In recent years, we are witnessing the introduction of electric drives and components on operating machines and tractors. The use of electric drives on operating machines has many advantages in terms of regulation and energy efficiency, while the electrification of tractors still has many critical issues
The use of electric motors in agriculture (as well as in the automotive sector) was first experienced in the early 1900s, when the introduction of the first tractors led to experimentation with a wide variety of technological solutions. Due to numerous technical limitations, mainly related to the absence of accumulators suitable for use in agriculture and the need to power electric motors via cable, these solutions have been supplanted by the use of endothermic engines. Nowadays, the use of electric motors on agricultural machinery is back in vogue thanks to the introduction of brushless motors, which guarantee greater reliability and a wide range of speed variation. These engines, powered by the tractor’s electrical circuit (therefore powered by the alternator), can be installed wherever it is necessary to act with high precision. The new frontier of mechanization, on the other hand, are electric tractors, which, however, still have to solve many critical issues that limit their professional use.
Electric drives on operating machinery
An excellent example of the application of electric drives on operating machines is certainly the use of electric motors on precision seed drills. In this case, there can be several electric drives in the seeding elements: the rotation of the seeding disc can be performed by a direct drive electric motor, while pressure and/or vacuum can be generated by pumps driven by electric motors. In this case, the advantages are greater seeding precision with a reduction in failures and multiple seeding, thus ensuring higher operating speeds. In addition to these advantages, the use of electric drives allows the installation of electrically driven seeding modules without the need for a power take-off connection.
In addition to the seeders, electric drives can also be seen on machines used for crop protection. In fact, many manufacturers have started to install electric fans to generate the flow to transport the phytoiatric mixture to the crop. In this case, in addition to the ability to precisely adjust the air speed so as to ensure perfect penetration into the vegetation, while at the same time reduce drift, the use of electric fans makes it possible to quickly interrupt the air flow at the headland during turning phases. On machines with ISOBUS functionality, this check can also be performed automatically, without the need for the operator to intervene manually. In addition, the absence of cardan joints connecting the hydraulic pump and fan allows to reduce overall dimensions and increase the volume of transportable mixture. Also, with regard to sprayers, some models equipped with nozzles with PWM (pulse with modulation) valves are available. These solenoid valves are able to modulate the quantity of product sprayed and are applied in the presence of sensors (sonar or laser) capable of detecting the volume of foliage to be sprayed.
Smart Synthesis sprayer by Caffini with electric fan and PWM solenoid valves
In addition to these applications, other applications are being developed: an example are the shredder E-Mulcher and the sprayer E-Sprayer by Nobili, which are electrically-driven operating machines powered by a 700 V connection supplied by an alternator carried on the front lift of the tractor. Although these applications always involve the electrical components being powered by the energy supplied by the tractor’s alternator (and therefore by diesel), it should be remembered that power transmission via mechanical components (cardan joints, pulleys, gears) has varying efficiencies depending on their number and position. Similarly, the use of hydraulic transmissions causes inefficiencies due to leaks (of design and use) that reduce energy efficiency and cause power losses. In contrast, the electrical potential drop in the transmission of electrical energy, although a function of distance and voltage, is almost negligible over such short cable lengths as in the case of agricultural machinery.
In the case of the use of mounted alternators to power electrical components, it should be remembered that in this configuration the endothermic engine can operate at constant operating speeds, guaranteeing maximum fuel use efficiency.
Electric tractors
In addition to applications on operating machines, electric motors are beginning to be applied as propulsion systems on tractors and self-propelled machines. These propulsion systems can be assisted or not by endothermic motors, guaranteeing different performances in terms of fuel economy and autonomy. The type defined as Mild Hybrid involves a conventional endothermic engine and an electric motor with a small accumulator. The latter recharges at high engine speeds and provides energy at low engine speeds. The drive in Mild Hybrid engines is therefore never fully electric. In contrast, the Full Hybrid type allows fully electric propulsion, even if only for limited periods. In fact, in this case there is a dedicated alternator powering a small battery system. Some tractors can be equipped with a cable charging system, which allows the accumulators to be recharged via the mains. In this case, the drive is called Plug-in Hybrid. These types of electrification systems for agricultural machinery are being developed by many manufacturers, who plan to install them on small tractors. Stage V/Tier 4 regulations have very stringent emission limits, which force designers to introduce complex and bulky filters and exhaust gas recirculation systems, which are hardly suitable for small tractors with limited engine capacity. Therefore, (hybrid) electrification may be a solution to reducing emissions in small tractors.
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Fully electric tractors
The most advanced level of electrification is represented by fully electric tractors equipped with large accumulators, mains recharging cable and no endothermic engine. In addition to the essence of the endothermic engine, fully electrically powered tractors and self-propelled operating machines make it possible to simplify vehicle architecture by reducing the mechanisms required to transmit motion. In fact, electric motors are able to develop high torque even at very low engine speeds, ensuring efficiency even when variable tractive effort is required. Although all-electric tractors make it possible to almost completely emancipate agricultural mechanization from the use of fossil fuels, there are numerous critical issues related to these solutions. The accumulators so far developed and used for electric mobility (mainly lithium-ion with liquid electrolyte or solid state batteries) have an energy density of around 0.2 – 0.3 kWh/kg, while the specific energy of diesel is 2.5 kWh/kg. However, it must be considered that endothermic engines are only able to convert into motion only a part of the chemical energy contained in diesel fuel (around 40%) due to thermal energy losses. An electric engine, on the other hand, can convert about 80% of the energy it receives into motion, with obvious advantages in terms of efficiency.
The main criticality of a fully electric propulsion system therefore remains the storage system. Although there are continuous innovations in accumulator technologies, it will still take a long time before we see accumulators suitable for use in agricultural machinery (both in terms of specific energy value and cost). In addition to this, it is necessary to rethink the electricity supply system at the level of national and European infrastructures, which will have to be adapted for the increased demands associated with the electrification of the agricultural sector.
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AEF standards for electrification
Under the auspices of the Agriculture industry Electronics Foundation (AEF), manufacturers of agricultural machinery and equipment have created a standard for electrical power transmission between tractor and implement. This standard (AEF High Voltage 030 – ISO 23316) provides for the transmission of electrical power up to 150 kW in 480 V alternating current or 700 V direct current. The standardized connector provides for high safety standards such as the complete isolation from the chassis of active lines and the use of interlock pins that instantaneously interrupt the current flow if disconnected.
Marco Sozzi
