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The price of fuel has almost doubled over the last year and a half. Up to this point, fuel has been relatively inexpensive. Most companies weren’t concerned about fuel conservation, fuel efficiency, mpg, or gph (gallons per hour) when operating a piece of construction equipment or a mobile vehicle. That has changed. Now the world has record high fuel prices and fuel conservation and fuel efficiency are openly being discussed.
This has led construction equipment manufacturers and any mobile vehicle suppliers, who normally are using full hydraulic systems, to try to find ways to make their hydraulic systems or mobile vehicles more fuel efficient. We all know about the Toyota Prius hybrid automobile, but what about the Caterpillar D9E electric dozer? There are now more mobile equipment manufacturers moving from full hydraulics to electro-hydraulic systems. A electro-hydraulic system is simply the integration of an electric motor or servo motor to a hydraulic pump, along with the necessary electronics and controls. This sounds somewhat simple to implement, and it can be straightforward, once the basics are understood behind an electro-hydraulic system.
There are similarities between a hydraulic and an electro-hydraulic system. Take, for example, the mechanical makeup of a hydraulic system. There is the oil reservoir, hydraulic pump, and control valves. Driving this could be an IC (internal combustion) engine of some type. The comparable electric system would be a battery (for storage), electric servo motor / generator, an amplifier (or inverter) for controlling the motor and, again, an IC engine. When comparing a hydraulic pump to an electric motor even the formulas for basic HP is similar.
HP = T (in-lbs) x RPM / 63025
HP = Pressure (psi) x Flow (gpm) / 1714
As can be seen, motor torque corresponds to hydraulic pressure, and motor rpm corresponds to pump flow. In a hydraulic-based, mobile system, a diesel engine (or any IC engine) drives a hydraulic pump to create hydraulic power. The hydraulic power is distributed throughout the vehicle. In an electrohydraulic system, the diesel engine will now drive an electric generator, which now generates electric power vs. hydraulic power. This electric power is distributed throughout the mobile vehicle. That electric power is now used to run an electric motor, which now drives a hydraulic pump. The electric power is also used to run any other electric device on the mobile vehicle, such as electric fans, AC units, or power steering pumps. Any excess electric energy is stored in batteries or capacitors for later use.
When a servo motor is provided with electric energy, it converts this electric energy into rotational motion. That same electric motor can also be a generator, whereas rotational energy back-drives the motor, and this rotation is converted into electrical energy. This electrical energy can now be stored in batteries, much like a hydraulic accumulator stores hydraulic energy.
It must be noted that not all mobile vehicles will implement a full electrohydraulic solution.
This is dependent upon the HP needed for a certain action. A typical frontend loader may use an electro-hydraulic solution to raise and lower the bucket. The power required to do this may be in the 50 – 80 HP range. That same vehicle would probably not use electro-hydraulic for vehicle propulsion. Some construction vehicles need power to propel the vehicle well in the 300 – 400 HP range; most electric motors can not supply that power easily.
It is common for most hydraulically experienced engineers to be very familiar with how a hydraulic system is configured, what the terminology is, and how to implement a hydraulic system. Those same engineers may not be familiar how the electric side of a hydro-electric system is configured, what the terminology is, or how to implement an electric motor, drive, and controls on a vehicle. The components of the electric side of hydro-electric will now be discussed.
Electric Motors
AC Induction Technology There are a few electric motor technologies in the market. The most common are AC induction motors, and the other technology is AC servo motors.
Most people are most familiar with an AC induction motor. That motor technology is very old and is by far the most common motor technology out there. It is simple to use. You merely plug the motor into a wall outlet and the motor runs. Most water pumps, house fans, and industrial-based hydraulic power supplies use this technology. The power output of an AC induction motor is from fractional HP to over 1000 HP. The disadvantages of AC induction motors are they are a low energy efficient motor, in the 70% - 80% range at best. Another deficiency of AC induction motors is they have a very poor power density (power (HP) / volume), meaning they are large and heavy relative to the amount of power it can produce. A drive or inverter can be connected to these motors to allow the motors to run at a variable speed. This drive is relatively inexpensive to operate an AC induction motor.
A major advantage to an AC induction motor is its low cost. AC induction motors use outer copper coils and iron bars on the motor’s rotor to cause motor rotation. There are no magnets in an AC induction motor. Due to the absence of magnets, this motor is relatively inexpensive, compared to a permanent magnet AC motor, which will be discussed next. Since there are so many of these motors produced yearly, it is a commodity item.
AC Servo Motor Technology
The permanent magnet AC servo motor, on the other hand, is a newer technology, is very efficient (90%+), and has a very high torque density.— it has a more torque relative to the size of the motor. The disadvantage to this design is it has lower power output. The power output of an AC servo motor is from fractional HP to 100 HP. This is the motor technology currently being produced by Parker’s Electromechanical Automation Division.
If we look at the cutaway of an AC servo motor, the copper coils are to the outside of the motor. The rotor, however, has permanent magnets bonded onto it. It is the magnets and the winding technology of the coils that give the motor its higher torque density compared to an AC induction motor. Yet due to the permanent magnets, the cost of this motor is higher.
Also notice the feedback device to the right of the rotor. All AC servo motors must have a feedback device in order to commutate the motor correctly. This feedback device, along with the necessary circuitry in the amplifier and the permanent magnets, makes this motor design more expensive. The following table compares the power density, weight, and price of the two motor technologies. Note how the power density of the AC servo motor is 2 – 3 times more than the AC induction motor and also 2 – 4 times lighter also. 
