SKAI Multi Converter System
- Case study
Low-consumption, low-emission tractors thanks to high-level-integration power electronics
A tractor manufacturer is introducing an electric power supply system for its Premium line of upper power-class tractors. The aim here is to reduce fuel consumption and noise emissions. This development also constitutes the introduction of the technological architecture needed for further electric drive applications in agricultural machinery. At the heart of the new electric power supply system is a high-integration multi-converter system from SEMIKRON.
Challenge / Motivation
Up till now secondary equipment in tractors was connected to the main drive mechanically via gears. This mechanical connection does not enable these functions to operate at the optimum operating point. This leads to poor overall efficiency and consequently to increased fuel consumption and pollutant emissions. The aim was therefore to disconnect the secondary equipment from the main drive; for this purpose a modified generator connected to the main drive was to generate electric power. This energy is then electrically converted to ensure optimum operation of the fan, air compressor, air conditioning equipment and the 14V on-board power-supply.
The following system-level aims were to be met:
- Reduced fuel consumption and pollutant emissions
- Increase in power for main drive thanks to optimised cooling for diesel engine
- Better reliability of the 14V on-board power-supply
- To prepare tractors for future electric equipment
- Greater overall efficiency hand in hand with lower fuel consumption
- Optimised system control and system properties
- New functions, e.g. 230V socket
A high-level-integration power electronics system from the SEMIKRON multi-converter system family was developed in line with customer specifications. The system comprises multiple converters used to control electric current flow under harsh ambient conditions. Different operating modes are possible, e.g. the system can be supplied with electric power by way of a 3-phase generator or an HVDC bus. The system is housed in a water-cooled case (IP67 protection rating) and communicates with the vehicle master controller via a CAN bus. The integrated semiconductor components come from the SEMIKRON’s tried and tested MiniSKiiP family (2nd generation). The signal interface features analogue and digital I/Os to allow for the connection of a wide variety of sensors, e.g. temperature sensors, resolver inputs.
The multi-converter system has a total of four power converters:
- Converter I: 3-phase 40kVA active front end converter
- Inverter II: 3- phase 20kVA drive inverter
- Inverter III: 3- phase 10kVA drive inverter
- DC/DC converter: 14V/300A or 28V/165A
At 1800 r.p.m., a generator flanged directly to the crankshaft produces as much as 20 kW of electric power, which the multi-converter system converts to operate the fan, air compressor and the air conditioning system. The new power supply system is also used to feed the 14 Volt on-board power supply system via a DC/DC converter, almost doubling the output of the on-board power supply. When the vehicle is stationary, electrical equipment can be operated via a 230 Volt outlet or a 230/400 V (5kW) three-phase socket outlet. These are often used in place of mobile power generators. The electric powered air compressor also ensures fast pressure increase even at low engine revolutions; the air conditioning system behaves similarly. The fan speed can be precisely adapted to the cooling power requirements and fan direction can be simply reversed for cooler cleaning. The fan speed, which is dependent on the engine revolutions, helps the combustion engine build up boost power even at just 1250 r.p.m., and at 1600 r.p.m. offers a higher torque than conventional models. The maximum power boost is also higher. The electric drive system used for secondary equipment improves the efficiency of such equipment, resulting in fuel savings.
The new electric power supply system is the basis for the introduction of ultra-precise, highly efficient electric drives for attachment and trailer equipment, as well as in final drive systems. The combination of different technological highlights results in far lower fuel consumption and reduced noise emissions, and ensures that future emissions limits are met into the bargain. All of the power transmission components were developed or optimised in order to improve overall efficiency. In sum, this constitutes a milestone on the path towards ultra-low consumption in upper power class tractors, which are normally prone to high consumption.