everything I’ve discussed so far but also because the components and their circuit symbols are relatively similar.
In hydraulics, we have a check valve. In electrics, we have a diode. Both allow flow in one direction while blocking in the reverse. In hydraulics, we have a flow control. In electrics, we have a resistor. Both restrict the flow of either oil or electrons, respectively. There are so many examples, in fact, that perhaps a list is more appropriate:
Hydraulic pump = power supply
Cylinder = linear motor
Accumulator = capacitor or battery
Lever valve = switch
Solenoid valve = transistor
Pressure-reducing valve = voltage regulator (or even Zener diode, perhaps?)
Additionally, many of the above technological cousins employ symbology that looks similar when you squint hard enough. Figure 1 shows four such symbols familiar to industry professionals. Modern circuit schematics do a great job of visually expressing the role of a component through simple lines and shapes. For the diode and check valve, it’s easy to see how their shapes allow for flow in the left direction only.
Looking at motor designs
I hope you now agree that the principles of electrics and hydraulics are pretty similar, so let’s get onto the subject at hand — electric versus hydraulic motors. Essentially, each device converts its fundamental energy into mechanical force in the form of torque (which is just force applied at a perpendicular distance from an axis). Other than an output shaft to appear similar, the two motors achieve force vastly differently.
An electric motor must first create a magnetic field, and it’s that field interaction that results in torque. They may use combinations of permanent and electro-magnets with various methods to generate and control magnetic fields. Electric motors may use Direct or Alternating Current, or even sometimes both.
A hydraulic motor creates torque when pressure is applied to a surface area suited to rotational motion. That surface area could be pistons, vanes or gears, but the larger the surface area and distance from the axis, the more tangential force created.
As expected, larger motors create more torque, but that’s not the sole defining predictor of torque output. Hydraulic motors are much simpler to analyze for torque efficiency, but perhaps I say that only as a hydraulic professional. The torque output of a hydraulic motor is a combination of pressure, displacement, volumetric efficiency and mechanical efficiency, although factors such as load, viscosity, temperature and speed also play into the equation.
For electric motors, the situation becomes more complex. Mechanically, you must still deal with the elements that create a better “lever,” such as armature length, rotor radius and stator/rotor geometry, but also with properties that affect how much magnetic field you can create. The number, configuration and cross-section of windings, permanent magnet strength, inductance of the windings and temperature all play a role in electric motor torque.
The primary difference between how an electric motor operates compared to a hydraulic motor is the use of magnetic fields. Hydraulic pumps create hydraulic pressure that creates a column of flow pushing into the hydraulic motor, which directly rotates its output shaft. Conversely, electric motors must count upon the strength of their magnetic fields, and how strong those magnetic fields are depends on the variables outlined above.
It’s generally been accurate to say that a hydraulic motor always produces more power than an electric one. For any given industrial electric motor, even today’s premium efficiency options, you can expect them to offer about 0.05-0.1 kilowatts of power per kilogram of motor weight. However, when you start talking about high-performance servo motors, you can expect to achieve up to 2kW/kg power density, which is pretty darn good.
If you’re a fan of electric vehicles and have read up on them, you’re probably aware there is a battle raging for electric motor power density supremacy. With the rapid innovation we’ve experienced in materials, construction and design, electric motors for vehicles have increased power density exponentially, and emerging technologies are pushing the boundaries toward hydraulic levels. The axial-flux motor, for example, has many startups producing models with over 10 kW/kg power density, and you can expect to see these in vehicles soon.
But is electric motor technology finally beating hydraulics? Not quite yet. Realistically, nobody in the fluid power industry necessarily focuses on power density as an end goal. We’re just out here making efficient and powerful actuators that are power-dense by default. You won’t find kW/kg in any hydraulic motor catalog either. But if you did, there would be some clear winners.
The bent-axis piston motor is hard to beat for power density. The Rexroth A2FM5 creates 10.35 kW/kg, or 26 kW or about 35 horsepower, continuously without breaking a sweat. No special cooling, exotic materials or technological advancements are required, and the modern design is based upon the technology Rexroth developed in the 70s. I know I’ve mentioned this motor series in the past, and I don’t mean to ignore all the other manufacturers here, but I’ve yet to see someone push the technology to this level.
“So what?” you might be saying, electric motors are already matching the A2FM’s little 5cc motor. Yes, maybe – but are they doing it in a housing that can fit into your empty Venti coffee cup? Indeed, this little motor is only six inches long, weighs just over five pounds and kicks out 35 horsepower.
And if you thought that was impressive, let’s talk about their 16 cc version of this motor. At just over twice the weight and melting faces with 115 horsepower continuous, it’s pegging the power density meter at nearly 20kW/kg. Will electric motors ever achieve this kind of power density? It’s hard to say — magnetic saturation prevents materials from improving much past a certain point, and efforts to increase power result in the law of diminishing returns. Plus, it’s just as likely that we’ll be unable to cool higher-density motor units, limiting how much power we can make in a small package.
Ultimately, hydraulic motors are like the old boxer fighting in long pants and a white tank top. The axial flux motor is the young mixed martial artist getting all the attention for all the right reasons. But when push comes to shove, the axial flux motor will be surprised by the unexpected dad strength, experience and unwavering demeanor of the hydraulic motor, who has lots of fight left in him.
Filed Under: Components Oil Coolers, Engineering Basics, EV Engineering, Mobile Hydraulic Tips, pumps, Pumps & Motors, Technologies
