By Josh Cosford, Contributing Editor
I never thought I’d be writing this article, even if you asked me a year ago. Mobile hydraulic machines are often high-powered beasts with insatiable power demands that traditionally disqualified battery power. However, the proliferation of electric passenger vehicles has funnelled investment dollars into battery technology, which has trickled down into other industries.
By removing the powertrain and ancillary components from an excavator, for example, and replacing them with compact electric motors and battery packs, the weight gain is nominal at worst. Placing the heavy battery packs at the back of the housing not only helps balance against the load but also places them where the packs are easily replaced with fresh ones if your machine is so equipped. Depending on the machine and work effort, you can see four or five hours of battery life, although, with intelligently applied design, you can expect extended battery swaps or reduced charge frequency.
Let me be clear that, whenever possible, these recommendations should be applied to every machine. Not considering the impact of wasted energy on both our environment and your pocketbook is simply ignorant. Luckily, all the big players in the mobile machinery game have been well aware of both regulatory and customer needs, and machines are already highly efficient, all things considered. But if you’re new to the industry, this is what you need to know.
Although it sounds easy when you say it fast, achieving your work task while using as few electrons as possible is your goal here. In other words, the work being done by your machine must be as closely matched in power as the combination of voltage and amperage coming from the battery pack. Respecting this efficiency relationship ensures your machine completes the most amount of work possible before running dry of battery charge.
Machinery idling is never a good thing
For starters, we can take a lesson from vehicle technology, especially the part where motors only turn while required. It seems we’ve previously accepted the fact that when the operator enters the cab, she’ll turn the key and let the engine run for the workday. We can take a lot from passenger vehicles, where EV’s electric drive motors are powered only when loaded to drive. Heck, even most new ICE (internal combustion engine) cars have automatic engine stop/start functions where the rpms drop to zero during stops. Why can’t mobile machinery offer the same utility to shut down the powertrain when demand is zero and utilize a high voltage starter is beyond me. But I digress — this is about electric-only machinery.
Just as how no current is directed to the drive motors of an EV until the throttle pedal is depressed, the electric motor-driven pump(s) of your battery-powered machine remains idle until a button, lever or joystick is actuated, which offers myriad benefits. Even a motor powered with no load still uses a surprising amount of power as it operates outside its most efficient speed and current draw range, so cutting power during no-load operation keeps the electrons snug in the battery.
Even if an idling motor were efficient, spinning a hydraulic pump at idle is definitely not. For the same reason engine stop/start technology saves energy, so too does shutting down your hydraulic pump when you don’t need it. Even a pressure-compensated pump still requires power to idle with no load, as its rotating mass, friction, leakage and pilot energy pull anywhere from 3-10% of the total capacity of the pump. Saving battery power is all about splitter hairs, and this is giraffe hair we’re talking about here.
If you’re skeptical about the amount of energy used by an idling pump, remove the case drain line at the reservoir side, pour it into a bucket and watch the amount of fluid lost without first offering any work. You’ll likely notice significant heat emitting from the fluid as it pours because any hydraulic fluid pressurized and then lost before creating work is surrendered as pure heat.
Sophisticated electronics to the rescue
You might be surprised to learn that we can now recreate the load-sensing, pressure-compensated hydraulic pump using sophisticated electronics. Consider what a pressure-compensated pump does to vary flow rates while maintaining pressure. The compensator is essentially a tiny relief valve that dictates the operation of the control piston, which decreases pump volume until pressure stabilizes at any given flow rate.
Imagine your pump is set to 3,000 psi and rated for up to 10 gpm of flow. When you actuate a valve, the open flow path creates a (slight) reduction in outlet pressure as fluid is directed to the actuator needed. The pump will force fluid from the outlet port to achieve what is essentially 3,000 psi of potential, whether the load pressure needs full pressure or not. This is why some people call these “constant pressure” systems. Should the flow demand exceed 10 gpm, the pump drops to load pressure until the downstream flow is reduced or stopped.