However, the AC servo is also 1.5 – 2 times more expensive than the AC induction motor.
Because of the superior power density and the lighter weight of the AC servo motors, this is the motor of choice for the electro-hydraulic vehicles which Parker has been involved in.
AC Amplifier / Inverter
An AC servo motor and an AC induction must use an amplifier (or inverter) to run the motors at a variable velocity. The size of the amplifier is dependent upon the current rating of the motor it is connected to. The servo amplifier usually has the same or greater output power than the motor it is attached to. As an example, an AC servo motor that requires 10 amps of rated current would normally have a 10 amp (or larger) amplifier to control it. Servo amplifiers can operate in various voltage levels from 12 - 640 volts DC, and come in various power levels. It is not possible to state a standard voltage for any mobile vehicles due to the variables in each vehicle and vehicle vendor. One vendor may specify 280 volts DC; the next may specify 600 volts DC.
| Motor Technology |
HP |
Power |
Density Weight |
Price |
| |
|
HP / in^3 |
lbs |
|
| AC Induction |
5 |
0,007 |
95 |
$970 |
| AC Induction |
10 |
0,006 |
170 |
$1561 |
| AC servo |
5 |
0,023 |
21 |
$1400 |
| AC servo |
10 |
0,013 |
89 |
$3100 |
However, when specifying an operating voltage range for a mobile system, there is one point to remember: The higher the voltage, the less current that is required. As an example, a function on a mobile vehicle may require 15 kW of power. If it is decided to use a 100 volt DC system, the amplifier must produce 150 amps of current, using the formula:
Kw = volts x ampsHowever, if the system can increase the voltage to 600 volts, the current requirement from the amplifier is now 25 amps. A rule of thumb when deciding to use a servo amplifier is this: “Voltage is cheap, current is expensive.” In other words, the higher the system voltage, the less expensive the amplifier, since it will require less current.
Most servo amplifiers are rated to run in an ambient environment of 40 degrees C. Knowing this, the amplifier temperature must not exceed this rating. This will require special cooling techniques such as forced air, water glycol cooling, or an AC cooling system for the system electronics. Since this device will be installed onto a mobile vehicle, special consideration must be given to the robustness of the servo amplifier. Amplifiers designed for the mobile market are rated for higher shock and vibration ratings. It is imperative that standard industrial-rated amplifiers not be used in mobile applications.
Batteries
Most mobile vehicles must use some type of energy storage device. The most common storage device knows is the battery.
There is no single type of battery technology to use in a mobile hybrid vehicle. There are three common battery technologies in use today: lithium Ion, nickel-metal hydride, and lead acid. Each of these can be graded upon power density, measured in w-hr/Kg (watt-hour per Kilogram), voltage, recharge time, weight, volume, and cost.
The following table compares the power density of all three motor technologies and the cost of each.
| Batty Tech |
Energy W-hr/Kg |
Cost $.Kw-hr |
| Lithium Ion |
140 |
$770 |
| Nickel-metal Hydride |
110 |
$880 |
| Lead Acid |
50 |
$100 |
Design News 8-28-08
Even though lithium ion batteries have a higher energy density, they are almost 8 times more expensive than the Lead Acid. However, the lithium ion has 3 times more energy per Kg than the lead acid, meaning it packs more energy per size. Lead acid batteries are more readily available than nickel-metal hydride and lithium-Ion. A local hardware store carries lead acid batteries, but the other technologies are more difficult to procure.
The batteries themselves are only part of the equation. You must have a way to charge the batteries using a battery charger, and you must have the BMS (Battery Management System) to do that. Independent of the battery technology, there are always multiple batteries in an
electrohydraulic system. In order to have a system voltage of 300 volts DC, and depending on the amount of current to be supplied, a lead acid batter configuration may take more than 25 batteries; a lithium-ion configuration could take more than 100. The BMS determines which batteries need to be charged in the system and it is also able to detect any battery shorts. A short in a single battery will draw all the current from the battery charger, leaving the other batteries to slowly lose their total charge. The BMS requires a connection to every battery terminal, so it does tend to be a wiring issue with a higher voltage system due to the number of batteries that must be used. The BMS is also a technology in itself.
The battery charger can be a simple plug-in unit. However, in certain applications, the conversion of kinetic to electrical energy, such as back-driving the motor, or if the IC engine is turning the generator, will also charge the batteries.There is another electrical storage device called a “super-cap.” This device is a high power electric capacitor (hence the name) and works like a hydraulic accumulator. The reason to use a super cap is it is able to take a recharge very quickly (less than one second) and it is also able to release that stored energy quickly. It is not meant for long-term electrical storage like a battery.
Implementation of an electro-hydraulic system
Now that the major electronic components are known for an electro-hydraulic system (servo motor, drive, batteries), the question must be raised as to why build an
electrohydraulic system. What are the benefits to this design vs. a standard straight engine driven hydraulic system?
There are many benefits, and each benefit has more weight depending upon the actual application:
1) Engine optimization with energy recovery and storage
2) Variable speed, fixed displacement efficiency
3) Power on demand
4) Reduced emissions.
Detile:
Electric motor in hydraulic system
Source: en.usp-ltd.com.ua
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