If, instead of using a pressure-compensated pump, we used a fixed-displacement pump driven by a variable-speed electric motor, we can achieve the same effect by adding a pressure transducer at the pump’s outlet. Your best bet is to use a compact, synchronous servomotor, which has low inertia and precise control over speed and torque. Either way, instead of controlling pump volume at constant motor speed, we control pump flow by varying angular velocity (rpm).
So now the machine controller (PLC) looks for 3,000 psi at the pump’s outlet, and any time pressure is less than the set point, it speeds up the pump to stuff more flow into the system to raise the pressure. Any time there is a sudden drop in pressure as a result of increased flow demand, the controller simply speeds up the pump to feed the demand.
The result of such a system is that the motor spins only when needed and as much as needed. I won’t pretend that servomotors are just as efficient at partial load and rpm, but it certainly beats just running an engine at 2,500 rpm for a 12-hour workday.
Variable displacement technology takes it further
The astute fluid power aficionados will undoubtedly point out that we can take this a step further by using a load-sensing hydraulic pump rather than a variable-speed fixed-displacement pump. The upside to a load-sensing pump is the reduced waste at the top end of pressure, which, instead of forcing 3,000 psi into the system when it may need less, simply provides around 300 psi extra oomph for the purpose of flow potential. It otherwise reads the pressure of work ports downstream of the valves to confirm the pump provides only enough pressure to supply the highest load and no more.
I’m hoping you same aficionados realized in the previous paragraph that we can achieve the same result using a network of pressure transducers instead of check and shuttle valves. By installing pressure transducers at the outlet ports of the control valves, you can measure the pressure drop as compared to the pump outlet sensor. Any valve or restriction along the way will be included in the pressure calculation, but primarily, the control orifice will need to be adjusted to suit the desired flow rate. But since a machine controller is doing all the work here, you can even have individual pressure drop programming for each actuator, even including a differential cylinder. Rather than feed the same flow rate (as dictated by pressure drop) to the rod side of a cylinder, you can attenuate flow so the same extend and retract velocity is electronically controlled. Yes, this can be done hydraulically, but it’s just so damn easy with electronics.
Bringing all the pump discussion together into one paragraph, we now have an on-demand flow system that creates only enough hydraulic energy as is required by the machine. When no function is required, only enough battery power is consumed to power the displays, lights and controllers.
Now that we’ve reduced the need to run an engine needlessly for hours on end, we can also consider ways to reduce the overall electrical demand of our hydraulic system. Low-wattage solenoid coils make for a quick win, as does a high-voltage control system. Both upgrades reduce the need for heavy gauge wire, reducing overall machine weight.
It may be less obvious to make the hard decisions, such as obsoleting anything but piston motors. Although on the pump side of things, an inside gear pump makes a great alternative to the piston style, on the actuator side of things, the alternatives kind of suck. Let it be known that low-speed, high-torque motors have budget-friendly designs that have no place in a machine that prioritizes efficiency. Their surplus of leakage that increases exponentially with pressure makes them poorly suited for battery-powered machinery.
“Why not just use electric motors?” you might ask. These are a solid option in many cases, as they avoid the 15-20% penalty of the motor-pump-motor series of energy transformation, each of which produces some waste. But if you already have a hydraulic pump running efficiently through smart machine controllers, there are advantages to something like hydraulic drive motors.
Some of the top manufacturers produce piston motors capable of 95% efficiency, and even though I risk looking hypocritical after mentioning low-wattage coils, these motors are simply too good to ignore. If your machine absolutely requires the most powerful and compact motor possible, you can achieve twice the torque in half the size on a reasonable budget.
That being said, switching to an efficient electric motor, in many cases, will still improve battery life. I can’t believe those words just came from my fingers, but “times they are a changin’.” There are plenty of electric motors that are less efficient than 95%, so be mindful of what you replace a hydraulic motor with, especially if your design requires weatherproof and/or submersible capability.
Many of the big players in the construction and agriculture game are already making battery-powered electric machines. Just as with EVs, they’re expensive and with limited capabilities with current technology. However, with intelligent design and attention to energy-saving detail, you can expect battery-powered electric machinery to grow in market share.
Filed Under: Engineering Basics, EV Engineering, Mobile Hydraulic Tips, pumps, Trending
